LIBRARY
OF THE
UNIVERSITY OF CALIFORNIA.
Class
RAMSDEH'S ELECTRICAL MACHINE.
BUNSEN'8 BATTERY
HANDBOOK
OF
NATURAL PHILOSOPHY.
BY
DIONYSIUS LARDNER, D.C.L.
FORMERLY
PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY IN UNIVERSITY COLLEGE, LONDON,
ELECTRICITY, MAGNETISM, AND ACOUSTICS.
EIGHTH THOUSAND.
EDITED BY
GEORGE CAREY FOSTER, B.A., F.C.S.
FELLOW OF, AND PROFESSOR OF PHYSICS IN, UNIVERSITY COLLEGE, LONDON.
WITH FOUR HUNDRED ILLUSTRATIONS.
LONDON:
JAMES WALTON,
BOOKSELLER AND PUBLISHER TO UNIVERSITY COLLEGE,
137 GOWER STREET.
1868.
LONDON: PRINTED BY
8POTTISWOODB AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
ADVERTISEMENT TO THE NEW EDITION.
THE extensive circulation which Lardner's HANDBOOK OP
NATURAL PHILOSOPHY has met with ever since its first
publication, and the large demand for it which still exists,
prove conclusively that it supplies the requirements of a
large number of students of Elementary Physics. Hence,
in preparing a New Edition of the volume which treats of
Electricity, Magnetism, and Acoustics, the Editor, while
endeavouring to bring it into harmony with the best scien-
tific teaching of the day, has adhered as closely as possible,
not only to the arrangement and general plan, but also
to the phraseology of the last edition published in the
Author's lifetime.
The changes which it has been thought desirable to
make have naturally been, in part, by way of addition, and,
in part, by way of substitution and alteration. Among
the more important additions to Book I. are a Section on
the phenomenon of the residual charge of the Leyden
jar, and a Chapter (XIV.) on Sources of Electricity other
than friction. The principal additions to Book II. relate
to Ohm's law of the intensity of currents, the tangent-
galvanometer, the measurement of conducting powers, the
rheostat, ozone, the polarisation of electrodes, the retarda-
tion of telegraphic signals by inductive action in sub-
marine cables, and the laws of the development of heat
in the voltaic circuit. In the same Book, in addition to
vi ADVERTISEMENT.
numerous smaller alterations, Chapter I. has been almost
entirely rewritten, as well as large parts of Chapters III.
and IV. and several Sections of Chapter XIII. The
changes in Books III. and IV. are less extensive, the
most considerable being in Section 647, on the velocity
of sound, in Sections 675 and 676, on the extremes of
high and low pitch, and in Section 694, on the theory
of organ-pipes.
In all cases where the new matter inserted by the present
Editor amounts to one or more whole paragraphs, it is
distinguished by being enclosed between square brackets
[ ] ; but smaller alterations and corrections are not thus
marked, except in a few cases where a slight change of
language produces an important change of meaning.
10th April, 1866.
PREFACE.
THIS work is intended for all who desire to attain an accu-
rate knowledge of Physical Science, without the profound
methods of Mathematical investigation. Hence the expla-
nations ar6 studiously popular, and everywhere accompanied
by diversified elucidations and examples, derived from
common objects, wherein the principles are applied to the
purposes of practical life.
It has been the Author's especial aim to supply a manual
of such physical knowledge as is required by the Medical
and Law Students, the Engineer, the Artisan, the superior
classes in Schools, and those who, before commencing a
course of Mathematical Studies, may wish to take the
widest and most commanding survey of the field of inquiry
upon which they are about to enter.
Great pains have been taken to render the work complete
in all respects, and co-extensive with the actual state of the
Sciences, according to the latest discoveries.
Although the principles are here, in the main, developed
and demonstrated in ordinary and popular language, mathe-
matical symbols are occasionally used to express results
more clearly and concisely. These, however, are never
employed without previous explanation.
A 4
viii PREFACE.
The present edition has been augmented by the introduc-
tion of a vast number of illustrations of the application
of the various branches of Physics to the Industrial Arts,
and to the practical business of life- Many hundred en-
gravings have also teen added to those, already numerous,
of the former edition.
For the convenience of the reader the series has been
divided into Four Treatises, which may be obtained sepa-
rately.
MECHANICS .... One Volume.
HYDROSTATICS, PNEUMATICS, and HEAT . One Volume.
OPTICS ..... One Volume.
ELECTRICITY, MAGNETISM, and ACOUSTICS . One Volume.
The Four Volumes taken together form a complete
course of Natural Philosophy, sufficient not only for the
highest degree of School education, but for that numerous
class of University Students who, without aspiring to the
attainment of Academic honours, desire to acquire that
general knowledge of these Sciences which is necessary
to entitle them to graduate, and, in the present state of
society, is expected in all well educated persons.
CONTENTS,
BOOK I.
Electricity.
CHAPTER I.
ELECTRICAL ATTRACTIONS AND REPUL-
SIONS.
Sect. Page
I. Electrical effects l
Origin of name of electricity - z
z. Positive and n> gative electricity - 3
3. Nature of electricity ... H>.
4. Mode of describing electrical effects ib.
5. Hypothesis of <i single electric fluid 4
6. Hypothesis of two electric fluids - ib.
7. The >econd hypothesis convenient ib.
8. Explanation of the effects already
described ib.
9. Electricity developed by various
bodies when submitted to fric-
tion ---.-- 5
No certain test for determining
which of the bodies submitted
to friction receives positive, and
which negative electricity - - ib.
to. Classification ot positive and nega-
tive substances 6
loa. Both electricities always produced
together . .... 7
II. Method of producing electricity by
glass and silk with amalgam - ib.
CHAP. II.
CON DUCTION.
IZ. Conductors and nonconductors - 7
13. Degrees of conduction - ib.
14. Insulators ----- 8
15. Insulating stools .... ib.
16 Electric^ and non-electrics obsolete
terms ------ ib.
17. Two persons reciprocally charged
with co trary electricities placed
on insulating stools ... 9
18. The atmo>phere a nonconductor - ib.
19. Effect of rarefying the air - - ib.
zo. Use of the silk string which sus-
pends pith balls - - - - 10
n. Water a conductor - - ib.
Sect. Page
zz. Insulators must be kept dry - - 10
zj. No certain test to distinguish con-
ductors from nonconductors - ib.
Z4. Conducting power variously af-
fected by temperature - ib.
Z5. Effects produced by touching an
electrified body with a conductor
which is not insulated - n
z6. Effect produced when the touching
conductor is inflated ... ib.
zy. Why the earth is called the com-
mon reservoir .... t'b.
z8. Electricity passes by preference on
the best conductors ... -iz_
CHAP. III.
INDUCTION.
Z9- Action of electricity at a distance - 12
30. Induction defined - - - - 13
31. Experimental exhibition of its ef-
fects ib.
33 Effects of sudden inductive action - 15
34. Example in the case of a frog - 16
35 Inductive shock of the human body ib
36. Development of electricity by in-
duction - - - - - ib.
CHAP. IV.
ELECTRICAL MACHINES.
37. Description of an electrical ma-
chine 17
Parts of electrical machines - - ib.
The rubber - - - - - ib.
39. The conductors - ... 16.
40. The common cylindrical machine - /A.
Explanation of its operation - 18
41. Nairne's cylinder machine - - ib.
4Z. Common plate machine, known as
Van M-irum's - - - - 19
43. Ramsden's plate machine - - zi
X
CONTENTS.
Sect. Page
44. Armstrong's hydro-electrical ma-
chine .---.- 22
45. Appendages to electrical machines 24
46. Insulating stools .... ib.
47. Discharging rods .... ib.
,8. Jointed dischargers - - - 15
^9. Universal discharger ... t'b.
CHAP. V.
CONDENSER AND ELBCTROPHORCS.
50. Reciprocal inductive effects of two
conductors ... - - 26
51. The condenser - 28
52. Dissimulated or latent electricity - tb.
53. Free electricity .... ib.
54. Construction of condensers - - 29
55. Collecting and condensing plates - iL.
56. Cuthbertson's condenser - - ib.
57. The electrophorus - - 30
CHAP. VI.
ELECTROSCOPES.
58. Electroscopes, their general prin-
ciples ...... 31
59. Pith-ball electroscope - - -31
66. The needle electroscope ... ib.
6t. Coulomb's electroscope - - - 33
62. Quadrant electrometer - - - ib
63. Gold-leaf electroscope - - - 34
64. The condensing electroscope - - ib.
CHAP. VII.
THE LEYDEN JAR.
The principle of the Leyden jar
The fulminating pane ...
Discovery of the electric shock
The Leyden jar ....
Effect of the metallic coatings
Experimental proof that the charge
adheres to the glass and not to
the coating .....
Improved form of the Leyden jar -
Lane's discharging electrometer
Cuthbertson's do. do.
Harris's circular electrometer
Charging a series of jars by cascade
Electric battery ....
Common do. ....
Manner of estimating the amount of
the charge .....
. Residual charge - -
CHAP. VIII.
LAWS OF ELECTRICAL FORCES.
77. Electric forces investigated by Cou-
lomb -_..._.
78. Proof plane -
79. Law of electrical force similar to
that of gravitation -
80. Distribution of the electric fluid on
conductors .....
Sect. Page
81. It is confined to their surfaces - 55
82. Intensity of an electrical charge
upon a conductor less in propor-
tion as the total surface of 'the
conductor is greater - - - 56
83. Faraday's apparatus - - - 57
84. How the distribution of the fluid
varies ..... 59
85. Distribution on an ellipsoid - - ib.
86. Effects of edges and points - - ib.
87. Distribution of electric fluid varied
by induction - - - 61
88. Experimental illustration of the
effects of a point - - - ib.
89. Rotation produced by the reaction of
points ..... 62
90. Experimental illustration of this
principle ... « - 6?
he electrical orrery
The electrical blow pipe
91.
02.
910. Explanation of foregoing effects - 65
- tb.
CHAP. IX.
MECHANICAL EFFECTS OF ELECTRICITY.
03. Attractions and repulsions of elec-
trified bodies .... 66
94. Action of an electrified body on a
nonconductor not electrified - 67
95. Action of an electrified body on a
nonconductor charged with like
electricity - - - - - ib.
96. Its action on a non-conductor
charged with opposite electricity - ib.
on. Its action on a conductor not elec-
trified ib.
98. Its action upon a conductor charged
with like electricity - - 68
99. Its action upon a conductor charged
with opposite electricity - - ib.
ico. Attractions and repulsions of pith
balls explained .... ib.
101. Strong electric charges rupture
imperfect conductors - .69
lot. Curious fact observed by M. Tre-
mery ... - 70
103. Wood and glass broken by discharge ib.
104. Electrical bells - 71
105. Repulsion of electrified threads - 72
ico. Curious effect of repulsion of pith
ball i*.
107. Electrical dance - - - - 73
108. Curious experiments on electrified
water ib.
109. Experiment with electrified sealing
wax -..--- 74
no. Electrical see-saw ... ib.
CHAP. X.
THERMAL EFFECTS OF ELECTRICITY.
Hi. A current of electricity passing
over a conductor raises its tem-
perature -
112. Experimental verification. Wire
ib. heated, fused, and burnt
113. Thermal effects are greater as the
t'J. conducting power is less - - ib.
114. Ignition of metals - ib.
55 115. Effect of fulminating silver - - 76
CONTENTS.
XI
Sect. Page
116. Electric pistol - 76
117. Ether and alcohol ignited - - 77
1 18. Resinous powder burned - - 78
119. Gunpowder exploded - - - ib.
120. Electric mortars - - - ib.
121. Kinnersley's thermometer - - ib.
CHAP. XL
LUMINOUS EFFECTS OP ELECTRICITY.
izi. Electric fluid is not luminous
123. Conditions under which light is
developed by an electric current
124. The electric spark -
124/1. Duration of the spark ...
12$. Electric brush
120. The length of the spark
127. Discontinuous conductors produce
luminous effects ....
128. Various experimental illustra-
tions ......
129. Effects of rarefied air -
130. Experimental imitation of the
auroral light -
131. Phosphorescent effect of the
spark > .....
132. Lichtenberg's figures -
1 33. Experiments indicating specific
differences between the two
fluids
134. Electric lightabove the barometric
column -
135. Cavendish's electric barometer -
136. Luminous effects produced by im-
perfect conductors ...
137. Attempts to explain electric light.
The thermal hypothesis -
138. Hypothesis of decomposition and
recomposition -
139. Cracking noise attending electric
spark -
CHAP. XII.
Sect. Page
140. Electric shock explained - - 88
141. Secondary shock ... - 89
142. Effect produced on the skin by
proximity to an electrified body - ib.
143. Effect of the sparks taken on the
knuckle ib.
144. Methods of limiting and regula-
ting the shock by a jar - - ib.
145. Effect of discharges of various
force ib.
146. Phenomena observed in the ex-
amination after death by the
shock i*.
147. Effects of a long succession of
moderate discharges - - - 90
148. Effects upon a succession of
patients receiving the same dis-
charge - - - - - ib.
149. Remarkable experiments of Nol-
let, Dr. Watson, and others - ib.
CHAP. XIII.
CHEMICAL AND MAGNETIC EFFECTS OF
ELECTRICITY.
150. Phenomena which supply the
basis of the electro-chemical
theory .... 91
151. Faraday's experimental illustra-
tion of this - - - - - ib.
152. Effect of an electric discharge on
a magnetic needle ... ib.
1 53. Experimental illustration of this - 92
154. Effect of an electric discharge on a
suspended magnet - ib.
CHAP. XIV.
SOURCES OF ELECTRICITY.
155. Sources of electricity classified - 93
156. Mechanical sources of electricity - i(>.
157. Development of electricity by heat 94
BOOK II.
Voltaic Electricity.
CHAPTER I.
SIMPLE VOLTAIC COMBINATIONS.
Sect. Pape
158. Discovery of galvanism - - 96
159. Galvani's theory -
160. Volta's theory
161. Electromotive force - - • 99
162. True explanation of results above
described - .... IQO
163. Development of electricity by
chemical action ... jot
164. Formation of an electric current - 102
165. Direction of the current - -103
: X
Sect. Page
166. Chemical changes accompanying
the production of the electric
current - - - - - 103
167. Effect of connecting the plates - 104
168. Direction of the current through
the liquid 105
169. The galvanic current is a circula-
tion of electricity ... ib.
170. Power of various galvanic com-
binations 106
171. Electro-chemical series - - ib.
172. Necessity for using a liquid In order
to produce a galvanic current - io»
XII
CONTENTS.
Sect
173-
174.
176
58-
181.
182.
183.
Page
A galvanic current may be pro-
dncea by the mutual action of
liquids ..... 108
Production of a current by the
combination of two gases - - 109
Conditions needed lor the pro-
duct ion of a constant current - ib.
Smee'sN system - no
Daniell's system - - - - in
Chemical theory of a Daniell's
cell
- 113
- 115
no
. ib.
184.
185.
186.
187.
188.
189.
190.
191
192.
194.
195.
196.
197.
199.
200.
201.
202.
203.
204
205.
200.
207.
208.
Grove's system -
Hanson's system -
Wneatstone's system
Bagration's system
Becquerel's system
CHAP. II.
VOLTAIC BATTERIES.
Volta's invention of the pile - 117
Explanation of the principle of
the pile - - - - - 118
Pole* of the pile - 119
Volta's first pile - - - - ib.
The couronne des tasses - - ib.
Cruii- shank's arrangement - - 120
Wollaston's anangement - - ib.
Vuncn's battery - - - - 121
Helical pile of Faculty of Sciences
at Paris ..... 122
Conductors connecting the ele-
ments ------ 124
Pile may be placed at any dis-
tance from pUce «'f experiment 125
Memorable piles. Davy's pile at
the Royal Institution - ib.
Napoleoh'f pile at Polytechnic
School - - - - - ib.
Children's great plate battery - ib.
Ha:e's deflagrator - - - ib.
Stratingh's deflagr.tor - - 126
Vepy's pile at the London Institu-
tion ------ ib.
These and all similar apparatus
h »ve fallen into disuse - - ib.
Dry piles ..... ib.
D-luc'spile ..... &•
Zamboni's pile -
Voltaic j<>ux de bague -
Piles of a single metal
Hitter's secondary piles
127
ib.
- 128
- t&
CHAP. III.
VOLTAIC CURRENTS.
209. The voltaic current - 129
210. Voltaic circuit - ijo
211. Case 'in which the earth com-
plefe> the circuit ... ib.
212. Methods "f connecting the poles
with the earth . - - -131
3*13. Various denominations of cur.
rents --.--- ib.
214. The electric fluid forming the
current not necessarily iu mo-
tion 16.
215. Resistance of conductors - - 132
132
ib.
Sect. Page
216. Difference between the electrical
machine and the voltaic battery
217. Laws of voltaic currents - -
218. The intensity of the current is the
same in every part of the same
circuit ------ jj j
219 Relation between strength of cur-
rent, electro-motive force, and
resistance : Ohm's law - - ib.
220. In'ernal and external resistance - 134
221. Effect of increasing the number of
cells ...... ib.
222. Effect of increasing the size of
the plates - - - - - 135
223. Method of coating the conducting
wires ---_.- 156
224. Supports of conducting wire* - ib.
225. Ampere's reotrope to reverse the
current - - - - - ib.
226. Pohl's reotrope - - - - 137
227. Electrodes - - - - - 138
228. Floating supports for conducting
wire ------ ib.
229. AmpSre's apparatus for supporting
movable currents ... ib.
230. Velocity of electricity ... 139
CHAP. IV.
RECIPROCAL INFLUENCE OF RECTILINEAR
CURRENTS AND MAGNETS.
231. Mutual action of magnets and cur-
rents .---._
232. Electro-magnetism -
233. Case of a needle free to oscillate
in a horizontal plane -
234. Rule by which the foregoing effects
may be remembered ...
235. Case of a needle oscillating in a
vertical plane - - - ~
236. Action of a vertical current on a
needle oscillating in a horizontal
plane ......
237. Direction of the force exerted by
a rectilinear current upon each
pole of a magnet -
238. Action of a rectilinear current upon
a magnet free to oscillate about
some point other than its centre
239. Apparatus to measure intensity of
this force - - - - -
240. Intensity varies inversely as the
distance ... _
241. Attractive force exerted upon a
magnet by aconduetor conveying
a current - - . - -
242. A cur lent tends to make a mag-
netic pole revolve round it - -
243. The forces which act between cur-
rents and magnets are mutual -
244 Apparatus to illustrate the electro-
magnetic rotation - -
245. To cause either pole of a magnet
to revolve round a fixed voltaic
current -
246. To cause a movable current to
revolve round the fixed pole of
a magnet - - . _ -
247. Ampere's method - - - -
248. To make a magnet turn on its own
axis by a current parallel to it -
145
144
145
1^5
tb.
147
CONTENTS.
CHAP. V.
RECIPROCAL INFLUKNCE OF CIRCULATING
Sect. Page
278. Conditions on which a needle is
magnetised positively and nega-
Sect. Page
140. Front and back of circulating cur.
279. Results o,"S;ivary's experiments - ib.
280. Magnetism imparted to the needle
250 Axis of a current - il>.
251. Reciprocal action of circulating
substance which surrounds it - 165
281. Formation of powerful electro-
252. Intensity of the force vanishes
when the distance of the pole
bears a very g eat ratio to the
diameter of the current - - 154
253. But the directive power of the
282. Conditions which determine the
force of the magnet - - -167
283. Electro, magnet of Faculty of
Sciences at Paris - - - ib.
284. Forces of electro-magnets in ge-
254 Spiral and helical currents - - 155
255. Expedients to render circulating
currents movable, 155 ; Ampdre
and Delarive's apparatus - - ib.
256. Rotatoiy morion imparted to cir-
cular current by a magnetic pole 156
257. Progressive motion imparted to it ib.
258. Reciprocal action of the current
285. Electromagnetic power applied as
a mechanical agent - - - ib.
286. Electro-motive power applied in
the workshop of M. Froment 168
287. Electro - motive machines con-
structed by him, 170; descrip-
tions of the same - - - 171
287*. The electro motive machine of
259. Action of a magnet on a circular
floating current - ib.
260. Reciprocal action of the current
on the magnet - - - - 157
261. Case of unstable equilibrium of
the current .... ib.
162. Case of a spiral current - - ib.
265. Circular or spiral currents exer-
cise the same action as a magnet 158
264 Case of a helical current -, - ib.
265. Method • f neutralising the effect
of the progressive motions of
288. Applied as a sonometer - - 175
289. Momentary current by induction ib.
290. Experimental illus ration - - 176
291. Momentary currents produced by
magnetic indut tion - 177
292. Experimental illustrations - - ib.
293. Inductive effects produced by a
permanent magnet revolving un-
der an electro-magnet - - 179
294. Use of a contact breaker - - 180
295. Magneto-electric machines - - ib.
296. Effects of this machine, its medical
266. Right handed and left handed
297. Clarke's apparatus - - -183
267. Front of current of each kind - ib.
268. Magnetic properties of helical
currents— their poles determined id.
269. Experimental illustration of these
299. Ruhmkorff s apparatus to produce
currents of tension - 185
300. Stratification of electric light - 186
joi. Peculiar pronerties of the direct
270. The front of a circulating current
has the properties of a south,
and the back those of a north,
302. Statham's apparatus - - - 188
303. Inductive effects of the successive
convolutions of the same helix - 189
171. Adaptation of any helical current
to Ampere's and Delarive's ap-
currents produced upon revolv-
ing metallic discs Researches
272. Action of a helical current on a
magnetic needle placed in its
axis - 161
and Faraday - - - - ib.
CHAP. VI.
ELECTRO-MAGNETIC INDUCTION.
273. Inductive effect of a voltaic cur-
rent upon a magnet, 162; soft
iron rendered magnetic by vol-
taic currents ; sewing needles
attracted bv current - - - 162
274. Magnetic induction of a helical
current - - - - - l6j
275. Pol irity produced by the induction
of helical currents ... /ft.
276. Consecutive points produced - ib.
277. Inductive action of common elec-
tricity produces polarity - - ib.
CHAP. VII.
INFLUENCE OP TERRESTRIAL MAGNETISM
ON VOLTAIC CURRENTS.
305. Direction of the earth's magnetic
attraction ----- 192
306. In this part of the earth it corre-
sponds to that of the boreal or
southern pole of an artificial
magnet - - - - - ib.
307. Manner of ascertaining the di-
rection of the force impressed
by terrestrial magnetism on a
curient - - - - - ib,
308. Ve tical current - - - - 19}
309. Horizontal current in plane of
magnetic meridian ... ib.
310 Horizontal current perpendicular
to magnetic meridian - - ib.
XIV
CONTENTS.
Sect. Page
311. Horizontal current oblique to
magnetic meridian - 193
312. Effect of the earth's magnetism
on a vertical current which turns
round a vertical axis ... 194.
313. Effect on a current vhich is capa-
ble of moving in a horizontal
plane ...... ib.
314. Experimental illustrations of these
effects. Pouillet's apparatus - ib.
315. Its application to show the effect
of terrestrial magnetism on a
horizontal current ... 156
316. Its effect on vertical currents
shown by Arape're's apparatus - ib,
317. Its effect on a circular current
shown by Ampere's apparatus - ib.
318. Its effects on a circular or spiral
current shown by Delarive's
floating apparatus ... iyj
319. Astatic currents formed by Am-
pdre's apparatus - - ib.
320. Effect of earth's magnetism on
spiral currents shown by Am-
pere's apparatus - ib.
321. Effect on a horizontal current
shown by Pouillet's apparatus - 198
322. Effect of terrestrial magnetism
on a helical current shown by
Ampere's apparatus - ion
323. The dip of a current illustrated
by Ampere's rectangle - - ib.
CHAP. VIII.
RECIPROCAL INFLUENCE OP VOLTAIC
CURRENTS.
324. Results of Ampere's researches - 200
325. Reciprocal action of rectilinear
currents - - - - - ib.
326. Action of a spiral or helical cur-
rent on a rectilinear current - 201
327. Mutual action of diverging or
converging rectilinear currents ib.
328. Experimental illustration of this 202
329. Mutual action of rectilinear cur-
rents which are not in the same
plane ------ 2x33
330. Mutual action of different parts of
the same current - ib.
331. Ampere's experimental verifica-
tion of this ----- 004
332. Action of an indefinite rectilinear
current on a finite rectilinear
current at right angles to it - ib.
333. Case in which the indefinite cur-
rent is circular - - - - 205
334. Experimental verification of these
principles ----- ib.
335. Way of determining in general
the action of an indefinite recti-
linear current on a finite recti-
linear current - - . - 206
336. Experimental illustration of these
principles ... - 209 |
337. Effect of a straight indefinite cur-
rent on a system of diverging or
converging currents - ib.
338. Experimental illustration of this
action - - - - - ib,
339. Consequences deducible from this
action ----- no
Sect. Page.
340. Action of an indefinite straight
current on a circulating current 211
341. Case in which the indefinite
straight current is perpendicular
to the plane of the circulating
current ----- n2
342. Case in which the straight cur-
rent is oblique to the plane of the
circulating current - - - 213
343. Reciprocal effects of curvilinear
currents - - - - . ib.
344. Mutual action of curvilinear cur-
rents in general - ... ib.
CHAP. IX.
VOLTAIC THEORY OF MAGNETISM.
345. Circulating currents have the
magnetic properties - - - 214
346. Magnetism of the earth may pro-
ceed from currents - 215
347. Artificial magnets explained on
this hypothesis - ... ib.
348. Effect of the presence or absence
of coercive force - ib.
349. All the phenomena of the mutual
action of magnets and voltaic cur-
rents are explicable on this hypo-
thesis " - ib.
CHAP. X.
REOSCOPES AND REOMETERS.
350. Instruments to ascertain the pre-
sence and to measure the inten-
sity of currents - ... 216
351. Expedient for augmenting the
effect of a feeble current - - 217
352. Method of constructing a reo-
scppe, galvanometer, or multi-
plier ------ ib.
353. Nobili's reometer - 219
354. Differential reometer - ib.
355. Great sensitiveness of these in-
struments illustrated - - - 220
3 s$a. Pouillet's tangent galvanometer - ib.
CHAP. XI.
PHOTOMAGNETISM AND DIAMAGNBTISM.
356. Faraday's discovery - 222
357. The photomagnetic phenomena - ib.
358. Apparatus for their exhibition - ib.
359. Photomagnetic phenomena - - 224
360. Effects on polarised solar light - 225
361. Diamagnetic phenomena - - ib.
362. Diamagnetism of solids - - 226
363. Various diamagnetic bodies - - 227
364. Diamagnetism varies with the
surrounding medium - - - ib.
365. Plvicker's apparatus • az8
366. The diamagnetic properties of li-
quid< exhibited - . 249
367. Diamagueti&m of flame - - 230
CHAP. XII.
THERMO-ELECTRICITY.
368. Disturbance of the thermal equi-
librium of conductors produces
a disturbance of the electric
equilibrium - - - - 231
CONTENTS.
Sect. P»KR
369. Thermo-electric current - - 231
370. Experimental illustration - - ib.
371. Conditions which determine the
direction of the current - -232
372. A constant difference of tempera-
ture produces a constant current ib.
373. Different metals have different
thermo-electric energies . - 233
374. Pouillet's thermo-electric appa-
ratus ...--- ib.
375. Relation between the intensity of
Ihe current and the length and
section of the conducting wire - 274
376. Conducting powers ot metals - 235
377. Wheatstone's method of measuring
conducting powers - ib.
377/1. The reostat ----- 236
378. Equivalent simple circuit - - 237
379. Ratio of intensities in two com-
pound circuits - - - - Z38
380. Intensity of the current on a
given conductor varies with the
thermo-electric energy of the
source - - - - - ib.
381. Thermo-electric piles - - - Z39
382. Thermo-electric pile of Nobili and
Melloni ib.
CHAP. XIII.
383
384.
385.
386.
387.
388.
389.
390.
391.
39Z.
393-
394-
395-
396.
397-
398.
399-
400.
401
403.
404.
405.
ELECTRO-CHEMISTRY.
Decomposing power of a voltaic
current
Electrolytes and electrolysis • - to.
Liquids alone susceptible of elec-
trolysis ib.
Faraday's electro-chemical no-
menclature - - - - - ib.
Positive and negative electrodes - ib.
Only partially accepted - - 242
Composition of water - - - ib.
Electrolysis of water - - - 243
Explanation of this phenomenon
by the electro-chemical hypo-
thesis ib.
Method of electrolysis which se
parates the constituents - - 244
How are the constituents trans-
ferred to the electrodes ? - - 245
Solution on the hypothesis of
Grotthus 246
Effect of acid and salt on the elec-
trolysis of water - t'6.
Secondary action of the hydrogen
at the negative electrode - - 248
Its action on bodies dissolved in
the bath ib.
Example of zinc and platinum
electrodes in water - ib.
Secondary effects of the current - 249
Influence of concentration of the
solution and size of the elec-
trodes - - - - - ib.
Electrolytic classification of the
.simple bodies .... il,.
Electro-negative bodies - - 250
Electro-positive bodies - - t'6.
The order of the series not cer-
tainly determined ... ib.
Electrolytes which have compound
constituents - ib
Sect. Page
406. According to Faraday electrolytes
whose constituents are simple
can only be combined in a single
proportion .... 451
407. Apparent exceptions explained by
secondary action - - - ib.
408. Secondary effects favoured by the
nascent state of the constituents;
results of the researches of Bec-
querel and Crosse ... ib.
409. The successive action of the same
current on different vessels of
water - 252
410. The same current has an uniform
electrolytic power - ib.
411. Voltameter of Faraday - - 253
412. Effect of the same current on diffe-
rent electrolytes*. — Faraday's
law
413. It 'comprises secondary results - f'6.
414. Practical example of its applica-
tion Z54
415. Sir H. Davy's experiments show-
ing the transfer of the consti-
tuents of electrolytes through
intermediate solutions - - ib.
416. While being transferred they are
deprived of their chemical pro-
perty 255
417. Exception in the case of producing
insoluble compounds - - 256
418. This transfer denied by Faraday ib.
419. Apparent transfer explained by
him on Grotthus's hypothesis - 257
420. Faraday thinks that conduction
and decomposition are closely
related 258
421. Maintains that non-metallic liquids
only conduct when capable of
decomposition by the current - ib.
422. Faraday's doctrine not universally
accepted.— Pouillet's observa-
tions ib.
423. Davy's experiments repeated and
confirmed by Becquerel - 259
424. The electrodes supposed to exer-
cise different electrolytic powers
by Pouillet - ... ib.
425. Case in which the negative elec-
trode alone acts ... 260
426. This unequal action of the elec-
trodes is only apparent - - ib,
427. Liquid electrodes.— Series of elec-
trolytes in immediate contact - ib.
428. Experimental illustration of this 261
429. Electrolysis of the alkalis and
earths 262
430. The series of new metals - - tto.
431. Schcenbein's experiments on the
passivity of iron - ib.
432. Other methods of rendering iron
passive 264
433. The tree of Saturn - - - ib.
434. Davy's method of preserving the
copper sheathing of ships - ib.
Peculiar properties of electrolytic
oxygen. — Ozone ... 265
Nature of ozone .... ib.
Effect of ozone in lessening the
quantity of gas evolved in a
voltameter .... ib.
438. Polarisation of the electrodes - 266
439. Reverse current due to polarisa-
tion of the electrodes - - ib.
435-
436.
437-
XVI
CONTENTS.
Sect. Page
440. Chemical processes which take
place in a voltaic battery - 267
441. Amount of chemical action in the
battery - - - - - 268
441. Advantages of using amalgamated
zinc -..--- ib.
CHAP. XIV.
ELECTRO-METALLURGY.
Origin of this art ... 268
The metallic constituent deposited
on the negative electrode - 269
Any body may be used as the ne-
gative electrode - ib.
Use of a soluble positive electrode
Conditions which affect the state
of the metaf deposited - -
The deposit to be of uniform
thickness - - - - - ib.
Means to prevent absorption of
the solution by the electrode - 270
Nonconducting coating used where
partial deposit is required
App'ication of these principles t
gilding, silvering, Ac. - -
Cases in which the coating is in-
adhesive - - - - -
Application to gilding, silvering,
or bronzing objects of art - 271
Production of metallic moulds
of articles - - - - - ib.
Production of objects in solid
metal ...... ft.
Reproduction of stereotypes and
engraved plates ... 272
Metallising textile fabrics - - ib.
Glyphography - ib.
Reproduction of daguerreotypes 273
Galvano-plastic apparatus - - ib.
443.
444.
445.
446.
447.
448.
449.
450.
451.
452.
453.
454.
455.
456.
457.
458
459.
460.
461.
462.
463.
464.
465.
- ib.
- ib.
ib.
avano-a aa - - .
Simple gal vano-plastic apparatus 274
Spencer's simple apparatus - 275
Fau's simple apparatus - - ib.
Brandely's simple apparatus - 276
Compound galvano-plastic appa-
ratus - - - - - ib.
CHAP. XV.
ELECTRO-TELEGRAPHY.
466. Common principle of all electric
telegraphs -----
467. Conducting wires -
468. The construction of telegraphs -
469. Methods for the preservation and
insulation of underground wires
470. Testing posts - - - -
471. Telegraphic signs ...
472. Signs made with the needle system
473. Telegraphs operating by an elec-
tro-magnet - - - -
474. Morse's system - ...
475. Electro-chemical telegraphs -
47ca.Retardation of the current in sub-
marine telegraph wires - -
CHAP. XVI.
CALORIFIC, LUMINOUS, AND PHYSIOLOGICAL
EFFECTS OF THB VOLTAIC CURRENT.
Sect. Page
476. Conditions on which calorific
power of current depends •• - 290
477. Calorific' effects - ... 292
478. Sources of the heat developed by
the current - - - - - ib.
479. Experimental illustration of the
conditions which effect calorific
power of a current - - - .293
480. Substances ignited and exploded
by the current - ib.
481. Applications of this in civil and
military engineering - - - ib.
482. Jacobi's experiments on conduc-
tion by water - - 294
483. Combustion of the metals - - 295
484. Spark produced by the voltaic
current - - - - - ib.
485. The electric light - ib.
486. Incandescence of charcoal by the
current not combustion - - 296
487. Electric lamps of Messrs. Fou-
cault, Deleuil, and Dubosc-
Soleil 097
488. Method of applying the heat of
charcoal to the fusion of refrac-
tory bodies and the decomposi-
tion of the alkalis ... 299
489. Physiological effects of the cur-
rent ------ ib.
490. Therapeutic agency of electricity 300
491. Duchenne's electro- voltaic appa-
ratus ------ 301
492. Duchenne's magneto-electric ap-
paratus ----- 303
493. Pulvermacher's galvanic chain - 304
494. Medical application of the voltaic
shock - - - - - - 305
495. Effects on bodies recently deprived
of life ------ ib.
496. Effect of the shock upon a leech - 306
497. Excitation of the nerves of taste - ib.
498. Excitation of the nerves of sight - ib.
499. Excitation of the nerves of hear-
ing .----- ib.
500. Development of electricity in the
animal organism - ,307
501. Electrical fishes - ib.
502. Properties of the torpedo ; ob-
servations of "Walsh - tb
503. Observations of Becquerel and
Breschet 308
504. Observations of Matteucci •• - ib.
505. The electric organ - ib.
506. The torpedo ... - ib.
507. The Silurus electricus - - - 310
508. The Gymnotus electricus - - ib.
Manner of capturing them - - 311
Electric organs - ... ib.
ELEMENTARY COURSE
ELECTRICITY, MAGNETISM, AND
ACOUSTICS.
BOOK THE FIRST.
ELECTRICITY.
CHAPTER I.
ELECTRICAL ATTRACTIONS AND REPULSIONS.
I . Electrical effects. — If a glass tube, being well dried, be
briskly rubbed with a dry woollen cloth, the following effects may
be produced : —
The tube, being presented to certain light substances, such as
feathers, metallic leaf, bits of light paper, filings of cork, or pith
of elder, will attract them.
If the friction take place in the dark, a bluish light will be seen
to follow the motions of the cloth.
If the glass be presented to a metallic body, or to the knuckle
of the finger, a luminous spark accompanied by a sharp cracking
sound, will pass between the glass and the finger.
On bringing the glass near the skin, a sensation will be produced
like that which is felt when we touch a cobweb.
The same effects will be produced by the cloth, with which the
glass is rubbed, as by the glass itself.
In an extensive class of bodies, when submitted to the same
kind of mutual friction, similar effects are produced.
ELECTRICITY.
The physical agency from which these and like phenomena arise
has been called electricity, from the Greek word tXenrpov (elec-
tron), signifying
amber, that sub-
stance having been
the first in which
the property was
observed by the
ancients.
To study the laws
which govern elec-
trical forces, let an
apparatus be pro-
vided, called an
electric pendulum,
consisting of a small
ball A, fig. i ., about
the tenth of an inch
in diameter, turned
from the pith of elder, and suspended, as represented in the
figure, by a fine silken thread attached to a convenient stand.
If the glass tube B, after being rubbed as above described, be
brought into contact successively with two pith balls thus sus-
pended, and then separated from them, a property will be imparted
to the balls, in virtue of which they will be repelled by the glass
tube when it is brought near them, and they will in like manner
repel each other when brought into proximity.
Fig. i.
V
^°B'
Fig. a.
Fig.j.
Thus, if the glass tube s, fig. 2, be brought near the ball B',
the ball will depart from its vertical position, and will incline itself
from the tube in the position B.
If the ,two balls, being previously brought into contact with the
tube, be placed near each other, as in fig. 3., they will incline from
POSITIVE AND NEGATIVE ELECTRICITY. 3
each other, departing from the vertical positions B and B', and
taking the positions b and b'.
2. Positive and negative electricity. — If the hand which
holds the cloth be covered with a dry silk glove, the cloth, after
the friction with the glass, will exhibit the same effects as above
described. If it be brought into contact with the balls and then
separated from them, it will repel them, and the balls themselves
will repel each other. It appears, therefore, that by the friction
the electric fluid is at the same time developed on the glass and on
the cloth. If, after friction, the glass be brought into contact with
one ball B, jig. 3, and the cloth with the other B', other effects
will be observed. The glass, when presented to the ball B', will
attract it, and the cloth presented to the ball B will attract it ; and
the balls when brought near each other, will now exhibit mutual
attraction instead of repulsion. It follows, therefore, that the
electricity developed by friction on the cloth differs from that
developed on the glass, inasmuch as instead of being characterised
by reciprocal repulsion they are mutually attractive.
3. [Nature of electricity. — In order to explain these and
many other effects, which will be described in the following
chapters, it was formerly supposed that a subtle and imponderable
fluid, called the electric fluid, was generated upon the surface of
glass and other bodies when they were rubbed with a woollen
cloth, and that the presence of this fluid was the cause of th<T~
phenomena which electrified bodies exhibit. It is, however, now
known that this supposition is incorrect ; and although it may be
impossible to say exactly what electricity, 01* the supposed cause of
electrical phenomena, really is, we know at least that it is not a
fluid or substance of any kind, but merely a condition or state of
ordinary matter, which can be brought about in the manner
already described, as well as in many other ways that we shall
have to study as we go on.
4. Mode of describing: electrical effects. — Nevertheless, a
great number of the most important effects of electricity can be
very conveniently described in language which is borrowed from
the supposition of an electric fluid ; for, notwithstanding that this
supposition is, as we have said, erroneous, the form which has been
given to it is such that very many phenomena are exactly what
they would be if it were true. The notion of an electric fluid,
therefore, facilitates considerably both the perception and ex-
pression of the general laws according to which electrical pheno-
mena are found to take place ; and, consequently, language
founded upon this idea is still used to a very great extent in
describing these phenomena and explaining their laws.
4 ELECTRICITY.
5. Hypothesis of a single electric fluid. — The supposition
that electrical effects are due to a peculiar substance, has taken
two somewhat different forms. Some philosophers, following the
hypothesis adopted by the celebrated Benjamin Franklin, have
supposed : (a), that there is only a single electric fluid, the particles
of which mutually repel each other, but attract those of material
bodies ; (&), that this fluid is present more or less abundantly in
all bodies in their natural or unelectrified state; and (c), that
when any body contains either more or less than its natural dose
or charge of electric fluid, this excess or deficiency causes the body
to possess various properties which are collectively expressed by
saying that it is electrified.
On this view, it is supposed that when a piece of glass is rubbed
with a woollen cloth, the cloth loses part of its natural charge of
electricity, and thus becomes electrified negatively or by defi-
ciency; while the electricity which the cloth loses is accumulated
on the glass, which therefore becomes electrified positively or
by excess.
6. Hypothesis of two electric fluids. — Others, again, have
supposed that there are two fluids concerned in the production of
electrical phenomena. These two fluids, like the single electric
fluid admitted by those who adopt the view stated in (5.), are
regarded as each of them self-repulsive, but as attracting each
other. Material bodies, in their usual non-electric state, are
supposed to owe their neutrality, not to the absence of electric
fluid, but to the fact of their containing both fluids in equivalent
quantity, so that the attraction or repulsion which one fluid exerts,
is exactly balanced and counteracted by the equally powerful
repulsion or attraction exerted by the other. In electrified bodies,
on the other hand, one of the fluids is supposed to be in excess ;
or, what comes to the same thing, there is a deficiency of the other
fluid.
In order to distinguish the two electric fluids, one of them is
called the positive or vitreous fluid, and the other the negative or
resinous fluid.
7. Fundamentally, these two hypotheses are only different ways
of expressing the same idea, so that there is no reason for abso-
lutely preferring one to the other ; but as some phenomena can
be described more simply on the hypothesis of two fluids, the lan-
guage of this hypothesis will be commonly employed in this work.]
8. Explanation of the effects already described. — Assuming
then, for convenience, the existence of two electric fluids, we may
say that when the glass tube and woollen cloth are submitted to
mutual friction, their natural electricities are decomposed, the
DEVELOPMENT OF ELECTRICITY. 5
positive fluid passing to the glass, and the negative to the cloth.
The glass thus becomes surcharged with positive, and the cloth
with negative, electricity.
The pith ball B {fig. 3.), touched by the glass, receives the posi-
tive fluid from it, and the pith ball B', touched by the cloth, receives
the negative fluid from it. The ball B therefore becomes positively,
and the ball B7 negatively, electrified by contact.
Since the contrary electricities are mutually attractive, the balls
B and B' in this case attract each other ; and, since like electricities
are mutually repulsive, the glass rod repels the ball B, and the
cloth repels the ball B'.
9. Electricity is developed by various bodies, when sub-
mitted to friction. — If a stick of resin or sealing wax be rubbed
by a woollen cloth, like effects will follow : but, in this case, the
electricity of the wax or resin will be contrary to that of the
glass, as may be rendered manifest by the pith balls. If B be
electrified by contact with the glass, and B' by contact with the
resin or wax, they will attract each other, exactly as they did
when B' was electrified by contact with the cloth rubbed upon the
glass. It appears, therefore, that while glass is positively, resin is
negatively, electrified by the friction of woollen cloth.
It was owing to this circumstance that positive electricity came
to be called vitreous, and negative electricity resinous.
This nomenclature, is, however, faulty ; inasmuch as there are
certain substances by the friction of which glass will be negatively
electrified, and others by which resin will be positively electrified.
When a woollen cloth is rubbed on resin or wax which, as has
been staged, it electrifies negatively, it is itself electrified posi-
tively ; since the natural fluid being decomposed by the friction,
and the negative element going to the resin, the positive element
must be developed on the cloth. Thus it appears that the woollen
cloth may be electrified by friction, either positively or negatively,
according as it is rubbed upon resin or upon glass.
There is no certain test to determine, previous to experiment,
which of the bodies submitted to friction receives positive, and
which negative, electricity. In general, when any two bodies are
rubbed together, electricity is developed, one of them being
charged with the positive, and the other with the negative, fluid.
A great number of experimental researches have from time to time
been undertaken, with a view to the discovery of the physical law,
which determines the distribution of the constituent electric fluids
in such cases between the two bodies, so that it might in all cases
be certainly known which of the two would be positively and
which negatively electrified. These inquiries, however, have
B 3
6 ELECTRICITY.
hitherto been attended with no clear or certain general conse-
quences.
It has been observed, that hardness of structure is generally
attended with a predisposition to receive positive electricity.
Thus, the diamond, submitted to friction with other stones or with
glass, becomes positively electrified. Sulphur, when rubbed with
amber, becomes negatively electrified, the amber being conse-
quently positive ; but if the amber be rubbed upon glass or dia-
mond, it will be negative.
It is also observed that when heat is developed by the friction
of two bodies, that which takes most heat is negatively, and the
other positively, electrified.
In short, the decomposition of the electricity and its distribution
between the rubbing bodies is governed by conditions infinitelv
various and complicated.
An elevation of temperature will frequently predispose a body
to take negative, which would otherwise take positive electricity.
An increase of polish of the surface produces a predisposition for
the positive fluid. The colour, the molecular arrangement, the
direction of the fibres in a textile substance, the direction in which
the friction takes place, the greater or less pressure used in pro-
ducing it, all affect more or less, in particular cases, the interchange
of the fluids and the relative electricities of the bodies. Thus," a
black silk ribbon rubbed on one of white silk takes negative elec-
tricity. If two pieces of the same ribbon be rubbed transversely,
one being stationary and the other moved upon it, the former takes
positive, the latter negative, electricity. JEpinus found that cop-
per and sulphur rubbed together, and two similar plates of
glass, evolved electricity, but that the interchange of the fluids
was not always the same. There are substances, disthene, for
example, which, when submitted to friction, develop positive elec-
tricity at some parts, and negative at other parts of their surface,
although their structure and the state of the surface be perfectly
uniform.
I O. Classification of positive and negative substances. —
Of all known substances, a cat's fur is the most susceptible oi
positive, and perhaps gun-cotton of negative, electricity. Between
these extreme substances others might be so arranged that any
substance in the list being rubbed upon any other, that which
holds the higher place will be positively, and that which holds the
lower place negatively, electrified. Various lists of this kind have
been proposed, one of which is as follows : —
1. Fur of a cat.
2. Flannel.
3. Ivory.
4. Rock-crystal.
5. Wool.
6. Glass.
7. Cotton.
8. White silk.
9. The dry hand.
10. Wood.
11. Sealing-wax.
12. Amber.
13. Sulphur.
14. Caoutchouc.
15 Gun-cotton.
CONDUCTION. 7
loa. [Both electricities always produced together. —
Although it is not always possible to say, of two substances which
are electrified by being rubbed together, which will be electrified
positively, and which negatively, it is a rule, from which there is
no exception, that whenever and however one kind of electricity
is produced, an exactly equal quantity of the opposite electricity is
always produced at the same time. Moreover, on the hypothesis
of two electric fluids, we must admit that one fluid can never be
imparted to a body without an exactly equal quantity of the other
fluid being removed at the same time ; so that the total quantity
of electric fluid which the body contains, remains always precisely
the same.]
1 1 . Method of producing1 electricity by glass and silk with
amalgam. — Experience has proved that the most efficient means
of developing electricity in great quantity and intensity is by the
friction of glass upon a surface of silk or leather smeared with an
amalgam composed of tin, zinc, and mercury, mixed with some
unctuous matter. Two parts of tin, three of zinc, and four of
mercury, answer very well. Let some fine chalk be sprinkled on
the surface of a wooden cup, into which the mercury should be
poured hot. Let the zinc and tin melted together be then poured
in, and the box being closed and well shaken, the amalgam may
be allowed to cool. It is then finely pulverised in a mortar,
and being mixed with unctuous matter, may be applied to the
rubber.
CHAP. II.
CONDUCTION.
12. Conductors and nonconductors. — Bodies differ from each
other in a striking manner in the freedom with which the electric
fluid moves upon them. If that fluid be imparted to the surface
of glass or wax, it will be confined to that portion of the surface
which originally receives it ; but if it be imparted to a portion of
the surface of a metallic body, it will instantaneously diffuse
itself over the entire extent of such metallic surface.
The former class of bodies, which do not give free motion to
the electric fluid on their surface, are called nonconductors; and
the latter, on which apparently unlimited freedom of motion pre-
vails, are called conductors.
13. Degrees of conduction. — Of all bodies the most perfect
conductors are the metals. These bodies transmit electricity in-
B4
8 ELECTRICITY.
stantaneously, and without any sensible obstruction, provided
their dimensions are not too small in relation to the quantity of
electricity imparted to them.
The bodies named in the following series possess the conducting
power in different degrees in the order in which they stand, the
most perfect conductor being first, and the most perfect noncon-
ductor last in the list. The black line divides the most imperfect
conductors from the most imperfect nonconductors ; but it must
be observed that the position of this line is arbitrary, the exact
relative position of many of the bodies composing the series not
being certainly ascertained. The series, however, will be useful
as indicating generally the bodies which have the conducting and
nonconducting property in a greater or less degree : —
AH the metals.
Moist earth and stones.
Dry vegetable bodies.
Well-burnt charcoal.
Powdered glass.
Baked wood.
Plumbago.
Flowers of sulphur.
Dry gases and air.
Concentrated acids.
__^_^
Leather.
Powdered charcoal.
Dry metallic oxides.
Parchment.
Dilute acids.
Oils, the heaviest the best.
Drv paper.
Saline solutions.
Ashes of vegetable bodies.
Feathers.
Metallic ores.
Ashes of animal bodies.
Hair.
Animal fluids.
Manv transparent crystals,
Wool.
Sea-water.
dry.
Dyed silk.
Spring- water.
Ice below 13° Fahrenheit.
Bleached silk.
Rain-water.
Phosphorus.
Raw silk.
Ice above lj° Fahrenheit.
Lime.
Transparent gems.
Snow.
Dry chalk.
Diamond.
Living vegetables.
Native carbonate of ba-
Mica.
Living animals.
rytes.
All vitrifactions.
Flame.
Lycopodium.
Glass.
Smoke.
Caoutchouc.
Jet.
Steam.
Camphor.
Wax.
Salts soluble in water.
Some siliceous and argilla-
Sulphur.
Rarefied air.
ceous stones.
Resins.
Vapour of alcohol.
Dry marble.
Amber.
Vapour of ether.
Porcelain.
Gum-lac.
14. Insulators. — Good nonconductors are also called insu-
lators, because when any body suspended by a nonconducting
thread, or supported on a nonconducting pillar, is charged with
electricity, such charge will be retained, since it cannot escape by
the thread or pillar, which refuses a passage to it in virtue of its
nonconducting quality. Thus, a globe of metal supported on a
glass pillar, or suspended by a silken cord, being charged with
electricity will retain the charge ; whereas, if it were supported on
a metallic pillar, or suspended by a metallic wire, the electricity
would pass away by its free motion over the surface of the pillar
or the wire.
1 5. Insulating: stools are formed with glass legs, so that any
body charged with electricity and placed upon them will retain its
electric charge.
1 6. Electrics and non-electrics obsolete terms. — Con-
ducting bodies were formerly called non-electrics, and noncon-
ducting bodies were called electrics, from the supposition that the
CONDUCTORS AND NONCONDUCTORS. 9
latter were capable of being electrified by friction, but the former
not.
The incapability of conductors to be electrified by friction was,
however, afterwards shown to be only apparent, and accordingly
the use of these terms has been discontinued.
If a rod of metal be submitted to friction, the electricity evolved
is first diffused over its entire surface in consequence of its con-
ducting property, and thence it escapes by the hand of the ope-
rator which holds it, and which, though not as perfect a conductor
as the metal, is a sufficiently good one to carry off the electricity,
so as to leave no sensible trace of it on the metal. But if the
metal rod be suspended by a dry silken thread (which is a good
nonconductor), or be supported on a pillar of glass, and then be
struck several times with the fur of a cat, it will be found to be
negatively electrified, the fur which strikes it being positively
electrified.
1 7. Two persons being: placed on Insulating: stools : if one
strike the other two or three times with the fur of a cat, he that
strikes will have his body positively, and he that is struck nega-
tively, electrified, as may be ascertained by the method already
explained, of presenting to them successively the pith ball ~B,fig- 2.,
previously charged with positive electricity. It will be repelled
by the body of him that strikes, and attracted by that of him who
is struck. But if the same experiment be made without placing
the two persons on insulating stools, the same effects will not
ensue ; because, although the electricities are developed as before
by the action of the fur, they immediately escape through the feet
to the ground.
1 8. The atmosphere is a nonconductor, for if it gave a
free passage to electricity, the electrical effects excited on the
surface of any body surrounded with it would soon pass away ;
and no electrical phenomena of a permanent nature could be pro-
duced, unless the bodies were removed from the contact of the air.
It is found, however, that resin and glass, when excited by friction,
retain their electricity for a considerable time.
1 9. [Effect of rarefying: the air. — An electrified body will
retain its electricity, if placed in the exhausted receiver of an air-
pump, quite as long or longer than in the open air, provided it has
received only a very feeble charge ; but if the charge is at all con-
siderable, it is liable to escape as a luminous discharge, as will be
described hereafter.]
20. Use of the silk string- which suspends pith balls. —
In the experiments described in (l) et seq. with the pith balls,
the silken string by which they are suspended acts as an insulator.
The pith of elder being a conductor, the electric fluid is diffused
10 ELECTRICITY.
over the ball; but the silk being a nonconductor, it cannot
escape. If the ball were suspended by a metallic wire attached
to a stand composed of any conducting matter, the electricity
would escape, and the effects described would not ensue. But if
the metallic wire were attached to a glass rod or other noncon-
ductor, the same effects would be produced. In that case the
electricity would be diffused over the wire as well as over the
ball.
21. Water a conductor. — Water, whether in the liquid or
solid form, is a conductor, though of an order greatly inferior to
the metals. This fact is of great importance in electrical pheno-
mena. The atmosphere always contains in suspension more or
less aqueous vapour, which is apt to condense on the surface of
any solid bodies exposed to it. Hence, electrical experiments
always succeed best in cold and dry weather, for the most perfect
nonconductors lose their virtue if their surface be moist, the elec-
tricity passing by the conducting power of the moisture.
22. Insulators must be kept dry. — This circumstance also
shows why it is necessary to dry previously the bodies on which
it is desired to develop electricity by friction. For the same
reason it is often needful, in experimenting on electricity, to wipe
the glass pillars, by which the different apparatus are usually sup-
ported, with a dry and warm cloth, so as to remove the film of
moisture which condenses upon them ; as well as to cover the glass
with a thin coating of shell-lac varnish, which to a great extent
prevents the deposition of moisture.
23. There is no certain test to 'distinguish conductors
from nonconductors. — It will be apparent from what has been
explained, that it would be more correct to designate bodies as
good and bad conductors in various degrees, than as conductors
and nonconductors. There exists no body which, strictly
speaking, is either an absolute conductor or absolute nonconduc-
tor ; the most perfect conductors offering some resistance to the
passage of electricity, and the most perfect nonconductors not
entirely preventing it.
24. The conducting: power is variously affected by tempe-
rature.— In the metals it is diminished by elevation of temperature;
but in all other bodies, and especially in liquids, it is augmented.
Some substances, which are nonconductors in the solid state, be-
come conductors when fused. Sir H. Davy found that glass raised
to a red heat became a conductor ; and that sealing-wax, pitch,
amber, shell-lac, sulphur, and wax become conductors when lique-
fied by heat. The manner in which electricity is communicated
from one body to another, depends on the conducting property of
the body imparting and the body receiving it.
CONDUCTORS AND NONCONDUCTORS. 1 1
2 5 Effects produced by touching- an electrified body by a
conductor which is not insulated. — If the surface of a non-
conducting body, glass, for example, be charged with electricity,
and be touched over a certain space, as a square inch, by a con-
ducting body which is not insulated, the electricity which is dif-
fused on the surface of contact will pass away by the conductor,
but no other part of the electricity with which the body is charged
will escape. A patch of the surface corresponding with the magni-
tude of the conductor will alone be stripped of its electricity. The
nonconducting property of the body will prevent the electricity,
which is diffused over the remainder of its surface, from flowing
into the space thus drained of the fluid by the conductor. But if
the body thus charged with electricity, and touched by a con-
ductor not insulated, be a conductor, the effects produced will be
very different. In that case, the electricity which covers the sur-
face of contact will first pass off; but the moment the surface of
contact is thus drained of the fluid which covered it, the fluid dif-
fused on the surrounding surface will flow in and likewise pass off,
find thus all the fluid diffused over the entire surface of the body
will rush to the surface of contact, and escape. These effects,
though, strictly speaking, successive, will be practically instan-
taneous ; the time which elapses between the escape of the fluid
which originally covered the surface of contact, and that which
rushes from the most remote parts to the surface of contact, being
inappreciable.
26. Effect produced when the touching conductor is In-
sulated. — If a conducting body, which is insulated and charged
with electricity, be brought into contact with another conducting
body, which is also insulated and in its natural state, the electricity
will diffuse itself over the surfaces of both conductors in propor-
tion to their relative magnitudes.
If 8 express the superficial magnitude of an insulated conducting body,
E the quantity of electricity with which it is charged, and s' the superficial
magnitude of the other insulated conductor with which it is brought into
contact, the charge E will, after contact, be shared between the two con-
ductors in the ratio of s to s' -, so that
E x^-j-p= the charge retained by s ;
Exs+s'= ^e cnarSe received by s'.
27. Why ttie earth is called the common reservoir.— If
the second conductor s' be the globe of the earth, s' will bear
a proportion to 8 which, practically speaking, is infinite; and
consequently the quantity of electricity remaining on s, ex-
press,ed by
ELECTRICITY.
will be nothing. Hence the body s loses its entire charge when
put in conducting communication with the ground.
An electrified body being a conductor, is therefore reduced to
its natural state when put into electric communication with the
ground, and the earth has been therefore called the common reser-
voir, to which all electricity has a tendency to escape, and to which
it does in fact always escape, unless its passage is intercepted by
nonconductors.
28. Electricity passes by preference on the best con-
ductors. — If several different conductors be simultaneously placed
in contact with an insulated electrified conductor so as to form a
communication between it and the ground, the electricity will
always escape by the best conductor. Thus, if a metallic chain
or wire be held in the hand, one end touching the ground and the
other being brought into contact with the conductor, no part of
the electricity will pass into the hand, the chain being a better
conductor than the flesh of the hand. But if, while one end of the
chain touch the conductor, the other be separated from the ground,
then the electricity will pass into the hand, and will be rendered
sensible by a convulsive shock.
CHAP. III.
INDUCTION.
29. Action of electricity at a distance. — If a body A,
charged with electricity of either kind, be brought into proximity
with another body B in its natural state, the fluid, with which A is
surcharged, will act by attraction and repulsion on the two con-
stituents of the natural electricity of B ; attracting that of the con-
trary, and repelling that of the same kind. This effect is precisely
similar to that produced on the natural magnetic fluid in a piece
of iron, when the pole of a magnet is presented to it, as will be
explained hereafter.
If the body B in this case be a nonconductor, the electric fluid
having no free mobility upon its surface, its decomposition will
be resisted, and the body B will continue in its natural state, not-
withstanding the attraction and repulsion exercised by A on the
constituents of its natural electricity. But if B be a conductor,
the fluids having freedom of motion on its surface, the fluid similar
INDUCTION. M
to that with which u is charged will be repelled to the side most
distant from B, and the contrary fluid will be attracted to the side
next to B. Between these regions a neutral line will separate
those parts of the body B, over which the two opposite fluids are
respectively diffused.
30. Induction is the action of an electrified body exerted at
a distance upon the electricity of another body, and is analogous,
in many respects, to that which produces similar phenomena in
the magnetic bodies.
3 I . Experimental exhibition of its effects. — To render it
experimentally manifest, let s and s', jig. 4., be two metallic balls
Fig. 4.
supported on glass pillars ; and let A A' be a metallic cylinder simi-
larly mounted, whose length is ten or twelve times its diameter,
and whose ends are rounded into hemispheres. Let s be strongly
charged with positive, and s' with negative electricity, the cylinder
A A' being in its natural state.
Let the balls s and s' be placed near the ends of the cylinder A A', their
centres being in line with its axis, as represented in the figure. The positive
electricity of s will now attract the negative, and repel the positive consti-
tuent of the natural electricity of A A', so as to separate them, drawing the
negative fluid towards the end A, and repelling the positive fluid towards the
end A'. The negative electricity of s' will produce a like effect, repelling the
negative electricity of A A' towards A, and drawing the positive towards A'.
Since the cylinder A A' is a conductor, and therefore the fluids have freedom
of motion on its surface, this decomposition will take effect, and the half o A
of the cylinder next to s will be charged with negative, and the half o A'
next to s' with positive electricity.
That such is in fact the condition of A A' may be proved by presenting a
pith ball (i.) pendulum charged with positive electricity to either half of the
cylinder. When presented to o A' it will be repelled, and when presented to
o A it will be attracted.
If the two balls s s' be gradually removed to increased but equal distances
from the ends A and A', the recomposition of the fluids will gradually take
place; and when the balls are altogether removed the cylinder A A' will
recover its natural state, the fluids which had been separated by the action
of the balls being completely recombined by their mutual attraction.
14 ELECTRICITY.
Let a metallic ring n',fg. 5., be supported on a rod or
hook of glass n, and let two pith balls b V be suspended
from it by fine wires, so that when hanging vertically
they shall be in contact. Let a ball of metal r, strongly
charged with positive electricity, be placed over the ring
«' at a distance of eight or ten inches above it. The
presence of this ball will immediately cause the pith
balls to repel each other, and they will diverge to in-
creased distances the nearer the ball r is brought to
the ring nt. If the ball r be gradually raised to greater
\ ^ ; distances from the ring, the balls b b' will gradually ap-
proach each other, and will fall to their position of rest
Fig. 5. vertically under the ring when the ball r is altogether
removed.
If the charge of electricity of the balls s and &,fig. 4., or of the ball r,
fig. 5., be gradually diminished, the same effect will be produced as when the
distance is gradually increased ; and, in like manner, the gradual increase of
the charge of electricity will have the same effect, as the gradual diminution
of the distance from the conductor on which the action takes place.
If the ring nf, the balls b b', and the connecting wire, be first feebly charged
•with negative electricity, and then submitted to the inductive action of the
ball r charged with positive electricity, placed, as before, above the ring, the
following effects will ensue. When the ball r approaches the ring, the balls
bb>, which previously diverged, will gradually collapse until they come into
contact. As the ball r is brought still nearer to n', they will again diverge,
and will diverge more and more, the nearer the ball r is brought to the ring.
These various effects are easily and simply explicable by the action of the
electricity of the ball r on that of the ring. When it approaches the ring,
the positive electricity with which it is charged decomposes the natural elec-
tricities of the ring, repelling the positive fluid towards the balls. This fluid
combining with the negative fluid with which the balls are charged, neutra-
lises it, and reduces them to their natural state: while this effect is gradually
produced, the balls b b' lose their divergence and collapse. But when the
ball r is brought still nearer to the ring, a more abundant decomposition of
the natural fluid is produced, and the positive fluid repelled towards the balls
is more than enough to neutralise the negative fluid with which they are
charged; and the positive fluid prevailing, the balls again diverge with
positive electricity.
These effects are aided by the attraction exerted by the positive electricity
of the ball r on the negative fluid, with which the balls b b> are previously
charged.
If the electrified ball, instead of being placed above the ring, be placed at
an equal distance below the balls b b1, a series of effects will be produced in
the contrary order, which the student will find no difficulty in analysing and
explaining.
If the ball r be charged with negative electricity, it will produce the same
effects when presented above the ring as when, being charged with positive
electricity, it is presented below it.
32. Let three copper cylinders, AB, A'B', A.»K",fig. 6., rounded at the ends,
be supported on insulating pillars, and the pith ball pendulums be inserted at
their extremities, the pith balls being supported by wires or other conducting
threads on rods which are also conductors. Let the cylinders, placed end
to end, as shown in the figure, be brought near to a conductor c. charged,
for example, with positive electricity ; the electricity of c will decompose the
INDUCTION I $
natural electricity of A, attracting to the end near it the negative, and re-
pelling to the remote end the positive fluid. The positive fluid thus collected
at the remote end B, will act by induction in a similar manner upon the
natural electricity of A' B' ; attracting the negative electricity to the near end,
and repelling the positive to the remote end, as indicated in the figure, where
+ indicates the positive, and — the negative electricity.
This distribution of the two fluids will be shown by the pith balls, as indi-
cated in the figure; the pith balls, charged with each kind of electricity,
being repelled by the rods similarly charged.
In all cases whatever, the conductor, whose electrical state has
been changed by the proximity of an electrified body, returns to
its primitive electrical condition when the disturbing action of
such body is removed ; and this return is either instantaneous or
gradual, according as the removal of the disturbing body is in-
stantaneous or gradual.
33. Effects of sudden inductive action. — It appears, there-
fore, that sudden and violent changes in the electrical condition
of a conducting body may take place, without any portion of elec-
tricity being either imparted to or abstracted from such body. The
electricity with which it is invested before the inductive action
commences, and after such action ceases, is exactly the same ;
nevertheless, the decomposition and recomposition of the consti-
tuent fluids, and their motion more or less sudden over it and
io ELECTRICITY.
through its dimensions, are productive often of mechanical effects
of a very remarkable kind. This is especially the case with im-
perfect conductors, which offer more or less resistance to the
reunion of the fluids.
34. Example in the case of a frog-. — Let a frog be sus-
pended by a metallic wire which is connected with an insulated
conductor, and let a metallic ball, strongly charged with positive
electricity, be brought under, without, however, touching it. The
effects of induction already described will ensue. The positive
fluid will be repelled from the frog towards the insulated con-
ductor, and the negative fluid will be attracted towards it, so that
the body of the frog will be negatively electrified ; but this, taking
place gradually as the electrified ball approaches, is attended with
no sensible mechanical effect.
If the electrified ball, however, be suddenly discharged, by con-
necting it with the ground by a conductor, an instantaneous re-
vulsion of the electric fluids will take place, between the body of
the frog and the insulated conductor with which it is connected ;
the positive fluid rushing from the conductor, and the negative
fluid from the frog, to recombine in virtue of their mutual at-
traction. This sudden movement of the fluids will be attended
by a convulsive motion of the limbs of the frog.
35. Inductive shock of the human body. — If a person
stand close to a large conductor strongly charged with electricity,
he will be sensible of a shock when this conductor is suddenly
discharged. This shock is in like manner produced by the sudden
recomposition of the fluids in the body of the patient, decomposed
by the previous inductive action of the conductor.
36. Development of electricity by induction. — A con-
ductor may be charged with electricity by an electrified body,
though the latter shall not lose any of its own electricity or impart
any to the conductor so electrified. For this purpose, let the con-
ductor to be electrified be supported on a glass pillar so as to
insulate it, and let it then be connected with the ground by a me-
tallic chain or wire. If it be desired to charge it with positive
electricity, let a body strongly charged with negative electricity be
brought close to it without touching it. On the principles already
explained, the negative electricity of the conductor will be repelled
to the ground through the chain or wire ; and the positive elec-
tricity will, on the other hand, be attracted from the ground to the
conductor. Let the chain or wire be then removed, and, after-
wards, let the electrified body by whose inductive action the effect
is produced be removed. The conductor will remain charged with
positive electricity.
It may in like manner be charged with negative electricity, by
the inductive action of a body charged with positive electricity
ELECTRICAL MACHINES. { 7
CHAP. IV.
ELECTRICAL MACHINES.
37. An electrical machine is an apparatus, by means of which
electricity is developed and accumulated, in a convenient manner
for the purposes of experiment.
All electrical machines consist of three principal parts, the rub-
ber, the body on whose surface the electric fluid is evolved, and
one or more insulated conductors, to which this electricity is trans-
ferred, and on which it is accumulated.
38. The rubber is a cushion stuffed with hair, bearing on its
surface some substance, which by friction will evolve electricity.
The body on which this friction is produced is glass, so shaped
and mounted as to be .easily and rapidly moved against the rubber
with a continuous motion. This object is attained by giving the
glass the form either of a cylinder revolving on its geometrical axis,
or of a circular plate revolving in its own plane on its centre.
39. The conductors are bodies having a metallic surface and
a great variety of shapes, and always mounted on insulating pillars,
or suspended by insulating cords.
40. The common cylindrical machine. — A hollow cylinder
of glass A B,Jig- 7., is supported in bearings at c, and made to
revolve by means of the wheels c and D connected by a band, a
handle B being attached to the greater wheel.
The cushion H, represented separately in fig. 8., is mounted on a glass
pillar, and pressed with a regulated force against the cylinder by means of
springs fixed behind it. A chain, fig. 7., connects the cushion with the
ground. A flap of black silk equal in width to the cushion covers it, and
is carried over the cylinder, terminating above the middle of the cylinder
on the opoosite side.
. 7- Fig. 8.
The conductor is a cylinder of thin brass M N, the ends &f which are parts
of spheres greater than hemispheres. It is supported by a glass pillar o p.
c
18 ELECTRICITr.
To the end of the conductor next the cylinder is attached a row of points
represented separately in fig. 9., which are pre-
sented close to the surface of the cylinder, but
without touching it. The extent of this row of
points corresponds with that of the rubber.
As the efficient performance of the machine
Fig. 9. depends in a great degree on the good insulation of
the several parts, and as glass is peculiarly liable
to collect moisture on its surface which would impair its insulating virtue, it
is usual to cover the insulating pillars of the rubber and conductor, and all
that part of the cylinder which lies outside the cushion and silk flap, with a
coating of resinous varnish, which, while its insulating property is more per-
fect than that of glass, offers less attraction to moisture.
To explain the operation of the machine, let us suppose that the
cylinder is made to revolve by the handle R. Positive electricity
is developed upon the cylinder, and negative electricity on the
cushion. The latter passes by the conducting chain to the ground.
The former is carried round under the flap, on the surface of the
glass, until it arrives at the points projecting from the conductor.
There it acts by induction (30.) on the natural electricity of the
conductor, attracting the negative electricity to the points and
repelling the positive fluid. The negative electricity issuing from
the points combines with and neutralises the positive fluid diffused
on the cylinder, the surface of which, after it passes the points, is
therefore restored to its natural state, so that when it arrives again
at the cushion it is prepared to receive by friction a fresh charge
of the positive fluid.
It is apparent, therefore, that the effect produced by the oper-
ation of this machine is a continuous decomposition of the natural
electricity of the conductors, and an abstraction from it of just so
much negative fluid as compensates for that which escapes by
the cushion and chain to the earth. The conductor is thus as it
•were drained of its negative electricity by a stream of that fluid,
which flowing constantly from the points passes to the cylinder,
and thence by the cushion and chain to the earth. The conductor
is therefore left surcharged with positive electricity.
41. Nairne's cylinder machine. — This apparatus, which is
adapted to produce at pleasure either positive or negative elec-
tricity, is similar to the last, but has a second conductor in con-
nection with the cushion.
A geometrical drawing in outline of this machine is shown in fig. 10. When
it is desired to collect positive electricity, the conductor M F is put in con-
nection with the ground, and the machine acts as that described above.
When it is desired to collect negative electricity, the conductor M' B is put
in connection with the ground, and the conductor M F is insulated. In this
case a stream of positive electricity flows continually from M F through the
cushion to the cylinder, and thence by the conductor M' B to the ground,
CYLINDER MACHINES
Fig. 10.
electricities to attract each other and combine.
leaving the conductor BI F charged with
negative electricity.
A perspective drawing of the same ma-
chine, with some unimportant modifica-
tions of form and arrangement, is given
in fig. ii. In this, c is the conductor
which carries the rubber D, and B that
which collects the positive electricity;
the cylinder A, between these, is worked
by a winch M having an insulating
handle. The rods attached to the posi-
tive and negative conductors, terminate
in copper balls, between which, when
brought near to each other, a series of
electric sparks constantly pass, proceed-
ing from the tendency of the opposite
Fig. ii
42. The common plate machine, known as Van Marum's, is
represented in geometrical outline in Jig. 1 2.
It consists of a circular plate of glass
A B, fig. 12., mounted as represented
in the figure. It is embraced between
two pair of cushions at E and E', a cor-
responding width of the glass being
covered by a silk sheathing extending to
F', where the points of the conductors are
presented. The handle being turned in
the direction of the arrow, and the cushions
being connected by conducting chains
with the ground, positive electricity is
developed on the glass, and neutralised as
in the cylinder machine, by the negative
Fig. 12. electricity received by induction from the
c a
20
ELECTRICITY.
conductors, which consist of a long narrow cylinder, bent into a form to
adapt it to the plate. It is represented at M N, a branch M o being carried
parallel to the plate and bent into the form MOPQ, so that the part PQ shall
be presented close to the plate under the edge of the silk flap. A similar
branch of the conductor extends on the other side, terminating just above
the edge of the lower silk flap.
The principle of this machine is similar in all respects to that of the com-
mon cylinder machine. With the same weight and bulk, the extent of
rubbing surface, and consequently the evolution of electricity, is much great er
than in the cylinder machines.
Fig. 14.
A perspective view of this machine is given in fig. 13., where the arc of
copper T Y', connected with the handle is placed vertically, and in fig. 14
PLATE MACHINES.
2\
the same arc x X' is exhibited horizontally, being then in contact with the
cushions. On the other side of the plate is the large copper ball o, standing
on an insulating pillar to which the arc x x'/«/. 13. and Y v fig. 14. is fixed,
being placed horizontally in fig. 13., and vertically in fig. 14.
When the two arcs Y Y' and xx' are placed as in fig. 13., Y Y' being vertical,
and x X' horizontal, the two branches x x' are in contact with the cushions,
while those of Y Y' approach the plate without touching it; consequently, if
by the aid of the handle the plate is turned, the cushions, which are nega-
tively electrified, charge the ball o with the negative fluid, while the positive
electricity of the plate, acting by induction upon Y Y', draws from the ground
the negative fluid, which it neutralises.
On the other hand, if the branches YY' and XX' be disposed as in fig. 14.,
the cushions communicating with the ground by x x' lose all their electricity,
Avhile the plate which is positively electrified, acting by induction upon Y Y',
and the ball o, drains them of the negative fluid, and leaves them posi-
tively electrified.
43. Ramsden's plate machine. — One of the earliest electric
apparatus of this form which was constructed is represented in
Fig. 15.— RAMSDEN'S ELECTRICAL MACHINE.
The large glass plate o, is mounted between wooden supports Mm, and turned
by a handle x. It is pressed between two pairs of rubbers, c c. In the direc-
tion of its horizontal diameter it passes between two curved brass tubes r>D',
which collect the electricity from it by points in the usual way. These are
22
ELECTRICITY.
connected with two large conductors B B', supported on insulating pillars
p p, and connected at the remote end by a cylindrical tube, from the middle
of which another tube E proceeds at right angles, terminated in a knob.
After what has been explained of the other machines the theory
of this will be readily understood.
44. Armstrong's hydro-electrical machine. — Anew species
of electric machine has resulted from the accidental observation
of an electric shock, produced by the contact of a jet of high
pressure steam issuing from a boiler at Newcastle-on-Tyne in
1840. Mr. Armstrong of that place took up the inquiry, and
succeeded in contriving a machine for the production and accu-
mulation of electricity, by the agency of steam. Professor Faraday
investigated the theory of the apparatus, and showed that the
origin of the electrical development was the friction of minute
aqueous particles, produced by the partial condensation of the
steam against the surface of the jet, from which the steam issued.
The hydro-electrical machine has since been constructed in
various forms and dimensions.
Fig. 16.
Let a cylindrical boiler a, fig. 16., whose length is about twice its diameter
be mounted on glass legs v, so as to be in a state of insulation.
HYDRO-ELECTRICAL MACHINE. 23
./'is the furnace door, the furnace being a tube within the boiler.
s is the safety-valve.
n is the water-gauge, a glass tube indicating the level of the water in the
boiler.
r a regulating valve, by which the escape of steam from the boiler may be
controlled.
t a tube into which the steam rushes as it escapes from r.
e three or more jet pipes, through which the steam passes from t, and from
the extremities of which it issues in a series of parallel jets.
d a condensing box, the lower half of which contains water at the common
temperature.
g the chimney.
g1 an escape pipe for the vapour generated in the condensing box d.
b the conductor which takes from the steam the electricity which issues
with it from the jet pipes e.
k the knob of the conductor from which the electricity may be received
and collected for the purpose of experiment.
The jet pipes e traverse the middle of the condensing box <?, above the
surface of the water contained in it. Meshes of cotton thread surround these
tubes within the box, the ends of which are immersed in the water. The
water is drawn up by the capillary action of these threads, so as to surround
the tubes with a moist coating, which, by its low temperature, produces
a slight condensation of the steam as it passes through that part of
the tube.
The fine aqueous particles thus produced within the tube are carried for-
ward with the ateam, and, on issuing through the jet pipe, rub against its
sides. This friction decomposes the natural electricity, the negative fluid
remaining on the jet, and the positive being carried out with the particles of
water, and imparted by them to the conductor b.
It will be apparent that in this arrangement the interior surface of the jet
plays the part of the rubber of the ordinary machine, and the particles of
water that of the glass cylinder or plate, the steam being the moving power
which maintains the friction.
In order to insure the efficiency of the friction, the conduit provided for
the escape of the steam is not straight but an-
gular. A section of the jet pipe near its extremity
is represented in fig. 17. The steam issuing from
the box d encounters a plate of metal m which in-
tercepts its direct passage to the mouth of the jet.
It is compelled to turn downwards, pass under the
edge of this plate, and, rising behind it, turn again
Fig. I7. into the escape pipe, which is a tube formed of
partridge wood enclosed within the metal pipe n.
It is found that an apparatus thus constructed, the length of the boiler
being 32 inches and its diameter 16 inches, will develop as much electricity
in a given time as three common plate machines, whose plates have a
diameter of 40 inches, and are worked at the rate of 60 revolutions per
minute.
A machine on this principle, and on a great scale of magnitude, was erected
by the Royal Polytechnic Institution of London, the boiler of which was 78
inches long, and 42 inches diameter. The maximum pressure of the steam
at the commencement of the operation was sometimes 90 Ibs. per sq. inch.
C4-
24. ELECTRICITY.
This, however, fell to 40 Ibs. or less. Sparks have been obtained from the
conductor at the distance of 22 inches.
Another view of this machine, rendered more distinct by shading, is shown
in Jig. 1 8.
Fig. 18. — ARMSTRONG'S HYDRO-ELECTRICAL MACHINE.
45. To facilitate the performance of experiments, various acces-
sories are usually provided with these machines.
46. Insulating: stools. — Insulating stools, constructed of
strong, hard wood, well baked and dried, and supported on legs
of glass coated with resinous varnish, are useful when it is re
quired to keep for any time any conducting body charged with
electricity. The body is placed on one of these stools while it is
being electrified.
Thus, two persons standing on two such stools, may be charged,
one with positive, and the other with negative, electricity. If,
when so charged, they touch each other, the contrary elec-
tricities will combine, and they will sustain a nervous shock
proportionate to the quantity of electricity with which they were
charged.
47. Discharging rods. — Since it is frequently necessary to
observe the effects of points and spheres, pieces such as figs. 1 9,
ACCESSORIES. 23
20. are provided, to be inserted in holes in the conductors ; also
metallic balls, Jigs. 21, 22., attached to glass handles for cases in
which it is desired to apply a conductor to an electrified body
without allowing the electricity to pass to the hand of the
Fig. 19. Fig. zo. Fig.zi. . Fig. zz.
operator. With these rods the electricity may be taken from
a conductor gradually by small portions, the ball taking by each
contact only such a fraction of the whole charge as corresponds
to the ratio of the surface of the ball to the surface of the con-
ductor.
48. Jointed dischargers. — To establish a temporary connec-
tion between two conductors, or between a conductor and the
ground, the jointed dischargers, figs. 23, 24., are useful. The
Fig.z?.
distance between the balls can be regulated at pleasure by means
of the joint or hinge by which the rods are united.
49. Universal discharger. — The universal discharger, an in-
strument of considerable convenience and utility in experimental
researches, is represented in Jig. 25. It consists of a wooden table
to which two glass pillars A and A' are attached. At the summit
of these pillars are fixed two brass joints capable of revolving in a
horizontal plane. To these joints are attached brass rods c c',
terminated by balls i> D', and having glass handles E E'. These
25 ELECTRICITY.
rods play on joints at B B', by which they can be moved in vertical
planes.
The balls DI/ are applied to a wooden table sustained on a
pillar capable of having its height adjusted by a screw T. On the
table is inlaid a long narrow strip of ivory, extending in the direc-
tion of the balls D D'. These balls D D' can be unscrewed, and
one or both may .be replaced by forceps, by which may be held
any substance through which it is desired to transmit the elec-
tric charge. One of the brass rods c is connected by a chain or
wire with the source of electricity, and the other with the
ground.
The electricity is transmitted by bringing the balls DD' with
the substance to be operated on between them, within such a dis-
tance of each other as will cause the charge to pass from one to the
other through the introduced substance.
CHAP. V.
CONDENSER AND ELECTROPHOEUS.
50. IF a conductor A, communicating with the ground, be placed
near another conductor B, insulated and charged with a certain
quantity of electricity E, a series of effects will ensue by the
reciprocal inductive power of the two conductors, the result of
which will be that the quantity of electricity with which B is
charged, will be augmented in a certain proportion, depending on
the distance between the two conductors through which the induc-
tive force acts. The less this distance is the more energetic the
induction will be, and the greater the augmentation of the charge
of the conductor B.
To explain this, we are to consider that the electricity E, acting on the
CONDENSERS. 27
natural electricity of A, repels a certain quantity of the fluid of the saiuo
name to the earth, retaining on the side of A next to B the fluid of the con-
trary name. This fluid of a contrary name thus developed in A reacts upon the
natural electricity of B, and produces a decomposition in the same manner,
augmenting the charge E by the fluid of the same name decomposed, and
expelling the other fluid to the more remote side of B. This increased fluid
in B again acts upon the natural electricity of A, producing a further decom-
position ; and this series of reciprocal inductive actions producing a succes-
sion of decompositions in the two conductors, and accumulating a tide of
contrary electricities on the sides of the conductors which are presented
towards each other, goes on through an indefinite series of reciprocal actions,
which, nevertheless, are accomplished in an inappreciable interval of time ;
so that, although the phenomenon in a strict sense is physically progressive,
it is practically instantaneous.
To obtain an arithmetical measure of the amount of the augmentation of
the electrical charge produced in this way, let us suppose that a quantity of
electricity on B, which we shall take as the unit, is capable of decomposing
on A a quantity which we shall express by m, and which is necessarily less
than the unit, because nothing short of actual contact would enable the
electricity of B to decompose an equal quantity of the electricity of A.
If, then, the unit of positive electricity act from B upon A, it will decompose
the natural electricity, expelling a quantity of the positive fluid expressed by
m, and retaining on the side next to B an equal quantity of the negative
fluid. Now this negative fluid m, acting on the natural electricity of B at the
same distance, will produce a proportionate decomposition, and will develop
on the side of B next to A an additional quantity of the positive fluid, just so
much less than m as m is less than 1. This quantity will therefore be mxwi,
or m*.
This quantity m2 of positive fluid, again acting by induction on A, will
develop, as before, a quantity of negative fluid expressed by m2x«*» ornt5.
And in the same manner this will develop on B an additional quantity of
positive fluid expressed by m5xm, or m4. These inductive reactions being
indefinitely repeated, let the total quantity of positive electricity developed
on B be expressed by p, and the total quantity of negative electricity deve-
loped on A by N, we shall have
P=l+m2-t-m4+m6+ ..... &C. ad inf.
N=m+m3-f7n5+7M7+ ..... &c. ad inf.
Each of these is a geometrical series; and, since m is less than 1, they are
decreasing series. Now it is proved in arithmetic, that although the number
of terms in such series be unlimited, their sum is finite, and that the sum of
the unlimited number of terms composing the first series is ^_my and that
of the second \_nfl We shall therefore have
1 m
In this case we have supposed the original charge of the conductor B to be
the unit. If it consist of the number of units expressed by E, we shall have
28 ELECTRICITY.
It follows, therefore, that the original charge E of the conductor B has been
augmented in the ratio of 1 — m2 to 1 by the proximity of the conductor A.
The less is the distance between the conductors A and B. the more nearly
will m be equal to 1, and therefore the greater will be the ratio of 1 to 1— m»,
and consequently the greater will be the augmentation of the electrical charge
of B produced by the presence of A.
For example, suppose that A be brought so near B, that the positive fluid
on B will develop nine tenths of its own quantity of negative fluid on A. In
that case m=^-=o-9. Hence it appears, that 1— 7ra2 = l— o-8i=o'i9; and,
consequently, the charge of B will be augmented in the ratio of 0-19 to 1, or
of 19 to 100.
5 1 . The condenser. — In such cases the electricity is said to be
condensed on the conductor B by the inductive action of the con-
ductor A, and apparatus constructed for producing this effect are
called condensers.
52. Dissimulated or latent electricity. — The electricity
developed in such cases on the conductor A is subject to the
anomalous condition of being incapable of passing away, though a
conductor be applied to it. In fact, the conductor A in the pre-
ceding experiment is supposed to be connected with the earth by
conducting matter, such as a chain, metallic column, or wire. Yet
the charge of electricity N does not pass to the earth, as it would
immediately do if the conductor B were removed.
In like manner, all that portion of the positive fluid p which is
developed on B by the inductive action of A, is held there by the
influence of A, and cannot escape even if a conductor be applied
in contact with it.
Electricity thus developed upon conductors and retained there
by the inductive action of other conductors, is said to be latent or
dissimulated. It can always be set free by the removal of the con-
ductors by whose induction it is dissimulated.
53. Free electricity is that which is developed independently
of induction, or which, being first developed by induction, is after-
wards liberated from the inductive action.
In the process above described, that part of the charge P of the
conductor B which is expressed by E, and which was imparted to
B before the approach of the conductor A, is/ree, and continues to
be free after the approach of A. If a conductor connected with
the earth be brought into contact with B, this electricity E will
escape by it ; but all the remaining charge of B will remain, so long
as the conductor A is maintained in its position.
If, however, E be discharged from B, the charge which remains
will not be capable of retaining in the dissimulated state so great
a quantity of negative fluid on A as before. A part will be ac-
cordingly set free, and if A be maintained in connection with the
ground it will escape. If A be insulated, it will be charged with
it still, but in a free state.
CONDENSERS. 2V
If this free electricity be discharged from A, the remaining
charge will not be capable of retaining in the latent state so large
a quantity of positive fluid on B as previously, and a part of what
was dissimulated will accordingly be set free, and may be discharged.
In this manner, by alternate discharges from the one and the
other conductor, the dissimulated charges may be gradually libe-
rated and dismissed, without removing the conductors from one
another or suspending their inductive action.
54. Condensers are constructed in various forms, according to
the strength of the electric charges they are intended to receive.
Those which are designed for strong charges require to have the
two conductors separated by a nonconducting medium of some
considerable thickness, since, otherwise, the attraction of the oppo-
site fluids diffused on A and B would take effect ; and they would
rush to each other across the separating space, breaking their way
through the insulating medium which divides them. In this case
the distance between A and B being considerable, the condensing
power will not be great, nor is it necessary to be so, since the
charges of electricity are by the supposition not small or feeble.
In case of feeble charges, the space separating the conductors
may be proportionally small, and, consequently, the condensing
power will be greater.
Condensers are usually constructed with two equal circular
plates, either of solid metal or having a metallic coating.
55. Collecting- and condensing plates. — The plate corre-
sponding to the conductor A in the preceding paragraphs is called
the condensing plate, and that which corresponds to B the collecting
plate. The collecting plate is put in communication with the body
whose electrical state it is required to examine by the agency of
the condenser, and the condensing plate is put in communication
with the ground.
56. Cuthbertson's condenser is represented in. fig. 26.
The collecting plate B is supported on a glass pillar,
and communicates by a chain attached to the hook i>
with the source of electricity under examination. The
condensing plate A is supported on a brass pillar,
movable on a hinge, and communicating with the
ground. By means of the hinge the disc A may be
moved to or from B. The space between the plates in
this case may be merely air, or, if strong charges are
used, a plate of glass may be interposed.
When used for feeble charges, it is usual to cover
the condensing plate with a thin coating of varnished
silk, or simply with a coating of resinous varnish. An
instrument thus arranged is represented in fig. 27.,
•where bb>, the condensing plate, is a disc of wood coated with varnished
•ulk tf. The collecting plate c c' has a glass handle m, by which it may be
ELECTRICITY.
Fig. 17.
raised, and a rod of metal a d by which it may
be put in communication with the source of
electricity under examination.
The condensing plate in this case has gene-
rally sufficient conducting power when formed
of wood, but may be also made of metal, and,
instead of varnished silk, it may be coated with
gum -lac, resin, or any other insulator.
When the plate ccf has received its accu-
mulated charge, its connection with the source
of electricity is broken by removing the rod ad; and the plate cc> being
raised from the condensing plate, the entire charge upon it becomes free, and
may be submitted to an electroscopic test.
57. The electrophorus is an expedient by which a small
charge of free electricity may be made to produce a charge of in-
definite amount, which may be imparted to any insulated conductor.
This instrument consists of a circular cake, composed of a mixture
of shell-lac, resin, and Venice turpentine, cast in a tin mould A
{fig. 29.). Upon this is laid a circular metallic disc u, rather less
in diameter than A, having a glass handle.
Before applying the disc B, the resinous surface ,is electrified
negatively by striking it several times with the fur of a cat. The
disc B being then applied to the cake A, and the finger being at
the same time pressed upon the disc B (fg. 28.), to establish a
Fig. 28 ELECTROPHOKUS.
communication with the ground through the body of the operator,
a decomposition takes place by the inductive action of the negative
fluid on the resin. The negative fluid escapes from the disc B
to the ground, and a positive charge remains in it. But the resin
being a nonconductor, the positive electricity of the disc cannot
penetrate it, so as to neutralise any of its negative electricity
except what resides quite at the surface. Below this, therefore,
the resin remains permanently charged with negative electricity.
ELECTROPHORUS. 31
When the disc B is thus charged with positive electricity kept
latent on it by the influence of the negative fluid on A, the finger
Fig. 49 — ELECTROPHORUS
being previously removed from the disc B, let it be raised from the
resin and the electricity upon it, before dissimulated, will become
free, and may be imparted to any insulated conductor adapted to
receive it.
The charge of negative electricity remaining undiminished on
the resin A, the operation may be indefinitely repeated ; so that an
insulated conductor nwy be strongly charged by giving to it the
electric fluid little by little thus evolved on the disc B by the
inductive action of A.
This is the origin of the name of the apparatus.
CHAP. VI.
ELECTROSCOPES
58. Electroscopes in general consist of two light conducting
bodies freely suspended, which hang vertically and in contact, in
their natural state. When electricity is imparted to them they
repel each other, the angle of their divergence being greater or
32 ELECTRICITY.
less according to the intensity of the electricity diffused on them.
These electroscopic substances may be charged with electricity
either by direct communication with the electrified body, in which
case their electricity will be similar to that of the body ; or they
may be acted upon inductively by the body under examination, in
which case their electricity may be either similar or different from
that of the body, according to the position in which the body is
presented to them. In some cases the electroscope consists of a
single light conductor, to which electricity of a known species is
first imparted, and which will be attracted or repelled by the body
under examination when presented to it, according as the elec-
tricities are like or unlike.
These instruments vary infinitely in form, arrangement, mode of
application, and sensitiveness, according to the circumstances under
which they are placed, and the intensities of the electricities of
which they are expected to detect the presence, measure the in-
tensity, or indicate the quality. In electroscopes, as in all other
instruments of physical inquiry, the most delicate and sensitive
is only the most advantageous, in those cases in which much deli-
cacy and precision are required. A razor would be an ineffectual
instrument for felling timber.
59. Pith, ball electroscope. — One of the most simple and
generally useful electroscopic instruments is the pendulous pith
ball already mentioned (l.), the action of which may now be
more fully explained. When an electrified body is presented to
such a ball suspended by a silken thread, it acts by induction upon
it, decomposing, its natural fluid, attracting the constituent of the
contrary name to the side of the ball nearest to it, and repelling
the fluid of the same name to the side most remote from it. The
body will thus act at once by attraction and repulsion upon the
two fluids ; but since that of a contrary name which it attracts is
nearer to it than that of the same name which it repels, and equal
in quantity, the attraction will prevail over the repulsion, and the
loall will move towards the electrified body. When it touches it,
the fluid of a contrary name, which is diffused round the point of
contact, combining with the fluid diffused upon the body, will be
neutralised, and the ball will remain charged with the fluid of the
same name as that with which the body is electrified, and will con-
sequently be repelled by it. Hence it will be understood why,
as already mentioned, the pith ball in its neutral state is first at-
tracted to an electrified body, and after contact with it repelled
by it.
60. The needle electroscope. — The electric needle is an
electroscopic apparatus, somewhat less simple, but more sensitive
than the pendulum. It consists of a rod of copper terminated by
ELECTROSCOPES.
Fig. jo.
two metallic balls B and B', Jig. 30., which
are formed hollow in order to render them
more light and sensitive. At the middle
point of the rod which connects them is a
conical cup, formed of steel or agate, sus-
pended upon a fine point, so that the needle
is exactly balanced, and capable of turning
freely round the point of support in a hori-
zontal plane, like a magnetic needle. A
very feeble electrical action exerted upon either of the balls B or B'
will be sufficient to put the needle in motion.
6l. Coulomb's electroscope. — The electroscope of Coulomb,
better known as the balance of
torsion, is an apparatus still more
sensitive and delicate, for indicat-
ing the existence and intensity of
electrical force. A needle gg', fig.
31., formed of gum-lac, is sus-
pended by a fibre of raw silk. At
one extremity it carries a small
disc e, coated with metallic foil,
and is so balanced at the point of
suspension, that the needle resting
horizontally is free to turn in
either direction round the point of
suspension. When it turns it pro-
duces a degree of torsion or twist
of the fibre which suspends it, the
reaction of which measures the
force which turns the needle. Up-
on the glass cage v v', which is cy-
lindrical, is a graduated circle dd',
which measures the angle through which the
needle is deflected. In the cover of the cage
an aperture is made, through which may be in-
troduced the electrified body whose force it is
desired to indicate and measure by the apparatus.
62. Quadrant electrometer. — This instru-
ment, which is generally used as an indicator on
the conductors of electrical machines, consists of
a pillar A B,^. 32., of any conducting substance,
terminated at the lower extremity by a ball B. A
rod, also a conductor, of about half the length, ter-
minated by a small pith ball D, plays on a centre
c in a vertical plane, having behind it an ivory
D
Fig 31
Fig. jz.
34 ELECTRICITY.
semicircle graduated. When the ball B is charged with electricity,
it repels the pith ball D, and the angle of repulsion measured on
the graduated arc supplies a rough estimate of the intensity of the
electricity.
63. Gold leaf electroscope. — A glass cylinder A BCD, fig.
33., is fixed on a brass stand E, and closed at the top by a
Fig. 33. Fig. 34.
circular plate A B. The brass top G is connected by a metallic rod
with two slips of gold leaf f, two or three inches in length, and
half an inch in breadth. In their natural state they hang in con-
tact, but when electricity is imparted to the plate G, the leaves
becoming charged with it indicate its presence, and in some degree
its intensity, by their divergence. On the sides of the glass
cylinder opposite the gold leaves are attached strips of tinfoil^
communicating with the ground. When the leaves diverge so
much as to touch the sides of the cylinder, they give up their elec-
tricity to the tinfoil, and are discharged. This instrument may
also be affected inductively. If an electrified body B (Jig. 34.),
be brought near to the knob A, its natural electricity will be de-
composed ; the fluid of the same name as that with which the body
is charged will be repelled, will accumulate in the gold leaves ee\
and will cause them to diverge.
64. [Condensing electroscope. — This instrument consists of
a gold-leaf electroscope connected with a condenser (51. and
54-56.). As usually made, the condenser is screwed on the top of
ELECTROSCOPES.
35
the electroscope, the condensing plate being in connexion with
the gold leaves, and the collecting plate being laid upon it. This
form of the instrument is represented infigs. 35. and 36., which
also show the manner of using it. The collecting plate P, fig. 35.,
being laid on the condensing plate, but prevented from touching
it by a thin sheet of glass or mica, F, or by a coating of varnish,
the body, M, whose, electricity is to be tested, is brought in contact
with the upper plate, and at the same time the lower plate is un-
insulated by touching it with the finger. Some of the electricity
of M is thus communicated to the plate P, and there, acting in-
ductively on the lower plate, repels thence into the ground a
portion of electricity of the same kind as itself, and attracts
thither an equal quantity of the opposite electricity. The lower
*
Fig. 35-
Fig. ?6
plate, being thus charged with the contrary electricity to that on
M and P, reacts inductively on P, as explained in (50.), enabling
it to receive a larger charge from M than it otherwise would do.
This additional charge, in its turn, causes a further accumulation
36 ELECTRICITY.
of the opposite electricity on the lower plate, and thus the two
plates act and react until equilibrium is established. The finger
is now removed from the lower plate, and then the source of
electricity to be tested is removed from the plate P. On after-
wards raising the collecting plate by its insulating handle, as
shown in jig. 36., the electricity accumulated in the lower plate,
and hitherto held disguised by the opposite electricity of the other
plate, becomes free and distributes itself over the gold leaves,
causing them to diverge.
Or, the body to be tested may be put in electrical communica-
tion with the lower plate, which then becomes the collector, while
the upper plate, which then becomes the condensing plate, is
touched with the finger. In this case the electricity with which
the leaves diverge is similar to that of the body M : in the first
way of using the instrument it is of the opposite kind.]
CHAP. VII.
THE LETDEN JAR.
65. THE inductive principle which has supplied the means, in the
case of the condenser, of detecting and examining quantities of
electricity so minute and so feeble as to escape all common tests,
has placed, in the Leyden jar, an instrument at the disposal of the
electrician, by which artificial electricity may be accumulated in
quantities so unlimited, as to enable him to copy in some of its
most conspicuous effects the lightning of the clouds.
To understand the principle of the Leyden jar, which at one
time excited the astonishment
of all Europe, it is only neces-
sary to investigate the effect
of a condenser of considerable
magnitude placed in connec-
tion, not with feeble, but with
energetic sources of electricity,
such as the prime conductor
of an electrical machine. In
such case it would be evi-
dently necessary, that the col-
lecting and condensing plates
should be separated by a non-
Fig. 38. conducting medium, of sum-
LEYDEN JAR. 37
cient resistance to prevent the union of the powerful charges, with
which they would be invested.
Let ~?,fig' 38., represent the collecting plate of such a con-
denser, connected by a chain /' with the conductor of an electric
machine ; and let p7 be the condensing plate connected by a chain
/ with the ground. Let A be a plate of glass interposed between
p and P'.
Let e express the quantity of electricity with which a superficial unit of
the conductor is charged. It .follows that e will also express the/ree elec-
tricity on every superficial unit of the collecting plate p ; and if the total
charge on each superficial unit of p, free and dissimulated, be expressed by
a, we shall, according to what has been already explained, have
The charge on the superficial unit of the condensing plate p' being ex
pressed by a', we shall have
mxe
which will be wholly dissimulated.
If s express the common magnitude of the two plates P' and p, and E
express the entire quantity of electricity accumulated on p, and E' that
accumulated on p', we shall have
It is evident, therefore, that the quantity of electricity with which the
plates P and p' will be charged, will, be augmented, firstly, with the magni-
tude (s) of the plates ; secondly, with the intensity («) of the electricity
produced by the machine upon the conductor ; and thirdly, with the thin-
ness of the glass plate A which separates the plates P' and p. The thinner
this plate is, the more nearly equal to 1 will be the number m, and conse-
quently the less will be 1 — m9, and the greater the quantity E.
When the machine has been worked until e ceases to increase, the charge
of the plates will have attained its maximum. Let the chains / and /'
be then removed, so that the plates P and p' shall be insulated, being
charged with the quantities of electricity of contrary names expressed by
E and E'.
If a metallic wire, or any other conductor, be now placed so as to con-
nect the plate p with the plate p', the free electricity on the former passing
along the conductor will flow to the plate p' where it will combine with or
neutralise a part of the dissimulated fluid. This last, being thus diminished
in quantity, will retain by its attraction a less quantity of the fluid on p' a
corresponding quantity of which will be liberated, and will therefore pass
along the wire to the plate p', where it will neutralise another portion of the
dissimulated fluid ; and this process of reciprocal neutralisation, liberation,
and conduction will go on until the entire charge E' upon the plate p' has
been neutralised by a corresponding part of the fluid E originally diffused on
the plate p.
38 ELECTRICITY.
Although these effects are strictly progressive, they are practically in-
stantaneous. The current of free electricity flows through the wire, neutra-
lises the charge E', and liberates all the dissimulated part of E in an interval
so short as to be quite inappreciable. In whatever point of view the power
of conduction may be regarded, a sudden and violent change in the electrical
condition of the wire must attend the phenomenon. If the wire be regarded
merely as a channel of communication, a sort of pipe or conduit through
which the electric fluid passes from p to p', as some consider it, so large an
afflux of electricity may be expected to be attended with some violent
effects. If, on the other hand, the opposite fluids are reduced to their na-
tural state, by decomposing successively the natural electricity of the parto
of the wire, and taking from the elements of the decomposed fluid the elec-
tricities necessary to satisfy their respective attractions, a still more power-
ful effect may be anticipated from so great and sudden a change.
It appears, from what has been stated, that all the negative
electricity collected upon the plate p' is dissimulated by the attrac-
tion of the positive electricity collected upon p ; and that, on
the other hand, the negative electricity on p', dissimulating a
proportionate quantity of the positive fluid on p, leaves the
excess free ; and this excess, acting upon the electric pendulum,
repels the ball from p. But if the apparatus be so arranged, as
Fig. 39-
shown in jig. 39., that the two plates may be withdrawn from
each other, and from the intermediate plate A, the chief part or
the whole of the fluids upon p and p' may be rendered free.
For this purpose, after the plates have been charged in the manner
described above, let the wire /', connecting p with the electrical
machine, and the wire /, connecting p' with the ground, be both
detached from the pillars, so as to leave the plates p and P' at once
insulated and charged. This being done, if the plates be removed
LEYDEN JAR. 39
from A, as shown in jig. 39., the electric pendulum on P', as well
as that on p, will be immediately repelled, showing that the nega-
tive fluid on P', or part of it, is rendered free by the removal
of the plate P.
The plates, P and p', being charged in the manner described,
and the wires f and f being detached, so as to leave them thus
charged upon the insulating pillars, they may be discharged either
by slow degrees or instantaneously.
To discharge them by slow degrees, let a metallic knob, which is
in connection with the ground, be applied to P, and it will draw oft
from it all the positive fluid which is not dissimulated by the
negative fluid on p'. But the plate P being at some distance, how-
ever small, from the plate P', can only dissimulate upon p' a portion
of fluid somewhat less than its own quantity.
It will, therefore, follow, that after the knob has been applied
to P, the quantity of negative fluid on p' will exceed the quantity
of positive fluid on p, and, consequently, a certain portion of the
negative fluid on p' will be free ; and this will be, accordingly,
rendered manifest by the repulsion of the electrical pendulum on p'.
Meanwhile all the positive electricity on p being dissimulated, the
pendulum on p will not be repelled.
It appears, therefore, that the relative electrical conditions of
the two plates p and p' have been interchanged, p' being now that
which repels the pendulum by its surplus free electricity, while P
does not affect it.
If the conducting knob connected with the ground be now ap-
plied to P', it will draw off the free electricity, and the pendulum
on P' will be no longer repelled. It will at the same time liberate
a portion of the electricity on p, which will be indicated by the
repulsion of the pendulum.
The same process may then be repeated upon p, and so on
alternately until all the electricity upon the two plates has been
drained off, as it were, drop by drop.
To discharge the plates instantaneously, it is only necessary to
connect them electrically by any conductor, such as a rod or wire
of metal placed in contact with each. The effect of such a con-
nection will be, to produce in an inappreciable instant of time all
the interchanges which have been just described. At first the free
electricity of p will rush towards P', and a portion of the dissimu-
lated fluid on p', being thus liberated, will rush towards p; a further
portion of the fluid on which being thereby liberated, will rush
towards p' ; and so on. Although these effects, regarded theoreti-
cally, must be considered as taking place successively, they will be
practically instantaneous, the whole interval of their accomplish-
ment being inappreciable.
D4
4o ELECTRICITY.
66. The fulminating- pane was one of the final and most simple
forms given to the condenser.
This consisted of a glass plate, fig. 40., enclosed in a frame, and having
a square leaf of tinfoil attached to each side of it, the leaf on one side
being connected with the frame by a ribbon of foil. To charge this, the
operator places the side on which the foil is connected with the frame by
the ribbon downwards, and connects the ribbon with the ground by a chain
or other conductor. He then connects the upper leaf of foil E with the prime
conductor of the machine by means of a jointed discharger c, as shown in
the figure. The machine being worked, the upper leaf becomes charged
with positive electricity, which, acting upon the natural electricities of the
lower leaf, decomposes them, and produces the same effects as have been
described in the case of the apparatus fig. 39. ; and the two leaves of tinfoil
will become charged with opposite electricities, as in the former case, and
may be discharged either gradually or instantaneously, in the manner already
described.
Fig. 40.
The class of phenomena evolved by these expedients has been
attended with some of the most remarkable effects presented in
the whole domain of physical research. If two such conductors
as the plates of tinfoil attached to the fulminating pane, being
strongly charged in the manner just described, be put in commu-
nication by the human body, which may be done by touching one
plate with the fingers of one hand, and the other with the fingers
of the other, the two electric fluids, in rushing towards each other,
pass through the body, producing the phenomenon now rendered
so familiar, called the electric shock, and which, though so little
LEYDEN JAR. 4 1
regarded at present, produced, when first experienced, the most
extraordinary impressions.
Like many other important scientific facts, the discovery of the
electric shock, and of the apparatus by which it is most commonly
produced, was the result of accident. In 1 746 the celebrated
Musschenbroeck, having fixed a metallic rod in the cork of a
bottle filled with water, he presented it to the electrical machine
for the purpose of electrifying the water, holding at the same time
the bottle in his hand by its external surface, without touching the
metallic rod by which the electricity was conducted to the water.
By this accidental circumstance a real condenser was formed, of
which the experimenter was totally unconscious, and the principle
of which was then wholly unknown. The water in contact with
the internal surface of the bottle, and receiving the electricity by
the metallic rod from the machine, corresponded to the plate P
(./%"• 38-)' an(i the metallic rod to the conducting wire./'. The
hand of the operator applied to the external surface of the bottle
corresponded to the plate P', and the body of the operator commu-
nicating with the ground corresponded to the wire f. In the
same manner exactly, therefore, as in the case of the apparatus
shown in jig. 39., the inside of the bottle acquired a strong charge
of positive, and the outside an almost equally strong charge of
negative, electricity. The operator, then ignorant of the effects,
withdrawing the bottle from the machine, and desiring to remove
from the mouth of it the wire by which it was charged, applied his
left hand to the latter for that purpose, still holding the bottle by its
exterior surface in his right hand. His arms and body, therefore,
becoming a conductor between the interior and exterior surfaces
of the bottle, the electric fluids, in reuniting, passed through him,
and inflicted, for the first time, the nervous commotion now known
as the electric shock. Nothing could exceed the astonishment and
consternation of the operator at this unexpected sensation, and in
describing it in a letter addressed immediately afterwards to
Reaumur, he declared that for the whole kingdom of France he
would not repeat the experiment.
The experiment, however, was soon repeated in different parts
of Europe, and the apparatus by which it was produced received a
more convenient form, the water being replaced by tinfoil attached
to the interior of the jar, which received the name of the Ley den
jar, or Ley den phial, the city of Leyden being the place where its
remarkable effects were first exhibited.
67. Tfce Xieylen jar. — In experimental researches, therefore,
the form which is commonly given to the apparatus, with a view
to develop the above effects, is that of a cylinder or jar, AH
(fig. 41.), having a wide mouth and a flat bottom
42
ELECTKICITY.
Fig. 41.
The shaded part terminating at c is a coating of tinfoil
placed on the bottom and sides of the jar, a similar coating
being attached to the corresponding parts of the interior sur-
face. To improve the insulating power of the glass, it is
coated above the edge of the tinfoil with a varnish of gum-
lac, which also renders it more proof against the deposition
of moisture. A metallic rod, terminated in a ball D, descends
into the jar, and is fixed in contact with the inner coating.
To understand the action of this apparatus it is only neces-
sary to consider the inner coating and the metallic rod as
representing the metallic surface p, fig. 38., and the outer
coating of the surface p', the jar itself playing the part of
the intervening nonconducting medium. If the ball D be
put in communication by a metallic chain with the con-
ductor of the electric machine, and the external coating
c B with the ground, the jar will become charged with electricity, in the
same manner and on the same principles exactly as has been explained in
the case of the metallic surfaces P and p', fig. 38.
If, when a charge of electricity is thus communicated to the jar, the com-
munication between D and the conductor be removed, the charge will remain
accumulated on the inner coating of the jar. If in this case a metallic com-
munication be made between the ball D and the outer coating, the two oppo-
site electricities on the inside and outside of the jar will rush towards each
other, and will suddenly combine. In this case there is no essential distinc-
tion between the functions of the outer and inner coating of the j ar, as may
be shown by connecting the inner coating \vith the ground and the outer
coating with the conductor. For this purpose it is only necessary to place
the jar upon an insulating stool, surrounding it by a metallic chain in contact
with its outer coating, which should be carried to the conductor of the
Fig. 41.
machine ; while the ball D, which communicates with the inner coating, is
connected by another chain to the ground. In this case the electricity will
LEYDEN JAR, 4.3
flow from the conductor to the outer coating, and will be accumulated there
by the inductive action of the inner coating, and all the effects will take
place as before.
If, after the jar is thus charged, the communication between the outer coat-
ing and the conductor be removed, and a metallic communication be made
be'tween the inner and outer coating, the electricities will, as before, rush
towards each other and combine, and the jar will be restored to its natural
state.
To charge the jar internally, it will be sufficient to hold it with the hand
in contact with the external coating, fig. 42., presenting the ball c to the
conductor of the machine. The electricity will flow from the conductor to
the inner coating, and the external coating will act inductively, being con-
nected through the hand and body of the operator with the earth.
Like the apparatus shown in fig. 38., the Leyden jar may be discharged
either gradually or instantaneously. To discharge it instantaneously, without
suffering the electric shock, let the jar A, fig. 43., be placed with its ex-
F'S-43-
ternal coating in communication with the ground, and let the operator, apply-
ing one knob c' of a jointed discharger D to the external coating, bring the
other c near to the knob B of the jar. Under these circumstances, the two
fluids rushing towards each other, along the arms of the discharger, will
reunite, and the jar will be discharged.
The process of slow discharge may be executed in the following manner.
The rod which enters the jar has attached to the top of it a small bell, i,
fig. 44. ; placed near the bottle, upon a convenient stand, is a metallic rod,
p, supporting a similar bell, E, level with I ; and an electric pendulum, con-
sisting of a small copper ball, suspended by a silken thread, hangs between
ELECTRICITY
the two bells, so that it
can be attracted and re-
pelled by the one and
the other. Supposing the
jar to be charged, and
its external coating con-
nected with p by a con-
ductor e, and the stand
to be insulated, the free
part of the positive elec-
tricity on the interior
of the jar will attract the
copper ball, which will
strike the bell I ; and be-
coming charged with po-
sitive electricity, will be
repelled by I, and at-
tracted by E; it will,
therefore, strike against
E, and will impart to it
the positive electricity,
and receive from it a
charge of negative elec-
tricity, proceeding from
the outside coating of
the jar through the pillar
p. The copper ball being
negatively electrified,
will then be repelled by
E, and attracted by i, against which it will strike, and will convey to the
interior of the jar the negative fluid which it carries, receiving in exchange
an equal charge of the positive fluid.
In this way the pendulum will oscillate between the two bells, conveying
successive portions of positive electricity from the interior to the exterior, and
of negative electricity from the exterior to the interior.
Effect qf tlie metallic coatings. — The metallic coatings
of the jar have no other effect than to conduct the electricity to
the surface of the glass, and when there to afford it a free passage
from point to point. Any other conductor would, abstractedly
considered, serve the same purpose ; and metallic foil is selected
only for the facility and convenience with which it may be adapted
to the form of the glass, and permanently attached to it. That
like effects would attend the use of any other conductor may be
easily shown.
68. Experimental proof that the charge adheres to the
glass, and not to the coating. — The electricity with which the
jar is charged in this case resides, therefore, on the glass, or on
the conductor by which it passes to the glass, or is shared by
these.
To determine where it resides, it is only necessary to provide
Fig- 44.
LEYDEN JAR.
means of separating the jar from the coating after it has been
charged, and examining the electrical state of the one and the
other. For this purpose let a glass jar B, fig. 45., be provided,
having a loose cylinder of metal c fitted to its interior, which can
be placed in it or withdrawn from it at pleasure, and a similar
loose cylinder A fitted to its exterior. The jar being placed in the
external cylinder A, and the internal cylinder c being inserted in
it, as shown at D, let it be charged with electricity by the machine
in the manner already described. Let the internal cylinder be
Fig. 45-
then removed, and let the jar be raised out of the external cy-
linder. The two cylinders, being then tested by an electroscopic
apparatus, will be found to be in their natural
state. But if an electroscope be brought within
the influence of the internal or external surface
of the glass jar, it will betray the presence of the
one or the other species of electricity. If the glass
jar be then inserted in another metallic cylinder
made to fit it externally, and a similar metallic
cylinder made to fit it internally be inserted in it,
it will be found to be charged as if no change
had taken place. On connecting by metallic
communication the interior with the exterior, the
opposite electricities will rush towards each other
and combine. It is evident, therefore, that the
seat of the electricity, when a jar is charged, is
not the metallic coating, but the surface of the
glass under it.
69. Improved form of the Leyden jar. —
An improved form of the Leyden jar is repre-
sented in fig. 46. Besides the provisions which
Fig. 46.
4.6
ELECTRICITY.
have been already explained, there is attached to this jar a
hollow brass cup c, cemented into a glass tube. This tube
passes through the wooden disc which forms the cover of the jar,
and is fastened to it. It reaches to the bottom of the jar. A com-
munication is formed between c and the internal coating by a brass
wire terminating in the knob D. This wire, passing loosely through
a small hole in the top, may be removed at pleasure for the purpose
of cutting off the communication between the cup and the inte-
rior coating. This wire does not extend quite to the bottom of
the jar, but the lower part of the tube is coated with tinfoil,
which is in contact with the wire, and extends to the inner coating
of the jar.
At the bottom of the jar a hook is provided, by which a chain
may be suspended so as to form a communication between the ex-
ternal coating and other bodies. When a jar of this kind is once
charged, the wire may be removed or allowed to fall out by in-
verting the jar, in which case the jar will remain charged, since no
communication exists between its internal and external coating ;
and as the internal coating is protected from the contact of the
external air, the absorption of humidity in this case is prevented.
An electric charge may thus be transferred from place to place,
and preserved for a considerable length of time.
In the construction of cylindrical jars it is not always possible to
obtain glass of uniform thickness, for which reason jars are some-
times provided of a spherical form.
70. Lane's discharging electrometer (./fc. 47-) consists of
a bent glass rod, ABC, at one
D end, c, of whiqh a socket is
placed, by which it may be
attached to a conductor, or
to the rod of a Leyden jar,
as shown in the figure. To
the other end is attached a
short cylindrical rod A pierced
by a hole, through which a
brass rod DE slides, having
balls D and E at its extremities.
When the instrument is used,
one of the balls, D for ex-
ample, is put in communica-
tion with the ground, or with
the external coating of the
jar. The rod D E is then ad-
vanced through the hole A
until it comes so near to the
fig. 47.
JAR.
47
ball of the jar that a spark passes between them, and the jar is
discharged. The force of the charge is estimated by the distance
between the balls at which the spark passes.
The indications of this instrument are modified by so many
causes, that as a measure of the electric force of the charge it has
but little value. The distance through which the spark will be
projected will vary with the hygrometric state of the air, with its
temperature, and probably with other physical conditions. It will
also vary with the magnitude and form of the conductor, or the
knob of the jar to which it is presented.
71. Cuthbertson's discharging- electrometer. — Fig. 48. con-
sists of two glass pillars supported on a wooden table ; upon these
Fig. 48.
are fixed two brass balls B and E. Through the ball B an opening
is cut, in which the lever c D' terminated in brass balls is inserted,
and in which it is balanced on a knife edge. A small sliding
weight L is placed on the arm BD', by the adjustment of which
any desired preponderance can be given to the opposite arm c B,
which is the heavier when BD' is unloaded. The arm BD' is gra-
duated to indicate the number of grains weight at the centre of
the ball D', which would be in exact equilibrium with the pre-
ponderance which c has in each position of L. Another arm B D,
fixed to the ball B, is terminated in a ball D, which is in contact
with D', when the lever CD' is horizontal. By the chain G the
balls c, D, and D' can be put in communication with the internal
coating of the jar, the free electricity of which will therefore
charge the balls D and D', and by the chain F the ball E is put in
communication with the external coating, the electricity of which,
being dissimulated, will not affect the ball E. The balls D and D',
being similarly electrified, will repel each other, and as soon as
the charge of the jar is so great that the repulsive force given to
4.8 ELECTRICITY.
the balls D and D' is sufficient to overcome the preponderance of
the ball c, the ball D' will be repelled by D ; and when the former
comes into contact with E, the jar will be discharged.
Another form of this instrument, with a quadrant electrometer
attached, is shown in Jig. 49., the corresponding parts being indi-
Fig. 49.
Fig. 50.
cated by the same letters. In this case D and D', receiving elec-
tricity from the inner coating, repel each other. The knife edge
is within B, and the repulsion depresses c until it touches E, when
the discharge is effected.
72. Harris's circular electrometer. — Fig. 50. is an instru-
ment which is often substituted with advantage for the quadrant.
It depends on the same principle, but is more sensitive and ac-
curate.
73. Charging a series of jars by cascade. — In charging a
single jar, an unlimited number of jars, connected together by
conductors, may be charged with very nearly the same quantity
ELECTRIC BATTERY 49
of electricity. For this purpose let the series of jars be placed on
insulating stools, as represented in Jig. 51. and let c be metallic.
Fig. 51.
chains connecting the external coating of each jar with the in-
ternal coating of the succeeding one. Let D be a chain connecting
the first jar with the conductor of the machine, and D' another chain
connecting the last jar with the ground. The electricity con-
veyed to the inner coating of the first jar A acts by induction on
the external coating of the first jar, attracting the negative elec-
tricity to the surface, and repelling the positive electricity through
the chain c to the inner coating of the second jar. This charge of
positive electricity in the second jar acts in like manner induc-
tively on the external coating of this jar, attracting the negative
electricity there, and repelling the positive electricity through the
chain c to the internal coating of the third jar ; and in the same
manner the internal coating of every succeeding jar in the series
will be charged with positive electricity, and its external coating
with negative electricity. If, while the series is insulated, a dis-
charger be made to connect the inner coating of the first with the
outer coating of the last jar, the opposite electricities will rush
towards each other, and the series of jars will be restored to their
natural state.
74. Electric battery. — When several jars are thus combined
to obtain a more energetic discharge than could be formed by a
single jar, the system is called an electric battery, and the method
of charging it, explained above, is called charging by cascade.
After the jars have been thus charged, the chains connecting
the outer coating of each jar with the inner coating of the suc-
ceeding one are removed, and the knobs are all connected one
with another by chains or metallic rods, so as to place all the in-
ternal coatings in electric connection, and the outer coatings are
similarly connected. By this expedient the system of jars is ren-
dered equivalent to a single jar, the magnitude of whose coated
surface would be equal to the sum of all the surfaces of the series
of jars. The battery would then be discharged, by placing a con-
ductor between the outer coating of any of the jars and one of the
kuobs.
50 ELECTRICITY.
[When an electric battery is charged by cascade, each jar re-
ceives a smaller charge than the one which precedes it, and a larger
charge than the following one : the charge of the second jar is in
fact only equal to what that of the first would be if the thickness
of the glass were doubled ; for the inductive action by which its
charge is produced takes place through two thicknesses of glass
instead of only one. Similarly, the charge of the third jar is pro-
duced by inductive action taking place through three thicknesses
of glass, and is therefore equal to what the first jar would receive
if the glass were made three times as thick : and so on of the
others.]
75. Common electric battery. — Hence, in order to charge
all the jars to the full extent, they are commonly placed in a box,
as represented in jig. 5 1 ., coated on the inside with tinfoil, so as
to form a metallic communication between the external coating
of all the jars. The knobs, which communicate with their in-
ternal coating, are connected by a series of metallic rods in the
manner represented in the figure ; so that there is a continuous
metallic communication between all the internal coatings. If the
Fig. 5J.
metallic rods which thus communicate with the inner coating be
placed in communication with the conductor of a machine, while
ELECTRIC BATTERY. 51
the box containing the jars is placed in metallic communication
with the earth, the battery will be charged according to the
principles already explained in the case of a single jar, and the
force of its charge will be equal to the force of the charge of a
single jar, the magnitude of whose external and internal coating,
would be equal to the sum of the internal and external coating of
all the jars composing the battery.
The manner in which a battery is charged by connecting it with
a conductor of an electric machine, is shown in jig. 53., an elec-
trometer being usually fixed on one of the pivots to indicate the
strength of the charge.
The method of discharging the battery and transmitting its
charge through an object submitted to experiment, is shown in
fig. 54. The object under experiment is placed on a convenient
stand between the knobs of two insulated conductors, one of which
communicates with the outside coating of one of the jars. The
other is put in communication with the inside coating of a jar, by
means of a jointed discharger.
76. To estimate tlie amount of the charge of a jar or bat-
tery, it is to be considered that the internal coating is, in effect, a
continuation of the conductor ; and if the jars had no external
coating, the communication of the internal coating with the con-
ductor would be attended with no other effect, than the distribution
of the electricity over the conductor and the internal coating,
according to the laws of electrical equilibrium ; but the effect of
the external coating is to dissimulate or render latent the electri-
city as it flows from the conductor, so that the repulsion of the
part of it which remains free is less than the expansive force of the
electricity of the conductor, and a stream of the fluid continues
to flow accordingly from the conductor to the internal coating ;
and this process continues until the increasing force of the free
t,z ELECTRICITY.
electricity on the internal coating of the jars becomes so great,
that the force of the fluid on the conductor can no longer over-
come it, and thus the flow of electricity to the jars from the con-
ductor will cease.
It follows, therefore, that during the process of charging the jars, the
depth or tension of the electricity on the conductor, is just so much greater
than that of the free electricity on the interior of the jars, as is sufficient to
sustain the flow of electricity from the one to the other ; and as this is
necessarily so extremely minute an excess as to be insensible to any measure
which could be applied to it, it may be assumed that the depth of electricity
on the conductor is always equal to that of the free electricity on the in-
terior of the jars. If e therefore express the actual depth of the electric
fluid at any time on the interior coating (l-»*2)xe will express the depth
of the free electricity ; and since, throughout the process, m does not change
its value, it follows that the actual depth of electricity, and therefore the
actual magnitude of the charge, is proportionate to the depth of free elec-
tricity on the interior of the jar, which is sensibly the same as the depth of
free electricity on the conductor. It follows, therefore, that the magnitude
of the charge, whether of a single jar or several, will always be proportionate
to the depth of electricity on the conductor of the machine from which the
charge is derived. If, therefore, during the process of charging a jar or
battery, an electrometer be attached to the conductor, this instrument will
at first give indications of a very feeble electricity, the chief part of the fluid
evolved being dissimulated on the inside of the jars; but as the charge in-
creases, the indications of an increased depth of fluid on the conductor
become apparent ; and at length, when no more fluid can pass from the con-
ductor to the jars, the electrometer becomes stationary, and the fluid evolved
by the machine escapes from the points or into the circumjacent air.
The quadrant electrometer, described in (62.), is the indicator
commonly used for this purpose, and is inserted in a hole on the
conductor. When the pith ball attains its maximum elevation, the
charge of the jars maybe considered as complete. The charge
.which ajar is capable of receiving, besides being limited by the
strength of the glass to resist the mutual attraction of the opposite
fluids, and the imperfect insulating force of that part of the jar
which is not coated, is also limited by the imperfect insulating
force of the air itself. If other causes, therefore, allowed an
unlimited flow of electricity to the jar, its discharge would at
length take place, by the elasticity of the free electricity within it
surmounting the resistance of the air, and accordingly the fluid of
the interior would pass over the mouth of the jar, and unite with
the opposite fluid of the exterior surface.
j6a. [Residual charge.— \Yhen a Leyden jar or an electric
battery has been discharged, in any of the ways above described,
it is usually found that, after the lapse of a few minutes, a second
discharge — called the residual discharge — can be obtained from it.
This discharge, though much weaker than the first, is often strong
ELECTRIC BATTERY. 53
enough, with a large battery, to produce a painful shock if it
passes through the body.
To understand this effect, we must remember that the coatings
on the two sides of the jar are charged with opposite electricities ;
that these, owing to their self-repulsive properties, tend not only
to escape from the coatings into the surrounding air, but also
to penetrate into the glass; and that this latter tendency is
strengthened by the attraction which the electricity of each coat-
ing exerts upon that of the opposite one. Consequently, since
glass does not entirely prevent the motion of electricity, but only
opposes so much resistance to it as to make it very slow, the two
electricities not only pass from the coatings to the surface of the
glass (68.), but actually penetrate gradually into its substance.
When the jar is discharged, one of the forces which caused the
penetration of the electricities into the glass, namely, the repulsion
of the electricity on the surface, is removed. Accordingly, the
repulsion which the several particles of each electricity exert upon
each other, causes the electricities to return gradually to the sur-
face of the glass ; for the mutual attraction of the electricities on
the opposite sides, which is now the only force tending to prevent
this return, is less powerful than the repulsion which tends to
produce it, inasmuch as it acts at a distance through a greater or
less thickness of glass. If, therefore, the two coatings are con-
nected by a conductor a few minutes after the first discharge, a
second discharge will be obtained, and sometimes indeed, after
H further interval, a third discharge may be obtained in- like
manner.
In working with a large Leyden jar, and especially with a
Lattery of several jars, it is very needful to be aware of this
phenomenon of the residual charge : for if an experimenter, sup-
posing that all the electricity had been removed from the jar
or battery by the first discharge, were soon afterwards to touch a
•conductor connected with the inside coating, while the outside
coating was in communication with the ground, or with some
other part of his body, he would receive a shock which would be
at least startling, if not painful.]
54 ELECTRICITY.
CHAP. VIII.
LAWS OF ELECTRICAL FORCES.
77. Electric forces investigated by Coulomb. — It is not
enough to ascertain the principles which govern the decomposition
of the natural electricity of bodies, and the reciprocal attraction
and repulsion of the constituent fluids. It is also necessary to
determine the actual amount of force exerted by each fluid in
repelling fluid of the like or attracting fluid of the opposite kind,
and how the intensity of this attraction is varied, by varying the
distance between the bodies which are invested by the attracting
or repelling fluids.
By a series of experimental researches, which rendered his name
for ever memorable, Coulomb solved this difficult and delicate pro-
blem, measuring with admirable adroitness and precision these
minute forces, by means of his electroscope or balance of torsion,
already described (61.).
78. Proof-plane. — The electricity of which the force was to
be estimated was taken up from the surface of the electrified body
upon a small circular disc c, fig. 55., coated with me-
tallic foil, and attached to the extremity of a delicate rod
or handle, AB, of gum-lac. This disc, called a proof -plane,
was presented to the ball suspended in the electrometer of
torsion (6 1.), and the intensity of its attraction or repul-
sion was measured, by the number of degrees through
which the suspending fibre or wire was twisted by it.
The extreme degree of sensibility of this apparatus may
be conceived, when it is stated that a force equal to the
Fig. 55. 34Oth part of a grain was sufficient to turn it through
360 degrees ; and since the reaction of torsion is propor-
tional to the angle of torsion, the force necessary to make the
needle move through one degree would be only the izz^oothpart
of a grain. Thus this balance was capable of dividing a force
equal to a single grain weight into 122400 parts, and rendering
the effect of each part distinctly observable and measurable.
79. Law of electrical force similar to that of gravitation.
— By these researches it was established that the attraction and
repulsion of the electric fluids, like the force of gravitation, and
other physical influences which radiate from a centre, vary accord-
ing to the common law of the inverse square of the distance ; that is
to say, the attraction or repulsion exerted by a body charged with
electricity, or, to speak more correctly, by the electricity with
which such a body is charged, increases in the same proportion as
LAWS OF ELECTRIC FORCE. 55
the square of the distance from the body on which it acts is dimi-
nished, and diminishes as the square of that distance is increased.
In general, if / express the force exerted by any quantity of
electric fluid, positive or negative, at the unit of distance, --^ will
express the force which the same quantity of the same fluid will
exert at the distance D.
In like manner, if the quantity of fluid, taken as the unit, exercise
at the distance D the force expressed by -^, the quantity expressed
by E, will exert at the same distance D the force F expressed by
These formulae have been tested by numerous experiments made
under every possible variety of conditions, and have been found to
represent the phenomena with the greatest precision.
So. The distribution of the electric fluid on conductors can
be deduced as a mathematical consequence of the laws of attraction
and repulsion, which have been explained above, combined with
the property in virtue of which conductors give free play to these
forces. The conclusions thus deduced may further be verified by
the proof-plane and electrometer of torsion, by means of which the
fluid diffused upon a conductor may be gauged, so that its depth
or intensity at every point may be exactly ascertained ; and such
Fig. 56.
depths and intensities have accordingly been found to accord per-
fectly with the results of theory.
8 1 . It is confined to their surfaces. — If an electrified con -
E4
ELECTRICITY.
ductor be pierced with holes, a little greater than the proof-plane,
(fis. ^6.) to different depths, that plane, inserted so as to touch
the bottom of these holes, will take up no electricity.
If a spheroidal metallic body
A (fg. 57.)? suspended by a silken
thread, be electrified, and two
thin hollow caps, B B and B' B',
made to fit it, coated on their
inside surface with metallic foil,
and having insulating handles
c c' of gum-lac, be applied to it,
on withdrawing them the sphe-
roid will be deprived of its elec-
Fig. 57. tricity, the fluid being taken off
by the caps.
The same experiment may be performed conveniently by the
apparatus shown in Jig. 58., consisting of a metallic spheroid sup-
Fig. 58
ported on an insulating pillar, and two hollow hemispheroids of
corresponding magnitude, with insulating handles.
82. The charge of electricity upon a conductor being therefore
superficial, it follows that its depth or intensity, other things being
the same, will be less in proportion as the total surface of the con-
ductor is greater. This may be very elegantly illustrated by
means of a band of metallic foil wound round an insulated cylin-
der,^. 59. A quadrant electrometer is mounted on the end of
the insulated cylinder to indicate the varying intensity. The
band of foil being completely rolled up, let the conductor be
strongly chargecl by means of a machine. The electrometer will
then show a strong charge, the ball being thrown up to 50° or 60°.
LAWS OF ELECTRIC FORCE.
57
The machine being then detached, let the band of foil be gradually
unrolled so as to enlarge the surface of the conductor. According
Fig. 59
as this takes place, the ball of the electrometer will fall to a less
and less angle ; and if the band be again coiled up, the ball will
be again repelled, showing that the intensity of the electricity
increases as the surface is diminished, and vice versa.
83. Faraday's apparatus (Jig.bo.) also illustrates the super-
ficial distribution of electricity in a striking manner. A conical
muslin bag, like a butterfly net, is attached to an insulated ring
of metallic wire. If it be electrified, it will be found that the elec-
tricity will be confined to its exterior surface. This may be as-
certained by the proof-plane. By means of two insulated silk
threads fixed to the apex of the cone, one within and the other
without, as shown in the figure, the bag may be turned inside out,
so that the exterior surface shall become the interior, and vice
versa. The electricity will always pass to the exterior surface,
the interior being free from it.
The same principle was illustrated by Faraday in several other
ways. A cylinder of metallic ga.uze, or a trellis of iron wire, the
ELECTRICITY.
meshes of which were not very close, was placed upon a hori-
zontal metal disc, resting on an insulated support. Electricity was
then communicated to its inner surface ; but on applying the
a
Fig. 60.
proof-plane it was found that the exterior surface alone was elec-
trified. An animal, such as a mouse, placed in the interior, did
not suffer any shock even when the entire apparatus was strongly
electrified, and vivid sparks taken from it.
A hollow metal cylinder was placed on an insulated metal disc,
having a diameter a little larger than its own ; being electrified,
its exterior surface alone gave signs of electricity. It was sur-
rounded externally with small brass columns, higher than itself,
resting by their bases on the same metal disc. The electricity
was immediately distributed upon the exterior surface of these
small columns.
Faraday, in his lectures, covers his most sensitive gold leaf
electroscopes with cotton or linen nets, having loose meshes to
protect them from the influence of the surrounding electricity.
Notwithstanding the vicinity of powerful electrical machines in
action, the sensitive electroscopes thus covered are never affected
by electricity, the fluid being exclusively confined to the exterior
surface of the tissue with which they are enveloped.
Although it follows, from these and other experimental tests,
as well as from theory, that the diffusion of electricity on con-
ductors is nearly superficial, it is not absolutely so. If one end of
a metallic rod, coated with sealing wax, be presented to any source
of electricity, the fluid will be received as freely from the other
end, as if its surface were not coated with a nonconductor. It
follows from this that the electricity must pass along the rod suffi-
ciently within the surface of the metal, which is in contact with the
EFFECTS OF POINTS. 59
wax, to be out of contact with the wax, which, by its insulating
virtue, would arrest the progress of the fluid.
84. How the distribution varies. — It remains, however, to
ascertain how the intensity of the fluid, or its depth on different
parts of a conductor, varies.
There are some bodies whose form so strongly suggests the
inevitable uniformity of distribution, as to render demonstration
needless. In the case of a sphere, the symmetry of form alone
indicates the necessity of an uniform distribution. If, then, the
fluid be regarded as having an uniform depth on every part of a
conducting sphere, exactly as a liquid might be uniformly diffused
over the surface of the globe, the total quantity of fluid will be
expressed by multiplying its depth by the superficial area of the
globe.
85. Distribution on an ellipsoid. — If the electrified conductor
be not a globe, but an elliptical spheroid, such as A A? (fg. 6 1 .),
the fluid will be found to be accumulated in
greater quantity at the small ends A and A',
than at the sides B B', where there is less cur-
vature. This unequal distribution of the fluid
Fig. 61 . 1S rePresente(i by the dotted line in the figure.
It follows from theory, and it is confirmed by
observation, that the depth of the fluid at A and A' is greater than
at B B', in the ratio of the longer axis A A'
of the ellipse to the shorter axis B B'.
;• If, therefore, the ellipsoid be very elon-
gated, as in jig. 62., the depth of the fluid
Fig. 6z. at the ends A and A' will be proportionally
greater.
If a metallic body formed, as shown in fig. 63., be supported
on an insulating pillar, it will be found by the proof-plane that
the depth of the electricity will gradually increase towards the
point B, and will decrease towards A.
86. Effects of edgres and points. — If the conductor be a flat
disc, the depth of the fluid will increase from its centre towards its
edges. The depth will, however, not vary sensibly near the centre,
but will augment rapidly in approaching the edge, as represented
in Jig. 64., where A and B are the edges, and c the centre of the
disc, the depth of the fluid being indicated by the dotted line.
It is found in general that the depth of the fluid increases in a
rapid proportion in approaching the edges, corners, and extre-
mities, whatever be the shape of the conductor. Thus, when a
circular disc or rectangular plate has any considerable magnitude,
the depth of the electricity is sensibly uniform at all parts not
contiguous to the borders ; and whatever be the form, whether
Oo ELECTRICITY.
round or square, if only it be terminated by sharp angular edges,
the depth will increase rapidly in approaching tnem.
Fig. 63.
If a conductor be terminated, not by sharp angular edges, but
by rounded sides or ends, then the distribution will become more
Fig. 64.
uniform. Thus, if a cylindrical conductor of considerable dia-'
meter have hemispherical ends, the distribution of the electricity
upon it will be nearly uniform ; but if its ends be flat, with sharp
angular edges, then an accumulation of the fluid will be produced
contiguous to them. If the sides of a flat plate of sufficient
thickness be rounded, the accumulation of fluid at the edges will
be diminished.
The depth of the fluid is still more augmented at corners where
the increases of depth, due to two or more edges, meet and are
EFFECTS OF POINTS.
61
combined ; and this effect is pushed to its extreme limit if any
part of a conductor have the form of a, point.
[Hence it follows, that the charge of electricity, which a con-
ductor of given superficial area is capable of retaining, must be
greater, the more nearly its form approaches to a sphere; for, if
the conductor have any other shape, the electricity will not
diffuse itself uniformly upon it ; and consequently its depth or ten-
sion at some parts will be suilicient to cause it to escape thence,
although at other parts its tension is considerably less.]
87. Distribution of electric fluid varied by induction — If
a cylindrical conductor with rounded ends be presented to an
electrified sphere (fig. 65.), its natural electricity will be decora-
Fig. 65.
posed by induction, the fluid of the same name being repelled,
and that of the contrary name attracted, by the sphere, as may be
indicated by electric pendulums.
88. Experimental illustration of the effect of a point. —
Let P, fig. 66., be a metallic point attached to a conductor c, and
let the perpendicular n express the thickness or density of the
electric fluid at that place ; this thickness will increase in ap-
proaching the point P, so as to be represented by perpendiculars
drawn from the respective points of the curve w, n', n" to A P, s«
that its density at P will be expressed by the perpendicular »" p.
Experience shows that, in ordinary states of the atmosphere, a very mo
derate charge of electricity given to the conductor c, will produce such u
density of the electric fluid at the point p, as to overcome the resistance of
the atmosphere, and to cause the spontaneous discharge of the electricity.
62
ELECTRICITY.
The following experiments will serve to illustrate this escape of electricity
from points.
Let a metallic point, such as A p, fig. 66., be attached to a conductor, and
let a metallic ball of two or three inches in diameter, having a hole in it
Fig. 66.
Fi£T. frj.
corresponding to the point p, be stuck upon the point. If the conductor be
now electrified, the electricity will be diffused over it, and over the ball
which has been stuck upon the point P. The electric state of the conductor
may be shown by a quadrant electrometer being attached to it (Jig. 67.).
Let the ball now be drawn off the point p by a silk thread attached to it for
the purpose, and let it be held suspended by that thread. The electricity of
the conductor c will now escape by the point P, as will be indicated by the
electrometer, but the ball suspended by the silk thread will be electrified as
before.
89. Rotation produced by the reaction of points. — Let
Fig. M.
Fig. 69.
two wires, AB and c D, Jig. 68., placed at right angles, be sup-
ported by a cap E upon a fine point at the top of an insulating
EFFECTS OF POINTS
stand, and let them communicate by a chain F with a conductor
kept constantly electrified by a machine. Let each of the four
arms of the wires be terminated by a point in a horizontal direc-
tion, at right angles to the wire, each point being turned in the
same direction, as represented in the figure. [When electricity is
imparted to the wires, it escapes from the points into the air,
causing the particles of the latter to repel each other, as well as
the arms of the apparatus ; a current of air is thus produced as
though issuing from the points, while the points themselves recede,
so as to make the wire spin round on its centre B.]
Other expedients for varying this experiment are shown in jigs.
69, 70, 71.
Lnjig. 70. this rod supports two sets (A and u) of points turned
in contrary ways, which will, therefore, revolve in contrary direc-
tions if both are free and independent ; but if they are connected
they will counteract each other and remain at rest.
In Jig. 71. a silk thread sustains a small ball of metal, which
strikes a series of bells as it revolves.
Fig. 70.
Fig. 71.
90. Another experimental illustration of this principle
is represented in jig. 72. A square wooden stand T has four rods
of glass inserted in its corners, the
rods at one end being less in height
than those at the other. The tops
of these rods having metal wires A B
and c D stretched between them,
across these wires another wire E F
is placed, having attached to it at
right angles another wire G H, hav-
ing two points turned in opposite
directions at its extremities, so that
Fig.7z.
when G H is horizontal these two points shall be vertical, one
64 ELECTRICITY.
Oemg presented upwards, and the other downwards. A chain
from A communicates with a conductor kept constantly electrified
by a machine.
The electricity coming from the conductor by the chain, passes
along the system of wires, and escapes at the points G and H. The
consequent recoil causes the wire G H to revolve round E F as an
axis, and thereby causes E F to roll up the inclined plane.
91. The electrical orrery is represented in fig. 73. A me-
tallic ball A rests upon an insulating stand by means of a cap wilhin
it, placed upon a fine metallic point
forming the top of the stand.
From the ball A an arm in A pro-
ceeds, the extremity of which is
turned up at E, and formed into a
fine point.
A small ball B rests by means of a
cap on this point, and attached to it
are two arms extending in opposite
directions, one terminated with a
small ball c, and the other by a point
p presented in the horizontal direc-
tion at right angles to the arm. Another point p', attached at
right angles to the arm D A, is likewise presented in the horizontal
direction. By this arrangement the ball A, together with the arm
D A, is capable of revolving round the insulating stand, by which
motion the ball B will be carried in a circle round the ball A.
The ball B is also capable at the same time of revolving on the
point which supports it, by which motion the ball c will revolve
round the ball B in a circle. If electricity be supplied by the
chain to the apparatus, the balls A and B and the metallic rods
will be electrified, and the electricity will escape at the points p
and P'. The recoil produced by this escape will cause the rod D A
to revolve round the insulating pillar, and at the same time the
rod £ c together with the ball B to revolve on the extremity of the
arm D A. Thus, while the ball B revolves in a circular orbit round
the ball A, the ball c revolves in a smaller cifcle round the ball B,
the motion resembling that of the moon and earth with respect to
the sun.
92. Tlie electrical blow pipe consists of a metallic point pro-
jecting from the conductor of a machine (./%•. 74-)» from which
an electric current issues, the effect of which is to produce a cur-
rent of air directed from the point so strong as to afiect the flame
of a candle, and even to blow it out.
This experiment may be varied by placing the candle upon the
conductor, and presenting to its flame a metallic point, as shown
EFFECTS OF POINTS. 6s
mJ*g- 75-» fr°lu which a stream of negative electricity will issue,
so as to produce a similar current of air.
Fig' 74-
gza. [Explanation of the foregoing effects. — All the facts
stated in this chapter, relative to the distribution of electricity on
conductors, and its tendency to escape from angular or pointed
surfaces, can be easily shown to be direct results of the fundamental
property of like electricities to repel, and of opposite electricities
to attract, each other.
It is an obvious consequence of this property that electricity
must always tend to spread itself out as far as possible, until
stopped by some nonconducting medium ; and therefore that it
will leave the interior of a conductor and accumulate upon its
surface, as the experiments described in 8 1., 82., and 83. prove
that it does.
For the same reason, in order that any portion of electricity
may remain at rest upon a conductor, the electricity which sur-
rounds it must be so distributed, that the force tending to move it
in any direction is equal to that tending to move it in the opposite
direction. In the case of a plane surface of unlimited extent, or
of a spherical surface, this condition is fulfilled when the electricity
is distributed uniformly over the whole surface. Hence the ten-
sion at every point of an electrified sphere is the same (84.) ; for,
if it were otherwise, the electricity could not remain at rest, the
forces tending to move it towards the parts where the tension was
F
66
ELECTRICITY.
least, being greater than those tending to move it away from such
parts.
But, on a conductor of any other form, there are points of the
surface so situated, that the extent of surface on one side of them
is greater than that on the opposite side (for instance, at any point
near the top of a cylindrical conductor placed vertically, the ex-
tent of surface above the point is less than the extent of surface
below it); hence, in order that the electricity may remain at rest
at such a point, the density of the charge must be greatest on that
side of it on which the extent of surface is least. Thus we see
why it is that electricity accumulates at the ends of cylindrical
conductors, and at the edges of flat plates.
Precisely similar considerations afford an explanation of the
action of points, in facilitating the escape of electricity from a
charged conductor. In proportion as the point is sharper, and
consequently has a smaller surface, the electricity upon it must
have a greater density, to enable it to keep that upon the rest of
the conductor in equilibrium. Hence, the density of the charge
at the extremity of a sharp point will have become great enough
to cause it to escape through the air, or other nonconducting
medium which surrounds it, when the density of the electricity
upon other parts of the conductor is very much smaller.]
CHAP. IX.
MECHANICAL EFFECTS OF ELECTRICITY.
93. Attractions and repulsions of electrified bodies. — If a
body charged with electricity be placed
near another body, it will impress up-
on such body certain motions, which
will vary according as the body thus
affected is a conductor or noncon-
ductor ; according as it is in its natural
state or charged with electricity ; and,
in fine, if charged with electricity, ac-
cording as the electricity is similar or
opposite to that with which the body
acting upon it is charged.
Let A, fig. 76., be the body charged
with electricity, which we shall sup-
pose to be a metallic ball supported
on an insulating column. Let B be
MECHANICAL EFFECTS. 67
the body upon which it acts, which we shall suppose to be a small
ball suspended by a fine silken thread. We shall consider suc-
cessively the eases above mentioned.
94. Action of an electrified body on a nonconductor not
electrified.— I °. Let B be a nonconductor in its natural state.
In this case no motion will be impressed on B. The electricity with which
A is charged will act by attraction and repulsion on the two opposite fluids,
which compose the natural electricity of B, attracting each molecule of one
by exactly the same force as it repels the molecule of the other. No de-
composition of the fluid will take place, because the insulating property of B
will prevent any motion of the fluids upon it, and will therefore prevent their
separation. Each compound molecule therefore being at once attracted and
repelled by equal forces, no motion will take place.
95. Action of an electrified body on a nonconductor
charged with like electricity. — 2°. Let B be charged with
electricity similar to that with which A is charged.
In this case B will be repelled from A. For, according to what has been
explained above, the forces exerted on the natural electricity of B will be in
equilibrium, but the electricity of A will repel the similar electricity with
which B is charged ; and since this fluid cannot move upon the surface of B
because of its insulating virtue, and cannot quit the surface because of the
resistance ottered by the surrounding air, it must adhere to the surface, and,
being repelled by the electricity of A, must carry with it the ball B in the
direction of such repulsion. The ball B therefore will incline from A, and
will rest in such a position that its weight will balance the repulsive force.
96. Its action on a nonconductor charged -with opposite
electricity. — 3°. Let B be charged with electricity opposite to
that with which A is charged.
In this case B will be attracted towards A, the distribution of the fluid upon
it not being changed, for the same reasons as in the last case.
97. Its action on a conductor not electrified. — 4°. Let B be
a conductor in its natural state.
In this case the action of the fluid on A attracting one constituent of the
natural electricity of B, and repelling the other, will tend to decompose and
separate them ; and since the conducting virtue of B leaves free play to the
movement of the fluids upon it, this attraction and repulsion will take effect,
the attracted fluid moving to the side of B nearest to A, and the repelled fluid
to the opposite side.
To render the explanation more clear, let us suppose that A is charged with
positive electricity.
In that case, the negative fluid of B will accumulate on the side next A,
and the positive fluid on the opposite side. The negative fluid will therefore
be nearer to A than the positive fluid ; and since the force of the attraction
and repulsion increases as the square of the distance is diminished (79.). and
since the quantity of the negative fluid on the side next A is equal to the
quantity of positive fluid on the opposite side, the attraction exerted on the
former will be greater than the repulsion exerted on the latter ; and since the
fluids are prevented from leaving B by the resistance offered by the air, the
F 2
68 ELECTRICITY.
fluids, carrying with them the ball B, will be moved towards A, and will rest in
equilibrium, when the inclination of the string is such that the weight of B
balances and neutralises the attraction.
If A were charged with negative electricity, the same effects would be pro-
duced, the only difference being that, in that case, the positive fluid on B
would accumulate on the side next A, and the negative fluid on the opposite
side. '
Thus it appears that a conducting body in its natural state is always
attracted by an electrified body, with whichever species of electricity it be
charged.
98. Its action upon a conductor charged with like electri-
city. — 5°. Let B be a conductor charged with electricity similar
to that with which A is charged.
In this case the effect produced on B will depend on the relative strength
of the charges of electricity of A and B.
The electricity of A will repel the free electricity of B, and cause it to
accumulate on the side of B most remote from A. But it will also decompose
the natural electricity of B, attracting the fluid of the contrary kind to the
side near A, and repelling the fluid of the same kind to the opposite side. It
will follow from this, that the quantity of the fluid of the same name accu-
mulated at the opposite side of B will be greater than the quantity of fluid
of the contrary name collected at the side near A. While, therefore, the latter
is more attracted than the former, by reason of its greater proximity, it is
less attracted by reason of its lesser quantity. If these opposite effects neu-
tralise each other, — if it lose as much force by its inferior quantity as it gains
by its greater proximity, the attractions 'and repulsions of A on B will neu-
tralise each other, and the ball B will not move. But if the quantity of
electricity with which B is charged be so small that more attraction is gained
by proximity than is lost by quantity, then the ball B will move towards A.
If, however, the quantity of electricity with which B is charged be so great
that the effect of quantity prevail over that of distance, the ball B will be
repelled.
It follows, therefore, from this, that in order to ensure the repulsion of the
ball B in this case, the charge of electricity must be so strong as to prevail
over that attraction which would operate on the bali B if it were in its natural
state. A very small electrical charge is, however, generally sufficient for this.
99. Its action upon a conductor charged with opposite
electricity. — 6°. Let B be charged with electricity of a contrary
name to that with which A is charged.
In this case B will always be attracted towards A, for the attraction exerted
on the fluid with which it is charged will be added to that which would be
exerted on it if it were in its natural state.
The free electricity on B will be attracted to the side next A, and the na-
tural fluid will be decomposed, the fluid of the same name accumulating on
the side most remote from A, and the fluid of the contrary name collecting on
the side nearest to A, and there uniting with the free fluid with which B is
charged. There is therefore a greater quantity of fluid of the contrary name
on that side, than of the same name on the opposite side. The attraction of
the former prevails over the repulsion of the latter therefore at once by
greater quantity and greater proximity, and is consequently effective.
I oo. Attractions and repulsions of pith balls explained. —
MECHANICAL EFFECTS. 69
What has been explained above will render more clearly under-
stood the attractions and repulsions manifested by pith balls, before
and after their contact with electrified bodies (i.). Before con-
tact, the balls, being in their natural state, and being composed of «
a conducting material, are always attracted, whatever be the elec-
tricity with which the body to which they are presented is charged
(97.) ; but after contact, being charged with the like electricity,
they are repelled (98.).
When touched by the hand, or any conductor which communi-
cates with the ground, they are discharged and restored to their
natural state, when they will be again attracted.
If they be suspended by wire or any other conducting thread,
and the stand be a conductor communicating with the ground, they
will lose their electricity the moment they receive it.
The electric fluid in passing through bodies, especially if they
Tae imperfect conductors, or if the space they present to the fluid
"bear a small proportion to its quantity, produces various and
remarkable mechanical effects, displacing the conductors some-
times with great violence.
I O I . Strong electric charges rupture imperfect conductors.
— Card pierced by discharge of jar. — A method of exhibit-
ing this effect is represented in^. 77. The chain A
communicates with the outside coating of the jar.
The card c is placed in such a position that two me-
tallic points touch it on opposite sides, terminating
near each other. The pillar G, being glass, intercepts
the electricity. The ball of the discharger, being put
in communication with the inside coating of the jar,
is brought into contact with the ball B, so that the
two points which are on opposite sides of the card,
being in connection with the two coatings of the jar,
are charged with contrary fluids, which exert on each
other such an attraction that they rush to each other,
penetrating the card, which is found in this case pierced
'£• 77- by a noie larger than that produced by a common pin.
It is remarkable that the burr produced on the surface of the
card is in this case convex on both sides, as if the matter producing
the hole, instead of passing through the card from one side to the
other, had either issued from the middle of its thickness, emerging
at each surface, or as if there were two distinct prevailing sub-
stances passing in contrary directions, each elevating the edges o^
the orifice in issuing from it.
The accordance of this effect with the hypothesis of two fluids
is apparent.
7o
ELECTRICITY.
Another method of exhibiting this phenomenon is shown in
fig- 78.
Fig. 78.
0 O2. Curious fact observed by XVI. Tremery. — A fact has
been noticed by M. Tremery for which no explanation has yet been
given. That observer found that when the two points on opposite
sides of the card are placed at a certain distance, one above the
other, the hole will not be midway between them. When the ex-
periment is made in the atmosphere, the hole will always be nearer
to the negative fluid. When the apparatus is placed under the
receiver of an air-pump, the hole approaches the positive fluid as
the rarefaction proceeds.
If several cards be placed between . the knobs of the universal
discharger (49.)? they may be pierced by a strong charge of a jar
or battery, having more than one square foot of coated surface.
103. Wood and glass broken by discharge. — A rod of wood
half an inch thick may be split by a strong charge transmitted in
the direction of its fibres, and other imperfect conductors pierced
in the same manner.
If a leaf of writing paper be placed on the stage of the dis-
charger, the electricity passed through it will tear it.
The charge of a jar will penetrate glass. An apparatus for
-
MECHANICAL EFFECTS. 71
exhibiting this effect is shown in fig. 79. It may also be exhibited
by transmitting the charge through the side of a phial, fig. 80.
Fig. 80.
Fig. 79.
A strong charge passed through water, scatters the liquid in all
directions around the points of discharge,^. 8 1.
104. Electrical bells. —The alternate attraction and repulsion
of electrified conductors is prettily illustrated by the electrical
bells.
L
Fig. 81.
DO
Fig. 8z.
AB and CD, fig. 82., are two metal rods supported on a glass
pillar. From the ends of these rods four bells A'B'C'D' are sus-
pended by metallic
chains. A central bell
G is supported on the
wooden stand which
sustains the glass pil-
lar EF, and this central
bell communicates by a
chain with the ground.
From the transverse
rods are also suspended,
by silken threads, four
small brass balls H. The
transverse rods being
72 ELECTRICITY.
put in communication with the conductor of an electrical machine,
the four bells A'B'C'D' become charged with electricity. They
attract and then repel the balls H, which when repelled strike the
bell G, to which they give up the electricity they received by
contact with the bells A'B'C'D', and this electricity passes to the
ground by the chain. The bells will thus continue to be tolled
as long as any electricity is supplied by the conductor to the bells
A'B'C'D'.
Another form of this apparatus is shown in. fig- 83.
105. Repulsion of electrified threads. — Let a skein of linen
thread be tied in a knot at each end, and let one end of it be
attached to some part of the conductor of the machine. When
the machine is worked the threads will become electrified, and
will repel each other, so that the skein will swell out into a form
resembling the meridians drawn upon a globe.
1 06. Curious effect of repulsion of pith ball. — Let a me-
tallic point be inserted into one of the holes of the prime conduc-
tor, so that, in accordance with what has been explained, a jet of
electricity may escape from it when the conductor is electrified.
Let this jet, while the machine is worked, be received on the
interior of a glass tumbler, by which the surface* of the glass will
become charged with electricity.
If a number of pith balls be laid upon a metallic plate com-
municating with the ground, and the tumbler be placed with
its mouth upon the plate, including the balls within it, the balls
Fig. 84.
will begin immediately leaping violently from the metal and
striking the glass, and this action will continue till all the
MECHANICAL EFFECTS. 73
electricity with which the glass was charged has been carried
away.
Another form of this apparatus is shown in Jig. 84.
This is explained on the same principle as the former experi-
ments. The balls are attracted by the electricity of the glass, and
when electrified by contact, are repelled. They give up their
electricity to the metallic plate, from which it passes to the ground ;
and this process continues until no electricity remains on the glass
of sufficient strength to attract the balls.
107. Electrical dance. — Let a disc of pasteboard or wood,
coated with metallic foil, be suspended by wires or threads of
linen from the prime conductor of an electrical machine, and let
a similar disc be placed upon a stand capable of being adjusted
to any required height. Let this latter disc be placed immediately
under the former, and let it have a metallic communication with
the ground. Upon it place small • coloured representations in
paper, of dancing figures, which are prepared for the purpose.
When the machine is worked, the electricity with which the upper
disc will be charged will attract the light figures placed on the
lower disc, which will leap upwards ; and after touching the upper
disc and being electrified, will be repelled to the lower disc, and
this jumping action of the figures will continue so long as the
machine is worked. An electrical dance is thus exhibited for the
amusement of young persons.
1 08. Curious experiments on electrified water. — Let a
small metallic bucket B,/#. 85., be
suspended from the prime con-
ductor of a machine, and let it
have a capillary tube CD of the
siphon form immersed in it ; or let
it have a capillary tube inserted in
the bottom; the bore of the tube
being so small that water cannot
escape from it by its own pressure.
When the machine is put in opera-
tion, the particles of water, becom-
ing electrified, will repel each
Fi 8 other, and immediately an abund-
ant stream will issue from the
tube ; and as the particles of water after leaving the tube still
exercise a reciprocal repulsion, the stream will diverge in the form
of a brush.
If a sponge saturated with water be suspended from the prime
conductor of the machine, the water, when the machine is first
worked will drop slowly from it ; but when the conductor becomes
74 ELECTRICITY.
strongly electrified, it will descend abundantly, and in the dark
will exhibit the appearance of a shower of luminous rain.
109. Experiment with electrified sealing-wax. — Let a
piece of sealing-wax be attached to the pointed end of a metallic
rod ; set fire to the wax, and when it is in a state of fusion blow
out the flame, and present the wax within a few inches of the
prime conductor of the machine. Strongly electrified myriads of
fine filaments will issue from the wax towards the conductor, to
which they will adhere, forming a sort of network resembling
wool. This effect is produced by the positive electricity of the
conductor decomposing the natural electricity of the wax; and
the latter being a conductor when in a state of fusion, the nega-
tive electricity is accumulated in the soft part of the wax near the
conductor, while the positive electricity escapes along the metallic
rod. The particles of wax thus negatively electrified, being at-
tracted by the conductor, are drawn into the filaments above
mentioned.
110. Tne electrical see-saw, a &, fig. 86., is a small strip of
wood covered over with silver leaf or tinfoil, insulated on c like a
balance. A slight preponderance is given
f> &» C__ j§— ft / to it at a, so that it rests on a wire having
?'« _^^Jy po/ a knob m at its top ; p is a similar metal
^^"^ I' nt ball insulated. Connect p with the inte-
Fi gg rior, , and m with the exterior coating of
the jar, charge it, and the see-saw motion
of a & will commence from causes similar to those which excited
the movements of the pith balls.
CHAP. X.
THERMAL EFFECTS OF ELECTRICITY.
1 1 1 . A current of electricity passing1 over a conductor
raises its temperature. — If a current of electricity pass over a
conductor, as would happen when the conductor of an electrical
machine is connected by a metallic rod with the earth, no change
in the thermal condition of the conductor will be observed, so
long as its transverse section is so considerable as to leave suffi-
cient space for the free passage of the fluid. But if its thickness
be diminished, or the quantity of fluid passing over it be aug-
mented, or, in general, if the ratio of the fluid to the magnitude of
THERMAL EFFECTS. 75
the space afforded to it be increased, the conductor will be found
to undergo an elevation of temperature, which will be greater the
greater the quantity of the electricity and the less the space
supplied for its passage.
112. Experimental verification. — Wire heated, fused, and
burned. — If a piece of wire of several inches in length be placed
upon the stage of the universal discharger (49.)? a feeble charge
transmitted through it will sensibly raise its temperature. By in-
creasing the strength of the charge, its temperature may be ele-
vated to higher and higher points of the thermometric scale ; it
may be rendered incandescent, fused, vaporised, and, in fine,
burned.
With the powerful machine of the Taylerian Museum at Haar-
lem, Van Marum fused pieces of wire above 70 feet in length.
Wire may be fused in water; but the length which can be
melted in this way is always less than in air, because the liquid
robs the metal of its heat more rapidly than air.
A narrow ribbon of tinfoil, from 4 to 6 inches in length, may
be volatilised by the discharge of a common battery. The me-
tallic vapour is in this case oxidised in the air, and its filaments
float like those of a cobweb.
113. Thermal effects are greater as the conducting
power is less. — The worst conductors of electricity, such as
platinum and iron, suffer much greater changes of temperature
by the same charge than the best conductors, such as gold and
copper. The charge of electricity, which only elevates the tem-
perature of one conductor, will sometimes render another incan-
descent, and will volatilise a third.
114. Ignition of metals. — If a fine silver wire be extended
between the rods of the universal discharger (49.), a strong charge
will make it burn with a greenish flame. It will pass off in a
greyish smoke. Other metals may be similarly ignited, each pro-
ducing a flame of a peculiar colour. If the experiments be made
in a receiver, the products of the combustion being collected,
will prove to be the metallic oxides.
If a gilt thread of silk be extended between the rods of the dis-
charger, the electricity will volatilise or burn the gilding, without
affecting the silk. The effect is too rapid to allow the time neces-
sary for the heat to affect the silk.
A strip of gold or silver leaf placed between the leaves of paper,
being extended between the rods of the discharger, will be vola-
tilized by a discharge from a jar having two square feet of coating.
The volatilized metal will in this case appear on the paper as a
patch of purple colour in the case of gold, and of grey colour in
that of silver.
76 ELECTRICITY
A spark from the prime conductor of the great Haarlem ma-
chine burnt a strip of gold leaf twenty inches long by an inch and
a half broad.
115. Effect on fulminating silver. — The heat developed in
the passage of electricity through combustible or explosive sub-
stances, which are imperfect conductors, causes their combustion
or explosion.
A small quantity of fulminating silver placed on the point of a
knife, explodes if brought within a few feet of the conductor of an
electrical machine in operation. In this case the explosion is pro-
duced by induction.
1 1 6. Electric pistol. — The electrical pistol or cannon is
charged with a mixture of hydrogen and ox}rgen gases, in the
proportion necessary to form water. A conducting wire termi-
nated by a knob is inserted in the touch hole, and the gases are
Fig. 88.
confined in the barrel by the bullet. An electric spark imparted
to the ball at the touch hole, causes the explosion of the gases.
THERMAL EFFECTS.
77
This explosion is produced by the sudden combination of the
gases, and their conversion into water, which, in consequence of
the great quantity of heat developed, is instantly converted into
steam of great elasticity, which, by its expansion, forces the bullet
from the barrel in the same manner as do the gases which result
from the explosion of gunpowder.
One of the forms of this apparatus is represented in section in
Jig. 87. It consists of a metallic vessel c, which is filled with the
mixture of the gases, and hermetically closed by a cork. An
opening A is made in the side, in which is inserted a metallic rod,
terminated in two balls, as shown 'mfig. 87., one interior, and the
other exterior, the rod being fixed in the tube by mastic, which,
being a nonconductor of electricity, prevents the fluid from es-
caping from the rod to the sides of the vessel. Thus prepared,
the vessel is placed, as shown in^. 88., upon a support, and the
ball A is put in electric connection with the conductor of a machine
in operation, from which a spark being received a similar spark
is transmitted between the internal knob B and the side of the
vessel. By this spark the mixture of gases is inflamed, and the
cork blown out.
1 17. Ether and alcohol ignited. — Ether or alcohol may be
fired by passing through it an electric discharge. Let cold water
be poured into a wine glass, and let a thin stratum of ether be
carefully poured upon it. The ether being lighter will float on
the water. Let a wire or chain connected with the prime con-
ductor of the machine be immersed in the water, and, while the
machine is in action, present a metallic ball to the surface of the
ether. The electric charge will pass from the water through the
ether to the ball, and will ignite the ether. Or, if a person stand-
ing on an insulating stool, and holding in one hand a metal spoon
filled with ether, pre-
sent the surface of the
ether to a conductor,
and at the same time ap-
ply the other hand to
the prime conductor of
a machine in operation,
the electricity will pass
from the prime conduc-
tor through the body of
the person to the spoon,
and from the spoon
through the ether to the
conductor to which the
ether is presented, and
in so passing will ignite the ether.
ELECTRICITY.
Another arrangement for performing this experiment is shown
in fig. 89.
. 1 1 8. Resinous powder burned. — The electric charge trans-
mitted through fine resinous powder, such as that of colophony,
will ignite it. This experiment may be performed either by
spreading the powder on the stage of the discharger (49.), or by
impregnating a hank of cotton with it ; or, in a still more striking
manner, by sprinkling it on the surface of water contained in an
earthenware saucer.
119. Gunpowder exploded. — Gunpowder may, in like manner
be ignited by electricity. This experiment is most conveniently
exhibited by placing the powder in a small wooden cup, and con-
ducting the electric charge along a moist thread, six or seven inches
long, attached to the arm of a discharger, which is connected with
the negative coating of a jar, and the charge, in
its passage from one rod of the discharger to the
other, will ignite the powder.
120. Electric mortars. — The electric mortar
(Jig. 90 .) is an apparatus by which the gun-
powder is ignited by
passing an electric
charge through it.
The mixed gases
may also be used in
this instrument.
Common air or
gas, not being ex-
plosive, is heated so
suddenly and in-
tensely by transmit-
ting through it an
electric charge, that
it will expand so as
to project the ball
from the mortar.
121. Kilmers-
ley's thermometer
(Jig. 92.) is an in-
strument intended
to measure the de-
gree of heat deve-
loped in the passage
of an electric charge
by the expansion of
air. The discharge m:
LUMINOUS EFFECTS. 79
, takes place between the two balls in the glass cylinder, and the air
confined in the cylinder being heated, expands, presses upon the
liquid contained in the lower part of the cylinder, and causes the
liquid in the tube to rise. The variation of the column of liquid
in the tube indicates the elevation of temperature.
CHAP. XL
LUMINOUS EFFECTS OF ELECTRICITY.
122. Electric fluid is not luminous. — An insulated conductor,
or a Leyden jar or battery, however strongly charged, is never
luminous so long as the electric equilibrium is maintained and the
fluid continues in repose. But if this equilibrium be disturbed,
and the fluid move from one conductor to another, such motion is,
under certain conditions, attended with luminous phenomena.
123. Conditions under which light is developed by an
electric current. — If the conductor of an ordinary electric ma-
chine, while in operation, be connected with the ground by a thick
metallic wire, the current of the fluid which flows along the wire
to the ground will not be sensibly luminous ; but if the machine
be one of great power, such, for example, as the Taylerian machine
of Haarlem, an iron wire of 60 or 70 feet long, communicating
with the ground and conducting the current, will be surrounded
by a brilliant light. The intensity of the electricity necessary to
produce this effect, depends altogether on the properties of the
medium in which the fluid moves. Sometimes electricity of feeble
intensity produces a strong luminous effect, while in other cases
electricity of the greatest intensity develops no sensible degree of
light.
It has been already explained that the electric fluid with which
an insulated conductor is charged is retained upon it by the sur-
rounding air being a nonconductor. According as the pressure
of the air is increased or diminished, the force necessary to enable
the electricity to escape through it is increased or diminished.
When a conductor A, in communication with the ground, ap-
proaches an insulated conductor, B, charged with electricity, the
natural electricity of B will be decomposed, the fluid of the same
name as that which charges A escaping to the earth, and the fluid
of the opposite name accumulating on the side of B next to A. At
the same time, according to what has been explained (97. ), the
fluid on A accumulates on the side nearest to B. These two tides
of electricity of opposite kinds exert a reciprocal attraction, and
So ELECTRICITY.
nothing prevents them from rushing together and coalescing,
except the resistance of the intervening air. They will coalesce,
therefore, so soon as their mutual attraction is so much increased
as to overcome the resistance of the air.
This increase of mutual attraction may be produced by several
causes. First, by increasing the charge of electricity upon the
conductor A, for the pressure of the fluid will be proportional to
its depth or density. Secondly, by diminishing the distance be-
tween A and B, for the attraction increases in the same ratio as
the square of that distance is diminished ; and, thirdly, by increas-
ing the conducting power of either or both of the bodies A and B,
for by that means the electric fluids, being more free to move
upon them, will accumulate in greater quantity on the sides of A
and B which are presented towards each other. Fourthly, by the
form of the bodies A and B, for according to what has been already
explained (86.) (920.), the fluids will accumulate on the sides
presented to each other in greater or less quantity, according as
the form of those sides approaches to that of an edge, a corner, or
a point.
When the force excited by the fluids surpasses the restraining
force of the intervening air, they force their passage through the
air, and rushing towards each other, combine. This movement
is attended with light and sound. A light appears to be produced
between the points of the two bodies A and B, which has been
called the electric spark, and this luminous phenomenon is accom-
panied by a sharp sound like the crack of a whip.
1 24. The electric spark. — The luminous phenomenon called
the electric spark does not consist, as the name would imply, of
a luminous point which moves from the one body to the other.
Strictly speaking, the light manifests no progressive motion. It
consists of a thread of light, which for an instant seems to connect
the two bodies, and in general is not extended between them in
one straight line, but has a zig-
zag form, resembling more or less
the appearance of lightning, fig,
93., and probably due to the dis-
charge leaping across between par-
Fig- 93 • tides of dust suspended in the air.
1 240. [Duration of the spark. — When we look at a bright
electric spark, such as that obtained on discharging a good-sized
Leyden jar, the impression made upon the eye does not cease at
once when the spark has passed ; consequently we seem to see the
spark for a longer time than it^really exists. The very short duration
of the spark itself can be proved by causing it to pass in front of a
rapidly revolving wheel, in a dark room. When the spark passes,
the wheel is brightly illuminated, but appears as though it were
LUMINOUS EFFECTS. 81
quite stationary, thus proving that it does not revolve 1o any
perceptible extent during the time which the spaik lasts.
Professor Wheatstone has however proved, by viewing the
electric spark in a very rapidly revolving mirror, that, although
it persists for only a very short time, it is not absolutely instan-
taneous. And it has been since ascertained by Feddersen that
what appears to the eye as a simple discharge between two points,
is in reality a succession of discharges which pass in alternate
directions between them.]
125. Electric brush. — If the part of either of the bodies A
or B, which is presented to the other, have the form of a point,
the electric fluid will escape, not in the form of a spark, but as a
brush of light, the diverging rays of which sometimes have the
length of two or three inches. A very feeble charge is sufficient
to cause the escape of the fluid when the body has this form (87.).
126. Tlie length of the spark. — If the knuckle of the finger
or a metallic ball at the end of a rod held in the hand be pre-
sented to the prime conductor of a machine in operation, a spark
will be produced, the length of which will vary with the power of
the machine.
By the length of the spark must be understood the greatest
distance at which the spark can be transmitted.
A very powerful machine will so charge its prime conductor
that sparks may be taken from it at the distance of 30 inches.
127. Discontinuous conductors produce luminous effects.
— Since the passage of the electricity produces light wherever the
metallic continuity, or more generally wherever the continuity of
the conducting material is interrupted, these luminous effects may
be multiplied by so arranging the conductors, that there shall be
interruptions of continuity arranged in any regular or desired
manner.
128. Various experimental illustrations. — If a number of
metallic beads be strung upon a thread of silk, each bead being
separated from the adjacent one by a knot on the silk so as to
break the contact, a current of electricity sent through them will
produce a series of sparks, a separate spark being produced be-
tween every two successive beads. By placing one end of such
a string of beads in contact with the conductor of the machine,
and the other end in metallic communication with the ground,
a chain of sparks can be maintained so long as the machine is
worked.
The string of beads may be disposed so as to form a variety of
fancy designs, which will appear in the dark in characters of light.
Similar effects may be produced by attaching bits of metallic
foil to glass. Sparkling tubes and plates are contrived in this
manner, by which amusing experiments are exhibited. A glass
ELECTRICITY
plate is represented in Jig. 94., by
which a word is made to appear
in letters of light in a dark room.
The letters are formed by attach-
ing lozenge-shaped bits of tinfoil
to the glass, disposed in the proper
form. In the same manner designs may be formed on the inner
surface of glass tubes, fig. 95., or plates,^. 96., or, in fine, of glass
vessels of any form,^. 97.
Fig. 95-
In these cases the luminous characters may be made to appear
in lights of various colours, by using spangles of different metab,
since the colour of the spark varies with the metal.
Fig. 96.
1 29. Effect of rarefied air. — When the electric fluid passes
through air, the brilliancy and colour of the light evolved depends
on the density of the air. In rarefied air the light is more
diffused and less intense, and acquires a reddish or violet colour.
Its colour, however, is affected, as has been just stated, by the
LUMINOUS EFFECTS.
nature of the conductors between which the current flows.
When it issues from gold the light is green, from silver red,
Fig. 97.
from tin or zinc white, from water deep yellow inclining to
orange.
It is evident that these phenomena supply the means of con-
structing electrical apparatus by which an infinite
variety of beautiful and striking luminous effects
may be produced.
When the electricity escapes from a metallic
point in the dark, it forms a brush, fig. 98.,
which will continue to be visible so long as the
machine is worked.
The luminous effect of electricity in rarefied
air is exhibited by an apparatus, Jig. 99. and
Jig. 100, consisting of a glass receiver, which can be screwed
upon the plate of an air-pump and partially exhausted. The
electric current passes between two metallic balls attached to
rods, which slide in air-tight collars in the covers of the receiver.
It is observed that the brushes formed by negative electricity
are never as long or as divergent as those formed by positive
electricity, an effect which has been supposed to indicate an
essential difference between the two electric fluids.
130. Experimental imitation of the auroral light. — This
phenomenon may be exhibited in a still more remarkable manner
by using, instead of the receiver, a glass tube two or three
inches in diameter, and about thirty inches in length. In this
G 2
84
ELECTRICITY.
case a pointed wire being fixed to the interior of each of the caps,
one is screwed upon the plate of the air pump, while the external
Fig. 99.
knob of the other is connected by a metallic chain with the prime
conductor of the electrical machine. When the machine is worked
in the dark, a succession of luminous phenomena will be produced
in the tube, which bear so close a resemblance to the aurora
borealis as to suggest the most probable origin of that meteor.
When the exhaustion of the tube is nearly perfect, the whole
length of the tube will exhibit a violet red light. If a small
quantity of air be admitted, luminous flashes will be seen to issue
1'rom the two points attached to the caps. As more and more air
is admitted, the flashes of light which glide in a serpentine form
down the interior of the tube will become more thin and white,
until at last the electricity will cease to be diffused through the
column of air, and will appear as a glimmering light at the two
points.
131. Phosphorescent effect of the spark. — The electric
spark leaves upon certain imperfect conductors a trace which
continues to be luminous for several seconds, and sometimes even
BO long as a minute after the discharge of the spark. The colour
LUMINOUS EFFECTS. 85
of this species of phosphorescence varies with the substances on
which it is produced. Tims white chalk produces an orange
light. With rock crystal the light, at first red, turns afterwards
white. Sulphate of barytes, amber, and loaf sugar render the
light green, and calcined oyster shell gives all the prismatic
colours.
132. Xiichtenb erg's figures. — The spark in many cases pro-
duces effects which not only seem to confirm the hypothesis of
two fluids, but have been thought to indicate a specific differ-
ence between them. The experiment known as Lichtenberg's
figures presents another example of this. Let two Ley den jars
be charged, one with positive, the other with negative electricity ;
and let sparks be given by their knobs to the smooth and well
dried surface of a cake of resin. Let the surface of the resin be
then slightly sprinkled with powder of semen lycopodii, or flowers
of sulphur, and let the powder thus sprinkled be blown off. A
part will remain attached to the spots where the electric sparks
were imparted. At the spot which received the positive spark,
the adhering powder will have the form of a radiating star ; and
at the point of the negative spark it will have that of a roundish
clouded spot.
133. Experiments indicating: specific differences between
tne two fluids. — If lines and figures be traced in like manner on
the cake of resin, some with the positive, and some with the
negative knob, and a powder formed of a mixture of sulphur
and minium be dusted over the cake through a flannel sieve, and
then blown off, the adhering powder will mark the traces of the
two fluids imparted by the knobs, the traces of the positive fluid
being yellow, and those of the negative red. [In this case the
sulphur is electrified negatively, and the minium positively, by
friction against the flannel ; the former, therefore, collects on the
parts of the resin charged with positive electricity, and the latter
on those charged with negative electricity.]
Let two Leyden jars, one charged with positive and the other
with negative electricity, be placed upon a plate of glass coated at
its under surface with tinfoil at a distance of six or eight inches
asunder, and let the surface of the glass between them be sprinkled
with semen lycopodii. Let the jars be then moved towards each
other, and let their inner coatings be connected by a discharging
rod applied to their knobs. A spark will pass between their outer
coatings through the powder, which it will scatter on its passage.
The path of the positive fluid will be distinguishable from that of
the negative fluid, as before explained, by the peculiar arrange-
ment of the powder ; and this difference will disappear near the
86
ELECTRICITY.
Fig. 101.
point where the two fluids meet, where a large round speck is
sometimes seen bounded by neither of the arrangements which
characterise the respective fluids.
134. Electric light above the barometric column. — The
electric light is developed in every form of
elastic fluid and vapour when its density is very
inconsiderable. A remarkable example of this
is presented in the common barometer. When
the mercurial column is agitated so as to oscil-
late in the tube, the space in the tube above the
column becomes luminous, and is visibly so in
the dark. This phenomenon is caused by the
effect of the electricity developed by the fric-
tion of the mercury and the glass upon the
atmosphere of mercurial vapour which tills the
space above the column in the tube.
135. Cavendish's electric barometer,
figllQl. — Two barometers are connected at the
top by a curved tube, so that the spaces above
the two columns communicate with each
other. [When the cistern of one barometer is
connected with the conductor of an electrical
machine, and that of the other with the
ground, electric light appears in the curved tube.]
136. Luminous effects produced by imperfect conductors.
— The electric spark or charge transmitted Dy means of the
universal discharger and Ley den jar or battery through various
imperfect conductors, produces luminous effects which are amusing
and instructive.
Place a small melon, citron, apple, or any similar fruit on the
stand of the discharger ; arrange the wires so that their ends are
not far asunder, and at the moment when the jar is discharged the
fruit becomes transparent and luminous. One or more eggs may
be treated in the same manner if a small wooden ledge be so con-
trived that their ends may just touch, and the spark can be sent
through them all. Send a charge through a lump of pipe-clay, a
stick of brimstone, or a glass of water, or any coloured liquid, and
the entire mass of the substance will for a short time be rendered
luminous. As the phosphorescent appearance induced is by no
means powerful, it will be necessary that these experiments should
be performed in a dark room, and indeed the effect of the other
luminous electrical phenomena will be heightened by darkening
the room.
137. Attempt to explain electric light, — the thermal hypo-
thesis. — No explanation of the physical cause of the electric
LUMINOUS EFFECTS. 87
spark, or of the luminous effects of electricity, has yet been pro-
posed which has commanded general assent. It appears certain,
for the reasons already stated, and from a great variety of pheno-
mena, that the electric fluids themselves are not luminous. The
light, therefore, which attends their motion must be attributed to
the media, or the bodies through which or between which the fluids
move. Since it is certain that the passage of the fluids through a
medium develops heat in greater or less quantity in such me-
dium, and since heat, when it attains a certain point, necessarily
develops light, the most obvious explanation of the manifestation
of light was to ascribe it to a momentary and extreme elevation of
temperature, by which that part of the medium, or the body tra-
versed by the fluid, becomes incandescent.
According to this hypothesis, the electric spark and the flash of
lightning are nothing more than the particles of air, through which
the electricity passes, rendered luminous by intense heat. There
is nothing in this incompatible with physical analogies. Flame we
know to be gas rendered luminous by the ardent heat developed in
the chemical combinations, of which combustion is the effect.
138. Hypothesis of decomposition and recomposition. —
According to another hypothesis, first advanced by Hitter, and
afterwards adopted by Berzelius, Oersted, and Sir H. Davy, the
electric fluids have strictly speaking no motion of translation what-
ever, and never in fact desert the elementary molecules of matter
of which, according to the spirit of this hypothesis, they form an
essential part. Each molecule or atom composing a body is
supposed to be primitively invested with an atmosphere of elec-
tric fluid, positive or negative, as the case may be, which never
leaves it. Bodies are accordingly classed as electro-positive or
electro-negative, according to the fluid attracted to their atoms.
Those atoms which are positive attract so much negative fluid, and
those which are negative so much positive fluid, as is sufficient to
neutralise the forces of their proper electricities, and then the
atoms are unelectrised and in their natural state.
When a body is charged with positive electricity, its atoms act by induc-
tion upon the atoms of adjacent bodies, and these upon the atoms next
beyond them, and so on. The fluids in the series of atoms through which
the electricity is supposed to pass, assume a polar arrangement such as that
represented in fig. 102.
Fig. loz.
The first atom of the series being surcharged with + electricity acts by induc-
tion on the second, and decomposes its natural electricity, the negative fluid
88 ELECTRICITY.
being attracted to the side near the first atom, and tbe positive repelled to
the side near the third atom. The same effect i.s produced by atom 2 on atom
3, by atom 3 on atom 4, and so on. The surplus positive fluid on 1 theu
combines with and neutralises the negative fluid on 2 ; and, in like manner,
the positive fluid on 2 combines with and neutralises the negative fluid on 3,
and so on until the last atom of the series is left surcharged with positive
electricity.
Such is the hypothesis of decomposition and recomposition which is at
present in most general favour with the scientific world.
The explanation which it affords of the electric spark and other luminous
electric effects, maybe said to consist in transferring the phenomenon to be
explained from the bodies themselves to their component atoms, rather than
in affording an explanation of the effect in question, inasmuch as the pro-
duction of light between atom and atom, by the alternate decomposition and
recomposition of the electricities, stands in as much need of explanation as the
phenomenon proposed.
139. Cracking- noise attending- electric spark. — The sound
produced by the electric discharge is obviously explained by the
sudden displacement of the particles of the air, or other medium
through which the electric fluid passes.
CHAP. XII.
PHYSIOLOGICAL EFFECTS OF ELECTRICITY.
1 40. Electric shock explained. — The material substances
which enter into the composition of the bodies of animals are
generally imperfect conductors. When such a body, therefore,
is placed in proximity with a conductor charged with electricity,
its natural electricity is decomposed, the fluid of a like name being
repelled to the side more remote from, and the fluid of the con-
trary name being attracted to the side nearest to, the electrified
body. If that body be very suddenly removed from or brought
near to the animal body, the fluids of the latter will suddenly
suffer a disturbance of their equilibrium, and will either rush
towards each other to recombine, or be drawn from each other,
being decomposed ; and owing to the imperfection of the con-
ducting power of the fluids and solids composing the body, the
i lectricity in passing through it will produce a momentary derange-
ment, as it does in passing through air, water, paper, or any other
imperfect conductor. If this derangement do not exceed the
power of the parts to recover their position and organisation, a
uonvulsive sensation is felt, the violence of which is greater or less
PHYSIOLOGICAL EFFECTS. 89
according to the force of electricity and the consequent derange-
ment of the organs ; but if it exceed this limit, a permanent
injury, or even death, may ensue.
141. Secondary shock. — It will be apparent 'from this, that
the nervous effect called the electric shock does not require that
any electricity be actually imparted to, abstracted from, or passed
through the body. The momentary derangement of the natural
electricity is sufficient to produce the effect with any degree of
violence.
The shock produced thus by induction, without transmitting
electricity through the body, is sometimes called the secondary
shock.
The physiological effects of electricity are extremely various,
according to the quantity and intensity of the charge, accord-
ing to the part of the body affected by it, and according to the
manner in which it is imparted.
142. Effect produced on the skin by proximity to an
electrified body. — When the back of the hand is brought near
"to the glass cylinder of the machine, at the part where it passes
from under the silk flap, and when therefore it is strongly charged
with electricity, a peculiar sensation is felt on the skin, resembling
that which would be produced by the contact of a cobweb. The
hairs of the skin, being negatively electrified by induction, are
attracted and drawn against their roots with a slight force.
143. Effect of the sparks taken on the knuckle. — The
effect of the shock produced by a spark taken from the prime con-
ductor by the knuckle is confined to the hand ; but with a very
powerful machine, it will extend to the elbow.
144. Methods of limiting and regulating the shock by
a jar. — The effects of the discharge of a Leyden jar extend
through the whole body. The shock may, however, be limited to
any desired part or member, by placing two metallic plates con-
nected with the two coatings of the jar, on opposite sides of the
part through which it is desired to transmit the shock.
145. Effect of discharges of various force. — The violence
of the shock depends on the magnitude of the charge, and may be
so intense as to produce permanent injury. The discharge of a
single jar is sufficient to kill birds, and other smaller species of
animals. The discharge of a moderate-sized battery will kill
rabbits, and a battery of a dozen square feet of coated surface will
kill a large animal, especially if the shock be transmitted through
the head.
146. Phenomena observed in the examination after death
by the shock. — When death ensues in such cases, no organic
k-sion or other injury or derangement has been discovered by
90 ELECTRICITY.
post mortem examination ; nevertheless, the violence of the con-
vulsions which are manifested when the charge is too feeble to
destroy life, indicates a nervous derangement as the cause of death.
147. Effects of a long: succession of moderate discharges.
A succession of electric discharges of moderate intensity, trans-
mitted through certain parts of the body, produce alternate
contraction and relaxation of the nervous and muscular organs,
by which the action of the vascular system is stimulated and the
sources of animal heat excited.
148. Effects upon a succession of patients receiving: the
same discharge. — The electric discharge of a Ley den jar may
be transmitted through a succession of persons placed hand in
hand, the first communicating with the internal, and the last with
the external coating of the jar.
In this case, the persons placed at the middle of the series
sustain a shock less intense than those placed near either ex-
tremity, in consequence of some of the electricity passing into the
ground by the feet of each person.
149. Remarkable experiments of KTollet, Dr. Watson,
and others. — A shock has in this manner been sent through a
regiment of soldiers. At an early period in the progress of elec-
trical discovery, M. Nollet transmitted a discharge through a
series of 1 80 men ; and at the convent of Carthusians a chain ot
men being formed extending to the length of 5400 feet, by means
of metallic wires extended between every two persons composing
it, the whole series of persons was affected by the shock at the
same instant.
Experiments on the transmission of the shock were made in London by
Dr. Watson, in the presence of the Council of the Royal Society, when a
circuit was formed by a wire carried from one side of the Thames to the
other over Westminster Bridge. One extremity of this wire communicated
with the interior of a charged jar, the other was held by a person on the
opposite bank of the river. This person held in his other hand an iron rod
which he dipped in the river. On the other side near the jar stood another
person, holding in one hand a wire communicating with the exterior coating
of the jar, and in the other hand an iron rod. This rod he dipped into the
river, when instantly the shock was received by both persons, the electric
liuid having passed over the bridge, through the body of the person on the
other side, through the water across the river, through the rod held by the
other person, and through his body to the exterior coating of the jar.
Familiar as such a fact may now appear, it is impossible to convey an ade-
quate idea of the amazement bordering on incredulity with which it was at
that time witnessed.
CHEMICAL AND MAGNETIC EFFECTS. 91
CHAP. XIII.
CHEMICAL AND MAGNETIC EFFECTS OF ELECTKICITY.
1 50. Phenomena which supply the basis of the electro-
chemical theory. — If an electric charge be transmitted through
certain compound bodies, they will be resolved into their consti-
tuents, one component always going in the direction of the positive,
and the other of the negative fluid. This class of phenomena has
supplied the basis of the electro-chemical hypothesis already briefly
noticed (138.). The constituent which goes to the positive fluid
is assumed \o consist of atoms which are electrically negative, and
that which goes to the negative fluid, as consisting of atoms elec-
trically positive.
151. Faraday's experimental illustration of tbis. — This
class of phenomena is more prominently developed by voltaic elec-
tricity, and will be more fully explained in the following Book.
For the present it will therefore be sufficient.-to indicate an ex-
ample of this species of decomposition by the electricity of the
ordinary machine. The following experiment is due to Professor
Faraday.
Lay two pieces of tinfoil T T', fig. 103., on a glass plate, one being con-
nected with the prime conductor of the machine, and the other with the
ground. Let two pieces of platinum wire p p', resting on the tinfoil, be
placed with their points on a drop of the solution of the sulphate of copper
c, or on a piece of bibulous paper wetted with sulphate of indigo and muriatic
acid, or with iodide of potassium and starch, or on a piece of litmus paper
wetted with a solution of common salt or of sulphate of soda, or upon
turmeric paper containing sulphate of soda.
In all these cases the solutions are decomposed : in the first, sulphuric acid
goes to the positive wire; in the second the indigo is bleached by the chlorine
discharged at the same wire; in the third, iodine is liberated at the same
Fig. ioj.
wire ; in the fourth the litmus paper is reddened by the acid evolved at the
positive wire, and when muriatic is used, it is bleached by the chlorine
evolved at the same wire; and, in fine, in the tirth case, the turmeric paper
is reddened by the alkali evolved at the negative wire.
152. Effect of an electric discharge on a magnetic
needle. — When a stream of electricity passes over a steel needle
92 ELECTRICITY.
or bar of iron, it produces a certain modification in its magnetic
state. If the needle be in its natural state it is rendered magnetic.
If it be already magnetic, its magnetism is modified, being aug-
mented or diminished in intensity, according to certain conditions
depending on the direction of the current and the position of the
magnetic axis of the needle ; or it may have its magnetism de-
stroyed, or even its polarity reversed.
This class of phenomena, like the chemical effects just mentioned,
are, however, much more fully developed by voltaic electricity ;
and we shall therefore reserve them to be explained in the follow-
ing Book. Meanwhile, however, the following experiments will
show how common electricity may develop them.
153. Experimental illustration of this. — Place a narrow
strip of copper, about two inches in length, on the stage of the
universal discharger, and over it a leaf of any insulating mate-
rial, upon which lay a sewing needle transversely to the strip of
copper. Transmit several strong charges of electricity through
the copper. The needle will then be found to be magnetised, the
end lying on the right of the current of electricity being its
north pole.
If the same experiment be repeated, reversing the position of
the needle, it will be demagnetised. But by repeating the electric
discharges a greater number of times, it will be magnetised with
the poles reversed.
154. [Effect of an electric discharge on a suspended
magnet. — This effect can be best exhibited by means of a delicate
reometer or galvanometer. If one end of the wire of this appa-
ratus, a description of which will be found in the next Book
(Chap. X.), be connected with the positive conductor of a good
cylinder electrical machine, and the other end with the negative
conductor, the needle will be deflected when the machine is worked,
and the direction of the deflexion will be altered by changing the
ends of the wire which are respectively in connexion with the two
conductors. The same experiment can be made with a plate
electrical machine by connecting one end of the wire of the
galvanometer with the prime conductor, and the other end with
the ground.]
SOURCES OF ELECTRICITY. 93
CHAP. XIV.
SOURCES OF ELECTRICITY.
155. [The only source of electricity which has been specially con-
sidered in the preceding chapters, is the friction of two dissimilar
substances against each other. There are, however, many other
modes of producing electricity, some of which are of very great
importance. In fact, every action whereby the state of equilibrium
of the particles of material bodies is disturbed, seems to be
attended with the development of electricity.
The chief sources of electricity may be classified as — 1st, Me-
chanical actions, including friction, pressure, cleavage, &c. ; 2nd,
Heat ; 3rd, Chemical action ; 4th, MagnetismJ]
156. [Mechanical sources of electricity. — The most import-
ant of these, namely, friction, has been already considered ; it is
therefore only necessary to describe here some of the other pro-
cesses of a mechanical kind by which electricity can be produced.
The simplest and one of the most remarkable of these is pres-
sure. Very many substances, after being pressed with moderate
force, are found to be electrified ; but this effect is most strikingly
shown by a fragment of Iceland spar having bright polished sur-
faces, such as are obtained when it is freshly broken. When
such a crystal is pressed between the finger and thumb, it is found
afterwards to be positively electrified, and if well insulated, it
will retain its charge for several days. Hence, a small crystal of
Iceland spar fastened at the end of a light rod of shell-lac, and
the whole suspended by a fibre of floss-silk, so as to be balanced
and free to move in a horizontal plane, forms a convenient electro-
scope, by means of which the nature of the electricity with which
any body is charged can be determined.
- Many other crystallized minerals, such as Brazilian topaz, fluor
spar, corundum, emerald, spinelle, &c., show similar phenomena,
but in a less marked degree.
Another mechanical process in which electricity is developed is
cleavage, and the separation of closely-adhering surfaces. If a
crystal of mica is separated into two laminae, and these are rapidly
torn asunder by means of insulating handles to which they are
attached by means of wax, one lamina becomes positively, and
the other negatively, electrified. In a dark room a flash of light
may be seen at the moment of separation. Similarly, on tearing
a playing-card into its two sheets, these are found to be oppositely
electrified. Again, if two sheets of writing paper are laid oue
upon the other and rubbed with india-rubber, they stick together
94 ELECTRICITY.
and appear strongly charged with opposite electricities after being
pulled asunder.]
157. [Development of electricity by heat. — There are two
distinct ways in which heat can give rise to electricity. Certain
crystals, so long as they are undergoing a change of temperature,
exhibit contrary electricities at their two extremities. Such
crystals are termed pyro- electric, and among substances in which
this property is most easily studied are crystals of tourmaline.
The two ends of a pyro-electric crystal are called its poles, but
the kind of electricity manifested at each depends upon whether
the temperature is rising or falling ; that one which shows positive
electricity while the temperature is rising, shows negative electri-
city while it falls, and vice versa. The pole at which positive
electricity appears with a rise of temperature, and negative elec-
tricity with a fall, is called the analogous pole ; the other one,
which is negative when the temperature is rising, and positive
when it is falling, is called the antilogous pole.
With regard to the connexion between the quantity of elec-
tricity developed and the amount of change of temperature, it is
found that the. quantity of electricity evolved is always the same
for the same alteration of temperature, whether this takes place
quickly or slowly ; and that the quantity of one kind of electricity
developed at one pole, during a rise of temperature of a given
number of degrees, is precisely equal to the quantity of the oppo-
site electricity developed there during a fall of temperature of the
same amount. In order to charge an electroscope with electricity
produced in this way, one end of a crystal of tourmaline should
be connected with the electroscope by an insulated wire, and the
other end should be in contact with a wire leading to the earth.
The crystal should, of course, be perfectly clean and dry, and it
should not be heated much above the temperature of boiling water.
The pyro-electric poles of a crystal of tourmaline are situated at
the two ends of its principal crystallographic axis. The quantity
of electricity which accumulates at each pole is proportional to the
area of the cross section of the crystal, and is only indirectly
affected by its length.
The second mode in which heat is capable of producing elec-
tricity is shown in the phenomena of
thermo-electricity. When a circuit is made
of two good conductors, a copper and an
iron wire, for example, joined together at
C each end, as c and i, (fig. 104.), there
Fig. 104 -will be a continual flow of electricity
round the whole circuit, so long as the two points of juncture
of the conductors are kept at different temperatures.
SOURCES OF ELECTRICITY. 95
The essential condition for the development of electricity in
this manner is not a change of temperature, as in the case of
pyro-electricity, but that the circuit should be formed of at least
two heterogeneous materials, and that there should be a difference
of temperature between the junctions. Additional details re-
lating to thermo-electricity will be found further on (368.^ seq.).
The development of electricity as the result, of Chemical action
will form a prominent subject of the next part of this treatise.
Its production by the action of magnets will also be described in
a subsequent chapter.]
BOOK THE SECOND.
VOLTAIC ELECTRICITY.
CHAPTER I.
158. [Discovery of galvanism. — In the year 1780, Galvani,
Professor of Anatomy in the University of Bologna, being engaged
in investigating the nature of nervous action, accidentally observed
the occurrence of convulsive movements in the limbs of a recently
killed frog, when an electrical machine at a little distance was dis-
charged. These movements were simply an effect of the secondary
shock (141.), a phenomenon with which it appears that Galvani
was well acquainted, notwithstanding the assertions that have been
made to the contrary. This observation suggested to him that
muscular motion in all cases, and nervous action in general, might
be due to electricity. With this idea, he devoted several years
to an elaborate investigation into the circumstances of the pheno-
menon he had witnessed.
In the course of this enquiry, Galvani desired to ascertain
whether the discharge of a thunder-cloud would produce the
same effect as that of an electrical machine, and he found that this
was the case. One day in 1786, however, having suspended to
the iron palisades outside his laboratory the lower-limbs of a frog,
prepared for the purpose of his experiments, by means of a copper
wire which passed through the spinal marrow, he was surprised
to see that, although there were no thunder-clouds about, the
frog's legs gave a convulsive jerk every time they happened to
touch the iron railing as they swung in the wind.
This observation was in its turn eagerly followed up by Gal-
vani, who soon found that the convulsive movements could be re-
produced almost at will upon the limbs of a recently killed frog,
by making a communication between the lumbar nerves and the
muscles of the leg by means of a metallic arc, as c D (^gc. 105.).]
GALVANI'S DISCOVERIES. 97
Fig. 105.
I 59; Galvaui's theory. — In order to explain these results,
Galvani supposed that the nerves of animals possessed an electri-
city peculiar to themselves, and that this vital fluid, as he called
it, was communicated to the muscles through the metallic arc,
and caused their convulsive contraction. He thus compared the
limbs and body of the frog to a Leyderi jar, the two coatings of
which were represented respectively by the nerves and muscles,
and which was charged with a fluid analogous to, but not identical
with, electricity, and which was afterwards named the galvanic
fluid.-]
1 60. [Volta's theory. — These discoveries of Galvani excited
universal attention amongst scientific men, and for a time his
explanation of them was admitted without question. Soon, how-
ever, Volt a, at that time Professor of Natural Philosophy at
Pavia, while repeating Galvani's experiments, was struck with the
necessity of using an arc composed of two different metals in
order to ensure the production of vigorous movements. Following
up this observation, he was led to abandon Galvani's explanation
of the phenomena, and to regard them as resulting from the action
of ordinary electricity generated, not in the body of the frog itself,
which he considered as acting simply the part of an electroscope,
but at the surface of contact of the two metals forming the arc of
communication. In support of this theory, he made a great
n
93
VOLTAIC ELECTEICITY.
number of experiments by which he endeavoured to demonstrate
directly that electricity is produced whenever two different metals
are in contact.
The most important of these may be described as follows : a
delicate gold-leaf electroscope E (fig. 106.) was provided with a
condenser (64.), formed of two copper plates separated by a thin
nonconducting stratum. The upper plate was touched with the
copper extremity of a compound bar c z (made by soldering to-
gether a piece of copper and a piece of zinc), while the zinc end
of the bar was held in the hand ; and the lower plate was at the
same time uninsulated by touching it with a finger of the other
hand. On withdrawing the finger and compound bar, and then
raising the upper plate of the condenser, the gold leaves were
found to diverge with positive electricity, thus showing that the
plate which had been in contact with the copper end of the bar had
received a negative charge.
Fig. 106.
This experiment, which was varied in many ways, proved con-
clusively that, under the circumstances in question, there was a
development of electricity capable of affecting an ordinary elec-
troscope formed of inorganic materials, and therefore that it was
no longer necessary to suppose, with Galvani, that the vital elec-
tricity of the fiog's limb was the cause of the phenomena he had
first observed. The result was that Galvani's theory was generally
ELECTROMOTIVE FORCE. 99
abandoned and Volta's contact theory was accepted as affording the
true explanation of the experiments that have been described.]
l6l. [Electromotive force. — It has been stated already (160.)
that Volta considered tlie mere contact of two heterogeneous
ruetals to be sufficient to cause a disturbance of their electrical
equilibrium. He supposed the surface of contact between them
to be the seat of a peculiar force, which he called electromotive
force, whereby positive electricity was caused to move in one
direction across the surface of contact, and negative electricity in
the opposite direction, so as to cause the metals at each side to be
charged, one with positive, and the other with negative electricity.
Thus, in the experiment described in (160.), positive electricity
was supposed to flow from the place where the copper and zinc
were soldered together, over the piece of zinc and through the
arms and body of the experimenter, to the lower plate of the
condenser, while negative electricity was supposed to flow from
the same point to the upper plate.
This motion of the two electricities was not, however, supposed
to continue indefinitely — at least, not when the two metals in con-
tact were insulated from other conductors. It is obvious that the
opposite electricities, accumulated at the two sides of the surface
of contact, would exert an attractive force upon each other, and
tend to recombine in opposition to the electromotive force which
tended to separate them. Consequently, when the accumulation
of the electricities upon the two metals had reached a certain
point, the force with which they tended to recombine would be
equal to the electromotive force, and a state of equilibrium would
be established in which no further motion of the electricities could
take place.
The intensity with which the two electricities attracted each
other across the surface of contact, or with which they tended to
pass off into other conductors, was thus a measure of the electro-
motive force subsisting between any two metals, and it could be
approximately estimated by observing the amount of divergence
of the gold leaves produced in experiments such as that described
in (160.)-
By measuring in this way the electromotive force of a great
many different pairs of metals, it was found that this force varied
both in intensity and direction, from one pair to another, but was
pretty nearly constant for the same pair. And it was likewise
discovered that the metals could be arranged in a series, such that
any one of them gave a positive charge to the plate of the con-
denser touched with it, when connected with a metal below it in
the series, and a negative charge when connected with one above
it, the condenser being always made of the same metal as that
H 2
IOO
VOLTAIC ELECTRICITY.
with which it was touched. The following table gives such series
as they have been constructed by Volta, and by Pfaff, Henrici, and
Peclet :
Henrici.
Peclet.
Zinc. Zinc.
Zinc.
Zinc.
Lrad. Lead.
Lead.
Lead.
Tin.
Cadmium.
Tin.
Tin.
Iron.
Tin.
Antimony.
Bismuth.
Copper.
Iron.
Bismuth.
Antimony.
Silver.
Bismuth.
Iron.
Iron.
Graphite.
Charcoal.
Cobalt.
Arsenic.
Brass.
Copper.
Copper.
Silver.
Crystallised
Amber.
Copper.
Antimony.
Platinum.
Silver.
Mercury.
Gold.
Gold.
Platinum.
Gold.
Platinum.
Mercury.
Silver.
Charcoal.
As might be supposed, from the mode of formation of these
series, the electromotive force of a couple composed of any two
metals is greater in proportion as the places of the metals in the
series are farther apart. Moreover, if three metals are connected
together — as, for instance, zinc, iron, and copper — the electromotive
force of the combination is found to be precisely the same as that
of the couple formed by connecting the first and third metals
without the intervention of the second. From this it follows, and
the consequence is confirmed by experiment, that if any number
of metals are connected together, one after another, the electro-
motive force of the whole combination is equal to that of the
couple formed by connecting the first metal directly with the last.]
162. [True explanation of the results above described.
The experimental results from which Volta inferred that the mere
contact of different metals was sufficient to call into existence an
electromotive force, or power capable of causing the movement of
the two electricities in contrary directions, have been confirmed
by all subsequent investigators ; and a very great number of con-
sequences, deduced by himself and others, as necessarily following
from the- existence of such a force, have likewise been found to be
in exact accordance with experiment. Nevertheless, there can be
no doubt that this fundamental supposition of Volta' s was incor-
rect, and that the true source of the electricity in the experiments
referred to was chemical action.
This was maintained by Wollaston and others, near the begin-
ning of this century, and the controversy which thus early arose
between the partisans of the " chemical theory " and those who
supported the " contact theory " of the origin of galvanic or vol-
taic electricity, has not even yet completely died out. It is not
possible, nor desirable, to enter in this place into the details of
this controversy : we must content ourselves with pointing out that
ELECTRICITY DUE TO CHEMICAL ACTIOX. 101
the supposed electromotive force of contact, being — as will be seen
from what is said in subsequent chapters concerning the proper-
ties of voltaic currents — a source of heat and of mechanical force,
unaccompanied with the expenditure of energy in any other form,
would involve the actual creation of energy ; and this is shown by
the combined evidence of all the results of scientific enquiry to be
what never occurs under any known combination of circumstances.
With regard to the particular experiment described in (i6o.)»
we must suppose that the electricity there manifested is the result
of chemical action taking place between the zinc end of the com-
pound bar and the moisture of the hand. The fact that such
chemical action can only occur to a very slight extent does
not constitute the smallest real objection to the adoption of this
explanation. This is amply proved by the following experiment
made by Faraday. That philosopher found that the chemical
action which took place on dipping a copper and a zinc wire, each
TV of an inch in diameter, and separated from each other by a
little more than a quarter of an inch, into four ounces of water
mixed with one drop of sulphuric acid, to the depth of -| of an
inch, for 3-5 seconds, developed as much electricity as was obtained
by the discharge of a Leyden battery of 1 5 jars, having altogether
3500 square inches of internal coated surface, when charged by
30 turns of a large plate electrical machine in excellent order.
This quantity of electricity is so enormous when compared with
that required to cause a slight divergence of the leaves of a deli-
cate electroscope, that the amount of chemical action, by which
the quantity needed to produce the latter result would be engen-
dered, must be quite inconceivably small.]
163. [Development of electricity by chemical action.
Although the numerous experiments by which Volta sought to
prove the existence of the electromotive force of contact, were,
in reality, so many proofs of the development of electricity by
chemical action, it may help to make the matter still clearer to
consider a little more closely the effects observable in a particular
experiment.
Let A and B (fig. 1 07.), be the plates of an electrical condenser,
and let A be connected with a very delicate electroscope, E, and B
with a similar electroscope, F ; further, let c be a plate of copper,
and z a plate of chemically pure zinc (or of ordinary zinc well
amalgamated), which dip, without touching each other, into dilute
acid — which, for simplicity, we will suppose to be hydrochloric acid
— contained in an insulated glass vessel. Now let c be put into
electrical communication with A, and z with B, either for an instant
or for a longer time : no change will yet be seen in the electro-
scopes, but on separating the condensing plates A and B (after
102
VOLTAIC ELECTRICITY.
having broken their connexion with c and z), the electroscope i
will show a charge of positive electricity, and the electroscope F
a charge of negative electricity.
If, after discharging the condenser and the electroscopes con-
nected with it, we recommence the experiment, we obtain precisely
the same results as before ; and tffis is the case however often and
however rapidly the process is repeated. This proves that the
wire connected with the copper plate c is, in some way or other,
kept constantly charged with positive electricity, and the wire
connected with the zinc plate z with negative electricity, so that
as soon as ever a portion of the charge is removed, its place is
instantly filled by a fresh supply.]
164. [Formation of an electric current. — This being the
case, we might expect that if the two wires were directly united
together, without the intervention of the condenser, there would
be a continuous passage of positive electricity from the plate c,
through the wire towards z, and of negative electricity from the
plate z through the wire towards c. If such a constant inter-
change of electricities, or current, really does take place along the
wire, it is not of course to be expected that the electricity will
affect an electroscope, there being a free passage for it throughout
the circuit : we must rather seek for the proof of its presence in
the manifestation of such effects as are produced by a conductor
along which a constant stream of electricity is passing from the
prime conductor of an ordinary electrical machine to the earth.
The most easily observed of these effects are the magnetic pheno-
mena described in (153.) and (l54-)> and these can be reproduced
at will by means of the wire connecting the plates c and z, with
even greater ease than by means of the electrical machine. Thus,
if the wire is twisted a few times round a glass tube, so as to
ELECTRIC CURRENTS. 103
make a short spiral coil, a sewing needle placed inside the glass
tube so as to be surrounded by the spiral, will be strongly magne-
tised ; and if the connexion between the plates c and z be made
through the wire of even a coarse galvanometer, the needle will
be strongly deflected as long as the connexion is maintained.
From these properties of the wire connecting the plates c and
z, and from others to be hereafter described, we are justified in
concluding that as long as the plates are immersed in the dilute
acid, a current of positive electricity flows along the wire from
the copper plate to the zinc, and a current of negative electricity
from the zinc plate towards the copper.]
165. [The direction of an electric current is always spoken
of as being the direction in which the positive electricity moves ;
hence in the above case the current is said to be from the copper
through the wire to the zinc ; but it must be remembered that
there can be no such thing as a current of positive electricity in
one direction, without an equal current of negative electricity in
the opposite direction.]
1 66. [Chemical changes accompanying; the production of
the electric current. — So long as there is no electrical commu-
nication between the zinc plate and the copper plate, except
through the dilute acid into which they both dip, no chemical
action takes place between them and the acid. But as soon as the
two plates are connected by a wire, the zinc begins to dissolve in
the acid, as chloride of zinc, while hydrogen gas is evolved in
minute bubbles in contact with the copper plate. If the wire is
cut, or removed from contact with either of the plates, the solu-
tion of the zinc and evolution of hydrogen immediately cease,
but begin again as soon as the connexion is reestablished. That
is to say, whenever a current of electricity is passing in the con-
necting wire, chemical action is taking place between the acid
and the metallic plates. In fact, the connexion between these
two phenomena is so intimate, that it is impossible not to regard
them as correlative, or to fail to see that the chemical action
which goes on between the acid and the metals, and the current of
electricity in the wire, are both parts of one process.
Although it may not be possible, in the present state of science,
to traCe accurately all the steps of this process, the knowledge we
already possess is sufficient to throw considerable light on the
probable nature of some of the most important of them. We
know, for instance, that the energy with which chlorine combines
with zinc, to form chloride of zinc, is greater than that with
which it combines with hydrogen to form chloride of hydrogen or
hydrochloric acid ; while the energy with which it combines with
copper, to form chloride of copper, is less than that with which
VOLTAIC ELECTRICITY.
it combines with hydrogen. Hence we may assume, as exceed-
ingly probable, that when a plate of zinc and a plate of copper
are placed opposite each other, with a column of hydrochloric acid
between them, the molecules of the acid arrange themselves in
such a way that the atom of chlorine contained in each is turned
towards the zinc, and the atom of hydrogen towards the copper,
Fig. 108.
as represented in fig. 108., where z represents a plate of zinc,
c a plate of copper, and Cl H, Cl H, &c., a string of molecules of
hydrochloric acid, reaching from the zinc to the copper.
1 67. Effect of connecting- the plates.— At the same time, as
we have already seen (163.), the wire a connected with the zinc
plate becomes charged with .negative electricity, and the wire b
connected with the copper plate becomes charged with positive elec-
tricity. This is, for the present, the only perceptible effect. But
if the wires a and b be now joined, a current of positive electri-
city immediately begins to circulate in them in the direction c z,
and a current of negative electricity in the direction z c (164.) ;
and at the same time the zinc begins to be converted into chloride
of zinc, by combining with the chlorine of the acid, and hydrogen
to be evolved as gas in contact with the copper. The chemical
part of this process may be conceived as taking place as follows.
The chlorine of the first molecule of hydrochloric acid combines
with the zinc, and at the same time its hydrogen combines with
the chlorine of the second molecule of acid ; the hydrogen of the
second molecule combines with the chlorine of the third molecule ;
the hydrogen of this with the chlorine of the fourth ; and so on,
till the hydrogen of the last molecule of acid is liberated in con-
tact with the copper, but not being able under the circumstances
to form a stable compound therewith, it assumes the gaseous form.
This stage of the action is represented mfig- 109 (next page).
Next we must suppose that the molecules of hydrochloric acid,
which are thus left with their chlorine-atoms facing the copper,
turn back again, in obedience to the attraction of the zinc for the
chlorine, into the position represented \nfig. 1 08., a fresh mole-
cule taking the place of the one decomposed in the part of the
process already described. Everything being now in the same
ELECTRIC CURRENTS.
105
state as at first, the same changes repeat themselves, over and
over a^ain, the result being a continuous solution of zinc in the
"
|l!?'C
© ®© ®@ ®
© ©
Fig. 109.
acid, in the form of chloride, and separation of hydrogen at the
copper plate, while a current of electricity flows along the wire
from c to z.]
1 68. [Direction of tbe current through the liquid. — It will
be seen that the changes above described amount to a constant
movement of the atoms of chlorine through the acid to the zinc
plate, and of the atoms of hydrogen through the acid to the copper
plate. If now we suppose that, through some cause or other —
the possible nature of which it is not now needful to consider—
the two electricities, which constitute the normal charge of each
molecule of hydrochloric acid in the neutral state, are distributed
unequally between its two constituent atoms, the chlorine- atom
having an excess of negative electricity, and the hydrogen- atom
a corresponding excess of positive electricity, this motion of the
chlorine towards the zinc plate will involve the movement of
negative electricity in that direction, while the motion of the
hydrogen towards the copper plate will involve a movement of
positive electricity towards the copper. Thus then there would
be a constant current of electricity flowing through the liquid
from the zinc to the copper, forming, in conjunction with the
current flowing along the wire from the copper to the zinc, a
closed circuit.
That such a current actually does exist, and that the amount of
electricity which passes in a given time from plate to plate through
the liquid, is precisely equal to the quantity which passes a long
the wire in the same time, is a fact that can be proved by the most
unquestionable experiments, and is entirely independent of the
suppositions above made, or of any others, as to the mode in
which it may arise.]
169. [The galvanic current is a circulation of electricity.
The existence of a movement of electricity across the liquid equal
to the current which traverses the wire, obliges us to modify
our conception of the galvanic current, and, instead of regarding
it as a mere flow of electricity along the wire from the copper to
106 VOLTAIC ELECTKICITY.
the zinc, to look upon it as a circulation of electricity round the
entire circuit — the positive electricity taking the direction : zinc,
acid, copper, wire, zinc ; and the negative electricity the direction :
zinc, wire, copper, acid, zinc.
Such a circulation of electricity is not only a consequence, but
a necessary condition of the chemical processes that have been sup-
posed above (167., 1 68.); for, the chlorine arriving at the zinc
charged with an excess of negative electricity, the zinc with which
it combines must be charged with a corresponding excess of posi-
tive electricity in order that the chloride of zinc formed by the
combination may be neutral ; similarly, the hydrogen arriving at
the copper with an excess of positive electricity, must receive from
the copper an equivalent quantity of negative electricity, to reduce
it to the neutral condition in which it escapes.]
1 70. [Power of various galvanic combinations. — What has
been said above as to the probable mode in which the current is
generated, in the case of a plate of zinc and a plate of copper
dipping into dilute hydrochloric acid, •will apply with but slight
alteration to the case of any other simple galvanic combination
composed of two metals and a single liquid. Thus, for instance,
if sulphuric acid were substituted for hydrochloric acid, the
hydrogen of the acid would travel to the copper plate, and be set
free there, while the remaining elements, sulphur and oxygen
(S O4), would travel to the zinc plate, and like the chlorine, com-
bine with zinc, only forming sulphate of zinc instead of chloride.
Similarly, without causing any essential difference in the action
of the apparatus, we might substitute a plate of iron for the zinc,
or a plate of silver or platinum for the copper.
But, although a current of electricity would still be generated,
and generated in the same way, after any or all of these altera-
tions had been made, the strength of the current, or quantity of
electricity passing round the circuit in a given time — as measured,
for instance, by its power of deflecting a magnetic needle — would
be different in each case. Tn order that a current may be pro-
duced, it is necessary that one of the metals should have a greater
tendency to combine chemically with one of the constituents of
the liquid than with the other, and that its tendency to combine
with that constituent should be greater than that of the other
metal. The greater the difference between the metals in this re-
spect— the greater the tendency of one of them to combine with
one constituent of the liquid, and the less the tendency of the
other to combine with the same constituent — the stronger will be
the current produced.]
171. [Electro-chemical series. — By determining the direction
of the currents which different pairs of metals yield when im-
ELECTRO -CHEMICAL SERIES. 107
mersed in the same liquid, the metals can be arranged in a so-called
electro-chemical series, similar to the electro-motive series already
given in (161.), such that when a metal is combined with any of
those below it in the series, the current is always in the same
direction, but in the contrary direction when the same metal is com-
bined with any of those above it. When the metals are thus
arranged, the current produced by the first and last metals of the
series is stronger than that which either of them produces with
any of the intermediate metals under the same circumstances;
and, in general, the farther apart any two metals are in the series,
the stronger is the current which they produce.
But since not only the metals, but the liquid in which they are
immersed, take part in the generation of the current, the current
which a given pair of metals can produce differs in strength, and
may even differ in direction, when different liquids are em-
ployed. This is illustrated by the following table, taken from
Faraday : —
Electro -chemical order of the Metals, frc.
In a mixture of 1 vol. hydrochloric
acid and 1 vol. water. In colourless sulphide of potassium.
+ +
i- Zinc i. Cadmium
2. Cadmium I. Zinc
3- T*n 6. Copper
4. Lead 3. Tin
5- Iron 10. Antimony
6. Copper 9. Silver
7. Bismuth 4. Lead
8. Nickel 7. Bismuth
9- Silver g. Nickel
10. Antimony c. Iron
Gold
Platinum
Rhodium
Graphite
Ferric oxide
Peroxide of manganese
Peroxide of lead.
The ten metals contained in the second column are identical
with the first ten of the first column, but it will be seen that they
follow a very different order in each. To facilitate comparison,
the same number is attached to each metal in both columns. Both
series are so arranged that the direction of the current, obtained
with any two metals, is from any metal through the liquid to a
io8 VOLTAIC ELECTRICITY.
metal below it in the series, and through the wire to one above it.
Consequently, each metal is said to be electropositive relatively to
those below it, and electronegative in relation to those above it.]
172. [Necessity for using: a liquid in order to produce a
galvanic current. — The explanation given in (166. to 1 68.), of
the mode in which the galvanic current is generated, when a plate
of zinc and a plate of copper are immersed in dilute acid,
suggests a reason for what hns been universally found in practice
to be a necessary condition for the production of a continuous
current; namely, that one at least of the three substances em-
ployed should be a liquid. For it is evident that when the stage
of the process represented infg. 109. has been reached, the action
cannot continue unless the molecules of the acid turn half-round,
so as to reproduce the state of things represented in Jig. 108., and
such a motion would be possible only in a fluid medium.]
173. [A galvanic current may be produced by the mutual
action of liquids. — Provided that the substances employed are
such that there is a predominating tendency for chemical action
to take place between them in one direction only, and that their
physical condition allows of such action taking place, an electric
current will be generated, even if all the substances which take
part in the action are liquids.
This may be proved by the following experiment. Place four
wine glasses in a row, and pour into the first and fourth some
solution of nitrate of potassium (saltpetre), into the second some
nitric acid, and into the third some solution of potash; place in the
first and last glass a strip of platinum connected by a wire with a
galvanometer (see chap. X.) ; connect the liquids in the first and
second and in the third and fourth glasses by pieces of cotton
lamp-wick, soaked in solution of nitrate of potassium ; and lastly,
connect the liquids in the second and third glasses by a piece of
lamp-wick previously moistened with the liquid contained in either
of them. The galvanometer will now show a continuous current
whose direction through the liquid is from the potash to the nitric
acid.
It is obvious that in this experiment the current is due ex-
clusively to the mutual action of the different liquids, and that the
platinum plates merely serve to establish a connection with the
galvanometer; for being perfectly similar and surrounded by
similar liquids, any tendency which one might have to generate a
current in one direction would be neutralised by the equal ten-
dency of the other to generate a current in the opposite direction.
The neutrality of the platinum plates may, moreover, be proved
by direct experiment : thus, if the first and fourth glasses be con-
GROVE'S GAS-BATTERY. 109
nected directly by a piece of lamp-wick soaked with nitrate of
potassium, the galvanometer will either show no current at all, or
else a weak current which soon subsides, due to accidental in-
equality in the two pieces of platinum, or in the liquids contained
in the two glasses.
The effect in this case is explicable on precisely similar prin-
ciples to those previously applied in ( 1 66.- 1 68.), but a full
discussion of the experiment would involve the introduction of
chemical considerations, which would be out of place here. The
chemical portion of the phenomenon may, however, be described
in general terms as consisting of the transference of potassium
towards the first glass, and of the radical of nitric acid (NO3)
towards the fourth glass.]
1 74. [Production of a current by the combination of two
gases. — Even two gases, such as oxygen and hydrogen, may be
substituted for the copper and zinc plates of our original experi-
ment (163. et seq.). This is done in the remarkable apparatus in-
vented by Mr. Grove, and known as Grove's gas-battery. Fig.
1 10. represents a usual form of a single cell
of this construction. The glass tubes h and o
are inverted in a vessel containing water, or
preferably dilute sulphuric acid, and h is
nearly filled with hydrogen, and o is about
half filled with oxygen. A strip of platinum
occupies the middle of each tube, extending
from the top, where it is connected with a
platinum wire melted through the glass, nearly
to the bottom. When a metallic connexion
is established between the two platinum wires,
the hydrogen and oxygen gradually disappear,
Fi 5 and a current of electricity circulates in the
apparatus from o through the wire to A, and
thence through the liquid to o. In this apparatus, the current is
generated by the mutual action of the gases and the water or
acid, the strips of platinum only acting as conductors.]
175. [Conditions needed for the production of a constant
current. — Not only must one of the substances engaged in the
production of the current be liquid, so that its molecules may be
free to move (172.), but, in order that the strength of the current
may remain constant, no appreciable change must take place in
the chemical nature of the surfaces in contact. Hence, a cell
charged with pure water, with plates formed of amalgamated zinc
and copper, gives a current which becomes exceedingly weak after
a few seconds, although at the first instant it is as strong as if the
no
VOLTAIC ELECTRICITY.
cell had been charged with acid ; the reason being that the chemi-
cal action which accompanies the production of the current con-
verts the surface of the zinc plate into oxide of zinc, which is
insoluble in water, and therefore prevents further contact between
the water and the zinc. If a little hydrochloric or sulphuric acid
is poured into the water, the strength of the current increases
considerably, and remains comparatively constant for some time,
because the result of the chemical action now is to form chloride
or sulphate of zinc, which, being soluble in water, is removed as
fast as it is formed, thus leaving the zinc plate always in free con-
tact with the liquid. But even in this case, the strength of the
current declines at a greater rate than can be due to the gradual
exhaustion of the acid. The cause of this was for a long time in-
volved in great obscurity, but it has at last been clearly traced to
the effect of the hydrogen set free at the copper plate. A part of
this hydrogen, instead of escaping through the liquid in bubbles,
remains as a film of gas adhering to the copper ; consequently, as
soon as this film has been formed, we have practically a plate of
hydrogen, instead of a plate of copper, opposed to the zinc, and,
as the position of hydrogen in the electro-chemical series is much
nearer to zinc than that of copper is, the force of the current is
reduced.
All the earlier galvanic or voltaic apparatus, as arrangements
for obtaining an electric current by chemical means are called,
consisted of plates of copper and zinc immersed in dilute acid ;
hence, in all of them there was a rapid dimi-
nution in the strength of the current after the
first. But within the last twenty or thirty
years, several forms of galvanic cell have been
invented in which this defect is greatly di-
minished, if not entirely got rid of. Some of
the most important of these will now be de-
scribed.]
176. [Smee's system. — A single cell of the
construction introduced by Mr. Smee is shown
in^g\ III. It consists of a glass or porcelain
vessel, A, containing sulphuric acid diluted
with ten or twelve times its bulk of water,
into which dips a plate of platinized silver, s, placed between
two plates of amalgamated zinc, z z. The plates are usually
attached to a bar of wood, a, but in such a manner that there
is no metallic connexion between the silver and the zinc except
through the conducting wire. The action of such an arrange-
ment is essentially identical with that of a couple consisting of
zinc and copper, but the finely divided platinum with which the
Fig. in.
DANIELL'S CONSTANT BATTERY.
Fig.iiz.
silver plate is coated facilitates the escape of the hydrogen, and
thus renders the current stronger, and more uniform.]
177. Daniell's system. — In this arrangement the metals used
are amalgamated zinc and copper, but the
separation of hydrogen upon the latter is
entirely prevented by chemical means. It
is often constructed as shown in jig. 1 1 2.,
where c c is a copper vessel, widening near
the top, a d; in this is placed a cylindri-
cal vessel of porous unglazed porcelain, p ;
and in this latter is placed a hollow
cylinder of zinc, z. The space between
the copper and porcelain vessels is filled with a saturated solution
of sulphate of copper, which is maintained in a state of satu-
ration by crystals of the salt
placed in the wide cup abed,
in the bottom of which is a
grating composed of wire car-
ried in a zigzag direction be-
tween two concentric rings,
as represented in plan at G.
The vessel j», containing the
zinc, is filled with a solution
of sulphuric acid, containing
from I o to 25 per cent, of acid
when great electro -motive
power is required, and from
I to 4 per cent, when more
moderate action is sufficient.
Another usual form is repre-
sented in perspective in fig. 113.,
where v is a cylinder of glass or
porcelain filled with the saturated
solution of sulphate of copper.
The copper cylinder c, the sides of which are pierced with holes, is immersed
in this. To the upper part of this cylinder is attached the annular gallery
v, the bottom of which is pierced with small holes, and which is immersed
in the solution. This gallery is filled with crystals of sulphate of copper,
which are being constantly dissolved, so as to keep the solution at the point
of saturation. In fine, in the interior of the cylinder c is contained a smaller
cylinder of unglazed porcelain, filled with water, acidulated with sulphuric
acid, or holding in solution common sea salt, in which is plunged the zinc
cylinder B, open at both ends and amalgamated. To the cylinders of zinc
and copper are attached, by clamping screws, two copper ribbons, by means
of which the current can be carried wherever it may be required.
178. [Cbemical theory of a Daniell's cell.— In discussing the
112
VOLTAIC ELECTRICITY.
chemical theory of this arrangement, it will be convenient to sup-
pose that we have a flat plate of amalgamated zinc immersed in
sulphuric acid, and opposite to it a flat plate of copper immersed
in sulphate of copper, separated from the acid by a porous parti-
tion ; for the cylindrical form in which the apparatus is commonly
constructed is not in any degree essential, and is only adopted as
a matter of convenience. Let then z {fig. 1 1 4.) represent the
zinc plate, c the copper plate, p the porous partition ; let S04H2,
SO4H2, . . . (the chemical formula of sulphuric acid), repre-
sent the sulphuric acid ; and SO4Cu, S04Cu . . . the sulphate
Fig. 114.
of copper. Then, in consequence of the tendency of zinc to combine
with the sulphur and oxygen of the sulphuric acid being greater
than its tendency to combine with hydrogen, and also greater than
the tendency of copper to combine with the same elements, the
molecules of the acid will arrange themselves, as in the figure, with
their hydrogen atoms turned away from the zinc, and the group
of atoms SO4 turned towards it. Hence all the molecules of
sulphuric acid, which are in contact with the porous partition /?,
present their hydrogen face to the solution of sulphate of copper
on the other side of the partition. Accordingly, we may con-
sider the sulphate of copper as being contained between a plate
of hydrogen and a plate of copper. Under these circumstances,
its molecules will arrange themselves, as in the figure, so that the
atom of copper of each molecule is turned towards the copper
(away from the hydrogen), and the group of atoms SO4 towards
the hydrogen. The wire connected with the zinc plate at the
same time becomes charged with negative electricity, and that
connected with the copper plate becomes charged with positive
electricity.
As soon as contact is made between the wires, a current of
electricity begins to circulate from the zinc plate through the
liquids to the copper, and thence along the wire to the zinc. At
the same instant the zinc begins to dissolve as sulphate of zinc
in the sulphuric acid ; that is to say, some of the atoms of zinc
at the surface of the plate combine with the sulphur and oxygen
THEORY OF DANIELL'S BATTERY. 113
of the neighbouring molecules of sulphuric acid, taking the place
of the hydrogen previously combined with them. The hydrogen
thus displaced seizes the sulphur and oxygen of the next layer of
molecules of acid, while the hydrogen of this layer passes on to
the third, and so on, till the layer of molecules is reached, which
is in contact with the sulphate of copper through the porous par-
tition. The hydrogen here acts on the molecules of sulphate of
copper in contact with it, converting them into sulphuric acid by
taking the place of their copper. This copper, like the hydrogen
displaced by the zinc, passes on to the sulphur and oxygen of the
next layer of molecules of sulphate, while the copper of this
layer passes on to the following one, and so on, till, at the end of
the series, the copper of the last layer of molecules separates in
the solid form upon the copper plate itself.
The arrangement of the molecules when these changes, which
have necessarily been described as successive, though in reality
they are strictly simultaneous, have taken place, is illustrated by
fig. 115. Immediately afterwards, the molecules of sulphuric
acid and of sulphate of copper turn back again into their first
positions, as represented in fig. 1 14., when the changes above des-
cribed are repeated, and so the process goes on continuously as
long as electrical communication is kept up between the plates.
SO* H'SO* H&O* tf
'I
Fig. 115.
The general result of the entire process is that a certain quan-
tity of zinc passes from the metallic state into the form of sulphate
at one side of the cell, while, at the other, an equivalent quantity
of copper passes from the form of sulphate into the metallic
state. The hydrogen of the sulphuric acid, strictly speaking,
never separates from combination at all, but is morely transferred
from one molecule of acid to another; and there being nothing but
copper deposited upon the copper plate, no change can take place
in its activity.]
179. [Grove's system. — This arrangement possesses great
energy as well as great constancy, and is, on the whole, the most
convenient when a powerful current is required to be maintained
114 VOLTAIC ELECTRICITY.
for a considerable time. One of the forms in which it is constructed
is shown inj?g>. 1 16. Here G L is a glass or porcelain vessel con-
f
Fig. 116.
Fig. 117
taining dilute sulphuric acid, (one measure of acid to ten* or
twelve measures of water) ; into this dips a plate of zinc z, bent
round into a cylindrical form and well amalgamated ; in the
middle of the zinc cylinder is a cylindrical vessel P, of porous
earthenware, containing strong nitric acid ; and inside this, dipping
into the acid, is a plate of platinum which is supported by the
cap a. In order to save room, the platinum plate is sometimes
bent into the form of an S, as shown apart in Jig. 1 1 7. The screws
b and c serve to attach wires to convey the current in any required
direction.
It will be observed that a Grove's cell differs from one of
Daniell's construction only in the substitution of nitric acid for
the solution of sulphate of copper, and of a plate of platinum
for one of copper. The chemical theory of the two arrangements
is also very similar, inasmuch as in both the evolution of hydrogen
is prevented by chemical means. In Grove's arrangement this is
effected by the nitric acid, which gives up to the hydrogen part
of its oxygen, thus converting it into water, and being itself re-
duced to nitrous acid, or even partially to nitric oxide. In conse-
quence of this action, suffocating fumes of peroxide of nitrogen or
nitrous acid arise from the apparatus, especially when it has been
long at work, which often cause considerable inconvenience, and
BUNSEN'S BATTERY.
make it desirable always to place it in a position where thorough
ventilation can be secured.]
1 80. [Bunsen's system. — This system is merely a modification
of the preceding, in which a cylinder of very dense charcoal is
Figs. 118 — uj.
substituted for the platinum plate. It has, therefore, the advan-
tage of being cheaper than Grove's system, at the same time that
it is equally energetic in its action,
but it does not usually last so long.
The several parts composing a Bun-
sen's cell are represented separately
in Jigs. 118-123, where A is the
outer vessel, made of glass or glazed
earthenware, B the zinc plate, c a
vessel of porous earthenware, and
D the cylinder of charcoal. The
liquids with which the cell is charged
are dilute sulphuric acid surrounding
the zinc plate, and nitric acid round
the charcoal.
Fig. 1 24 shows the apparatus with
all its parts combined ; here E is the
vessel containing the dilute sulphu-
ric acid and the zinc plate z, p is *'ig. 1*4-
the porous cylinder, and A the cylinder of charcoal.
Very many other arrangements have been proposed, but none
T 2
16
VOLTAIC ELECTRICITY.
of them have come into such general use for experimental pur-
poses as those that have been described. A few of them are
briefly mentioned below.]
1 8 1 . Wheatstone's system. — Professor Wheatstone has pro-
posed the combination represented in Jig. 125. A cylindrical
vessel v v, of unglazed and half-baked red earthenware, is placed
in another and larger one v v, of glazed porcelain or glass.
The vessel v v is filled with a pasty amalgam of
zinc, and the space between the two vessels is filled
with a saturated solution of sulphate of copper. In
the latter solution is immersed a thin cylinder of
copper c c. A rod or wire of copper N is plunged in
the amalgam. The electro-motive forces of this sys-
tem are directed from the amalgam to the copper
solution ; so that p proceeding from the copper cylin-
der is the positive, and N proceeding from the amalgam
is the negative pole.
The action of this system is said to be con-
stant, like that of Daniell, so long at least as
the vessel v v allows equally free passage
to the two fluids, and the state of saturation of the copper solution
is maintained.
182. Bagration's system. — A voltaic arrangement suggested
by the Prince Bagration, and said to be well adapted to galvano-
plastic purposes, consists of parallel hollow cylinders, (j%. 126.) of
zinc and copper, immersed in sand contained in a porcelain vessel.
The sand is kept wet by a solution of hydrochlorate of ammonia.
Fig. 125.
Fig. 126. Fig. 127.
183. Becquerel's system.— M. Becquerel has applied the
principle of two fluids and a single metal explained in (173.) in
the following manner : —
A porcelain vessel v, fig. 127., contains concentrated nitric acid.
A glass cylinder x, to which is attached a bottom of unglazed
VOLTAIC PILE. 117
porcelain, is immersed in it. This cylinder contains a solution of
common salt. Two plates of platinum are immersed, one in the
nitric acid, and the other in the solution of salt. The electro-
motive forces take effect, the conduction being maintained through
the porous bottom of the glass vessel T, the positive pole being
that which proceeds from the nitric acid, and the negative that
which proceeds from the salt
CHAP. II.
VOLTAIC BATTERIES.
184. Volta's invention of the pile. — Whatever may be the
efficacy of simple combinations of electromotors compared one with
another, the electricity developed even by the most energetic
among them is still incomparably more feeble than that which
proceeds from other agencies, and indeed so feeble that without
some expedient by which its power can be augmented in a very
high ratio, it would possess very little importance as a physical
agent. Volta was not slow to perceive this ; but having also a
clear foresight of the importance of the consequences that must
result from it if its energy could be increased, he devoted all the
powers of his invention to discover an expedient by which this
object could be attained, and happily not without success.
He conceived the idea of uniting together in a connected and
continuous series, a number of simple electro-motive combina-
tions, in such a manner that the positive electricity developed by
each should flow towards one end of the series, and the negative
towards the other end. In this way he proposed to multiply the
power of the extreme elements of the series, by charging them with
all the electricity developed by the intermediate elements.
In the first attempt to realise this conception, circular discs of
silver and copper of equal magnitude (silver and copper coin
served the purpose), were laid one over the other, having inter-
posed between them equal discs of cloth or pasteboard soaked in
an acid or saline solution. A pile was thus formed which was
denominated a voltaic pile; and although this arrangement was
speedily superseded by others found more convenient, the original
name was retained.
Such arrangements are still called voltaic piles, and sometimes
voltaic batteries, being related to a simple voltaic combination in
the same manner as a Leyden battery is to a Leyden jar.
118
VOLTAIC ELECTRICITY.
185. Explanation of the principle of the pile. — To explain
the principle of the voltaic battery, let us suppose several simple
voltaic combinations, z'l/c1, z2L2c2, z3 L3 c3, z4 L4 c4, fig. 128., to
be placed, so that the negative poles z shall all look to the left
and the positive c to the right. Let the metallic plates c be ex-
tended, and bent into an arc, so as to be placed in contact with
the plates z. Let the entire series be supposed to stand upon any
insulating support, and let the negative pole z1 of the first com-
bination of the series be put in connection with the ground by a
conductor.
If we express by E the quantity of positive electricity develo
the negative fluid escaping by the conductor, this fluid E will pass to c1, and
from thence along the entire series to the extremity c4. The combination
Z*LICI acts in this case as the generator of electricity in the same manner as
the cushion and cylinder of an electrical machine, and the remainder of the
series z2L8c2, &c., plays the part of the conductor, receiving the charge of
fluid from ZILICI.
The second combination z8L8ca being similar exactly to the first, evolves
an equal quantity of electricity E, the negative fluid passing through ZILICI,
and the conductor to the ground. The positive fluid passes from z8L2c2 to
the succeeding combinations to the end of the series.
In the same manner, each successive combination acts as a generator of
electricity, the negative fluid escaping to the ground by the preceding com-
binations and the conductor, and the positive fluid being diffused over the
succeeding part of the series.
It appears, therefore, that the conductor p connected with the last combi-
nation of the series must receive from each of the four combinations an equal
charge E of positive fluid ; so that the depth or quantity of electricity upon
it will be four times that which it would receive from the single combination
z4L4c* acting alone and unconnected with the remainder of the series.
In general, therefore, the intensity of the electricity received by a con
ductor attached to the last element of the series, will be as many times greater
than that which it would receive from a single combination, as there are
combinations in the series. If the number of combinations composing the
series be n, and E be the intensity of the electricity developed by a single
combination, then n x E will be the intensity of the electricity produced at the
extremity of the series.
It has been here supposed that the extremity z1 of the series is connected
by the conductor N with the ground. If it be not so connected, and if the
entire series be insulated, the distribution of the fluids developed will be dif-
ferent. In that case, the conductor p will receive the positive fluid propa-
gated from each of the electro-motive surfaces to the right, and the conductor
N will receive the negative fluid propagated from each of these surfaces to tie
VOLTAIC PILE.
left, and each will receive as many times more electricity than it would
receive from a single combination, as there are simple combinations in the
series. If, therefore, E' express the quantity of fluid which each conductor
p and N would receive from a single combination ZILICI, then n x E' will be
the quantity it would receive from a series consisting of n simple combinations.
Since two different metals generally enter with a liquid into
each combination, it has been usual to call these voltaic combi-
nations pairs ; so that a battery is said to consist of so rnany^azrs.
On the Continent these combinations are called elements;- and
the voltaic pile is said to consist of so many elements, each ele-
ment consisting of two metals arid the interposing liquid.
1 86. [Poles of the pile. — The final plates of metal at each end
of the pile are called its poles, the one by which the current of
positive electricity issues being called the positive pole, and that
by which the negative current issues, the
negative pole. Sometimes the poles are named
from the metals composing them; thus, the
negative pole is sometimes spoken of as the
zinc pole, and the positive as the copper pole,
platinum pole, carbon pole, &c. Sometimes also
the name pole is transferred from the final
plates themselves to the conducting wires
attached to them, which may in fact be con-
sidered as mere extensions of the plates.]
187. Volta's first pile. — The first pile
canstructed by Volta was formed as follows: —
A disc of zinc was laid upon a plate of glass.
Upon it was laid an equal disc of cloth or
pasteboard soaked in acidulated water. Upon
this was laid an equal disc of copper. Upon
the copper were laid in the same order three
discs of zinc, wet cloth, and copper, and the
same superposition of the same combinations
of zinc, cloth, and copper was continued until
the pile was completed. The highest disc (of
copper) was then the positive, and the lowest
diSc (of zinc) the negative pole, according to
the principles already explained.
It was usual to keep the discs in their places
by confining them between rods of glass.
Such a pile, with its conducting wires, is shown in^g-. 1 29.
1 88. The oouronne des tosses. — The next arrangement pro-
posed by Volta formed a step towards the form which the pile defi-
nitely assumed, and is known under the name of the couronne des
tosses (ring of cups) : this is represented in Jig. 1 30., and consists of
a series of cups or glasses containing the acid solution. Rods of
Fig. 119.
I2O
VOLTAIC ELECTRICITY.
zinc and copper zc, soldered together end to enjj, are bent into
the form of arcs, the ends being immersed in two adjacent cups,
Fig. TJO.
so that the metals may succeed each other in one uniform order.
A plate of zinc, to which a conducting wire N is attached, is im-
mersed in the first ; and a similar plate of copper, with a wire P,
in the last cup. The latter wire will be the positive, and the
former the negative, pole.
189. Cruiksliank's arrangement. — The next form of vol-
taic pile proposed was
that of Cruikshank, re-
presented in Jig. 131.
This consisted of a
trough of glazed earth-
enware divided into pa-
rallel cells corresponding
in number and magni-
tude to the pairs of zinc
and copper plates which
were attached to a bar
of wood, and so con-
nected that, when im-
mersed in the cells, each
copper plate should be in connection with the zinc plate of the
next cell. The plates were easily raised from the trough when the
battery was not in use. The
trough contained the acid so-
lution.
190. Wollaston's arrange-
ment. — In order to obtain
within the same volume a
greater extent of electro-
motive surface, Dr. Wollaston
doubled the copper plate round
the zinc plate, without however
allowing them to touch. In
this case the copper plates have
Fj i z twice the magnitude of the
WOLLASTON'S AND HUNCH'S BATTERIES. 121
zinc plates. The system, like the former, is attached to a bar of
wood, and being similarly connected, is either let down into a
trough of earthenware divided into cells, as represented in jig.
132., or into separate glass or porcelain vessels, as represented
in fig. 1 3 3 . The latter method has the advantage of affording
greater facility for discharging and renewing the acid solution.
Another view of this form of mounting a battery is shown in
134-
191. launch's battery. — Professor Munch of Strasbourg has
simplified the form of Wollaston's battery as shown m .fig. 135.,
by plunging all the couples in a single wooden trough varnished
on the interior. The manner in which the plates of the couples
are combined is shown in the figure. This pile has the advantage
of small bulk, but its action is not of long continuance.
J22
VOLTAIC ELECTRICITY.
192. Helical pile of Faculty of Sciences at Paris. —
The helical pile is a voltaic arrangement adapted to produce
*'ig- »35-
electricity of low tension in great quantity. This pile, as con-
structed for the Faculty of Sciences at Paris under the direction
of M. Pouillet, consists of a cylinder of wood b,fig. 136., of about
four inches diameter and fifteen inches long, on
which are rolled spirally two thin leaves of zinc
and copper separated by small bits of cloth, and
pieces of twine extended parallel to each other,
having a thickness a little less than the cloth.
A pair is formed in this manner, having a surface
of sixty square feet. A single combination of
this kind evolves electricity in large quantity,
and a battery composed of twenty pairs is an
agent of prodigious power.
The method of immersing the combination in the acid solution
is represented in fig- 137.
1 93 . Piles are formed by connecting together a number of any
of the simple electro-motive combinations described in the last
chapter, the conditions under which they are connected being
always the same, the positive pole of each combination being put
in metallic connection with the negative pole of the succeeding one.
Fig. ij6.
BUNSEN'S BATTERY.
When the combinations are cylindrical, it is convenient to set
them in a framing, which will prevent the accidental fracture or
Fig. ij8.
Fig. IJ7-
strain of the connections. A battery of ten pairs of Grove's or
Bunsen's is represented with its proper connections \nfig. 138.
A similar battery upon Bunsen's principle is shown in fig. 139.
Ftg. 139.
In fig. 140. is represented a convenient form of Dan iell's bat-
tery, consisting of four pairs. The jars are here made flat, a form
I24 VOLTAIC ELECTRICITY.
which is more convenient when zinc is used, which is generally
manufactured in sheets. The diaphragms are made either of sail
cloth, or gold beater's leaf. Each pair is placed in connection
by a wire extending from the zinc of one pair to the copper of the
other. The terminal wire D attached to the zinc of the first pair
is the negative pole, and the wire E attached to the copper of the
last pair is the positive pole.
. 140-
194. Conductors connecting1 the elements. — Whatever be
the form or construction of the pile, its efficient performance re-
quires that perfect metallic contact should be made and main-
tained between the elements composing it, by means of short and
good conductors. Copper wire, or, still better, strips cut from
sheet copper from half an inch to an inch in breadth, are found
the most convenient material for these conductors, as well as for
the conductors which carry the electricity from the poles of the
pile to the objects to which it is to be conveyed. In some cases,
these conducting wires or strips are soldered to metallic plates,
which are immersed in the exciting liquid of the extreme elements
of the pile, and which, therefore, become its poles. In some cases,
small mercurial cups are soldered to the poles of the pile, in which
the points of the conducting wires, being first scraped, cleaned,
and amalgamated, are immersed. Many inconveniences, however,
attend the use of quicksilver, and these cups have lately been very
generally superseded by simple clamps constructed in a variety of
forms, by means of which the conducting wires or strips may be
fixed in metallic contact with the poles of the pile, with each
other, or with any object to which the electricity is required to be
conveyed. Where great precaution is considered necessary to
MEMORABLE PILES. 125
secure perfect contact, the extremities of the conductors at the
points of connection are sometimes gilt by the electrotyping
process, which may always be done at a trifling cost. I have not,
however, in any case found this necessary, having always obtained
perfect contact by keeping the surfaces clean,
and using screw clamps of the form in^g-. 1 40 b.
This is represented in its proper magnitude.
195. Pile may be placed at any dis-
tance from place of experiment. — It is
generally found to be inconvenient in practice
to keep the pile in the room where the ex-
F. <r ~ periments are made, the acid vapours being
injurious in various ways, especially where
nitric acid is used. It is therefore more expedient to place it in
any situation where these vapours have easy means of escaping
into the open air, and where metallic objects are not exposed to
them. The situation of the pile may be at any desired distance
from the place where the experiments are made, communication
with it being maintained by strips of sheet copper as above de-
scribed, which may be carried along walls or passages, contact
between them being made by doubling them together at the ends
which are joined, and nailing the joints to the wall. They
should of course be kept out of contact with any metallic object
which might divert the electric current from its course. I have
myself a large pile placed in an attic connected by these means
with a lower room in the house, by strips of copper which measure
about fifty yards.
196. Memorable piles: Davy's pile at the Royal Insti-
tution. — Among the apparatus of this class which have ob-
tained celebrity in the history of physical science, may be men-
tioned the pile of 2000 pairs of plates, each having a surface of
32 square inches, at the Royal Institution, with which Davy
effected the decomposition of the alkalies, and the pile of the
Royal Society of nearly the same magnitude and power.
197. Napoleon's pile at Polytechnic School. — In 1 808,
the Emperor Napoleon presented to the Polytechnic School at
Paris, a pile of 600 pairs of plates, having each a square foot of
surface. It was with this apparatus that several of the most im-
portant researches of Gay Lussac and ^henard were conducted.
198. Children's great plate battery, consisted of 1 6 pairs
of plates constructed by Wollaston's method, each plate measuring
6 feet in length and 2f feet in width, so that the copper surface
of each amounted to 32 square feet; and when the whole was
connected, there was an effective surface of 5 1 2 square feet.
199. Hare's defiagrator was constructed on the helical prin-
126 VOLTAIC ELECTRICITY.
ciple, and consisted of 80 pairs, each zinc surface measuring
54 square inches, and each copper 80 square inches.
200. Stratingh's deflagrator consisted of-ioo pairs on Wol-
laston's method. Each zinc surface measured 200 square inches.
It was used either as a battery of I oo pairs, or as a single combi-
nation (191.), presenting a total electro-motive surface of 227
square feet of zinc and 544 of copper.
10 I. Pepys' pile at London Institution consisted of ele-
ments each of which was composed of a sheet of copper and one
of zinc, measuring each 50 feet in length and 2 feet in width.
These were wound round a rod of wood with horsehair between
them. Each bucket contained 55 gallons of the exciting liquid.
202. These and all similar apparatus, powerful as they have
been, and memorable as the discoveries in physics are to which
several of them have been instrumental, have fallen into disuse,
except in certain cases, where powerful physiological effects are
to be produced, since the invention of the piles of two liquids,
which, with a number of elements not exceeding 40, and a
surface not exceeding 100 square inches each, evolve a power
equal to the most colossal of the apparatus above described.
The most efficient voltaic apparatus are formed by combining
Daniell's, Grove's, or Bunsen's single batteries, connecting their
opposite poles with strips of copper as already described. Grove's
battery, constructed by Jacobi of St. Petersburgh, consists of 64
platinum plates, each having a surface of 36 square inches ; so
that their total surface amounts to 16 square feet. This was at
the time the most powerful voltaic apparatus ever constructed.
According to Jacobi's estimate, its effect is equal to a Daniell's
battery of 266 square feet, or to a Hare's deflagrator of 5500
square feet.
203. Dry piles. — The term dry pile was originally intended
to express a voltaic pile composed exclusively of solid elements.
The advantages of such an apparatus were so apparent, that
attempts at its invention were made at an early stage in the pro-
gress of electrical science. In such a pile, neither evaporation nor
chemical action taking place, the elements could suffer no change ;
and the quantity and intensity of the electricity evolved would be
absolutely uniform and invariable, and its action would be per-
petual.
204. Deluc's pile. — The first instrument of this class con-
structed was the dry pile of Deluc, subsequently improved by
Zamboni. This apparatus is prepared by soaking thick writing-
paper in milk, honey, or some analogous animal fluid, and attaching
to its surface by gum a thin leaf of zinc or tin. The other side of
the paper is coated with peroxide of manganese. Leaves of thus
DRY PILES
127
are superposed, the sides similarly coated being all presented in
the same direction, and circular discs are cut of an inch diameter
by a circular cutter. Several thousands being laid over one
another, are pressed into a close and compact column by a screw,
and the sides of the column are then thickly coated with gum-lac.
[Even in this apparatus, notwithstanding that it was constructed
with the idea that no chemical change could occur in it, the pro-
duction of electricity must be attributed to chemical action taking
place between the metal foil, the moisture of the paper, and the
oxide of manganese.]
205 Zamboni's pile. — Piles, having two elements only, have
been constructed by Zamboni. These consist of one metal and
one intermediate conductor, either dry or moist. If the former,
the discs are of silvered paper laid with their metal faces all looking
the same way ; if the latter, a number of pieces of tinfoil, with one
end pointed and the other broad, are laid in two watch-glasses
which contain water, in such a manner, that the pointed part lies
in one glass and the broad part in the other. After some time,
they develop at their poles a feeble electricity, which they retain
for several days, the metal pole being positive in the dry pile, and
the pointed end of the zinc in the moist one.
Fig. 141.
206. Voltaic jeux de bague. — A pretty voltaic toy has been
constructed upon the principle of dry piles, as shown \njig. 141.
128 VOLTAIC ELECTRICITY.
Two columns of copper a and b, are connected within a circular box on
which they stand, by a powerful dry pile placed horizontally between them,
the pillar a being its positive, and b its negative pole. Upon a central pivot c
is an ivory cup I, with which are connected two horizontal rods at right
angles to each other, which support four wires, carrying birds, horses, or
boats, upon which stand small figures, holding in their hands rods, aimed, as
they pass, at a ring suspended from another 6gure standing on the same box.
From the four extremities of the horizontal rods little flags are suspended,
upon which metallic leaf is attached, and as the column revolves these leaves
are alternately attracted and repelled by the ball at the top of the columns
a and b, and by this attraction and repulsion the apparatus is kept in con-
stant revolution. Galvanic toys constructed on this principle, which con-
tinue moving for several years, may be seen in the shops of the opticians.
207. Piles of a single metal. — Piles of a single metal have
been constructed by causing one surface to be exposed to a che-
mical action different from the other. This may be effected by
rendering one surface smooth and the other rough. A pile of
this kind has been made with sixty or eighty plates of zinc of four
square inches surface. These are fixed in a wooden trough
parallel to each other, their polished faces looking the same way,
and an open space of the tenth to the twentieth of an inch being
left between them, these spaces being merely occupied by atmo-
spheric air. If one extremity of-this apparatus be put in commu-
nication with the ground, the other pole will sensibly affect an
electroscope.
In this case, the electro-motive action takes place between the
air and the metal.
208. Hitter's secondary piles. — The secondary piles, some-
times called Hitter's piles, consist of alternate layers of homo-
geneous metal plates, between which some moist conducting sub-
stance is interposed. When they stand alone, no electro- motive
force is developed ; but, if they be allowed to continue for a cer-
tain time in connection with the poles of a battery, and then
disconnected, positive electricity will be found to be accumulated
at that end which was connected with the positive pole, and nega-
tive electricity at the other end ; and this polar condition will
continue for a certain tune, which will be greater, the less the
electrical tension imparted. [This phenomenon is due to the
decomposition by the current of the battery of the moisture be-
tween each' pair of plates, whereby that one nearest the positive
pole of the battery becomes coated with oxygen, and the one
nearest the negative pole with hydrogen. This effect will be
better understood after the chemical action of the current has been
described. (See 439.)]
VOLTAIC CURRENTS.
129
CHAP. III.
VOLTAIC CURRENTS.
209. The voltaic current. — The voltaic pile differs from the
electrical machine, inasmuch as it has the power of constantly
reproducing whatever electricity may be drawn from it by con-
ductors placed in connection with its poles, without any manipu-
lation, or the intervention of any agency external to the pile
itself. So prompt is the action of this generating power, that
the positive and negative fluids pass from the respective poles
through such conductors, in a continuous and unvarying stream,
as a liquid would move through pipes issuing from a reservoir.
The pile may indeed be regarded as a reservoir of the electric
tluids, with a provision by which it constantly replenishes itself.
If two metallic wires be connected at one end with the poles P and N,
fig. 142., of the pile, and at the other with any conductor o, through which
it is required to transmit the electricity evolved in the pile, the positive fluid
will pass from P along the wire to o, and the negative fluid in like
Fig. 14*.
from N to o. The positive fluid will therefore form a stream or current from
p through o to N, and the negative fluid a contrary current from N through
o to P.
It might be expected that the combination of the two opposite fluids in
equal quantity would reduce the wire to its natural state ; and this would, in
fact, be the case, if the fluids were in repose upon the wire, which may be
proved by detaching at the same moment the. ends of the wires from the
poles P and N. The wires and the conductor o will, in that case, show no
indication of electrical excitement. If the wire be detached only from the
negative polo N, it will be found, as well as the conductor o, to be charged
with positive electricity ; and if it be detached from the positive pole p, they
will be charged with negative electricity, the electricity in each case being
in repose. But when both ends of the wire are in connection with the poles
p and N, the fluids, being in motion in contrary directions along the wire and
intermediate conductors, imoart to these, qualities which show that they are
not in the natural or unelectrificd state, but which have nothing in common
with the qualities, which belong to bodies charged with the electric fluid in
repose. Thus, the wire or conductor will neither attract nor repel pith balls,
VOLTAIC ELECTRICITY.
nor produce any electro?copic effects. They will, however, produce a great
variety of other phenomena, which we shall presently notice.
The state of the electricities in thus passing between the poles
of the piles, through a metallic wire or other conductor exterior
to the pile, is called a voltaic current.
210. Voltaic circuit. — When the poles are thus connected
by the conducting wire, the voltaic circuit is said to be complete,
and the current continually flows, as well through the pile. as
through the conducting wire. In this state the pile constantly
evolves electricity at its electro-motive surfaces, to feed and sus-
tain the current ; but if the voltaic circuit be not completed by
establishing a continuous conductor between pole and pole, then
the electricity will not be in motion, no current will flow ; but the
wire or other conductor which is in connection with the positive
pole will be charged with positive, and that in connection with
the negative pole will be charged with negative electricity, of a
certain feeble tension, and in a state of repose. Since, in such
case, the electricity with which the pile is charged has no other
escape than by the contact of the surrounding atmosphere, the
electro-motive force is in very feeble operation, having only to
make good that quantity which is dissipated by the air. The
moment, however, the voltaic circuit is completed, the pile enters
into active operation, and generates the fluid necessary to sustain
the current.
These are points which it is most necessary that the student
should thoroughly study and comprehend ; otherwise, he will find
himself involved in great obscurity and perplexity as he attempts
to proceed.
211. Case in which the earth completes the circuit. —
If the conduct ing wires connected with the poles P and N, instead
of being connected with the conductor o, Jig. 143., be connected
Fig. 143.
with the ground, the earth itself will take the place and play the
part of the conductor o in relation to the current. The positive
VOLTAIC CURRENTS. 131
fluid will in that case flow by the wire P E, fig. 143., and the
negative fluid by the wire N E to the earth E; and the two fluids
will be transmitted through the earth E E in contrary directions,
exactly in the s&me manner as through the conductor o. In this
case, therefore, the voltaic circuit is completed by the earth itself.
212. Methods of connecting: the poles with the earth. —
In all cases, in completing the circuit, it is necessary to ensure
perfect contact wherever two different conductors are united.
We have already explained the application of mercurial cups and
metallic clamps for this purpose, where the conductors to be con-
nected are wires or strips of metal. When the earth is used to
complete the circuit, these are inapplicable. To ensure the un-
obstructed flow of the current in this case, the wire is soldered to
a large plate of metal, having a surface of several square feet,
which is buried in the moist ground, or, still better, immersed in
a well or other reservoir of water.
In cities, where there are extensive systems of metallic pipes
buried for the convenience of water or gas, the wires proceeding
from the poles P and N may be connected with these.
There is no practical limit to the distance over which a voltaic
current may in this m, inner be carried, the circuit being still
completed by the earth. Thus, if while the pile PN, fig. 143., is
at London, the wire PE is carried to Paris or Vienna (being insu-
lated throughout its entire course), and is put in communication"
with the ground at the latter place, the current will return to
London through the earth E E, as surely and as promptly as if the
points BE were only a foot asunder.
213. Various denomination of currents. — Voltaic currents
which pass along wires are variously designated, according to the
form given to the conducting wire. Thus they are rectilinear
currents when the wire is straight; indefinite currents when it is
unlimited in length ; closed currents when the wire is bent so as
to surround or enclose a space ; circular or spiral currents when
the wire has these forms.
2 1 4. The electric fluid forming- the current not necessarily
In motion. — Although the nomenclature, which has been adopted
to express these phenomena, implies that the electric fluid has a
motion of translation along the conductor, similar to the motion
of liquid in a pipe, it must not be understood that the existence
of such motion of the electric fluid is necessarily assumed, or that
its nonexistence, if proved, could disturb the reasoning or shake
the conclusions which form the basis of this branch of physics.
Whether an actual motion of translation of the electric fluid along
the conductor exist or not, it is certain that the effect which would
such a motion is propagated along the conductor; and this
132 VOLTAIC ELECTRICITY.
is all that is essential to the reasoning. It has been already stated,
that the most probable hypothesis which has been advanced for
the explanation of the phenomena, rejects the motion of trans-
lation, and supposes the effect to be produced by a series of de-
compositions and recompositious of the natural electricity of the
conductor (138.)-
215. [Resistance of conductors. — It has already been stated
(22.) that the most perfect conductors of electricity offer some
resistance to its passage. When we are dealing with the electri-
city produced by the electrical machine, the resistance of any of
the ordinary metals is in most cases scarcely appreciable ; but in
many experiments with galvanic or voltaic apparatus, the resist-
ance offered by metallic conductors becomes very apparent, as
well as the fact that the resistance of some metals is greater than
that of others.]
216. [Difference between the electrical machine and the
voltaic battery. — This apparent difference in the behaviour of
conductors in relation to Irictioiial and voltaic electricity is due to
the fact that the former is usually obtained of high tension, but in
small quantity, while the latter commonly has a much lower ten-
sion, but is obtained in much larger quantity. The difference
between the electrical machine and a voltaic battery, as sources of
electricity, may be illustrated by comparing the former to a very
scanty spring of water, situated high up on a mountain side, and
the latter to an abundant spring at only a slight elevation. Any
pipes not of very small diameter would suffice to convey the whole
of the water from a small spring on the mountain down into the
plain, and when there it would exert a pressure sufficient to
force it up again to the height from which it had come. But in
order to convey away the whole of the water from the more
abundant spring, none but the largest pipes would suffice, and if
smaller pipes were used the difference between them would be
apparent from the different quantities of water they allowed to
pass. Just so it is with conducting wires applied to a source of
electricity. If the quantity of electricity is small and its tension
great, it will esc«pe along a small conductor, or one offering a
considerable resistance, as well as along a larger or more perfect
conductor. But a larger quantity of electricity of low tension
requires a large conductor, and one offering but little resistance.]
217. [laws of voltaic currents. — It will be evident, from what
has been said, that the strength of a voltaic current must in all
cases depend upon the relation which the force producing the
current, that is, the electro-motive power of the battery, bears to
the resistance which the entire circuit opposes to its passage.
LAWS OF VOLTAIC CURRENTS. 133
These relations can only be investigated experimentally when we
have obtained some method of measuring the intensity of currents
and the resistance which various conductors offer to them; we
shall, therefore, return once more to the consideration of them
when such methods have been described, (see 375- et seq.). Here
we shall merely state some of the most general conclusions de-
ducible from such investigations, taking the experimental results
for granted.]
2 1 8 [The intensity of the current is the same in every
part of the same circuit. — This is the most fundamental of all
the laws regulating the strength of voltaic currents. It amounts
to this, that when the poles of a voltaic battery are connected by
a succession of different conductors — for instance, first a thick bar
of copper, then a fine iron wire, then a piece of platinum, next a
tube containing a solution of some salt — the strength of a current
passing through all these various conductors at the same time,
will be precisely the same in every part, not greater where it is
traversing a good conductor than where it traverses a bad one,
but everywhere just the same as it is in the battery itself.]
219. [Relation between strength of current, electro-
motive force and resistance: Ohm's law. — The relation which
the strength or intensity of the current bears to the electro-
motive force of the battery, and the resistance of the circuit, was
first accurately ascertained by Professor Ohm, and the law by
which he found that it could be expressed is consequently known
as Ohm's law.
This law states that the strength of any current is directly
proportional to the electro-motive force by which it is produced,
and inversely proportional to the resistance opposed to its passage
by the circuit which it has to traverse. It may be expressed in
form of an equation, thus
T_E
-R'
I denoting the intensity of the current, as measured by methods
to be hereafter described; E the electro-motive force of the
battery, depending on the chemical nature of the metals and
liquids employed in its construction ; and R the resistance which
the entire circuit offers to the passnge of the current. This last
quantity depends, as will be further explained in a subsequent
chapter, on the length, sectional area, and nature of the con-
ductors composing the circuit.
This formula shows that if, without altering the electro-motive
force of the battery, we increase the resistance of the circuit, as
maybe done, for instance, by increasing the length or diminishing
134 VOLTAIC ELECTRICITY.
the section of the conductors, the intensity or strength of the
current will be diminished ; and similarly that the intensity will
be increased if the resistance is lessened.]
220. [internal and external resistance. — In applying (his
formula to particular cases, it is necessary to distinguish between
the resistance which the current mee(s with inside the battery
itself, and which may be distinguished as internal resistance, and
the resistance of the remainder of the circuit, which may be
called the external resistance. The internal and external resist-
ance together are all that the current has to overcome, and hence,
denoting the former by Rj and the latter by Re, we have
221. [Effect of increasing the number of cells. — If any
number, w, of galvanic cells, say of Daniell's construction, are con-
nected together in series, so as to form a battery, the zinc plate
of one cell being joined to the copper plate of (he next, and so
on, the electro-motive force of the battery will of course be just
so many times greater than that of a single cell, as there are
cells composing 11(185.); but at the same time the internal
resistance will be increased in precisely the same proportion, for
the current has now n cells to traverse instead of only one. If,
therefore, E denote the electro-motive force, and Rj the internal
resistance of a single cell, the intensity of the current produced
by the battery will be
T— *E E
- -
The last form of this expression shows that if the external
resistance of the circuit, Re, is very small compared to the internal
resistance — as it is when the poles of the battery are joined by a
short thick wire — the current produced by a number of cells is
scarcely more intense than that of a single cell ; for if Re is very
small, — -, which is still smaller, may be neglected without ap-
preciably altering the value of the expression, which would then
F E
become I = jr = -5- In such a case, therefore, there is no ad-
vantage in using more than a single cell.
If, however, the external resistance is considerable, the same
formula shows that the intensity of the current will be increased
OHM'S LAW. 135
E
by increasing the number of cells; for the fraction Rj-f Re must
E
always exceed the fraction -p p •, and will exceed it the more
the greater Re is in proportion to R. : that is, the advantage of
increasing the number of cells will be so much greater, the
greater is the external resistance of the circuit, and the smaller
the internal resistance of each cell.]
222. [Effect of increasing: the size of the plates.— If with-
out otherwise altering the construction of a galvanic cell we
increase the active surface of the plates, the only effect is a dimi-
nution of its internal resistance. The same result may be obtained
with a number of similar cells, by connecting together all the
positive (zinc) plates on the one hand, and all the negative
(copper, &c.) plates on the other hand. If there are m cells, the
m zinc plates connected together will be equivalent to one large
plate m times the size of a single one ; and so also of the copper
plates. With m cells thus connected, the intensity of the current
will therefore be
E mE
J_ — — i> . i> — — v> , -f~*
From this formula we see that when Re is small compared to Rh
there is an advantage in connecting several cells abreast, or, what
comes to the same thing, in using plates of larger size ; but when
Re has a considerable value, the advantage of such a proceeding
is proportionately less.
Hence, the most advantageous way of connecting a given num-
ber of cells will depend upon the resistance of the circuit which
the current is to traverse. If the external resistance is great, the
most powerful current will be obtained by connecting the cells in
series ; if it is small, the greatest intensity will be obtained by
connecting them abreast. If the resistance is of intermediate
value, the best disposition of the cells will be when both these
modes of connexion are combined, so as to make the total re-
sistance of the battery as nearly as possible equal to that of the
external portion of the circuit. For instance, if we have twelve
cells, they may be arranged either in a single series, or in 2
series of 6 cells, in 3 series of 4 cells, in 4 series of 3 cells, in 6
series of 2 cells, or lastly, they may all be connected abreast so
as to be equivalent to a single cell of twelve times the size. It is
plain that in each case the total internal resistance will be directly
I36
VOLTAIC ELECTRICITY.
proportional to the number of cells in each series, and inversely
proportional to the number of series.]
223. Method of coating: th« conducting: wires. — When the
wires by which the current is conducted are liable to touch other
conductors, by which the electricity may be diverted from its
course, they require to be coated with some nonconducting sub-
stance, under and protected by which the current passes. Wires
wrapped with silk or linen thread may be used in such cases, and
they will be rendered still more efficient if they are coated with
a varnish of gum-lac.
When the wires are immersed in water, they may be protected
by enclosing them in caoutchouc or gutta percha.
If they are carried through the air, it is not necessary to sur-
round them with any coating, the tension of the voltaic electricity
being so feeble, that the nonconducting quality of the air is suf-
ficient for its insulation.
224. Supports of conducting: wire. — When the wire is carried
through the air to such distances as would render its weight too
great for its strength, it requires to be supported at convenient
intervals upon insulating props. Rollers of porcelain or glass,
attached to posts of wood, are used for this purpose in the case of
telegraphic wires.
225. Ampere's reotrope to reverse the current. — In
experimental inquiries respecting the effects of currents, it is
frequentlv necessary to reverse the direction of a current, and
sometimes to do so suddenly, and many times in rapid succession.
An apparatus for accomplishing this, contrived by Ampere, and
which has since undergone various modifications, has been deno-
minated a commutator, but may be more appropriately named a
reotrope, the Greek words freos (reos) signifying a current, and
rp6iros (tropos), a turn.
Let two grooves rr' (fig. 144.)' about half an inch in width and depth, be
cut in a board, and between them let
four small cavities v, t, v', f be formed.
Let these cavities be connected diagon-
ally in pairs by strips of copper //' and
m TO', having at the place where they
cross each other a pie^e of cloth or other
nonconducting substance between them,
so as to prevent the electricity from
passing from one to the other. Let the
grooves r and r', and the four cavities,
be varnished on their surfaces with resin,
so as to render them nonconductors.
These grooves and cavities being filled
Fig. 144. with mercury, let the apparatus repre-
VOLTAIC CURRENTS
137
sented in fig. 145. be placed upon the board. A horizontal axis a a' moves
in two holes o o' made in
the upright pieces pp'. It
carries four rectangular
pieces of metal r, c', d, d>, so
adapted that when they are
pressed downwards one leg
of each will dip into the
mercury in the groove, and
the other into the adjacent
cavitv. The arms uniting
Fig. 145.
the rectangular metallic pieces are of varnished wood, and are therefore non-
conductors. When this apparatus is in the position represented in the figure,
it will connect the groove r with the cavity v, and the groove T> with the ca-
vity t'. When the ends d d' are depressed, and therefore c c' elevated, it Avill
connect the groove r with the cavity t, and the groove r' with the cavity v.
The conductor which proceeds from the positive pole of the pile is im-
mersed in the mercury in r, and that which comes from the negative pole is
immersed in the mercury in r1. Two strips of copper b, b' connect the
mercury in the cavities t and v' with the wire w w' which carries the current.
The apparatus being arranged as represented in fig. 145., the current will
pass from the pile to the mercury in r; thence to v by the conductor c;
thence to »' by the diagonal strip of metal //'; thence to w by the metal b',
and will pass along the wire as indicated by the arrows to 6; thence it will
pass to the mercury in t ; thence by the diagonal strip m> m to t' ; thence by
the conductor c' to the mercury in the groove r'; and thence, in fine, to the
negative pole of the pile.
If the ends d d1 be depressed, and the ends cc' elevated, the course of the
current may be traced in like manner, as follows: — from r to <; thence by
b to w' ; thence along the conducting wire in a direction contrary to that of
the arrows to U\ thence to »'; thence to r'; and thence to the negative pole
of the pile.
226. Pohl's reotrope. — Various forms have more recently
been given to reotropes, one of the most convenient of which is
that of Pohl, in which the use of mercury is dispensed with.
Four small copper columns A, B, c, r>,fig. 146., about £ inch diameter, are
set in a square board, and connected diagonally, A with D, and B with c, by
two bands of copper, which intersect without
contact. These pillars correspond to the four
cavities r, t?', t, t' in Ampere's reotrope. A
horizontal axis crosses the apparatus similar to
Ampere's ; the ends of which are copper, and the
centre wood or ivory. On each of the copper
ends a bow a c, b d of copper rests, so formed,
that when depressed on the one side or the other,
it falls into contact with the copper pillars
A, B, c, D. Two metallic bands connect the
pillars A and B with clamps or binding screws p and m, to which the ends of
the wire carrying the current are attached. The ends of the horizontal axis
are attached to conductors which proceed from the poles of the pile. The
course of the current may be traced exactly as in the reotrope of Ampere.
Fig. 146.
1 3 8 VOLTAIC ELECTRICITY.
The arrangement and mode of operation of the me-
tallic bows, by depressing one end or the other of which
tne directi°n of tne current is changed, is represented
yaj — | in/9. 147., where a c is the bow. A and o the two cop-
p. per pillars with which it falls into contact on the one
side or the other, and p the binding screw connected
with the wire which carries tha current.
227. Electrodes. — The designation of poles being usually
limited to the extreme elements of the pile, and the necessity
often arising of indicating a sort of secondary pole, more or less
remote from the pile by which the current enters and leaves
certain conductors, Dr. Faraday has proposed the use of the term
electrodes to express these. Thus in the reotrope of Ampere,
the electrodes would be the mercury in the grooves rr', Jig. 14.4.
In the reotrope of Pohl, the electrodes would be the ends of the
horizontal axis p and M.
This term electrode has reference, however, more especially to
the chemical properties of the current, as will appear hereafter.
228. Floating supports for conducting: wire. — It happens
frequently in experimental researches, respecting the effects of
forces affecting voltaic currents or developed by them, that the
wire upon which the current passes requires to be supported or
suspended in such a manner, as to be capable of changing its
position or direction in accordance with the action of such forces.
This object is sometimes attained by attaching the wire, together
with a small vessel containing zinc and copper plates immersed in
dilute acid, to a cork float, and placing the whole apparatus on
water or other liquid, on which it will be capable of floating and
assuming any position or direction, which the forces acting upon it
may have a tendency to give to it.
229. Ampere's apparatus for supporting: movable cur-
rents. — A more convenient and generally useful apparatus for
this purpose, however, is that contrived by Ampere ; which consists
of two vertical copper rods v v' fig. 148., fixed in a wooden stage
T x', the upper parts being bent at right angles and terminated in
two mercurial cups y y\ one below the other in the same vertical
line. The horizontal parts are rolled with silk or coated with
gum-lac, to prevent the electricity passing from one to the other.
Two small cavities r r' filled with mercury, being connected with
the poles of a battery, become the electrodes of the apparatus.
These may be connected at pleasure with two mercurial cups s *',
which are in metallic communication with the rods v v'. The
reotrope may be applied to this apparatus, so as to reverse the
connections when required.
The wire which conducts the current is so formed at its extre-
VOLTAIC CURRENTS.
'39
raities as to rest on two points in the cups y y\ and to balance
itself so as to be capable of revolving freely round the vertical
line passing through yy' as an axis.
A wire thus arranged is represented in Jig. 149., having its
T'
Fig. 148.
Fig. 149.
ends resting in the cups y y', the current passing from the cup y
through the wire, and returning to the cup yf. If the reotrope be
reversed, it will pass from y' through the wire and return to y.
230. Velocity of electricity. — Numerous experiments have
been made, to determine the velocity with which the voltaic cur-
rent is propagated on a conducting wire. In 1834 Professor
Wheatstone made a series of experiments for this purpose with
revolving reflectors, from which it resulted that a current trans-
mitted along a brass wire the twelfth of an inch in diameter was
propagated with a velocity of 286000 miles per second, being
greater than the velocity of light in the ratio of 286 to 192.
In 1 849 Mr. Walker, of the United States, made a series of
experiments with a view to solve the same problem by means of
the conducting wires of the electric telegraph. It resulted from
his researches that the velocity of the current was not more than
18000 miles per second, being nearly 1 6 times less than the
velocity determined by Professor Wheatstone.
In 1850 Messrs. Fizeau and Gounelle made a similar series of
experiments with the telegraphic wires in France, from which the
following results were deduced: —
1°. The velocity on an iron wire the fifth of an inch in diameter
was 62700 miles per second.
2°. On a copper wire the tenth of an inch in diameter it was
1 10000 miles per second.
3°. The two fluids, positive and negative, are propagated with
the same velocity.
140 VOLTAIC ELECTRICITY.
4°. The force of the pile and the intensity of the current have
no influence on the velocity of propagation.
5^ Conductors composed of different substances do not give
velocities proportional to their conducting powers.
CHAP. IV.
RECIPROCAL INFLUENCE OF RECTILINEAR CURRENTS AND
MAGNETS.
231. Mutual action of magnets and currents. — When a
voltaic current is placed near a magnetic needle, certain motions
are imparted to the needle or to the conductor of the current, or
to both, which indicate the action of forces exerted by the current
on the poles of the needle, and reciprocally by the poles of the
needle on the current. Other experimental tests show that the
magnets and currents affect each other in various ways ; that the
presence of a current increases or diminishes the magnetic inten-
sity, imparts or effaces magnetic polarity, produces temporary
magnetism where the coercive force is feeble or evanescent, or
permanent polarity where it is strong ; that magnets reciprocally
affect the intensity and direction of currents, and produce or arrest
them.
232. Electro-mag-netism. — The body of these and like pheno-
mena, and the exposition of the laws which govern them, constitute
that branch of electrical science which has been denominated
electro - magnetism.
To render clearly intelligible the effects of the mutual action of
a voltaic current and a magnet, it will be necessary to consider
separately the forces exerted between the current and each of the
magnetic poles ; for the motions which ensue, and the forces
actually manifested, are the resultants of the separate actions of
the two poles.
In approaching the study of these phenomena, it will be con-
venient to begin by examining the action of a rectilinear current
upon a freely suspended magnetic needle. We will consider first
the action of a rectilinear current on a magnetic needle free to oscil-
late in a plane parallel to the current.
233. [Case of a needle free to oscillate in a horizontal plane.
This action is most easily studied experimentally with a magnetic
needle, either hung by a fibre of floss-silk, or supported at its
centre by means of an agate cap upon a fine steel point, so as to
oscillate freely in a horizontal plane.
ELECTRO-MAGNETISM.
141
Such a needle, if left to itself, will assume a particular position,
one end pointing about 20° to the west of north, and the other end
pointing as much to the east of south. In consequence of this
property of a magnetic needle, which will be more fully discussed
in the next Book, the end or pole which points in a northerly
direction is called the north pole, and that which points towards
the south, the south pole.
Let N s (fig. 1 50) be a magnetic needle suspended as above de-
scribed, N being the north pole and s the south pole, and let a
conducting wire, A B, be brought under it, so that when the needle
has come to rest the wire is parallel to it. Now let a current be
sent through the wire in the direction A B; the needle will be im-
mediately deflected, as shewn by the arrows in the figure, and,
Fig. 150. Fig. 151.
after a few oscillations, will take a new position of rest with the
south pole towards the left hand, and the north pole towards the
right hand of a person looking along the direction of the current.
If the conductor be placed above the needle, instead of below
it, the needle will be deflected in the opposite direction, the south
pole moving towards the right, of a spectator looking along the
current, and the north pole towards his left, as represented in
ftg-W-
If the direction of the current is reversed while the conductor
is still below the magnetic needle, the direction in which the latter
- 151- Fig. 155.
is deflected will also be reversed ; so that a current passing below
or above the needle respectively, in the direction B A, will affect it in
the same way as a current passing above or below it in the direc-
tion A B, as shewn in Jigs. 152 and 153]
142 VOLTAIC ELECTRICITY.
234. [Rule by which the foregoingr effects may be remem-
bered.— It is easy to keep in mind the various positions taken by
the needle in all these cases, by help of the following rule. Let
the experimenter imagine himself swimming head-foremost in the
direction of the current, and with his face towards the magnetic
needle : then the deflection of the needle will, in all cases, be such
that the north pole moves towards his left hand.
Frcm the inversion which takes place in the deflection of the
needle when the conductor is removed from underneath to above
it, or vice versa, it is obvious that in an intermediate position, that
is, in the plane of oscillation of the needle itself, it will exert no
effect upon it.]
235. [Case of a needle oscillating: in a vertical plane. —
Precisely similar effects to those above described are obtained
when a conducting wire is placed parallel to a magnetic needle,
balanced about a horizontal axis, so as to be capable of oscillating
in a vertical plane. For instance, let the reader imagine himself
looking towards such a needle, balanced horizontally so that the
south pole is towards his left, and the north pole towards his
right ; if a conducting wire be now brought into the same hori-
zontal plane with the needle, between it and the observer, a cur-
rent passing through the wire from left to right will cause the
north pole of the needle to rise and the south pole to fall. And
if the current be reversed, or the wire placed behind the needle
instead of in front, the needle will be deflected in the opposite
way. If both these changes be made at once, the deflection will
be the same as in the first case. In all cases the deflection of the
needle is in accordance with the rule given above (234.) ; as in
fact it must be, for the positions we have supposed would all be
obtained if figs 150. to 153. be imagined to revolve through an
anjrle of 90° about the conducting wire as an axis, and since no
change would be hereby occasioned in the relative positions of the
magnetic needle and electric current, their mutual action must
remain the same as before.]
236. [In passing now to the consideration of the action of an
electric current, upon a magnetic needle free to oscillate in a
plane perpendicular to the direction of the current, it will be
sufficient to take the —
Case of a vertical current and a needle oscillating: in
a horizontal plane. — Let A u (Jig. I 54 ) be a long vertical con-
ducting wire in which a current is passing in the direction A to B,
and let NS be a magnetic needle suspended near it by a silk fibre.
In this case also the needle will be deflected in accordance with
the rule given in (234.), the two poles moving in the directions
indicated by the arrows in the figure. If the directive force of
the earth upon the needle N s be neutralised by placing in the
ELECTRO-MAGNETISM.
H3
proper position near it a fixed magnet, the needle will come to
rest under the influence of the current at right angles to the per-
pendicular drawn from the conductor to the middle point of its
axis. This state of things is represented inj%. I 5 5., where NS is the
needle seen from above, c a ho-izontal section of the conductor,
and c o a line drawn perpendicularly to the conductor from it to
the middle point of the axis of the needle. As already stated, the
needle will come to rest in such a position that its axis N s is at
Hg. 154.
I5S-
In this experiment the final position of the needle, relatirely to
the current, is precisely the same as that which it would have
assumed in all the cases previously considered if it were acted upon
solely by the current; but, in practice, it will never place itself
quite at right angles to the current unless the effect of the earth's
magnetism be counteracted by one or other of the expedients
to be hereafter described.]
237. [Direction of the force exerted by a rectilinear cur-
rent upon each pole of a magnet. — 'J he directions of the
resultant forces exerted by the current upon the two poles of the
magnet can easily be deduced from the result of the last experi-
ment. In the first place, they mu?t necessarily lie in the plane
containing the magnet N s and the perpendicular c o ; for the con-
ductor c (./(i,r I 55 or A B j%. 154.) is continued in a straight line
above and below the magnet so far that its extremities are not near
enough to produce any sensible effect upon the magnet, and hence
its whole effect may be considered as proceeding from the portion
contained in the same horizontal plane with N s. Let then the
force exerted by the current upon the north, pole N be repre-
i44 VOLTAIC ELECTRICITY.
sentod by the line N e, making with the line c N the angle e N c,
the value of which it remains for us to determine. If there were
at N a south pole of precisely equal strength with the north pole
that is there, the current would exert upon it a force equal to N <?,
but opposite in direction ; let such a force be represented by N d,
in the same straight line with N e. In the position of equilibrium
of the needle, the pole s is at the same distance from the wire as
the pole N ; the force s b exerted upon it will therefore be equal to
>• rf, and it will be inclined to the line c s, drawn from the conduc-
tor to the pole, at an angle b s c equal to that at which the force
N d is inclined to the line N c. Hence, the angle bsc is equal to the
angle disc. But since, when the needle is in equilibrium, the
tendency of the forces NC and sb to turn it about its axis must be
equal and opposite, the angles e N c and b s c must also be equal.
That is, the angles ex c and dye must be equal to one another;
consequently, the angle eye and the equal angle b s c are both
right angles.
The resultant of all the forces exerted by an infinitely long
rectilinear conductor, traversed by a current, upon a magnetic
pole is therefore perpendicular to the plane passing through the
conductor and the pole : or, in other words, it is a tangent to the
circle drawn about that point of the conductor which is nearest
to the pole as a centre, with the perpendicular distance from the
conductor to the pole as radius.]
238. [Action of a rectilinear current upon a magnet free
to oscillate about some point other than its centre. — If any
one point of a magnet be fixed, the magnet will place itself, when
under the influence of a current, so that the resultant of all the
forces exerted upon it by the current will pass through the fixed
point, whether that point is the centre of the magnet or not ; for,
if it did not do so, the magnet would rotate about the fixed point
in the direction of the resultant.
It follows from this that if one of the poles of a magnetic
needle be fixed, the other being free to rotate about it, the needle
will place itself, under the influence of a current circulating in a
conductor situated at a little distance, so that the axis of the
needle is perpendicular to the shortest line drawn from the free
pole to the conductor ; for in no other position of the needle
would the resultant of the forces exerted by the current upon
both poles pass through the fixed point.] x g \v
This law can be demonstrated experi-
mentally as follows. Let a light bar, fig. \ 56.,
of ivory, or any other substance not suscep-
tible of magnetism, made Hat at the upper
surface, be balanced like a compass needle on FI*. 156.
a fine point, so as to be free to move round it in a horizontal plane.
ELECTRO-MAGNETISM.
^ X
i
Fig. 157.
Let a magnetic needle, N s, be placed upon one arm of it, so that
one of the poles, the south pole for example, be exactly over the
point of support ; and let a counterpoise, w, be placed upon the
other arm. Let the magnet be rendered astatic, so as not to be
affected by the earth's magnetism, by one or other of the methods
which will be hereafter explained.
Let the needle thus suspended be supposed to play round s,fig. 157., in the
plane of the paper, and let a voltaic current pass downwards along a wire
perpendicular to the paper, c representing •
the intersection of such wire with the paper. j?_ -^
The needle, after some oscillations, will come
to rest in a position s N, so that its direc-
tion shall be at right angles to the line c N,
drawn from the current to the pole N, and
so that the centre s shall be to the left of
N as viewed from c.
It follows, from what has been already
explained, that the force exerted by the
current c on the pole N has the direction
indicated by the arrow from s to N. This
force is, therefore, directed to the right of
N as viewed from c.
If the wire carrying the current be moved
round the circle c c' c" c'", the pole N will follow it, assuming always such
positions N', N", N'", that s N', s N", s N'" shall be at right angles to c' N',
c" N", c'" N'". It follows, therefore, that whatever position may be given to
the current, it will exert a force upon the north pole N of the magnet, the
direction of which will be at right angles to the line drawn from the current
to the pole, and to the right of the pole as
viewed from the current. p"
If the position of the needle be reversed,
the pole N being placed at the centre of mo-
tion, the same phenomena will be manifested,
but in this case the needle will place itself
to the right of the pole s as viewed from the
current c, as represented in fig. 158. It
follows, therefore, in this case, that what-
ever position be given to the current, it will
exert a force upon the south pole of the
magnet, the direction of which will be at
right angles to the line drawn from the
current to the pole, and to the left of the
pole as viewed from the current.
If the conductor is placed at a less distance from the centre of
rotation than the length N s, it will be impossible for the needle
to take any position which will satisfy the above conditions. In
this case it will rotate until it strikes one side or other of the wire.
239. Apparatus to measure intensity of tnis force. —
Having indicated the conditions which determine the directions of
the forces exerted upon a magnetic pole by a current, it is neces-
sary to explain those which affect their intensity.
L
1
I
C'"
Fig. 158.
1 46 VOLTAIC ELECTRICITY.
Let s N, Jig. 1 59., be an astatic needle affected by the current c,
whose direction is perpendicular to the paper, as already ex-
plained. If N be displaced it will oscillate on the one side and the
other of its position of rest, and its oscillations will be governed
by the laws explained in the case of the pendulum. The intensity
Fig. 159-
of the force impressed on it in the direction of the arrows, by the
current c, will be proportional to the square of the number of
vibrations per minute.
240. Intensity varies inversely as the distance. — If the
distance of c from N be varied, it will be found that the square of
the number of vibrations per minute will increase, in the same
proportion as the distance c N is diminished, and vice versa. It
follows, therefore, that the force impressed by the current on the
pole is increased in the same ratio, as the distance of the current
from the pole diminishes, and vice versa.
In the case here contemplated, the length of the wire carrying
the current being considerable, each part of it exercises a separate
force on N, and the entire force exerted is consequently the result-
ant of an infinite number of forces, just as the weight of a body
is the resultant of the forces separately impressed by gravity on
its component molecules. Laplace has shown that the indefinitely
small parts into which the current may be supposed to be divided,
exert forces which are to each other in the inverse ratio of the
squares of their distances from the pole, and that by the compo-
sition of these a resultant is produced, which varies in the inverse
proportion of the distances, as indicated by observation.
The force which each smallest portion of a current exerts upon
a magnetic pole is, moreover, proportional to the sine of the angle
which its direction makes with the straight line drawn from it to
the pole.
241. [Attractive force exerted upon a magnet by a con-
ductor conveying a current. — The phenomena we have been
hitherto considering, in which an electric current causes a mag-
netic needle to assume a particular direction, are not the only
proofs that may be given of its exerting a peculiar force upon a
magnet. If a magnetic needle be suspended horizontally from
the arm of a balance, and counterpoised by a weight attached to
ELECTRO-MAGNETISM. 1 47
the other arm, it will be found to be attracted or repelled by a
conductor placed below and at right angles to it, according to
the direction in which the current conveyed by the conductor is
moving. To facilitate the description of the phenomenon, we will
suppose that the magnet points in its natural direction of approxi-
mately north and south, and that a conducting wire passes below
it, at right angles to it, or nearly east and west. If now a current
be sent through the wire in the direction east to west, the magnet
will be attracted down towards it ; but if the current goes from
west to east, the magnet will be repelled by it and move upwards.
If the wire be placed above the needle, a current from east to west
will repel it, and one from west to east will attract it. In every
case attraction will be converted into repulsion, and vice versa by
reversing the poles of the magnet.
That such attraction and repulsion must take place will be
easily seen from /%•. 1 60, in which N e and s b represent, as has been
already shewn (236.), the resultants of the
forces exerted by the current in the con-
ductor c upon the two poles of the magnet N s.
These two resultants, being equal and equally
inclined to N s, must have a common resultant
acting along the line o c, that is, a force tend-
ing to move the magnet towards the conduc-
tor. If the current were reversed, or if the
poles of the magnet changed places, the re-
sultant would still act along the line o c, but
in the contrary direction, and would therefore
tend to move the magnet away from the con-
rig. IOO.
ductor.
Again, if a magnetic needle be floated on water by means of a
piece of cork, and a conductor carrying a current be brought
over it, the first effect will be that (neglecting the effect of the
earth upon it) the needle will place itself at right angles to the
conductor, as already explained ; and will then move until its
middle point is directly under the conductor. This effect, like the
last, can be easily deduced from what has been stated above
(237.) respecting the direction of the force exerted by a current
upon the poles of a magnet.]
242. [A current tends to make a magnetic pole revolve
round it. — It has been stated (237.) that the resultant of the forces
which a rectilinear current exerts upon a magnetic pole is tangen-
tial to the circle drawn through the pole with the nearest point of
the conductor as centre. Kence, if the pole is free to move in
obedience to this force, but so as to remain always at the same
distance from the current, it will be caused to revolve round the
L 2
148
VOLTAIC ELECTRICITY.
current in a circle ; for the force exerted upon it will in this case
always remain the same in amount, and directed in the same way
relatively to the positions of the pole and the current. The direction
in which a current moving along the conductor c (jig. 161.) from
p to N would cause a north pole to revolve, is shewn by the
arrows. Fig. 162 shows the direction in which a south pole
would tend to move round a similar current.]
Fig. i6z.
243. [The forces which act between currents and mag-
nets are mutual. — We have hitherto considered only the move-
ments impressed by currents upon magnets. But, since in every
case action and reaction are equal and opposite, if the magnet had
been fixed and the currents movable in the foregoing experiments,
the latter would have moved so that their final positions relatively
to the magnets would have been the same as under the converse
conditions that have been previously supposed.]
244. Apparatus to illustrate electro-magnetic rotation.
A variety of interesting and instructive apparatus has been con-
trived to illustrate experimentally the reciprocal forces manifested
between currents and magnets. These may be described generally
as exhibiting a magnet revolving round a current, or a current
revolving round a magnet, or each revolving round the other.
It will be conducive to brevity, in describing these effects, to
designate a motion of rotation which is from left to right, or
according to that of the hand of a watch, as direct rotation, and
the contrary as retrograde rotation. Hence, if N and s express
the north and south poles of the magnet, and A and D express
an ascending and descending current, the rotation of each round
the other in every possible case will be as follows : —
ELECTRO-MAGNETISM.
149
N'D1 Direct.
S, A J
*' * j Retrograde.
We shall classify the apparatus according to the particular
manner in which they exhibit the action of the forces.
245. To cause either pole of a magnet to revolve round
a fixed voltaic current. — Let two bar magnets be bent into
the form shown in jig. 169., so that a small part at the middle
of their length shall be horizontal. Under this part an agate cup
is fixed, by which the magnet is supported on a pivot. Above the
horizontal part a small cup containing mercury is fixed. The
magnets are thus free to revolve on the pivots. A small circular
canal of mercury surrounds each magnet a little below the rect-
angular bend, into which the amalgamated point of a bent wire
dips. These wires are connected with
two vertical rods, which turning at right
angles above, terminate in a small cup
containing, mercury. Two similar mer-
curial cups communicate with the circu-
lar mercurial canals. If the upper cup
be put in communication with the posi-
tive pole of a battery, and the lower
cups with the negative pole, descending
currents will be established on the ver-
tical rods ; and if the upper cup be put
in communication with the negative, and
the lower with the positive, the currents
will ascend. The two magnets may be
placed either with the same or opposite
poles uppermost. The currents pass from
Fig. 169.
the vertical rods to the mercury in the circular canals, thence to
the lower cups, and thence to the negative poles.
When the descending current passes on the rods, the north pole
of the magnet revolves with direct, and the south pole with retro-
grade motion. When the current ascends, these motions are re-
versed.
246. To cause a movable current to revolve round the
fixed pole of a magnet. — Let a glass vessel, fig. 1 70., be
nearly filled with mercury. Let a metallic wire suspended from a
hook over its centre be capable of revolving while its end rests
upon the surface of the mercury. A rod of metal enters at the
bottom of the vessel, and is in contact with a magnetic bar fixed
vertically in the centre of the vessel. When one of the poles of
VOLTAIC ELECTRICITY.
*ig. 170.
the battery is put in communication with the movable
wire, and the other with tha fixed wire connected with
the magnet, a current will pass along the movable
wire, either to the mercury or from it, according to
the connection made with the poles of the battery ;
and the movable wire will revolve round the magnet,
touching the surface of the mercury with a motion
direct or retrograde, according as the current descends
or ascends, and according to the name of the magnetic
pole fixed in the centre (244.).
Let zz\ fig. 171., represent a section of a circular
trough containing mercury, having an opening at the
centre in which is inserted a metallic rod, terminating
at the top in a mercurial cup c. A wire at a b V a' is
bent so as to form three sides of a rectangle, the width b bf corre-
sponding with the diameter of the circular trough z z'. A point
/ ' is attached to the middle of b b', which rests in
the cup c, so that the rectangle is balanced on
the rod £, and capable of revolving on the pivot
as a centre. •
If the mercury in the circular trough be con-
nected by a wire with the negative, while the
cup c is connected with the positive pole of a
battery, descending currents will be established
along the vertical wires b a and b' 'a' '; and if the
connections be reversed, these currents will as-
cend.
If, when these currents are established, the pole of a magnet be
applied under the centre P, it will act upon the vertical currents,
and will cause the rectangular wire a b b' a' to revolve round c,
with a motion direct or retrograde, according
to the direction of the current and the name of
the magnetic pole (244.).
The points of contact of the revolving wires
with the mercury may be multiplied by at-
taching the ends a a' of the wires to a metallic
hoop, the edge of which will rest in contact
with the metal ; or the wires a b and a' b' may
be altogether replaced by a thin copper cylinder
balanced on a point in the cup at c.
Another apparatus for illustrating this is re-
presented in^/%1. 172. A bar magnet is fixed
vertically in the centre of a circular trough
containing mercury. A light and hollow cy-
Fig. i7x. Under of copper is suspended on a point resting
ELECTRO-MAGNETISM.
in an agate cup placed on the top of the magnet, and having a
vertical wire proceeding from it, which terminates in a small
mercurial cup p at the top. Another wire connects the mercury
in the trough with a mercurial cup N. When the cups p and N
are put in communication with the poles of the battery, a current
is established on the sides of the copper cylinder c c, and rotation
takes place as already described.
A double apparatus of this kind, erected on the two poles of a
horse shoe magnet, is represented in fig. 173.
247. Ampere's method. — Ampere adopted the following
method of exhibiting the revolution of a current round a magnet.
A double cylinder of copper c c, fig. 174., about 2^ inches dia-
Fig. 174.
meter and 2^ inches high, is supported on the pole of a bar
magnet by a plate of metal passing across the upper orifice of the
inner cylinder. A light cylinder of zinc z z, supported on a wire
arch A, is Introduced between the inner and outer cylinders of
copper, a steel point attached to the wire arch resting upon the
plate by which the copper cylinders are supported. On intro-
ducing dilute acid between the copper cylinders, electro-motive
iction takes place, the current passing from the zinc to the acid,
tience to the copper, and thence through the pivot to the zinc.
Tie zinc being in this case free to revolve, while the copper is
fix'd, and the current descending on the former, the rotation will
be lirect or retrograde according as the magnetic pole is north or
souh.
It the copper were free to revolve as well as the zinc, it would
turn in the contrary direction, since the current ascends upon it,
whileit descends on the zinc. Mr. J. Marsh modified Ampere's
apparatus, so as to produce this effect by substituting a pivot.
152
VOLTAIC ELECTRICITY.
resting in a cup at the top of the magnet, for the metallic arch by
which, in the former case, the copper vessel was sustained.
A double arrangement of this kind is given inj^g-. 175., where
the double cylinders are supported on pivots on the two poles of a
horse shoe magnet. The rotation of the corresponding cylinders
on the two opposite magnetic poles will be in contrary directions.
248. To make a magnet turn on its own axis by a cur-
rent parallel to it. — The tendency of the conductor on which
a current passes to revolve round a magnet will not the less exist,
though the current be so fixed to the magnet as to be incapable
of revolving without carrying the magnet with it. ^Lnjig. 176.
the magnet M is sunk by a platinum weights; its upper end
being fixed to the copper cylinder ww, a current passing from
p to N causes the cylinder to rotate, carrying with it the magnet.
Since a magnetic bar is itself a conductor, it is not necessary to
introduce any other; and a current passing along the bar will give
rotation to it. An apparatus for exhibiting this effect is repre-
sented in fig. 177., where a magnetic bar is supported in the
Fig. 175.
Fig. 176.
Fig. 177.
vertical position between pivots which play in agate cups. A
circular mercurial canal is placed at the centre of the magnet
and another round the lower pivot. Mercurial cups communicjte
with these two canals. When these cups are put in commuii-
cation with the poles of a battery, the current will pass between
the two canals along the lower pole of the magnet, in the one
direction or the other, according to the mode of connection ; and
the magnet will turn on its own axis with a direct or retro/rade
rotation, according to the name of the pole on which the cirrent
runs, and to the direction of the current.
CIRCULATING CURRENTS ANJ» MAGNETS. 153
CHAP. V.
RECIPROCAL INFLUENCE OF CIRCULATING CURRENTS AND
MAGNETS.
IF a wire P A B c D N (figs. 178, 1 79.) be bent into the form of any
geometrical figure, the extremities being brought near each other
C ^»s B
Fig. 179-
without actually touching, a current entering one extremity and
departing from the other, is called a circulating current.
249. Front and back of circulating current. — If such a
current be viewed on opposite sides of the figure formed by the
wire, it will appear to circulate in different directions, on one side
direct, and on the other retrograde (244.). That side on which it
appears direct is called the front^ and the other the back of the
current.
250. Axis of current. — If the current have a regular figure
having a geometrical centre, a straight line drawn through this
centre perpendicular to its plane is called the axis of the current.
251. Reciprocal action of circulating current and mag-
netic pole. — To determine the reciprocal influence of a circulating
current and a magnetic pole placed anywhere upon its axis, let
the axis be x c x' (fig- 1 80.), the plane of the current being at
right angles to the paper, A
KM
r x
being the point where it as-
cends, and D the point where
it descends through the paper.
i°. Let N be a north mag-
netic pole placed in front of
the current.
The part of the current at
r will exert a force on N in the direction N M' at right angles to
D N, and the part at A will exert an equal force in the direction
I8°-
A
i 54 VOLTAIC ELECTRICITY,
N M at right angles to A N. These two forces being compounded
will be equivalent to a single force NO* directed from N along
the axis towards the current.
It may be shown that the same will be true for every two points
of the current which are diametrically opposed.
2°. Let a south magnetic pole s ( jig. 1 8 1 .), be similarly placed
in front of a circulating current. The part D will exert upon it
-jj a force in the direction s M
M^ perpendicular to s D and to
"^ § .' \ ,. the left of s as viewed from D,
g 7*- — -f" and the part A will exert an
^^ ,\/ equal force in the direction
M * s M' to the right of s as viewed
from A. These two equal
forces will have a resultant
s o directed from the current ; and the same will be true of every
two points of the current which are diametrically opposed.
If the magnetic pole be placed at the lack of the current, the
contrary effects ensue.
The same inferences may be deduced with respect to any cir-
culating current which has a centre, that is, a point within it
which divides into two equal parts all lines drawn through it,
terminating in the current.
It may therefore be inferred generally that when a magnetic
pole is placed upon the axis of a circulating current, attraction or
repulsion is produced between it and the current; attraction when a
NORTH pole is before, or a SOUTH pole BEHIND, and repulsion when
a SOUTH pole is before, or a north pole BEHIND.
252. Intensity of the force vanishes when the distance
of the pole bears a very great ratio to the diameter of
current. — Since the intensity of the attraction between the com-
ponent parts of the current and the pole decreases as the square
of the distance is increased, and since the lines NM and NM',^.
1 80., and SM and SM', jig. 1 8 1., form with each other a greater
angle as the distance of the pole from the current is increased, it
is evident that when the diameter AD of the current bears an
inconsiderable ratio to the distance of the pole N or s from it, the
attraction or repulsion ceases to produce any sensible effect.
253. But the directive power of the pole continues. —
This, however, is not the case with relation to the directive power
of the pole upon the current. The tendency of the forces im-
pressed by the pole upon the current is always to bring the plane
of the current at right angles to the line drawn from the pole to
* « Mechanics " (148.).
CIRCULATING CURRENTS AND MAGNETS. 155
its centre. There is, in short, a tendency of the line of direction
of the pole to take a position coinciding with or parallel to the
axis of the current, and this coincidence may be produced either
by the change of position of the pole or of the plane of the cur-
rent, or of both, according as either or both are free to move.
254. Spiral and helical currents. — The force exerted by a
circulating current may be indefinitely augmented by causing the
current to circulate several times round its centre or axis. If the
wire which conducts the current be wrapped with silk or coated
with any nonconducting varnish, so as to prevent the electricity
from escaping from coil to coil when in contact, circulating cur-
rents may be formed round a common centre or axis in a ring, a
spiral, a helix, or any other similar form, so that the forces exerted
by all their coils on a single magnetic pole may be combined by
the principle of the composition of force ; and hence an extensive
class of electro-magnetic phenomena may be educed, which supply
at the same time important consequences and striking experimental
illustrations of the laws of attraction and repulsion which have
been just explained.
255. Expedients to render circulating currents movable.
— Ampere's and Delarive's apparatus. — Two expedients have
been practised to render a circulating current movable.
i. By the apparatus of Ampere already described (229.), the wire conduct-
ing the current being bent at the ends, as represented in Jig. 182., may be
supported in the cups yyf as represented in _/?<;. 148., so that its plane being
vertical, it shall be capable of revolving round the line y yi as an axis. By
this arrangement the plane of the current can take any direction at right
angles to a horizontal plane, but it is not capable of receiving any progres-
sive motion.
— .= £17 1 M. : -i -
i
Fig. i8z.
?.. The latter object is attained by the floating apparatus of M. Delarive.
Let a coated wire be formed into a circular ring composed of several coils.
Let one end of it be attached to a copper cell, fig. 183., and the other to a slip
of zinc which descends into this cell. The cell being filled with acidulated
water, a current will be established through the wire in the direction of the
156 VOLTAIC ELECTRICITY.
arrows. The copper cell may be inclosed in a glass vessel, or attached to a
cork so as to float upon water, and thus be free to assume any position which
tlie forces acting upon the current may tend to give it.
256. Rotatory motion imparted to circular current by a
magnetic pole. — If a magnetic north pole be presented in front
of a circular current, fig. 182., suspended on Ampere's frame,
fig. 148., the ring will turn on its points of suspension until its
axis pass through the pole. If the pole be carried round in a
circle, the plane of the ring will revolve with a corresponding
motion, always presenting the front of the current to the pole, the
axis of the current passing through the pole.
If a south magnetic pole be presented to the back of the cur-
rent, like effects will be produced.
If a north magnetic pole be presented to the back, or a south
to the front of the current, the ring will, on the least disturbance,
make half a revolution round its points of suspension, so as to
turn its front to the north and its back to the south magnetic
pole.
257. Progressive motion imparted to it. — If c,j%. 184., re-
present a floating circular current, a
north magnetic pole placed anywhere
on its axis will cause the ring con-
ducting it to move in that direction in
which its front is presented ; for if the
pole be before it at A it will attract
the current, and if behind it at B it
will repel it (251.). In either case
the ring will move in the direction in
which its front looks.
If a south magnetic pole be similarly placed, it will cause the
current to move in the contrary direction ; for if it be placed
before the current at A it will repel it, and if behind it at B it will
attract it. In either case the ring will move in the direction to
which the back of the current looks.
258. Reciprocal action of the current on the pole. — If the
magnetic pole be movable and the current fixed, the motion im-
pressed on the pole by the action of the current will have a
direction opposite to that of the motion which would be impressed
on the current, being movable, by the pole being fixed. A north
magnetic pole placed on the axis of a fixed circular current will
therefore be moved along the axis, in that direction in which the
back of the current looks, and a south magnetic pole in that
direction in which the front looks.
259. Action of a magnet on a circular floating- current. —
If a bar magnet SN, fig. 185., be placed in a fixed position with
SPIRAL CURRENTS. 157
the magnetic axis in
the direction of a float-
ing circular current
A, its north pole N
Fig. 185. , . ,. ,
being directed to the
front of the current, the current will be attracted by N and re-
pelled by s ; but the force exerted by N will predominate in con
sequence of its greater proximity to A, and the current will
accordingly move from A towards N. After it passes N, the bar
passing through the centre of the ring, it will be repelled by N
and also by s (251.); but so long as it is between N and the
centre c of the bar, as at B, the repulsion of N will predominate
over that of s in consequence of the greater proximity of N, and
the current will move towards c. Passing beyond c to B', the
repulsion of s predominates over that of N, and it will be driven
back to c, and after some oscillations on the one side and the
other, it will come to rest in stable equilibrium, with its centre at
the centre of the magnet, its plane at right angles to it, the front
looking towards s and the back towards N.
260. Reciprocal action of the current on the magnet. —
If the current be fixed and the magnetic bar movable, the latter
will move in a direction opposite to that in which the current
would move, the bar being fixed. Thus, if the current were fixed
at A, the bar would move to it in the direction of N A, and the
pole N passing through the ring, the bar would come to rest, after
some oscillations, with its centre at the centre of the ring.
261. Case of unstable equilibrium of the current. — If
the ring were placed with its centre at c and its front directed to
N, it would be in unstable equilibrium, for if moved through any
distance, however small, towards N or s, the attraction of the pole
towards which it is moved would prevail over that of the other
pole which is more distant, and the ring would consequently be
moved to the end of the bar and beyond that point, when, being
still attracted by the nearest pole, it would soon be brought to
rest. It would then make a half revolution on its axis and return
to the centre of the bar, where it would take the position of stable
equilibrium.
All these are consequences which easily follow
from the general principles of attraction and repul-
sion established in (251.).
262. Case of a spiral current. — If the wire
which conducts the current be bent into the form
of a spiral, Jig. 1 86., each convolution will exert
the force of a circular current, and the effect of
the whole will be the sum of the forces of all the
158 VOLTAIC ELECTRICITY.
convolutions. Such a spiral will therefore be subject to the con-
ditions of attraction and repulsion which affect a circular current
(251.).
263. Circular or spiral currents exercise the same action
as a mag-net. — In general it may be inferred that circulating
currents exercise "on a magnetic pole exactly the same effects as
would be produced by another magnet, the front of, the current
playing the part of a south pole, and the back that of a north pole.
264. Case of helical current. — It has been shown that a
helix or screw is formed by a point which is at the same time
affected by a circular and progressive motion, the circular motion
being at right angles to the. axis of the helix, and the progressive
motion being in the direction of that axis.* In each convolution
the thread of the helix makes one revolution, and at the same
time progresses in the direction of the axis through a space equal
to the distance between two successive convolutions.
265. Method of neutralising: the effect of the progressive
motion of such a current. — If a current therefore be trans-
mitted on a helical wire, it will combine the characters of a
circular and rectilinear current. The latter character, however,
may be neutralised or effaced by transmitting a current in a con-
trary direction to the progression of the screw, on a straight wire
extended along the axis of the helix. This rectilinear current
being equal, parallel, and contrary in direction to the progressive
component of the helical current, will have equal and contrary
magnetic properties, and the forces which they exert together on
any magnetic pole within their influence will counteract each other.
266. Right-handed and left-handed helices. — Helices are of
two forms : those in which the wire turns like the thread of a
corkscrew, that is, in the direction of the hands of a watch, fig. 187.;
and those in which it turns in a contrary direction,^-. 1 88.
Fig. 187. Fig. 188.
267. Front of current on each kind. — If a current traverse a
right-handed helix, its front will be directed to the end at which it
enters, and in the left-handed helix to the end at which it departs.
268. Magnetic properties of helical currents.— Their
poles determined. — Hence it follows that in a right-handed heli-
cal current, the end at which the current enters, and which is tne
positive pole, has the magnetic properties of a south pole ; and in
the left-handed helix this end has the properties of a north pole.
* '"Mechanics "(484-).
SPIRAL CURRENTS.
'59
269. Experimental illustration of these properties. — The
magnetic properties of spiral and helical currents may be illus-
trated experimentally by means of Ampere's arrangement, Jig.
148., or by a floating apparatus constructed on the same principle
as that represented in Jig. 183.
The manner of forming spiral currents adapted to Ampere's apparatus is
represented \nfigs. 189. and 190. In fig. 189. the spirals are both in the same
Fig. 190.
plane, passing through the axis of suspension yy'. Infiq. 190. they are in
planes parallel to this axis, and at right angles to the line joining their
centres, which is therefore their common axis.
270. The front of a circulating: current has the proper-
ties of a south, and the back those of a north, magnetic
pole. — According to what has been explained, the front of such a
spiral current will have the properties of a south magnetic pole,
and will therefore attract and be attracted by the north, and repel
and be repelled by the south pole of a magnet. If the spirals in
Jig. 189., therefore, be so connected with the poles of a voltaic
system, as to present their fronts on the same side, they will be
both attracted by the north pole and both repelled by the south
pole of a magnet presented
A B to them, that which is nearer
to the magnet being more
attracted or repelled than
the other. If the magnetic
pole be equally distant from
them, they will be in equi-
librium, and the equilibrium
will be stable if they are both
repelled, and unstable if they
are both attracted by the
magnet.
To demonstrate this, let 8, fg,
191., be the south pole of a mag-
net placed in front of the two
B
i6o
VOLTAIC ELECTRICITY.
spirals, whose centres are at A and B, equally distant from s. It is evident
that a perpendicular s o drawn from s to A B will in this case pass through
the middle of A B. The pole s will, therefore, according to what has been
already explained, repel the two spirals with equal forces. If the spirals be
removed from this position to the positions A' B', A', being nearer to s than
B', will be repelled by a greater force, and therefore A' will be driven back
towards A, and B' towards B. In like manner, if they were removed to the
positions A"B", the force repelling B" would be greater than that which repels
A", and therefore B" will be driven back to B, and A" to A.
It follows, therefore, that the position of equilibrium of A B is in this case
such that the system will return to it after the slightest disturbance on the
one side or the other, and is therefore stable.
If the pole s were the north pole, it would attract both currents, and in
that case A' would be more strongly attracted than B', and B" than A", and
consequently the spirals would depart further from the position A after the
least disturbance. The equilibrium would therefore be unstable.
It will be found, therefore, that when a north pole is presented before, or a
south pole behind, such a pair of spiral currents, the system,^. 189., will, on
the least disturbance from the position of 'unstable equilibrium, turn on its
axis y y> through half revolution, presenting the fronts of the currents to
the south pole, and will there come to rest after some oscillations.
In the position of stable equilibrium, the front of the currents must therefore
be presented to the south pole of the magnet, or the back to the north pole.
271. Adaptation of a helical current to Ampere's and
Delarive's apparatus. — The manner of adapting a helical
current to Ampere's arrangement,^-. 148., is represented injtfg-.
192., and the manner of adapting it to the floating method is
represented in^g-. 193.
Fig. I9Z.
The positive wire is carried down from y,fig. 192., and then coiled into an
helix from the centre to the extremity. Thence it is carried in a straight
direction through the centre of the helix to the other extremity, from whence
it is again conducted in helical coils back to the centre, where it is bent up-
wards and terminates at the negative pole y>. In one half of the helix the
current, therefore, enters at the centre and issues from the extremity, and in
the other half it enters at the extremity and issues from the centre
SPIRAL CURRENTS. ibi
If the helices be both right handed, therefore, the end from which the cur-
rent issues will have the properties of a north, and that at which it enters
those of a south, magnetic pole. If they be both left handed, this position of
the poles will be reversed (268.).
The wire which is carried straight along the axis neutralises that com-
ponent of the helical current, which is parallel to the axis, leaving only the
circular elements effective (265.).
These properties may be experimentally verified by presenting either pole
of a magnetic bar to one or the other end of the helical current. The same
attractions and repulsions will be manifested as if the helix were a magnet.
272. Action of a helical current on a magnetic needle
placed in its axis. — If HH' (Jig. 194.) represent a helical
current, the front of which
looks towards A, a north mag-
netic pole placed anywhere
in its axis, either within the
limits of the helix or beyond
its extremities, will be urged by a force directed from A towards c.
Between A and H it will be attracted by the combined forces of
the fronts of all the convolutions of the helix. Between H and H'
it will be attracted by the fronts of those convolutions which are
to the left of it, and repelled by the backs of all those to its right.
Beyond H' towards c, it will be repelled by the backs of all the
convolutions. In all positions, therefore, it will, if free, be moved
from right to left, or in a direction contrary to that towards which
the front of the current is directed.
If the pole were fixed and the current movable, the helix would
move from left to right, or in that direction towards which the
front of the current looks.
If the magnetic needle SN,^?g\ 195., be placed in the centre of
the axis of a helical current, with its poles equidistant from the
extremities, the south pole
F^ ^B s being presented towards
that end F to which the
front of the current looks,
it will be in equilibrium,
the pole N being repelled
towards B, and the pole s
Fig ,9J towards rby equal forces;
for in this case the pole N
will be attracted towards B by all the convolutions of the helix
between N and B, and will be repelled in the same direction by all
the convolutions between N and F ; while the pole s will in like
manner be attracted towards F by all the convolutions between
s and F, and repelled in the same direction by all the convolutions
between s and B.
1 62 VOLTAIC ELECTRICITY.
The needle s N, being thus impelled by two equal forces directed
from its centre, will be in stable equilibrium.
If the directions of the poles were reversed, they would be im-
pelled by two equal forces directed from its extremities towards
its centre, and the equilibrium would be unstable.
[When the magnetic needle is sufficiently light, and the helical
current sufficiently powerful, a curious effect may be observed.
The helix being placed with its axis vertical, and the needle at
the bottom, leaning against the side of it, so as to be nearly up-
right, the needle will leap up to nearly the centre of the helix,
and will remain there as long as the current passes, resting against
the wire. This experiment is often wrongly described as though
the needle would remain freely suspended at the axis of the helix
when the latter is horizontal.]
CHAP. VI.
ELECTRO-MAGNETIC INDUCTION.
273. Inductive effect of a voltaic current upon a mag--
net. — The forces which a voltaic current impresses upon the poles
of a permanent magnet, being similar in all respects to those with
which the same poles would be affected by another magnet, it may
be expected that the natural magnetism of an unmagnetised body
would be decomposed, and polarity imparted to it by the approach
of a voltaic current, in the same manner as by the approach of a
magnet. Experiment accordingly confirms this consequence of
the analogy suggested by the phenomena. It is, in fact, found that
a voltaic current is capable of decomposing the natural magnetism
of magnetic bodies, and of magnetising them as effectually as the
most powerful magnets.
Soft iron rendered magnetic by voltaic currents. — If the
wire upon which a voltaic current flows be immersed in filings of
soft iron, they will collect around it, and attach themselves to it in
the same manner as if it were a magnet, and will continue to
adhere to it so long as the current is maintained upon it ; but the
moment the connections with the battery are broken, and the
current suspended, they will drop off.
Sewing: needles attracted by current. — Light steel sewing
needles being presented to the wire conducting a current will
instantly become magnetic, as will be apparent by their assuming
ELECTRO-MAGNETISM.
163
a position at right angles to the wire, as a magnetic needle would
do under like circumstances. When the current is suspended or
removed, the needles will in this case retain the magnetism im-
parted to them.
274. magnetic induction of a helical current.
JL — To exhibit these phenomena with greater effect and
certainty, the needles should be exposed to the influence
not of one, but of several currents, or of several parts of
the same current flowing at right angles to them. This
is easily effected by placing them within a helical
current.
Let a metallic wire coated with silk or other nonconductor be
rolled helically on a glass tube, fig. 196., and the current being
made to pass along the wire, let a needle or bar of steel or hard
iron be placed within the tube. It will instantaneously acquire all
the magnetism it is capable of receiving under these circum-
stances.
On testing the needle it will be found that its boreal or south
pole is at that end to which the front of the current is presented ;
and, consequently, for -a right-handed helix, it will be towards
the positive, and for a left-handed helix towards the negative
pole. It appears, therefore, that the needle acquires a polarity
identical with that which the helix itself is proved to possess.
Polarity produced by the induction of helical cur-
rent.— In the case of the right-handed helix,
,t represented in Jig. 196., the current passes in
the direction indicated by the arrows, and con-
sequently the austral pole will be at a and the
boreal pole at b. In the case of the left-handed he-
lix^. 197., the position of these poles a and b is
reversed in relation to the direction of the cur-
rent, but the boreal pole b is in both cases at
that end to which the front of the current looks.
^ 276. Consecutive points produced. — If the
helix be reversed once or oftener in passing
along the tube, being alternately right-handed
and left-handed, as represented \n fig. 198., a
consecutive point will be produced upon the bar
at each change of direction of the helix.
277. Inductive action of common elec-
tricity produces polarity. — It is not only by
Fig. 197. Fig. 198. tjie induction of the voltaic current that mag-
netic polarity may be imparted. Discharges of common elec-
tricity transmitted along a wire, especially if it have the form
of a helix, will produce like effects. If the wire be straight, the
influence is feeble. Sparks taken from the prime conductor pro-
M 2
Fig. 196.
275.
-I
104 VOLTAIC ELECTRICITY.
cluce sensible effects on very fine needles ; but if the wire be
placed in actual contact with the conductor at one end and the
cushion at the other, so that a constant current shall pass along it
from the conductor to the cushion, no effect is produced. The
effect produced by the spark is augmented as the spark is more
intense and taken at a greater distance from the conductor.
If the wire be formed into a helix, magnetic polarity will be
produced by a continuous current, that is, by actually connecting
the ends of the wire with the conductor and the cushion : but
these effects are much more feeble than those produced under like
circumstances by the spark.
All these effects are rendered much more intense when the dis-
charge of a Leyden jar, and still more that of a Ley den battery,
is transmitted along the wire. When these phenomena were first
noticed, it was assumed that the polarity thus imparted by
common electricity must necessarily follow the law which prevails
in the case of a voltaic current, and that in the case of helices the
boreal or south pole would be presented towards the front of the
current. Savary, however, showed that the effects of common
electricity are modified by various circumstances, such as the
length of the helix and the intensity of the discharge.
278. Conditions on which a needle is magnetised posi-
tively and negatively. — When an electric discharge is trans-
mitted along a straight wire, a needle placed at right angles to the
wire acquires sometimes the polarity of a magnetic needle, which
under the influence of a voltaic current would take a like posi-
tion ; that is to say, the north pole will be to the right of an
observer who looks at the needle from the current, his head being
in the direction from which the current flows. The needle is in
this case said to be magnetised positively. When the opposite
polarity is imparted to the needle, it is said to be magnetised
negatively.
279. Results of Savary's experiments. — Savary showed
that needles are magnetised by the discharge of common elec-
tricity, positively or negatively, according to various conditions,
depending on the intensity of the discharge, the length of the
conducting wire, supposing it to be straight, its diameter, the
thickness of the needles, and their coercive force. In a series of
experiments, in which the needles were placed at distances from
the current increasing by equal increments, the magnetisation
was alternately positive and negative; when the needle was in
contact with the wire, it was positive ; at a small distance nega-
tive; at a greater distance no magnetisation was produced; a
further increase of distance produced positive magnetism; and
ELECTRO-MAGNETISM. i o 5
after several alternations of this kind, the magnetisation ended in
being positive, and continued positive at all greater distances.
The number and frequency of these alternations are dependent
on the conditions above mentioned, but no distinct law showing
their relation to those conditions has been discovered. In general
it may be stated that the thinner the wire which conducts the
current, the lighter and finer the needles, and the more feeble
their coercive force is, the less numerous will be those periodical
changes of positive and negative magnetisation. It is sometimes
found that when these conditions are observed, the magnetisation
is positive at all distances, and that the periodic changes only
affect its intensity.
Similar effects are produced upon needles placed in tubes of
wood or glass, upon which a helical current is transmitted. In
these cases, the mere variation in the intensity of the discharge
produces considerable effect.
280. Magnetism imparted to the needle affected by the
nonmagnetic substance which surrounds it. — Savary also
ascertained a fact which, duly studied, may throw much light on
the theory of these phenomena. The quantity of magnetism im-
parted to a needle by an electric discharge, and the character of
its polarity, positive or negative, are affected by the nonmagnetic
envelope by which the needle is surrounded. If a needle be in-
serted in the axis of a very thick cylinder of copper, a helical
current surrounding the cylinder will not impart magnetism to it.
If the thickness of the copper envelope be gradually diminished,
the magnetisation will be manifested in a sensible degree, and it
will become more and more intense as the thickness of the copper
is diminished. This increase, however, does not continue until
the copper envelope disappears, for when the thickness is reduced
to a certain limit, a more intense magnetisation is produced than
when the uncovered needle is placed within the helix.
Envelopes of tin, iron, and silver placed around the needle are
attended with analogous effects, that is to say, when they consist
of very thin leaf metal they increase the quantity of magnetism
which can be imparted to the needles by the current ; but when
the metallic envelope is much thicker, they prevent the action of
the electric discharge altogether. Cylinders formed of metallic
filings do not produce these effects, while cylinders formed of
alternate layers of metallic and nonmetallic substances do produce
them. It is inferred from this that solutions of continuity at right
angles to the axis of the needle, or to that of the cylinder, have an
influence on the phenomena.
281. Formation of powerful electro-magnets. — The in-
ductive effect of a spiral or helical current on soft iron is still
1 66
V7OLTAIC ELECTKICJTY.
more energetic than on steel or other bodies having more or less
coercive force. The property enjoyed by soft iron, of suddenly
acquiring magnetism from any external magnetising agent, and as
suddenly losing its magnetism upon the suspension of such agency,
has supplied the means of producing the temporary magnets which
are known under the name of electro-magnets.
The most simple form of electro-magnet is represented in fig.
199. It is composed of a bar of soft iron bent into the form of
Fig. 199.
a horse shoe, and of a wire wrapped with silk, which is coiled first
on one arm, proceeding from one extremity to the bend of the
horse shoe, and then upon the other from the bend to the other
extremity; care being taken that the convolutions of the spiral
shall follow the same direction in passing from one leg to the
other, since, otherwise, consecutive points would be produced.
An armature is applied to the ends of the horse shoe which will
adhere to them so long as a voltaic current flows upon the wire,
ELECTRO-MAGNETISM.
167
but which will drop off the moment that such current is dis-
continued.
282. Conditions which determine the force of the magnet.
— The force of the electro-magnet will depend on the dimensions
of the horse shoe and the armature, the intensity of the current, and
the number of convolutions with which each leg of the horse shoe JB
wrapped.
283. Electro-magnet of Faculty of Sciences at Paris. —
In 1 830 an electro-magnet of extraordinary power was constructed
under the superintendence of M. Pouillet at Paris. This ap-
paratus, represented in fig. 200., consists of two horse shoes, the
legs of which are presented to each other, the bends being turned
in contrary directions. The superior horse shoe is fixed in the
frame of the apparatus, the infe-
rior being attached to a cross piece
which slides in vertical grooves
formed in the sides of the frame. To
this cross piece a dish or plateau
is suspended, in which weights are
placed, by the effect of which the at-
traction which unites the two horse
shoes is at length overcome. Each
of the horse shoes is wrapped with
10000 feet of covered wire, and they
are so arranged that the poles of
contrary names shall be in contact.
With a current of moderate inten-
weight of several
Fig. zoo.
sity the apparatus is capable of supporting
tons.
284. Form of electro-magnets in general. — It is found
more convenient generally to construct electro-magnets of two
straight bars of soft iron, united at one end by a straight bar
transverse to them, and attached to them by screws, so that the
form of the magnet ceases to be that of a horse shoe, the end at
which the legs are united being not curved but square. The con-
ductor of the helical current is usually a copper wire covered
with silk.
285. Electro-magnetic power applied as a mechanical
agent. — The property of electro-magnets, by which they are
capable of suddenly acquiring and losing the magnetic force, has
supplied the means of obtaining a mechanical agent which may be
applied as a mover of machinery. An electro-magnet and its
armature, such as that represented in fig. 1 99., or two electro-
magnets, such as those represented in fig- 200., are placed so that
when the electric current is suspended they will rest at a certain
168 VOLTAIC ELECTRICITY.
distance asunder, and when the current passes on the wire they
will be drawn into contact by their mutual attraction. When the
current is again suspended they will separate. In this manner, by
alternately suspending and transmitting the current on the wire
which is coiled round the electro-magnet, the magnet and its
armature, or the two magnets, receive an alternate motion to
and from each other similar to that of the piston of a steam en-
gine, or the foot of a person who works the treddle of a lathe.
This alternate motion is made to produce one of continued rotation
by the same mechanical expedients as are used in the application
of any other moving power.
The force with which the electro- magnet and its armature
attract each other determines the power of the electro-motive
machine, just as the pressure of steam on the piston determines the
power of a steam engine. This force, when the magnets are given,
varies with the nature and magnitude of the galvanic pile which is
employed.
286. Electro-motive power applied in the workshop of
M. Froment. — The most remarkable and beautiful application of
electro-motive power as a mechanical agent which has been hitherto
witnessed, is presented in the workshops of M. Gustave Froment,
of Paris, so celebrated for the construction of instruments of pre-
cision. It is here applied in various forms to give motion to the
machines contrived by M. Froment, for dividing the limbs of astro-
nomical and surveying instruments and microscopic scales. The
pile used for the lighter description of work is that of Daniell, con-
sisting of about 24 pairs. Simple arrangements are made by
means of commutators, reometers, and reotropes, for modifying
the current indefinitely in quantity, intensity, and direction. By
merely turning an index or lever in one direction or another, any
desired number of pairs may be brought into operation, so that a
battery of greater or less intensity may be instantly made to act,
subject to the major limit of the number of pairs provided. By
another adjustment the copper elements of two or more pairs, and
at the same time their zinc elements, may be thrown into connec-
tion, and thus the whole pile, or any portion of it, may be made to
act as a single pair, of enlarged surface. By another adjustment
the direction of the current can be reversed at pleasure. Other
adjustments, equally simple and effective, are provided, by which
the current can be turned on any particular machine, or directed
into any room that may be required.
The pile used for heavier work is a modification of Bunsen's
charcoal battery, in which dilute sulphuric acid is used in the
porous porcelain cell containing the charcoal, as well as in the ceil
containing the zinc. By this expedient the noxious fumes of the
ELECTRO-MAGNETIC MACHINES. 169
nitric acid are removed, and although the strength of the battery
is diminished, sufficient power remains for the purposes to which
it is applied.
The forms of the electro-motive machines constructed by M.
Froment are very various. In some the magnet is fixed and the
armature movable ; in some both are movable.
In some there is a single magnet and a single armature. The
power is in this case intermittent, like that of a single acting
steam engine, or of the foot in working the treddle of a lathe, and
the continuance of the action is maintained in the same manner
by the inertia of a flywheel.
In other cases two electro-magnets and two armatures are
combined, and the current is so regulated that it is established on
each, during the intervals of its suspension on the other. This
machine is analogous in its operation to the double acting steam
engine, the operation of the power being continuous, the one
magnet attracting its armature during the intervals of suspension
of the other. The force of these machines may be augmented
indefinitely by combining the action of two or more pairs of
magnets.
Another variety of the application of this moving principle pre-
sents an analogy to the rotatory steam engine. Electro-magnets
are fixed at equal distances round a wheel, to the circumference of
which the armatures are attached at corresponding intervals. In
this case the intervals of action and intermission of the currents
are so regulated, that the magnets attract the armatures obliquely
as the latter approach them, the current, and consequently the
attraction, being suspended the moment contact takes place. The
effect of this is, that all the magnets exercise forces which tend to
turn the wheel, on which the armatures are fixed, constantly in the
same direction., and the force with which it is turned is equal to
the sum of the forces of all the electro-magnets which act simul-
taneously.
This rotatory electro-motive machine is infinitely varied, not
only in its magnitude and proportions, but in its form. Thus in
some the axle is horizontal, and the wheel revolves in a vertical
plane $ in others the axle is vertical, and the wheel revolves in a
horizontal plane. In some the electro- magnets are fixed, and the
armatures movable with the wheel ; in others both are movable.
In some the axle of the wheel which carries the armatures is itself
movable, being fixed upon a crank or excentric. In this case the
wheel revolves within another, whose diameter exceeds its own
by twice the length of the crank, and within this circle it has a
hypocycloidal motion.
Each of these varieties of the application of this power, as yet
170
VOLTAIC ELECTRICITY.
novel in the practical operations of the engineer and manufacturer,
possesses peculiar advantages or convenience, which render it
more eligible for special purposes.
287. Electro-motive machines constructed by him. — To
render this general description of M. Froment's electro-motive
machines more clearly understood, we shall add a detailed expla-
nation of two of the most efficient and useful of them.
In the machine represented in fig. 201., a. and b are the two legs of the
electro-magnet ; c d is the transverse piece uniting them, which replaces the
Fig. 201.
beud of the horse shoe; e/is the armature confined by two pins on the sum-
mit of the leg a (which prevent any lateral deviation), the end f being
jointed to the lever g A, which is connected with a short arm projecting from
an axis k by the rod t. When the current passes round the electro-magnet,
the lever J is drawn down by the attraction of the leg 6, and draws with it
ELECTRO-MAGNETIC MACHINES. 1 7 1
the lever g h, by which i and the short lever projecting from the axis k are
also driven down. Attached to the same axis k is a longer arm m, which
acts by a connecting rod n upon a crank o and a fly wheel t>. When the ma-
chine is in motion, the lever g h and the armature f attached to it recover
their position by the momentum of the fly wheel, after having been attracted
downwards. When the current is again established, the armature/ and the
lever g h are again attracted downwards, and the same effects ensue. Thus,
during each half-revolution of the crank o, it is driven by the force of the
electro-magnet acting on /, and during the other half-revolution it is carried
round by the momentum of the fly wheel. The current is suspended at
the moment the crank o arrives at the lowest point of its play, and is re-
established when it returns to the highest point. The crank is therefore
impelled by the force of the magnet in the descending half of its revolution,
and by the momentum of the fly wheel in the ascending half.
The contrivance called a distributor, by which the current is alternately
established and suspended at the proper moments, is represented in fig. 202..
where y represents the transverse section of the axis of the
fly wheel ; r, a spring which is kept in constant contact,
with it ; a*, an excentric fixed on the same axis y, and re-
volving with it ; and r' another spring similar to r, which is
acted upon by the excentric, and is thus allowed to press
against the axis y, during half the revolution, and removed
from contact with it during the other half-revolution. When
the spring r> presses on the axis y, the current is established ;
Fig. aoa. and when it is removed from it the current is suspended.
It is evident that the action of this machine upon the
lever attached to the axis k is exactly similar to that of the foot on the
treddle of a lathe or a spinning wheel ; and as in these cases, the impelling
force being intermittent, the action is unequal, the velocity being greater
during the descending motion of the crank o than during its ascending mo-
tion. Although the inertia of the fly wheel diminishes this inequality by
absorbing a part of the moving power in the descending motion, and re-
storing it to the crank in the ascending motion, it cannot altogether efface it.
Another electro-motive machine of M. Froment is represented in elevation
in fig. 203., and in plan in fig. 204. This machine has the advantage of pro-
ducing a perfectly regular motion of rotation, which it retains for several
hours without sensible change.
A drum, which revolves on a vertical axis x y, carries on its circumference
eight bars of soft iron a placed at equal distances asunder. These bars are
attracted laterally, and always in the same direction, by the intermitting
action of six electro-magnets 6, mounted in a strong hexagonal frame of cast
iron, within which the drum revolves. The intervals of action and suspen-
sion of the current upon these magnets are so regulated, that it is established
upon each of them at the moment one of the bars of soft iron a is approach-
ing it, and it is suspended at the moment the bar begins to depart from it.
Thus the attraction accelerates the motion of the drum upon the approach of
the piece a towards the magnet 6, and ceases to act when the piece a arrives
in front of b. The action of each of the six impelling forces upon each of
the eight bars of soft iron attached to the drum is thus intermitting. During
each revolution of the drum, each of the eight bars a receives six impulses,
and therefore the drum itself receives forty-eight impulses. If we suppose
the drum to make one revolution in four seconds, it will therefore receive a
172
VOLTAIC ELECTRICITY.
Fig.
succession of impulses at intervals of the twelfth part of a second, which ia
practically equivalent to a continuous force.
Fig. zo4.
The intervals of intermission of the current are regulated by a simple and
ingenious apparatus. A metallic disc c is fixed upon the axis of rotation.,
ELECTKO-MAGNETIC MACHINES. j?3
Its surface consists of sixteen equal divisions, the alternate divisions being
coated with nonconducting matter. A metallic roller h, which carries the
current, presses constantly on the surface of this disc, to which it imparts the
current. Three other metallic rollers e f g press against the edge of the
disc, and, as the disc revolves, come alternately into contact with the con-
ducting and nonconducting divisions of it. When they touch the conducting
divisions, the current is transmitted ; when they touch the nonconducting
divisions, the current is interrupted.
Each of these three rollers efg is connected by a conducting wire with the
conducting wires of two electro-magnets diametrically opposed, as is indi-
cated in Jig. 204., so that the current is thus alternately established and sus-
pended on the several electro-magnets, as the conducting and nonconduct-
ing divisions of the disc pass the rollers e,ft and g.
M. Froment has adapted a regulator to this machine, which plays the part
of the governor of the steam engine', moderating the force when the action
of the pile becomes too strong, and augmenting it when it becomes too
feeble.
Fig. zo5.
A divided circle m n, fig. 203., has been annexed to the machine at the
suggestion of M. Pouillet, by which various important physical experiments
may be performed.
Another form of this machine, in which the drum carrying the bars of soft
iron revolves upon a horizontal axis, is shown in Jig. 205.
H and o are the points where the current enters and leaves the machine,
*74
VOLTAIC ELECTRICITY.
these being connected by wires with the voltaic battery ; A B c D are four
pairs of powerful electro-magnets ; F the bars of soft iron upon which thev
act.
287*. The electro-motive machine of IVI. Bourbouze, fig.
206., consists of four hollow cylinders A a, B 6, round which the
conducting wire is coiled. Into the cores of these cylinders pass
Fig. zo6.
four rods of soft iron attached to the cross pieces A a and sb.
These cross pieces are themselves attached at their middle points
by the rods R and p to the extremities of the working beam F.
One arm of this beam, being prolonged, is jointed at i to a con-
necting rod IH, which is connected with a crank at H. Upon the
axis of this crank a fly wheel is fixed by which the varying effect
of the crank is equalised. Upon the other extremity of the axis
another crank Y is fixed, which is joined by a horizontal connecting
rod with a plate which slides to and fro in grooves made in the
top of the box N s.
The four soft iron rods attached to the cross pieces A a and ~&b extend less
than half way down the axes of the four cylinders. Four other similar cast
ELECTRO-MOTIVE MACHINE. 175
iron rods are similarly connected below by cross pieces E, and pass up the
axes of the cylinders less than half way, so that a space remains between
the extremities of the two sets of rods above and below.
The sliding plate u consists of a piece of metal in the middle, and slips
of ivory at the ends, the middle being always in connection with the positive
pole of the voltaic battery. Two conducting wires, each of which is con-
nected with the negative pole of the battery, are connected with the spiral
coils which are fixed upon the base ; and the ends of these coils are so placed
that they press constantly on the sliding plate u. When this plate slides to
the right, the end of the wire of the left hand coil rests upon the ivory, and
its connection with the battery is broken ; but that of the right hand coil
rests upon the metal, and its connection with the battery is completed. When
the plate u moves to the left, the connections are reversed, and the left hand
coil is connected with the battery, the right hand coil being disconnected.
In this way the current is alternately transmitted and suspended on the
two wires proceeding from the coils. These wires are connected respectively,
one with the wire coiled upon the cylinders A a, and the other with the wire
coiled on the cylinders B b. The current is therefore transmitted alternately
through the coils upon the pairs of cylinders placed under each extremity of
the beam, and renders momentarily magnetic the rods of soft iron inse'rted in
their cores. The coils are so arranged, that the poles of the upper and lower
electro-magnets presented to each other have contrary names, and they con-
sequently attract each other. The lower rods being fixed, draw the upper
rods towards them'when the current passes, and disengage them when it is
suspended. In this way the ends of the beam F are alternately drawn down,
and a motion of continuous rotation is imparted to the crank shaft, which is
equalised by the fly wheel.
288. Applied as a sonometer. — This machine has been ap-
plied with much success as a sonometer, to ascertain and register
directly the number of vibrations made by sonorous bodies in a
given time.
289. Momentary current by induction. — If a wire A, on
which a voltaic current is transmitted, be brought into proximity
with and parallel to another wire B, the ends of which are in me-
tallic contact either with each other, or with some continuous
system of conductors, so as to form a dosed circuit, the electric
equilibrium of the wire B will be disturbed by the action of the
current A, and a current will be produced upon B in a direction
opposite to that which prevails on A. This current will, however,
be only momentary. After an instant the wire B will return to
its natural state.
If the wire A, still carrying the current, be then suddenly re-
moved from the wire B, the electric equilibrium of B will be again
disturbed, and as before, only for a moment ; but in this case the
current momentarily produced on B will have the same direction
as the current on A.
If the contact of the extremities of the wire B, or either of
i/6 VOLTAIC ELECTRICITY.
them with each other, or with the intermediate system of con-
ductors which complete the circuit, be broken, the approach or
removal of the current A will not produce these effects on the
wire B.
If, instead of moving the wire A to and from B, the wires, both
in their natural state, be placed parallel and near to each other,
and a current be then suddenly transmitted on A, the same effect
will be produced on B as if A, already bearing the current, had
been suddenly brought into proximity with B ; and in the same
way it will be found that if the current established on A be sud-
denly suspended, the same effect will be produced as if A, still
bearing the current, were suddenly removed.
These phenomena may be easily exhibited experimentally, by
connecting the extremities of the wire A with a voltaic pile, and
the extremities of B with the wires of a reoscope. So long as the
current continues to pass without interruption on A. the needle
of the reoscope will remain at rest, showing that no current passes
on B. But if the contact of -A with either pole of the pile be
suddenly broken, so as to stop the current, the needle of the
reoscope will be deflected for a moment in the direction which
indicates a current similar in direction to that which passed on
A, and which has just been suspended ; but this deflection will
only be momentary. The needle will immediately recover its
position of rest, indicating that the cause of the disturbance has
ceased.
If the extremity of A be then again placed suddenly in contact
with the pile, so as to re-establish the current on A, the needle of
the reoscope will again be deflected, but in the other direction,
showing that the current produced on B is in the contrary direc-
tion to that which passes on A, and, as before, the disturbance
will only be momentary, the needle returning immediately to its
position of rest.
These momentary currents are therefore ascribed to the in-
ductive action of the current A upon the natural electricity of
the wire B, decomposing it and causing for a moment the positive
fluid to move in one direction, and the negative in the other. It
is to the sudden presence and the sudden absence of the current
A, that the phenomena must be ascribed, and not to any action
depending on the commencement of the passage of the current
on A, or on its discontinuance, because the same effects are pro-
duced by the approach and withdrawal of A while it carries the
current, as by the transmission and discontinuance of the current
upon it.
290. Experimental illustration. — The most convenient form
of apparatus for the experimental exhibition of these momentary
MOMENTARY CURRENTS. 177
currents of induction, con-
sists of two wires wrapped
with silk, which are coiled
round a cylinder or roller of
wood or metal, as represented
in fig. 207. The ends are
separated on leaving the roller,
so that those of one wire may
Fig. 207. be can>ie(i to the pile, and
those of the other to the reo-
scope. The effect of the inductive action is augmented in pro-
portion to the length of the wires brought into proximity, other
things being the same. It is found that the wire B, which receives
the inductive action, should be much finer and longer than that,
A, which bears the primary current. Thus, for example, while
150 feet of wire No. 18. were used for A, 2000 feet of No. 26.
were used for B.
The effect of the induction is greatly augmented by introducing
a cylinder of soft iron, or, still better, a bundle of soft iron wires,
into the core of the roller. The current on A renders this mass
of soft iron magnetic, and it reacts by induction on the wires con-
ducting the currents.
291. momentary currents produced by magnetic induc-
tion. — Since, as has been shown, a magnetic bar and a helical
current are interchangeable, it may naturally be inferred that if a
helical current produces by induction momentary currents upon
a helical wire placed in proximity with it, a magnet must pro-
duce a like effect. Experiment has accordingly confirmed this
inference.
292. Experimental illustrations. — Let the extremities of a
covered wire coiled on a roller, Jig. 208., be connected with a
reoscope, and let the pole of a magnet be suddenly inserted in the
core of the coil.
A momentary deflection of the needles will be produced, similar to that
which would attend the sudden approach of the end of a helical current
having the properties of the magnetic pole which is presented to the coil.
Thus the south pole will produce the same deflection as the from ami the
north pole as the back of a helical current.
In like manner, the sudden removal of a magnetic pole from proximity
with the helical wire will produce a momentary current on the wire, simi-
lar to that which would be produced by the sudden removal of a helical
current having like magnetic properties.
The sudden presence and absence of the magnetic pole within the coil
of wire on which it is desired to produce the induced current may be caused
more conveniently and efficiently by means of the effects of magnetic in-
178
VOLTAIC ELECTRICITY.
duction on soft iron. The manner of applying this principle to the pro-
duction of the induced current is as follows : —
Fig. 108.
Let a b, Jig. 209., be a powerful horse shoe mag-
net, over which is placed a similar shoe of soft iron,
round which the conducting wire is coiled in the
usual manner, the direction of the coils being re-
versed in passing from one leg of the horse shoe to
the other, so that the current in passing on each
leg may have its front presented in opposite di-
rections. The extremities of the wire are con-
nected with those of a reoscope at a sufficient dis-
tance from the magnet to prevent its indications
from being disturbed by the influence of the mag-
net.
If the poles a b of the magnet be suddenly
brought near the ends of the legs of the horse shoe
men, the needle of the reoscope will indicate the
existence of a momentary current on the coil of wire, the direction of which
will be opposite to that which would characterise the magnetic polarity
imparted bv induction to the horse shoe men. If the magnet a b be then
suddenly removed, so as to deprive the horse shoe men of its magnetism,
the reoscope will again indicate the existence of a momentary current, the
direction of which will now, however, be that which characterises the po-
larity imparted to the horse shoe men.
It appears, therefore, as might be expected, that the sudden decomposition
Fig. 209.
MAGNETO-ELECTRIC EFFECTS. 179
and recomposition of the magnetic fluids in the soft iron contained within
the coil has the same effect as the sudden approach and removal of a
magnet.
293. Inductive effects produced by a permanent magnet
revolving under an electro-magnet. — If the magnet a b were
mounted so as to revolve upon a vertical axis passing through the
centre of its bend, and therefore midway between its legs, its poles
might be made to come alternately under the ends of the horse
shoe wen, the horse shoe men being stationary. During each
revolution of the magnet a&, the polarity imparted by magnetic
induction to the horse shoe would be reversed. When the north
pole a passes under m, and therefore the south pole under w, m
would acquire south and n north polarity. After making half
a revolution b would come under rw, and a under n, and m would
acquire by induction north and n south polarity. The momen-
tary currents produced in the coils of wire would suffer correspond-
ing changes of direction consequent as well on the commencement
as on the cessation of each polarity, north and south.
To trace these vicissitud-es of the inductive current produced
upon the wire, it must be considered that the commencement of
north polarity in the leg m, and that of south polarity in the leg
M, give the same direction to the momentary inductive current,
inasmuch as the wire is coiled on the legs in contrary directions.
In the same manner it follows that the commencement of south
polarity in wi, and of north polarity in n, produce the same induc-
tive current.
The same may be said of the direction of the inductive currents
consequent on the cessation of north and south polarity in each
of the legs. The cessation of north polarity in m, and of south
polarity in n, or the cessation of south polarity in w?, and of north
polarity in n, produce the same inductive current. It will also
follow, from the effects of the current and the reversion of the coils
in passing from one leg to the other, that the inductive current
produced by the cessation of either polarity on one leg of men
will have the same direction as that produced by the commence-
ment of the same polarity in the other.
If the magnet a b were made to revolve under m c n, it would
therefore follow that during each revolution four momentary cur-
rents would be produced in the wire, two in one direction during
one semi-revolution, and two in the contrary direction during the
other semi-revolution. In the intervals between these momentary
currents the wire would be in its natural state.
It has been stated that if the extremities of the wire were net in
metallic contact with each other, or with a continuous system of
conductors, these inductive currents would not be produced. This
i8o VOLTAIC ELECTRICITY.
condition supplies the means of producing in the wire an inter-
mitting inductive current constantly in the same direction. To
accomplish this, it will be only necessary to contrive means to
break the contact of either extremity of the coil with the inter-
mediate conductor during the same half of each successive revolu-
tion of the magnet. By this expedient the contact may be
maintained during the half revolution in which the commencement
of north polarity in the leg m. and of south in the leg w, and the
cessation of south polarity in the leg w, and of north in the leg
n, respectively take place. All these changes produce momentary
currents having a common direction. The contact being broken
during the other semi-revolution, in which the commencement of
south polarity in m, and of north in 7i, and the cessation of
north polarity in wi, and of south in w, respectively take place,
the contrary currents which would otherwise attend these changes
will not be produced.
294. Use of a contact breaker. — If it be desired to reverse
the direction of the intermitting current, it wil1 be only necessary
to contrive a contact breaker, which will admit of such an adjust-
ment that the contact may be maintained at pleasure, during either
semi-revolution of the magnet a b, while it is broken during the
other.
295. Magneto-electric machines. — Such are the principles
on which is founded the construction of magneto-electric machines,
one form of which is represented in Jig. 210. The purpose of this
apparatus is to produce by magnetic induction an intermitting
current constantly in the same direction, and to contrive means by
which the intervals of intermission shall succeed each other so
rapidly that the current shall have practically all the effects of a
current absolutely continuous.
A powerful compound horse shoe magnet A is firmly attached by bolts
and screws upon an horizontal bed, beyond the edge of which its poles a and
b extend. Under these is fixed an electro-magnet XY, with its legs ver-
tical, and mounted so as to revolve upon a vertical axis. The covered wire
is coiled in great quantity on the legs XT, the direction of the coils being
reversed in passing from one leg to the other ; so that if a voltaic current
were transmitted upon it, the ends x and Y would acquire opposite po-
larities. «
The axis upon which this electro-magnet revolves has upon it a small
grooved wheel/, which is connected by an endless cord or band n, with a
large wheel K driven by a handle m. The relative diameters of the wheels
R and /is such that an extremely rapid rotation can be imparted to XT by
the hand applied at m.
The two extremities of the wire proceeding from the legs x and Y are
pressed by springs against the surfaces of two rollers, c and d, fixed upon
the axis of the electro-magnet. These rollers themselves are in metallic
MAGNETO ELECTRIC MACHINE.
181
connection with a pair of handles p and N, to which the current evolved in
the wire of the electro-magnet XY will thus be conducted.
If the electro-magnet XY be now put in rotation by the handle m. the
Fig. no
handles p and N being connected by any continuous conductor, a system of
intermitting and alternately contrary currents will be produced in the
wire and in the conductor by which the handles p and N are connected.
But if the rollers c and d are so contrived that the contact of the ends of
the wire with them shall be only maintained during a semi-revolution, in
which the intermitting currents have a common direction, then the current
transmitted through the conductor connecting the handles p and N will be
intermitting, but not contraiy ; and by increasing the velocity of rotation
of the electro-magnet XY, the intervals of intermission may be made to
succeed each other with indefinite celerity, and the current will thus acquire
all the character of a continuous current.
The contrivances by which the rollers c and d are made to break the
contact, and re-establish it with the necessary regularity and certainty, are
various. They may be formed as excentrics, so as to approach to and recede
from the ends of the wire as'they revolve, touching them and retiring from
them at the proper moments. Or, being circular, they may consist alter-
nately of conducting and nonconducting materials. Thus one half of the
182
VOLTAIC ELECTRICITY.
surface of such roller may be metal, while the other is wood, horn, or ivory.
When the end of the wire touches the latter the current is susp«ided, wLen
it touches the former it is maintained.
296. Effects of this machine — Its medical use. — All the
usual effects of voltaic currents may be produced with this appara-
tus. If the handles P and N be held in the hands, the arms and
body become the conductor through which the current passes from
Fig. MI.
p to N. If x Y be made to revolve, shocks are felt, which become
insupportable when the motion of x Y acquires a certain rapidity.
CLARKE'S APPARATUS.
183
If it be desired to give local shocks to certain parts of the body,
the hands of the operator, protected by nonconducting gloves,
direct the knobs at the ends of the handles to the parts of the
body between which it is desired to produce the voltaic shock.
297. Clarke's apparatus. — In another form of this apparatus,
as constructed by Mr. Clarke, of London, the magnet M,^/^. 211.,
is placed vertically, and the electro -magnets B E' revolve on a
horizontal axis, upon which the contact breaking apparatus a c is
fixed. In other respects this does not differ in principle from that
described above.
The manner of applying it to the decomposition of water is
shown in^o-. 212. This phenomenon will be more fully explained
hereafter.
Fig. ^l^.
To produce and apply physiological effects the wire rolled upon
the electro-magnet must be very fine, and have a total length of
nearly 2000 feet. To produce physical effects, on the contrary, the
Fig. zij.
Fig. 114.
wire should be thick, about I oo feet being rolled on each arm of
the electro-magnet. In^. 213. is shown the arrangement of the
commutator necessary to show the effect of the current in setting
184
VOLTAIC ELECTRICITY.
fire to ether, and in^g-. 214. the arrangement necessary to show
its effect in rendering metallic wire incandescent. These pheno-
mena will be explained more fully hereafter.
298. IKatteucci's apparatus. — This apparatus serves to exhi-
bit experimentally currents produced by induction, not only by the
electricity of the pile, but also those produced by the electricity of
the machine.
It consists of two circular discs of glass, N and M (fig. 215.)? each about 14
inches diameter, mounted in brass frames, and placed vertically on movable
Fig. 215.
stands, so as to be capable of being moved towards or from each other. Upon
the face of the plate N a copper wire, wrapped with silk, about the twelfth
of an inch in diameter, is rolled spirally, its extremities being passed through
two holes in the plate, one at the centre and the other at the circumference at
the top of the disc. To insulate still more effectually the current, each circuit
of the spiral is covered with a thick coating of gum- lac, a condition which,
though not necessary for the voltaic current, is indispensable when the appa-
ratus is used to exhibit the effects of a current produced by the discharge of a
Leyden jar.
A similar wire, but much finer, is coiled spirally upon the face of the other
plate M, which looks towards that of y ; and its extremities are brought in
like manner through holes at the centre and circumference of the plate, as
shown at a and &.
The arrangement shown in the figure is that which is necessary to ex-
hibit the effect of the current produced by the discharge of a Leyden jar.
Two wires, cf and d', clamped to the extremities of the spiral wire on y, are
connected, one with the inner coating of the Leyden jar, and the other placed
MATTEUCCI AND RUHMKORFFS APPARATUS. 185
so that the operator can touch it at will with a discharger, such contact
producing immediately the transmission of the electric charge of the jar
through the spiral wire on the disc N. At the moment the contact is made,
the positive fluid on the inside of the jar rushes along the conducting wire c',
and from thence to the extremity of the spiral wire which passes through
the centre of the plate N, and then circulating round the spiral, passes along
the wire d to the outer coating of the jar.
If the plate M be brought near and parallel to the plate N, and at the same
time the extremities, a and b, of the spiral wire upon it be connected, as
shown in the figure, by a person holding the conducting handles of the wires
c and d, an inductive current will be produced in the circuit of the wire upon
M, which will impart a corresponding shock to the person holding the
handles.
The intensity of the shock thus imparted may be varied at pleasure, by
moving the discs N and M nearer to or further from each other.
To exhibit the inductive current similarly produced by voltaic electricity,
it is only necessary to connect the wire c' and d' with the voltaic battery, and
the wires c and d with a reoscope, when the existence, direction, and intensity
of the induced current will be immediately indicated by the deflection of the
needle.
299. Ruhmkorffs apparatus to produce currents of ten-
sion. — By this apparatus inductive currents are produced which
have a tension bearing more analogy to that evolved by the elec-
trical machines than to ordinary voltaic currents.
The apparatus which is shown in Jig. 216. consists of a powerful bobbin c,
placed vertically upon a thick plate of glass, which insulates it. This bobbin,
which is about 14 inches high, is composed of two wires, one about the eighth
of an inch in diameter, making 300 coils, and the other the fiftieth of an inch
rolled upon the former, making 10000 coils. These wires are not only
wrapped with silk, but each coil is insulated from the adjacent oqes by a coat
of gum-lac. A current produced by one couple of Bunsen's battery is trans-
mitted through the thicker wire. The positive pole being in communication
with the wire p o, the current passes from it through E to the commutator r>,
from which it descends along the metallic plate to a ribbon of copper, which
conducts it to one of the extremities, a, of the thick wire of the bobbin. The
other extremity of this wire, being connected with one of the copper legs
which support the plate of glass, the current coming out of the bobbin
passes to a second ribbon c, from whence it mounts along an iron column,
6 B. Thence it arrives at an oscillating hammer, e, which is sometimes in
contact with d, and sometimes removed from it. When the contact takes
place, the current follows the conductors, d and F, and mounts to the com-
mutator D, from whence it returns to the pile.
The alternate motion of the hammer e is produced by a cylinder of soft
iron, placed in the axis of the bobbin. When the current of the pile passes
along the thick wire, this rod of soft iron becomes magnetic, and attracts
upwards the little hammer e, which is also iron. The current being then in-
terrupted, and not being capable of passing to the piece d, the rod of soft iron
loses its magnetism, and the hammer e falls back upon d. The current then
recommences, the hammer e being again raised, and so on.
While the current in this way passes with intermission along the thick
wire of the bobbin at each interval of suspension an inductive current is
1 86
VOLTAIC ELECTRICITY.
produced in the fine wire in alternately opposite directions. This being com-
pletely insulated, the induced current acquires a tension so great as to be
capable of producing various phenomena similar to those produced by the
common electrical machine. Thus, the current being imparted to two ron-
l-'ig. 216.
dncting wires h i and k I, which are connected with the two rods of such a globe
A as has been already described, the same electric light will be produced
as was produced by the electrical machine as described in (129.)-
The apparatus with the hammer above described, placed under the great
bobbin c, is represented on a larger scale to the left of the upper part of the
figure, where e represents the hammer, and Ae the wire which conducts the
current to it. It oscillates between the pieces/and d. It will be observed
in this experiment that the greatest brightness will be at the positive pole-
where the light will have a fiery red colour, that at the negative pole having
a violet tint, and being much more feeble. It will be further observed that
while the light round the positive pole is confined to its extremity, that
round the negative pole is extended along the metal rod to the point where
it enters the globe.
300. Stratification of electric light. — Experiments made
with the above apparatus by M. Quet exhibited the following
remarkable phenomena. If the rarefaction of the interior of the
globe is preceded by the introduction of the vapour of turpentine,
pyroligneous acid, alcohol, sulphuret of carbon, &c., the appear-
ance of the light is modified in a remarkable manner. It assumes
then the form of a series of horizontal zones, alternately bright
and dark, ranged one above the other, as shown in jig. 21 J.
In this experiment the light is not continuous, but consists of a
DIRECT AND INVERSE CURRENTS.
187
succession of discharges which follow each other more or less ra-
pidly according to the rate of the oscil-
lation of the hammer a, Jig. 216. The
luminous zones, Jig, 217., then appear
animated with a double movement of
gyration and undulation, which however
M. Quet considers as an optical illusion,
since by causing the hammer a to oscil-
late slowly with the hand, the zones appear
distinct and fixed. It may, however, be
objected that in that case the develop-
ment of the light is too momentary to
render manifest the effects in question.
As to the quality of the light developed
in this experiment, though that round the
positive pole is most frequently red, and
that round the negative pole violet, this
is subject to some variation, depending on
the nature of the vapour or gas which has
been introduced into the globe.
It has been observed by M. Despretz,
that the phenomena exhibited by MM.
Ruhmkorff and Quet, with an intermitting
current, are also produced with a common
continuous current, but with this important difference, that the
continuous current requires a strong battery consisting of many
pairs of Bunsen's system, while the intermitting current requires
only a single pair. It is worthy of remark also that the effect of
an intermitting current is very little increased by increasing the
power of the battery.
No satisfactory explanation appears to have been hitherto pro-
posed for these phenomena.
301. Peculiar properties of the direct and inverse in-
duced currents. — Notwithstanding the momentary character
and consequent intermission of induced currents, they are found
to possess all the physical properties of ordinary voltaic currents.
Thus they impart the same shock to the nervous system, they pro-
duce the same luminous, thermal, and chemical phenomena, they
impart magnetism to soft iron, they affect the reoscope in the same
manner, and, in fine, reproduce other currents of induction.
The shock produced by induced currents is however much more
intense than that which results from common voltaic currents.
To render the shock imparted by the latter sensible, a battery con-
sisting of many pairs is necessary, while a single pair with the
apparatus above described is sufficient to produce a shock, the
Fig. zi7-
j 88 VOLTAIC ELECTRICITY.
continuance of which would be insupportable with an induced
current.
The effects of the direct and inverse induced currents have been
compared by means of commutators, by which they can be sepa-
rately exhibited. So far as respects their effects upon the reoscope
they are nearly alike ; but while the direct current produces a
strong shock, that produced by the inverse current is scarcely
sensible. In like manner, while the direct current is capable of
imparting strong magnetism, the inverse current imparts none.
302. Statham's apparatus. — This consists of a copper wire
AB (fig. 218.), covered with a thick coating of sulphuretted
gutta percha.
Fig. 218.
At the end of some months a stratum of sulphuret of copper, having a
conducting power for the current, is formed at the surface of contact of the
metal and its envelope. If at any point whatever of the circuit a section be
made through the upper half of the envelope, so as to divide the wire, and
remove about a quarter of an inch of its length, as shown at a b, an intense
current, which being transmitted along the wire would be interrupted at ab,
finds its way nevertheless at that point along the coating of sulphuret of
copper not divided by the section ; and because of its imperfect conducting
power this part of the envelope becomes incandescent, so that it would
ignite gun cotton or other inflammable substance.
To perform this experiment with an ordinary current a powerful battery
is necessary ; but an induced current produced by a single pair of Bunsen
and Ruhmkorff's apparatus will be sufficient for it.
[A still more certain method of firing gunpowder or similar
combustible substances, by means of the induction spark, has
been discovered by Mr. Abel, who has constructed fuses charged
with a compound of phosphorus and copper, which ignite when
even a very small spark from an induction coil, or from a common
electrical machine, is sent through them. By means of these
fuses, properly arranged, as many as ten or a dozen separate
charges of powder may be fired at almost absolutely the same
instant. For this purpose, one of the terminals of the induction
coil and one of the wires connected with each fuse must commu-
nicate with the ground, and the other terminal of the coil must
communicate with the second wire of each fuse. It was by help of
these fuses and a frictional electrical machine that the south wall
of the Great Exhibition building of 1862 was overthrown.]
MOMENTARY INDUCTIVE CURRENTS. 189
303. Inductive effects of the successive convolutions of
the same helix. — The inductive effect produced by the com-
mencement or cessation of a current upon a wire, forming part of
a closed circuit placed near and parallel to it, would lead to the
inference that some effect may be produced by one coil of a.
helical current upon another at the moment when such current
commences or ceases. At the moment when the current com-
mences, it might be expected that the inductive action of one coil
upon another, having a tendency to produce a momentary current
in a contrary direction, would mitigate the initial intensity of the
nctual current, and that at the moment the current is suspended
the same inductive action, having a tendency to produce a mo-
mentary current in the same direction, would, on the contrary,
have a tendency to augment the intensity of the actual current.
The phenomena developed when the contact of a closed circuit
is made or broken, are in remarkable accordance with these an-
ticipations.
If the wires which connect the poles of an ordinary pile, con-
sisting of a dozen pairs, be separated or brought together, a very
feeble spark will be visible, and no sensible change in the intensity
of this spark will be produced when the length of the wire com-
posing the circuit is augmented so much as to amount to 150 or
200 yards. If this wire be folded or coiled in any manner, so
long as the parts composing the folds or coils are distant from
each other bv a quarter of an inch or more, no change of intensity
will be observed. But if the wire be coiled round a roller or
bobbin, so that the successive convolutions may be only separated
from each other by the thickness of the silk which covers them, a
very remarkable effect will ensue. The spark produced when the
extremities of the wire are brought together will still be faint ;
but that which is manifest when, after having been in contact,
they are suddenly sepnrated, will have an incomparably greater
length, and a tenfold or even a hundredfold greater splendour.
The shock produced, if the ends of the wire be held in the hands
when the contact is broken, has also a greater intensity.
304. Effects of momentary inductive currents produced
upon revolving: metallic discs. — Researches of Aragro, Her-
schel, Babbagre, and Faraday. — It was first ascertained by
Arago that if a circular disc of metal revolve round its centre in
its own plane under a magnetic needle, the needle will be de-
flected from the magnetic meridian, and the extent of its deflec-
tion will be augmented with the velocity of rotation of the disc.
By increasing gradually that velocity, the needle will at length be
turned to a direction at right angles to the magnetic meridian.
If the velocity of rotation be still more increased, the needle will
igo
VOLTAIC ELECTRICITY.
receive a motion of continuous rotation round its centre in the
same direction as that of the disc, Jig. 219.
Fig. 219.
That this does not proceed from any mechanical action of the
disc upon the intervening stratum of air, is proved by the fact
that it is produced in exactly the same manner, where a screen of
thin paper is interposed between the needle and the disc.
Sir John Herschel and Mr. Babbage made a series of experi-
ments to determine the relative power of discs composed of dif-
ferent metals to produce this phenomenon. Taking the action of
copper, which is the most intense, as the unit, the following are
the relative forces determined for discs of other metals : —
Copper -
Zinc
Tin
0-97
0-46
Lead
Antimony
Bismuth -
- 025
- 009
- ffoz
Professor Barlow ascertained that iron and steel act more ener-
getically than the other metals. The force of silver is considerable,
that of gold very feeble. Mercury holds a place between anti-
mony and bismuth.
Herschel and Babbage found that if a slit were made in the
direction of a radius of the disc it lost a great part of its force ;
but that when the edges of such a slit were soldered together
with any other metal, even with bismuth, which itself has a very
feeble force, the disc recovered nearly all its force.
The motion of rotation of the needle, is an effect which would
result from a force impressed upon it parallel to the plane of the
disc and at right angles to its radii. It was also ascertained, how-
ever, that the disc exercises on the needle forces parallel to its
.MOMENTARY INDUCTIVE CURRENTS. 191
own plane in the direction of its radii, and also perpendicular to
its plane.
A magnetic needle, mounted in the manner of a dipping needle,
so as to play on a horizontal axis in a vertical plane, was placed
over the revolving disc, so that the plane of its play passed through
the centre of the disc. The pole of the needle which was pre-
sented downwards was attracted to or repelled from the centre
of the disc according to its distance from that point. Placed
immediately over the centre, no effect, either of attraction or
repulsion, was manifested. As it was moved from the centre along
a radius, attraction to the centre was manifested. This attraction
was diminished rapidly as the distance from the centre was in-
creased, and, at a certain point, it became nothing, the pole of the
needle resting in its natural position. Beyond this distance re-
pulsion was manifested, which was continued even beyond the
limits of the disc. These phenomena indicate the action of a force
directed parallel to the plane of the disc and in the direction of
its radii.
A magnetic needle was suspended vertically by one of its ex-
tremities, and, being attached to the arm of a very sensitive
balance, was accurately counterpoised. It was then placed suc-
cessively over different parts of the disc, and was found to be
everywhere repulsed, whichever pole was presented downwards.
These phenomena indicate the action of a repulsive force directed
at right angles to the plane of the disc.
All these phenomena have been explained with great clearness
and felicity by Dr. Faraday, by the momentary inductive currents
produced upon the disc by the action of the poles of the magnet,
and the reaction of those currents on the movable poles them-
selves. By the principles which have been explained (285.), it
will be apparent that upon the parts of the disc which are ap-
proaching either pole of the magnet, momentary currents will be
produced in directions contrary to those which would prevail upon
an electro-magnetic helix substituted for the magnet, and having
a similar polarity ; while upon the parts receding from the pole,
momentary currents will be produced, having the same direction.
These currents will attract or repel the poles of the magnet
according to the principles explained and illustrated in (285.) ;
and thus all the motions, and all the attractions and repulsions
described above, will be easily understood.
1 92 VOLTAIC ELECTRICITY.
CHAP. VII.
INFLUENCE OF TERRESTRIAL MAGNETISM ON VOLTAIC CURRENTS.
305. Direction of the earth's magnetic attraction. — The
laws which regulate the reciprocal action of magnets and currents
in general being understood, the investigation of the effects pro-
duced by the earth's magnetism on voltaic currents becomes easy,
being nothing more than the application of these laws to a par-
ticular case. It has been shown that the magnetism of the earth
is such, that in the northern hemisphere the north pole of a
magnet freely suspended is attracted in the direction of a line
drawn in the plane of the magnetic meridian, and inclined below
the horizon at an angle which increases gradually in going from
the magnetic equator, where it is nothing, to the magnetic pole,
where it is 90°. In this part of Europe the direction of the lower
pole of the dipping needle, and therefore of the magnetic attrac-
tion of the earth, is .that of a line drawn in the magnetic meridian
at an angle of about 70° below the horizon, and therefore at an
angle of about 20° with a vertical line presented downwards.
306. In this part of the earth it corresponds to that
of the southern pole of an artificial magnet. — Since the
magnetism of the earth attracts the north pole of the needle,
to determine, therefore, its effects upon currents, it will be suffi-
cient to consider it as a southern magnetic pole, placed below the
horizon in the direction of the dipping needle, at a distance so
great that the directions in which it acts on all parts of the same
current are practically parallel.
307. To ascertain the direction of the force impressed by
terrestrial magnetism on a current, let a line be imagined to
be drawn from any point in the current parallel to the dipping
needle, and let a plane be imagined to pass through this line and
the current. According to what has been explained of the reci-
procal action of magnets and currents, it will follow that the
direction of the force impressed on the current, will be that of a
line drawn through the same point of the current perpendicular
to this plane.
Let cd, jig. 220., be the line of direction of the current, and draw OP
parallel to the direction of tne dip. Let LOR be a line drawn through o, at
right angles to the plane passing through op andcc/. This line will be
the direction of the force impressed by the magnetism of the earth on the
current cd. If the current pass from c to c', this force will be directed
from o towards L, since the effect produced is that of a southern mag-
netic pole placed in the line OP. If the current pass from d to c, the direc-
EFFECTS OF TERRESTRIAL MAGNETISM. 193
tion of the force impressed on it will be
from o towards R (437. 243.)
It follows, therefore, that the force
which acts upon the current is always in
a plane perpendicular to the dipping
needle. This plane intersects the hori-
zontal plane in a line directed to the
magnetic east and west, and therefore
perpendicular to the magnetic meridian ;
and it intersects the plane of the mag-
netic meridian in a line directed north
and south, making, in this part of the
earth, an angle with the horizon of 20°
Fig. zzo. elevation towards the north, and de-
pression towards the south.
308. If the current be vertical, the plane passing through
its direction and that of the dipping needle will be the magnetic
meridian. The force impressed upon the current will therefore
be at right angles to the plane of the magnetic meridian, and
directed eastward when the current descends, and westward when it
ascends.
309. If the current be horizontal, and in the plane of the
magnetic meridian, and therefore directed in the line of the mag-
netic north and south, the force impressed on it will be directed to
the magnetic east and west, and will therefore be also horizontal.
It will be directed to the east, if the current pass from north to
south ; and to the west, if it pass from south to north. This will be
apparent, if it be considered that the effect of the earth's mag-
netism is that of a south magnetic pole placed below the current.
310. If the current be horizontal and at right angles to
the magnetic meridian, the force impressed on it will be directed
north and south in the plane of the magnetic meridian, and
inclined to the horizontal plane at an angle of 20° in this part of
the earth. This may be resolved into two forces, one vertical
and the other horizontal. The former will have a tendency to
remove the current from the horizontal plane, and the latter will
act in the horizontal plane in the direction of the magnetic north
and south. It will be directed from the south to the north, if the
current pass from west to east, and from the north to the south, if
the current pass from east to west. This will also be apparent, by
considering the effect produced upon a horizontal current by a
south magnetic pole placed below it.
311. Zf a horizontal current have any direction inter-
mediate between the magnetic meridian and a plane at right
angles to it, the force impressed on it, being still at right angles
to the dipping needle, and being inclined to the horizontal plane
at an angle less than 20°, may be resolved into other forces,
o
194 VOLTAIC ELECTRICITY.
one of which will be at right angles to the current, and will be
directed to the left of the current, as viewed from below by an
observer whose head is in the direction from which the current
passes.
312. Effect of the earth's magnetism on a vertical current
which turns round a vertical axis. — It follows, from what has
been here proved, that if a descending- vertical rectilinear current
be so suspended as to be capable of turning freely round a vertical
axis, the earth's magnetism will impress upon it a force directed
from west to east in a plane at right angles to the magnetic meri-
dian ; and it will therefore move to such a position, that the plane
passing through the current and the axis round which it moves
shall be at right angles to the magnetic meridian, the current being
to the east of the axis.
If the current ascend, it will for like reasons take the position
in the same plane to the west of the axis, being then urged by a
force directed from east to west.
313. Effect on a current which is capable of moving in a
horizontal plane. — If a vertical current be supported in such a
manner that, retaining its vertical direction, it shall be capable of
moving freely in a horizontal plane in any direction .whatever, as
is the case when it floats on the surface of a liquid, the earth's
magnetism will impart to it a continuous rectilinear motion in a
direction at right angles to the plane of the magnetic meridian,
and directed eastward if the current descend, and westward if it
ascend.
If a horizontal rectilinear current be supported, so as to be
capable of revolving in the horizontal plane round one of its ex-
tremities as a centre, the earth's magnetism will impart to it a
motion of continued rotation, since it impresses on it a force always
at right angles to the current, and directed to the same side of it.
If in this case the current flow towards the centre round which it
revolves, the rotation imparted to it will be direct ; if from the
centre, retrograde, us viewed from above.
314. Experimental illustrations of these effects. — Pouillet's
apparatus.— A great variety of experimental expedients have
been contrived to verify these consequences of the principle of the
influence of terrestrial magnetism on currents.
To exhibit the effects of the earth's magnetism on vertical currents, M.
Pouillet contrived an apparatus consisting of two circular canals, repre-
sented in their vertical section in Jig. 221., one placed above the other, the
lower canal having a greater diameter than the upper. In the opening in
the centre of these canals a metallic rod t is fixed in a vertical position,
supporting a mercurial cup c. A rod h h', composed of a nonconducting
substance, is supported in the cup c by a point at its centre. The vertical
wires vv1 are attached to the ends of the rod hh', and terminate in points,
EFFECTS OF TERRESTRIAL MAGNETISM.
k'
which are turned downwards, w> as to dip into the liquid contained in the
upper canal, while their lower extremities dip into the liquid contained in
the lower canal. A bent wire connects the mercury
contained in the cup c with the liquid in the upper
canal.
The liquid in the upper and lower canals is acidu-
lated water or mercury. If the liquid in the lower
*| canal be put in communication with the positive, and
I the rod t with the negative pole, the current will pass
from that canal up the two vertical wires vv', thence
to the liquid in the upper canal, thence by the con-
necting wire to the mercury in the cup c, and thence
by the rod t to the negative pole.
By this arrangement the two vertical currents v V,
which both ascend, are movable round the rod t as
an axis.
Fig. 2il.
When this apparatus is left to the influence of the earth's magnetism, the
currents vv' will be affected by equal and parallel forces directed westward
at right angles to the magnetic meridian (308.), The equal and parallel
forces, being at equal distances from the axis t, will be in equilibrium in all
positions, and the wires will therefore be astatic ; that is to say, not affected
by the earth's magnetism.
If the point of the wire v1 at h' be raised from the upper canal, the current
on v1 will be suspended. In that case, the wire v being impelled by the
terrestrial magnetism westward at right angles to the magnetic meridian
the system will take a position at right angles to that meridian, the wire on
which the current passes being to the west of the axis t. If the point at h'
be turned down so as to dip into the liquid, and the point at h be turned up
so as to suspend the current on h and establish that on h', the system will
make half a revolution and will place the wire h' on which the current runs
to the west of t.
If by the reotrope the connections with the poles of the battery be re-
versed, the currents on vv' will descend instead of ascending. In that case
the system will be astatic as before, so long as both currents are established
on the wires vv'. But if the connection of either with the superior canal be
removed, the wire on which the remaining current passes being impelled
eastwards, the system will take a position perpendicular to the plane of the
magnetic meridian, the wire on which the current runs being east of the axis, t.
When the currents on the wires vv1 are both passing, the system will be
astatic only so long as the currents are equally intense, and both in the same
plane with the axis t. If while the latter condition is fulfilled one of the
wires be even in a email degree thicker than the other, it will carry a
stronger current, and in that case it will turn to the magnetic east or west,
according as the currents descend or ascend, just as though the current on
the other wire were suppressed ; for in this case the effective force is that
due to the difference of the intensities of the currents acting on that which
is the stronger.
If the two wires be not in the same plane with the axis, the forces which
act upon them being equal, and perpendicular to the plane of the magnetic
meridian, the position of equilibrium will be that in which the plane passing
through them will be parallel to the latter plane.
The position of equilibrium will be subject to an infinite variety of changes,
according as the plane of the w;res v v', their relative thickness, and their
196 VOLTAIC ELECTRICITY.
distances from the axis of rotation are varied, and in this way a great num-
ber of interesting experiments on the effects of the earth's magnetism may
be exhibited.
315. Its application to show the effect of terrestrial mag-
netism on a horizontal current. — To show experimentally the
effect of the earth's magnetism on a horizontal current, M. Pouillet
contrived an arrangement on a similar principle, consisting of a
circular canal, the vertical section of
/~ |i x ir"\ which is represented in jig. 222. A
V/ horizontal wire a b is supported by a
point at its centre which rests in a
_.. mercurial cup fixed upon a metallic
rod, like t,fig. 221. Two points a and #,
project from the wire, and dip into the liquid in the canal, the
small weights e and d being so adjusted as to keep the wire a b
exactly balanced.
If the central rod be connected with the positive, and the liquid
in the canal with the negative pole, the current will ascend on the
central rod, and will pass along the horizontal wire in both direc-
tions from its centre to the points a and i, by which it will pass to
the liquid in the canal, and thence to the negative pole. If by the
reotrope the connections be reversed and the names of the poles
changed, the current will pass from a and b to the centre, and
thence by the central rod to the negative pole.
In the former case, the wire a b will revolve with retrograde,
and in the latter with direct rotation, in accordance with what has
been already explained (313.).
316. Its effect on vertical currents shown by Ampere's
apparatus. — If a rectangular current, such as that represented
in^or. 149., be suspended in Ampere's frame, fig. 148., it will, when
left to the influence of terrestrial magnetism, take a position at
right angles to the magnetic meridian, the side on which the cur-
rent descends being to the east. For in this case the horizontal
currents which pass on the upper and lower sides of the rectangle,
being contrary in direction, will have a tendency to revolve, one
with direct, and the other with retrograde motion round yy'.
These forces, therefore, neutralise each other. The vertical de-
scending current will be attracted to the east, and the ascending
current to the west (312.).
317. Its effect on a circular current shown by Ampere's
apparatus. — If a circular current, such as that represented in
Jig. 182., be suspended in Ampere's frame, fig. 148., and sub-
mitted to the influence of terrestrial magnetism, each part of it
may be regarded as being compounded of a vertical and horizontal
component The horizontal components in the upper semicircle.
EFFECTS OF TERRESTRIAL MAGNETISM.
197
flowing in a direction contrary to those in the lower semicircle,
their effects will neutralise each other. The vertical components
will descend on one side and ascend on the other. That side on
which they descend will be attracted to the east, and that at which
they ascend to the west ; and, consequently, the current will place
itself in a plane at right angles to the magnetic meridian, its front
being presented to the south.
318. Its effect on a circular or spiral current shown by
Delarive's floating: apparatus. — If a circular or spiral current
be placed on a floating apparatus, it will assume a like position at
right angles to the magnetic meridian, with its front to the south ;
and the same will be true of any circulating current.
319. Astatic currents formed by Ampere's apparatus. —
To construct a system of currents adapted to Ampere's frame,
which shall be astatic, it is only necessary so to arrange them that
there shall be equal and similar horizontal currents running in
contrary directions, and equal and similar vertical currents in the
same direction, and that the latter shall be
at equal distances from the axis on which
the system turns ; for in that case the hori-
zontal elements, having equal tendencies to
make the system revolve in contrary direc-
tions, will equilibrate, and the vertical ele-
ments being affected by equal and parallel
forces at equal distances from the axis of
rotation, will also equilibrate.
By considering these principles, it will be
evident that the system of currents repre-
sented in jig. 223., adapted to Ampere's
frame, jig. 148., is astatic.
320. Effect of earth's magnetism on spiral currents
shown by Ampere's apparatus. — If the arrangement of spiral
currents represented in jig. 1 89. be so disposed that the current
after passing through one only of the two spirals shall return to
the negative pole, the earth's magnetism will affect it so as to
bring it into such a position that its plane will be at right angles
to the magnetic meridian. If the descending currents be on the
side of the spiral more remote from the axis of motion, the system
will arrange itself so that the spiral on which the current flows
shall be to the east of the axis. If the descending currents be on
the side nearer to the axis, the spiral on which the current flows
will throw itself to the west of the axis. In each case, the front of
the current is presented to the magnetic south, and the descending
currents are on the east aide of the spiral.
If the current pass through both spirals in jig. \ 89., and their
I I
it=J
Fig.
198 VOLTAIC ELECTRICITY.
fronts be on the same side, the earth's magnetism will throw them
into the plane at right angles to the magnetic meridian, their
fronts being presented to the south.
If their fronts be on different sides, the system will be astatic,
and will rest in any position independent of the earth's magnetism,
which in this case will produce equal and contrary effects on the
two spirals.
If the system of spiral currents represented in jig. 1 89. be sus-
pended in Ampere's frame, subject to the earth's magnetism, the
fronts of the currents being on the same side of the two spirals, it
will take such a position that the centres of the two spirals will be
in the magnetic meridian, their planes at right angles to it, and
the fronts of the currents presented to the south. If in this case
the fronts of the currents be on opposite sides, the system will be
astatic.
321. Effect on a horizontal current shown by Pouillet's
apparatus. — The rotation of the horizontal current produced
with the apparatus, Jig. 222., may be accelerated, retarded, ar-
rested, or inverted by presenting the pole of an artificial magnet
above or below it, at a greater or less distance. A south magnetic
pole placed below it, or a north magnetic pole above, producing
forces identical in direction with those produced by terrestrial
magnetism, will accelerate the rotation in a greater or less degree,
according to the power of the artificial magnet, and the greater or
less proximity of its pole to the centre of rotation of the current.
A north magnetic pole presented below, or a south pole above
the centre of rotation, producing forces contrary in their direction
to those resulting from the earth's magnetism, will retard, arrest,
or reverse the rotation according as the forces exerted by the
magnet are less than, equal to, or greater than
those impressed by terrestrial magnetism.
If the system of currents represented in Jig.
224., be suspended on Pouillet's apparatus, re-
presented in Jig. 221., it will receive a motion
of continued rotation from the influence of the
earth's magnetism. In this case the vertical
currents being in the same direction will be in
equilibrium (314.) ; and the horizontal currents
passing either from the centre of the upper
horizontal wire to the extremities, or vice versa,
Fig. 104. according to the mode of connection, will receive
a motion of rotation direct or retrograde (3 1 5.)-
This motion of rotation may be affected in the manner above de-
scribed, by the pole of a magnet applied in the centre of the lower
circular canal, jig. 221.
HELICAL CURRENTS.
199
322. Effect of terrestrial magnetism on a helical
current shown by Ampere's apparatus. — A helical cur-
rent, such as that represented in Jig. 192., being mounted on
Ampere's frame, or arranged upon a floating apparatus, Jig. 193.,
will be acted on by the earth's magnetism. The several convo-
lutions will, like a single circulating current, take a position at
right angles to the magnetic meridian, their fronts being pre-
sented to the south. The axis of the helix will consequently be
directed to the magnetic north and south; and it will, in fine,
exhibit all the directive properties of a magnetic needle, the end
to which the front of the currents is directed being its south pole.
If such a current were mounted on a horizontal axis at right
angles to the plane of the magnetic meridian, it would, under the
influence of the earth's magnetism, take the direction of the
dipping needle, the front of the currents corresponding in direc-
tion to the south pole of the needle.
323. The dip of a current illustrated by Ampere's rect-
angle. — The phenomenon of the dip may also be experimentally
illustrated by Ampere's electro-magnetic rectangle, Jig. 22$.,
M
Fig. 145.
which consists of a horizontal axis x v, which is a tube of wood or
other non-conductor, at right angles to which is fixed a lozenge-
shaped bar a z, composed also of a non-conductor. Upon this
cross is fixed the rectangle A B D c, composed of wire. The rect-
angle rests by steel pivots at M and N on metallic plates, which
communicate by wires with the mercurial cups at s and R. These
latter being placed in connection with the poles of u voltaic
200 VOLTAIC ELECTRICITY.
battery, the current will pass from the positive cup s up the pillar
and round the rectangle, as indicated by the arrows. At x it
passes along a wire through the tube xv to v, and thence by the
steel point, the plate M, and the pillar, to the negative cup E.
The axis MN being placed at right angles to the magnetic
meridian, and the connections established, the rectangle will be
immediately affected by the earth's magnetism, and after some
oscillations, will settle into a position at right angles to the direc-
tion of the dipping needle.
In this case the forces impressed by the earth's magnetism on
the parts of the current forming the sides AC and BD, will pass
through the axis MN, and will therefore be resisted. The forces
impressed on AB and CD will be equal, and will act at the middle
points a and z, at right angles to AB and CD, and in a plane at
right angles to the direction of the dip. These forces will there-
fore be in directions exactly opposed to each other when the line
az takes the direction of the dip, and will therefore be in equi-
librium.
CHAP. VIII.
RECIPROCAL INFLUENCE OF VOLTAIC CURRENTS.
324. Results of Ampere's researches. — The mutual attraction
and repulsion manifested between conductors charged with the
electric fluids in repose, would naturally suggest the inquiry
whether any analogous reciprocal actions would be manifested by
the same fluids in motion. The experimental analysis of this
question led Ampere to the discovery of a body of phenomena
which he had the felicity of reducing to general laws. The
mathematical theory raised upon these laws has supplied the
means by which phenomena, hitherto scattered and unconnected,
and ascribed to a diversity of agents, are traced to a common
source.
Although the limits, within which a treatise so elementary as
this manual is necessarily confined, exclude any detailed expo-
sition of these beautiful physico-mathematical researches, they
cannot be altogether passed over in silence. We shall therefore
give as brief an exposition of them as is compatible with their
great importance, and that clearness without which all exposition
would be useless.
325. Reciprocal action of rectilinear currents. — If two
RECTILINEAR CURRENTS.
201
rectilinear currents be parallel, they will attract or repel each
other according as they flow in the same or opposite directions.
This is verified experimental!}' by the apparatus represented in fig. 226.,
which is on the principle of Ampere's frame. The mercurial cup marked +
receives the current from the positive
pole. The current passes as indicated
by the arrows upwards on the pillar t,
and thence to the cup x, from which
it flows round the rectangle, returning
to the cup y, and thence to the pillar
t>, by which it descends to the cup
which is connected with the negative
pole.
If the rectangle thus arranged be
placed with its plane at an angle with
the plane of the pillars t and v, upon
•which the ascending and descending
currents pass, it will turn upon its axis
until its plane coincides with the plane
of the pillars t and r, the side of the
rectangle d e on which the current
Fig. zz6.
ascends being next the pillar t, on which it ascends. If by means of the
reotrope (226.) the connection be reversed, so that the current shall descend on
t and d e, and shall ascend on v and b c, it will still maintain its position. But
if the connections at x and y be reversed, the connections of the cups + and_
remaining unchanged, the current will descend oned while it ascends on t, and
will ascend on be while it descends on v. In this caie fwill repel de and
attract b c, and v will repel b c and attract d e, and accordingly the rectangle
will make a half revolution, and b c will place itself near t, and de near v.
326. Action of a spiral or helical current on a rectili-
near current. — A sinuous, spiral, or helical current, provided
its convolutions are not considerable in magnitude, impresses on
another current in its neighbourhood the same force as a straight
current would produce, whose direction would coincide with the
axis of the sinuous or spiral current. This is proved experi-
mentally by the fact that a spiral current which has a returning
straight current passing along its axis, will exercise no force
either of attraction or repulsion on a straight current parallel to
it. Now since on suspending the spiral current the straight
current will attract or repel a parallel straight current, it follows
that the spiral current exactly neutralises the effect of the
straight current flowing in the opposite direction, and conse-
quently it will be equivalent to a straight current flowing in the
same direction.
327. Mutual action of diverging or converging rectilinear
current*. — Rectilinear currents which diverge from or converge
to a common point mutually attract. Those, one of which di-
verges, and the other converges, mutually repel ; that is to say,
202
VOLTAIC ELECTRICITY.
if two rectilinear currents cc' and cc', fig. 227., which intersect
at o, both flow towards or from o, they will mutually attract ; but
if one flow towards, and the other from o, they will mutually
Fig. ^^•^.
repel. The currents, being supposed to flow in the direction of
the arrows, oc and oc will mutually attract, as will also oc' and
o cr ; while o c' and o c will repel, as will also o c and o cf.
If the wires conducting the currents were movable on o as a
pivot, they would accordingly close, the angle coc diminishing
until they would coincide.
328. Experimental illustration of this. — This may be ex-
perimentally illustrated by the apparatus
represented in fig. 228. in plan, and in
fig. 229., in section, consisting of a cir-
cular canal filled with mercury or acidu-
lated water separated into two parts by
partitions at a and b. Two wires c d and
e /, suspended on a central pivot, move
freely one over and independent of the
other, like the hands of a watch, the
points being at right angles, so as to dip
into the canal. The mercurial cup x being
supposed to be connected with the posi-
tive, and y with the negative pole, the
current passing to the liquid will flow along the wires as indicated
by the arrows from the liquid in one section to that of the other,
and will pass to the negative cup y. When
the wires cd and ef thus carrying the
current are left to their mutual influence*
the angle they form will close, and the
directions of the wires will coincide, so
Fig. 128.
Fig. 229.
that the currents shall flow in the same direction upon them.
In these and all similar experiments, the phenomena will neces-
sarily be modified by the effects produced by the earth's mag-
netism. In some cases the apparatus can be rendered astatic;
and in others, the effect due to the terrestrial magnetism being
known, can be allowed for, so that the phenomena under exa-
mination may be eliminated.
RECTILINEAR CURRENTS. 203
329. Mutual action of rectilinear currents which are not
in the same plane. — If two rectilinear currents be not in the
same plane, their directions cannot intersect although they are
not parallel. In this case a line may always be drawn, which is
at the same time perpendicular to both. To assist the imagination
in conceiving such a geometrical combination, let a vertical rod
be supposed to be erected, and from two different points of this
rod let lines be drawn horizontally, but in different directions,
one, for example, pointing to the north, and the other to the east.
If voltaic currents pass along two such lines, they will mutually
attract, when they flow both to or both from the vertical rod ; they
will mutually repel, when one flows to the vertical rod and the
other from it.
In either case the mutual action of such currents will have a
tendency to turn them into the same plane and to parallelism.
If they mutually attract, their lines of direction turning round
the vertical line will take a position parallel to each other, and at
the same side of that line. If they mutually repel, they will turn
on the vertical line in contrary directions, and will take a position
parallel to each other, but at opposite sides of it.
In Jig. 230., AB and CD represent two currents which are not in
the same plane. Let p o be the line which in-
tersects them both at right angles, and let
planes be supposed to pass through their di-
rections respectively, which are parallel to each
other, and at right angles to PO. If, in this
case, CD be fixed and AB movable, the latter
will be turned into the direction ab parallel to
Fig.ajo. CD; or if CD were free and AB fixed, CD
would take the position cd; if both were free
they would take some position parallel to each other ; and if free
to change their planes, they would mutually approach and coalesce.
It follows from this, that if the direction of either of the two cur-
rents be reversed, the directions of the forces they exert on each
other will be also reversed ; but if the directions of both currents
be reversed, the forces they exert on each other will be un-
altered.
330. Mutual action of different parts of the same cur-
rent. — Different parts of the same current exercise on each other
a repulsive force. This will follow immediately as a consequence
of the general principle which has been just established. Since a
repulsive action takes place between oc and oc',Jig. 227., and
such action is independent of the magnitude of the angle c o </, it
will still take place, however great that angle may be, and will
therefore obtain when the angle coc' becomes equal to 1 80°;
204
VOLTAIC ELECTRICITY.
that is, when o c' forms the continuation of c o, or coalesces with
o c'. Hence, between o c and o c' there exists a mutually repul-
sive action.
331. Ampere's experimental verification of this. — Inde-
pendently of this demonstration, M. Ampere has reduced the
repulsive action of different parts of the same rectilinear current
to the following experimental proof: —
Let ABCD, fig. 231., be a glass or porcelain dish, separated into two
divisions by a partition A c, also of glass ; and let it be tilled with mercury
on both sides of A c. Let a wire,
wrapped with silk, be formed into
two parallel pieces united, by a
semicircle whose plane is at right
angles to that of the straight parallel
parts, and let these two parallel
straight parts be placed floating on
Fig. zji. the surface of the mercury at each
side of the partition A c, over which
the semicircle passes. The mercury in the divisions of the dish is in metallic
communication with the mercurial cups E and F placed in the direction of
the straight arms of the floating conductor. When the cups E and F are put
in connection with the poles of a voltaic batter}', a current will pass from the
positive cup to the end of the floating conductor, from that along the arm of
the conductor, then across the partition by the semicircle, then along the
other floating arm, and from thence through the mercury to the negative cup.
There is thus on each side of the partition a rectilinear current, one part of
which passes upon the mercury, and the other part upon the straight arm of
the floating conductor. When the current is thus established, the floating
conductor will be repelled to the remote side of the dish. This repulsion is
effected by that part of the straight current which passes upon the mercury
acting on that part which passes along the wire.
332. Action of an indefinite rectilinear current on a finite
rectilinear current at right angles
to it. — A finite rectilinear current a 5,
Jig. 232., which is perpendicular to an
indefinite rectilinear current c d lying
all at the same side of it, will be acted
on by a force tending to move it pa-
rallel to itself, either in the direction of
the indefinite current, or in the con-
trary direction, according to the rela-
tive directions of the two currents.
Fig. ^l^.
If the finite current do not meet the indefinite current, let its
line of direction be produced till it meets it at a. Take any two
points c and d on the indefinite current at equal distances from a,
and draw the lines cb and db to any point on the finite current.
First case. Let the finite current be directed toward'} the indefi-
nite current. Hence the point b will be attracted by d and re-
RECTILINEAR CURRENTS. 205
pelled by c (327.) ; and since db = cb, the attraction will be equal
to the repulsion. Let the equal lines b e and bf represent this
attraction and repulsion. By completing the ' rectangle, the dia-
gonal b g will represent the resultant of these forces ; and this line
b g is parallel to c d, and the resultant is contrary in direction to
the indefinite current.
The same may be proved of the action of all points on the in-
definite current on the point ft, and the sum of all these resultants
will be the total action of the indefinite current on b.
The same may be proved respecting the action of the definite
current on all the points of the indefinite current.
Hence the current a b will be urged by a system of forces acting
at all points parallel to c d, and in a contrary direction.
Second case. Let the finite current be directed from the inde-
finite current. The point b will then be attracted by c and re-
pelled by c?, and the resultant bg' will be contrary to its former
direction.
Hence the current a b will be urged by a system of forces pa-
rallel to c d, and in the same direction as the indefinite current.
Since the action of the two currents is reciprocal, the indefinite
current will be urged by a force in its line of direction, either
according or contrary to its direction, as the finite current runs
from or towards it.
333. Case in which the indefinite current is circular. —
If the indefinite current c d be supposed to be bent into a circjular
form so as to surround a cylinder, on the side of which is placed
the vertical current aft, it is evident that the same reciprocal
action will take place ; but in that case the motion imparted will
be one of rotation round the axis of the cylinder as a centre.
334. Experimental verification of these principles.—
These principles are experimentally verified by the apparatus,
jig. 233., where azsb re-
presents a ribbon of copper
coated with silk and carried
round the copper circular
canal v. A conductor con-
nects the mercurial cup c
with the central metallic
pillar which supports a mer-
curial cup p. In this cup
the metallic point m is
placed. The mercurial cup
Fi«- *J3- d is in metallic communica-
tion with the acidulated water in the circular canal ». A hoop of
metal h is supported by the point m by means of the rectangular
zo6 VOLTAIC ELECTRICITY.
wire, and is so adjusted that its lower edge dips into the liquid in
the canal v.
Let the mercury in a be connected with the positive pole of the
battery, and the mercury in d with the negative pole. The cur-
rent entering at a will pass round the circular canal upon the
coated ribbon of copper, and, arriving at Z>, it will pass to c by a
metallic ribbon or wire connecting these cups. From c it will
pass to the central pillar and thence to the cup p. It will then
pass from m as a centre in both directions on the wire, and will
descend to the hoop h. from which it will pass into the liquid in
the canal a, and thenco to the cup c?, with which the liquid is in
metallic communication, and, in fine, from d it will pass to the
negative pole of the battery.
By this arrangement, therefore, a circular current flows round
the exterior surface of the vase v, while two descending currents
constantly flow upon the wire at right angles to this circular
current. The circular current being fixed, and the vertical
currents being movable, the latter will receive a motion of con-
tinued rotation by the action of the former ; and in the case here
supposed, this rotation will be in a direction contrary to the
direction of the circular current. If the connections be reversed
by the reotrope, the direction of the circular current will be re-
versed, but at the same time that of the vertical currents on the
wire will be .also reversed; and, consequently, no change will
take place in the direction of the rotation. These changes of
direction of the two currents neutralise each other. But if, while
d is still connected with the negative pole, b be connected with
the positive pole, the connection between Z> and c being removed,
and a connection between a and c being established, then the
direction of the circular current being from s to z will be re-
versed, while that of the vertical currents remains still the same ;
the direction of the rotation will therefore be reversed.
335. To determine in general the action of an indefinite
rectilinear current on a finite rectilinear current. — First.
Let it be supposed that the finite current AB,
Jig. 234., has a length so limited that all its
points may be considered as equally distant from
the indefinite current, and therefore equally
acted on by it. In this case the current AB may
be replaced by two currents, AD perpendicular
and AC parallel to the indefinite current, and the
action of the indefinite current on AB will be
equivalent to its combined actions on A D and
AC.
If A be supposed to be the positive end of the finite current, it
RECTILINEAR CURRENTS. 207
will also be the positive end of the component currents AD and
AC. Supposing the indefinite current parallel to AC to run in the
same direction as AC, then AD will be urged in the direction AC
(332.), and AC in the direction AC' by forces proportional to AD
and AC. Hence, if AD' = AD, and AC' = AC, AD' and AC' will
express in magnitude and direction the two forces which act on
the component currents. The resultant of these two forces AD'
and A c' will be the diagonal A B', which is evidently perpendicular
to AB and equal to it.
Secondly. Let the finite current have any proposed length, and
from its positive end A, Jig. 235., let a line AO be drawn perpen-
dicular to the indefinite
current x'x, this current
being supposed to run
from x' to x.
If the distance o A be
greater than AB, that cur-
rent AB, whatever be its
position, will lie on the
same side of x' x, and the
action of x'x on every
small element of AB will
be perpendicular to An,
as has been just demon-
strated. The current AB will therefore be acted on by a system
of parallel forces perpendicular to its direction. The resultant
of these forces will be a single force equal to their sum, and
parallel to their common direction. Hence the indefinite current
x'x will act on the finite current AB by a single force R in the
direction CD.
If the current AB be supposed to assume successively different
positions, B,, B,, BS, &c., around its positive end A, the line CD
will represent in each position the direction of the action of the
current x'x upon it.
It is evident that when the indefinite current runs from x' to x,
the action on the finite current is such as would cause it to turn
round its positive end A with a direct, or round its negative end
B with a retrograde rotation.
If the indefinite current run from x to x', the direction of its
action on AB, and the consequent motions of AB, would be re-
versed.
The point c of the current AB at which the resultant R acts
will vary with the position of the current A B, approaching more
towards x'x as AB approaches the position AB,; but in every
position this resultant must be between A and B. The force
2o8 VOLTAIC ELECTRICITY.
producing the rotation therefore having a varying moment, the
rotation will not be uniform.
If the distance OA be very great compared with AB, the re-
sultant B will be sensibly constant, and will act at the middle
point of AB.
In this case, if the middle point of A B be fixed, no rotation can
take place.
If the distance OA be less than AB, the current AB will in cer-
tain positions intersect x'x, fig. 236., and a part will be at one
Fig. aj6.
side and a part at the other. In this case the action on AB, in all
positions in which it lies altogether above x'x, is the same as in
the former case.
When it crosses x'x, as in the positions ABS, AB,, AB4, the
action is different. In that case the forces which act on ATW, and
those which act on ms, are in contrary directions, and their re-
sultant is in the one direction or in the other, according as the
sum of the forces acting on one part is greater or less than the
sum of the forces acting on the other part. If A m be in every
position of AB greater than WB, then the resultant will be in
every position in the same direction as if the current A B did not
cross x'x; and if the point A were fixed, a motion of continued
rotation would take place, in the same manner as in the former
case, except that the impelling force would be diminished as the
line A B would approach the position A BS.
A But if A o be less than
half A B, the circum-
stances will be different.
In that case there will
> ^\ ^xi' be two positions A B2 and
\ \ AB4, fig. 237., at equal
\-'1; \, *2 distances from AB3, at
B which the line A B will be
bisected by x'x.
In all positions of AB
not included between AB2 and AB,, the action of the indefinite
RECTILINEAR CURRENTS.
209
current upon it takes place in the same direction as in the former
cases.
But in the positions A B' and A B", where m B' and m B" are
greater than m A, the forces acting on m B' and m B" exceed those
acting in the contrary direction on m A, and consequently the re-
sultant of the forces on A B in all positions between A B2 and A B4,
is contrary to its direction in every other position of the line A B.
In the positions A B2 and A B4 the resultant of the forces in one
direction on Am is equal and contrary to the resultant of the
forces on B m. There will in these positions be no tendency of
n n the current A B to move except
round its middle point.
If the indefinite current x' x
X. pass through A, jig. 238., the re-
/ \* sultants of its action on A B will be
B/ \B in contrary directions above and
Fj below x' x, and will in each case
tend to turn the current A B round
the point A so as to make it coincide in direction with% the inde-
finite current x'x.
336. Experimental illustration of these principles. —
These effects may be illustrated experimentally by means of the
apparatus,^. 233., already described. The circular current sur-
rounding the canal v being removed, and the currents on the wire
m being continued, let an indefinite rectilinear current be con-
ducted under the apparatus at different distances from the vertical
line passing through the pivot, and the effects above described will
be exhibited.
337. Effect of a straight indefinite current on a system
of diverging or converging currents. — If any number of finite
rectilinear currents diverge from or converge to a common centre,
the system will be affected by an indefinite current near it, in the
same manner as a single radiating current would be affected.
Thus if a number of straight and equal wires have a common
extremity, and are traversed by currents flowing between that
extremity and the circumference of the circle in which their
other extremities lie, an indefinite current x' x placed in the plane
of the circle, as represented in jig. 239., will cause the radiating
system of currents to revolve in the one direction or the other, as
indicated by the arrows in the figures.
338. Experimental illustration of this action. — These
actions may be shown experimentally, by putting a vertical wire,
fig. 240., in communication with the centre of a shallow circular
metallic vessel of mercury v, and another wire w, communicating
p
VOLTAIC ELECTRICITY.
wit.h the outside of the vessel, into communication with the poles
of a battery : diverging currents will be transmitted through the
Fig. 140.
mercury in the one direction or the other, according to the con-
nection ; and if a straight conducting wire
c D, conveying a powerful electric cur-
rent, is brought near the vessel, a rotation
will be imparted to the mercury, the di-
rection of which will be in conformity with
the principles just explained. Davy used
a powerful magnet instead of the straight
wire.
339. Consequences deducible from
this action. — The following consequences
respecting the action of finite and indefinite rectilinear currents
will readily follow from the principles which have been established.
When a finite vertical conductor A B, movable round an axis o o',
is subjected to the action of an indefinite horizontal current MN,
the plane ABO'O will place itself in the position OO'B'A', when
the vertical current descends, and the horizontal current runs
from N to M, jig. 241.
If the direction of the vertical or horizontal current be reversed,
the position of equilibrium of the former will be OO'B A; but if
the direction of both be reversed, the position of equilibrium will
remain unaltered.
When two vertical conductors AB and A'B' are movable round
a vertical axis oo', and connected together, they will remain in
equilibrium, whatever be their position, if they are both traversed
RECTILINEAR CURRENTS.
211
by currents of the same intensity in the same direction, provided
that the indefinite rectilinear current which acts upon them be at
such a distance and in such a position that its distances from the
Fig. 141.
points B and B' may be considered always equal. When the wires
A B and A' B' are traversed by currents in opposite directions, one
ascending and the other descending, the system will then turn on
its axis oo' until the vertical plane through A B and A' B' becomes
parallel to M N, the descending current being on that side from
which the indefinite current flows.
340. Action of an indefinite straight current on a cir-
culating current. — The circulating current A, fig. 242., is
Fig. 444.
affected by the indefinite current p N in the same manner as the
rectangular current B would be affected. The current p N affects
the descending side a by a force contrary to, and the ascending side
b by an equal force according with, its own direction (332.). In
the same manner it affects the sides c and d with forces in contrary
directions, one towards, and the other from p N. But the side c,
being nearer to P N than d, is more strongly affected ; and conse-
quently the attraction, in the case represented in fig. 242., will
prevail over the repulsion. If the direction of either the recti-
linear or circulating current be reversed, the repulsion will prevail
over the attraction.
Thus it appears, that an indefinite current flowing from right to
left, under a circulating current having direct rotation, or one
moving from left to right under a circulating current having retro-
212
VOLTAIC ELECTRICITY.
grade rotation, will produce attraction ; and two currents moving
in the contrary directions will produce repulsion.
If the current A be fixed upon a horizontal axis a b on which
it is capable of revolving, that side c at which the current moves
in the same direction as P N will be attracted downwards, and the
plane of the current will take a position passing through P N, the
side c being nearest to that line.
If the current A be fixed upon the line cd as an axis, it will turn
into the same position, the side b on which the current ascends
being on the side towards which the current P N is directed.
341. Case in which the indefinite straight current is
perpendicular to the plane of the circulating: current. —
If the rectilinear current AB, fig. 243., be perpendicular to the
circular current Q N N, and within it, and be movable round the
central line o o', a motion of rotation will be impressed upon it
contrary to that of the circular current. This may be experimen-
tally verified by an apparatus constructed on the principles repre-
sented in fig. 244., consisting of a wire frame supported and
Fig. 244.
balanced on a central point in a mercurial cup. The current
passing between this point and the liquid in a circular canal will
ascend or descend on the vertical wires according to the arrange-
ment of the connections. The circular current may be produced
by surrounding the circular canal with a metallic wire, or ribbon
coated with a nonconductor, upon which the current may be
transmitted in the usual way. The wire frame will revolve upon
the central point with direct or retrograde rotation, according to
the directions of the currents. If the current ascend on the wires,
they will revolve in the same direction as the circular current ; if
it descend, in the contrary direction.
The circular current may also be produced by a spiral current
placed under the circular canal, and the wire frame may be replaced
by a light hollow cylinder, supported on a central point. The
spiral in this case may be movable and the cylinder fixed, or vice
and the reciprocal actions will be manifested.
CURVILINEAR CURRENTS.
213
342. Case in which the straight current is oblique to
the plane of the circulating: current. — Like effects will
be produced when the rec-
tilinear current, instead of being
perpendicular to the plane of
the circular current, is oblique
to it.
Let the rectilinear current a c,
jig. 245., be parallel to the plane
of the circular current NQ. If
the current flow from a to c, the
part a b which is within the cir-
cle will be affected by a force op-
posite to the direction of the
nearest part of the current N Q,
and the part b c outside the cir-
cle will be affected by a force in the same direction. If the current
flow from c to a, contrary effects will ensue.
If in this case the straight current be limited to a &, and be
capable of revolving round a in a plane parallel to that of the
circle, it will receive a motion of rotation in the same or in a con-
trary direction to that of the circulating current, accordingly as it
flows from b to a, or from a to b. If the straight current be limited
to b c, it will, under the same circumstances, receive rotation in the
contrary direction. If, in fine, it extends on both sides of the
circle, it will rotate in the one direction or the other, according
as the internal or external part predominates.
343. Reciprocal effects of curvilinear currents. — The
mutual influence of rectilinear and curvilinear currents being
understood, the reciprocal effects of curvilinear currents may be
easily traced. Each small part of such current may be regarded
as a short rectilinear current, and the separate effects of such ele-
mentary parts being ascertained, the effects of the entire extent of
the curvilinear currents will be the resultants of these partial
forces.
344. Mutual action of curvilinear currents in general. —
An endless variety of problems arises from the various forms that
curvilinear currents may assume, the various positions they may
have in relation to each other, and the various conditions which
may restrain their motions. The solution of all such problems,
however, presents no other difficulties than those which attend the
due application of the geometrical and mechanical principles,
already explained in each particular case.
To take as an example one of the most simple of the infinite
variety of forms under which such problems are presented, let the
214 VOLTAIC ELECTRICITY.
centres of two circular currents be fixed ; the planes of the cur-
rents being free to assume any direction whatever, they will turn
upon their centres until they come to the same plane, the parts of
the currents which intersect the line joining their centres flowing
in the same direction. It is evident that upon the least disturb-
ance from this position, they will be brought back to it by the
mutual attraction of the parts of the circles on the sides which are
near each other. This is therefore their position of stable equili-
brium, and it is evident that the fronts of the currents in this
position are on opposite sides of their common plane.
CHAP. IX.
VOLTAIC THEORY OF MAGNETISM.
345. Circulating: currents have tbe magnetic properties. —
From what has been proved, it is apparent that a helical current
has all the properties of a magnet. Such currents exert the same
mutual attraction and repulsion, have the same polarity, when
submitted to the influence of terrestrial magnetism have the same
directive properties, and exhibit the same phenomena of variation
and dip as are manifested by artificial and natural magnets. And
it is evident that these properties depend on the circulating and
not on the helical character of the current, inasmuch as the effect of
the progression of the helix being neutralised, by carrying the cur-
rent back in a straight direction along its axis, the phenomena
instead of being disturbed are still more regular and certain.
These properties of circulating currents have been assumed by
Ampere as the basis of his celebrated theory of magnetism, in
which all the magnetic phenomena are ascribed to the presence of
currents, circulating round the constituent molecules of natural
and artificial magnets, and round the earth itself.
Let a bar magnet be supposed to be cut by a plane at right
angles to its length. Every molecule in its section is supposed to
be invested by a circulating current, all these currents revolving
in the same direction, and consequently their fronts being pre-
sented to the same extremity of the bar. The forces exerted by
all the currents thus prevailing round the molecules of the same
section may be considered as represented by a single current cir-
culating round the bar; and the same being true of all the
transverse sections of the bar, it may be regarded as being sur-
rounded by a series of circulating currents all looking in the same
direction, and circulating round the bar. That end of the bar
THEORY OF MAGNETISM. 215
towards which the fronts of the currents are presented will liavc
the properties of a south or borenl pole, and the other end those
of a north or austral pole.
346. Magnetism of the earth may proceed from currents.
— In this theory the globe of the earth is considered to be tra-
versed by electric currents parallel to the magnetic equator. The
forces exerted by the currents circulating in each section of the
earth, like those in the section of an artificial magnet, are con-
sidered as represented by a single current equivalent in its effect,
and which is called the mean current of the earth, at each place
upon its surface. The magnetic phenomena indicate that the
direction of this mean current at each place is in a plane at right
angles to the dipping needle, and that it is directed in this plane
from east to west, and at right angles to the magnetic meridian.
347. Artificial magnets explained on this hypothesis. —
In bodies such as iron or steel, which are susceptible of magnetism,
but which are not magnetised, the currents which circulate round
the constituent molecules are considered to circulate in all pos-
sible planes and all possible directions, and their forces thus
neutralise each other. Such bodies, therefore, exert no forces of
attraction or repulsion on each other. But, when such bodies are
magnetised, the fronts of some or all of these currents are turned
in the same direction, and their forces, instead of being opposed,
are combined. The more perfect the magnetism is, the greater^
proportion of the currents will thus be presented in the same
direction, and the magnetisation will be perfect when all the
molecular currents are turned towards the same direction.
348. Effect of the presence or absence of coercive force.
— If the body thus magnetised be destitute of all coercive force,
like soft iron, the currents which are thus temporarily turned by
the magnetising agent in the same direction will fall into their
original confusion and disorder when the influence of that agent
is suspended or removed, and the body will consequently lose the
magnetic properties which had been temporarily imparted to it.
If, on the contrary, the body magnetised have more or less coer-
cive force, the accordance conferred upon the direction of the
molecular currents, is maintained with more or less persistence
after the magnetising agency has ceased ; and the magnetic pro-
perties accordingly remain unimpaired until the accordance of the
currents is deranged by some other cause.
349. [All the phenomena of the mutual action of magnets
and voltaic currents are explicable on this hypothesis. —
Although it may perhaps never be possible to prove by actual
demonstration the existence of these circulating molecular cur-
rents in magnetic bodies, the theory which supposes them to exist
216 VOLTAIC ELECTRICITY.
has received almost every other conceivable confirmation. It has
been proved by the most careful experiments that, in every case
of the mutual action of magnets and voltaic currents, the result
remains absolutely the same, not only in kind but in degree,
whether a magnet is used, or whether a current, such as upon this
theory is equivalent to it, be substituted for it. Ampere's theory
of magnetism must accordingly be considered as one of the most
remarkable theories in the whole range of physical science, for
the completeness with which it represents the phenomena it was
proposed to explain.]
CHAP. X.
REOSCOPES AND REOMETERS.
350. Instruments to ascertain the presence and to measure
the intensity of currents. — It has been shown that when a
voltaic current passes over a magnetic needle freely suspended, it
will deflect the needle from its position of rest, the quantity of this
deflection depending on the force, and its direction on the direction
of the current.
If the needle be astatic, and consequently have no directive
force, it will rest indifferently in any direction in which it may be
placed. In this case the deflecting force of the current will have
no other resistance to overcome than that of the friction of the
needle on its pivot ; and if the deflecting force of the current be
greater than this resistance, the needle will be deflected, and will
take a position at right angles to the current, its north pole being
to the left of the current (234., 236.)
If the needle be not astatic, it will have a certain directive
force, and, when not deflected by the current, will place itself in
the magnetic meridian. If, in this case, the wire conducting the
current be placed over and parallel to the needle, the poles will be
subject at once to two forces ; the directive force tending to keep
them in the magnetic meridian, and the deflecting force of the
current tending to place them at right angles to that meridian.
They will, consequently, take an intermediate direction, which
will depend on the relation between the directive and deflecting
forces. If the latter exceed the former, the needle will incline
more to the magnetic east and west ; if the former exceed the
latter, it will incline more to the magnetic north and south. If
these forces be equal, it will take a direction at an angle of 45°
REOSCOPES AND REOMETERS. 217
with the magnetic meridian. The north pole of the needle will, in
all cases, be deflected to the left of the current (234.).
If while the directive force of the needle remains unchanged the
intensity of the current vary, the needle will be deflected at a
greater or less angle from the magnetic meridian, according as the
intensity of the current is increased or diminished.
351. Expedient for augmenting- the effect of a feeble cur-
rent. — It may happen that the intensity of the current is so feeble,
as to be incapable of producing any sensible deflection even on the
most sensible needle. The presence of such a current may, never-
theless, be detected, and its intensity measured, by carrying the
wire conducting it first over and then under the needle, so that
each part of the current shall exercise upon the needle a force
tending to deflect it in the same direction. By this expedient the
deflecting force exercised by the current on the needle is doubled.
Such an arrangement is represented mjig. 246. The wire passes
from n to z over, and from y to x
1 under the needle ; and it is evident,
from what has been explained (233.,
234.), that the part z n and the
part y x exercise deflecting forces
in the same direction on the poles of
the needle, both tending to deflect
the north or austral pole a to the~
Fig. 246. left of a person who stands at z and
looks towards n. It may be shown
in like manner that the vertical parts of the current g x and y z
have the same tendency to deflect the north pole a to the left of a
person viewing it from z (236.)
352. Method of constructing- a reoscope, galvanometer,
or multiplier. — The same expedient may be carried further.
The wire upon which the current passes may be carried any number
of times round the needle, and each successive coil will equally
augment its deflecting force. The deflecting force of the simple
current will thus be multiplied by twice the number of coils. If
the needle be surrounded with a hundred coils of conducting
wire, the force which deflects it from its position of rest will be
two hundred times greater than the deflecting force of the simple
current.
The wire conducting the current must in such case be wrapped
with silk or other nonconducting coating, to prevent the escape
of the electricity from coil to coil.
Such an apparatus has been called a multiplier, in consequence
of thus multiplying the force of the current. It has been also
218
VOLTAIC ELECTRICITY.
denominated a galvanometer, inasmuch as it supplies the means of
measuring the force of the galvanic current.
We give it by preference the name reoscope or reometer, as
indicating the presence and measuring the intensity of the
current.
To construct a reometer, let two flat bars of wood or metal be
Ing. 10.
united at the ends, so as to leave an open space between them of
sufficient width to allow the suspension and play of a magnetic
REOSGOPES AND REOMETERS. 219
needle. Let a fine metallic wire of silver or copper, wrapped with
silk, and having a length of eighty or a hundred feet, be coiled
longitudinally round these bars, leaving at its extremities three or
four feet uncoiled, so as to be conveniently placed in connection
with the poles of the voltaic apparatus from which the current
proceeds. Over the bars on which the conducting wire is coiled,
is placed a dial, upon which an index plays, which is connected
with the magnetic needle suspended between the bars, and which
has a common motion with it, the direction of the index always
coinciding with that of the needle. The circle of the dial is
divided into 360°, the index being directed to o° or 1 80°, when
the needle is parallel to the coils of the conducting wire.
Such an instrument, mounted in the usual manner and covered
by a bell glass to protect it from the disturbance of the air, is
represented in fig. 247., and in another form, with its appendages
more complete, in fig. 248.
The needle is usually suspended by a single filament of raw silk
If the length of wire necessary for a single coil be six inches, fifty
feet of wire will suffice for a hundred coils. To detect the presence
of very feeble currents, however, a much greater number of coils
is frequently necessary, and in some instruments of this kind
there are several thousand coils of wire.
353. Nobili's reometer. — Without multiplying inconveniently
the coils of the conducting wire, Nobili contrived a reoscope which
possesses a sensibility sufficient for the most delicate experimental
researches. This arrangement consists of two magnetic needles
fixed upon a common centre parallel to each other, but with their
poles reversed as represented in fig. 249. If the directive forces
of these needles were exactly equal,
such a combination would be astatic ;
and although it would indicate the
presence of an extremely feeble cur-
rent, it would supply no means of
* measuring the relative forces of two
such currents. Such an apparatus
would be reoscopic, but not reometric.
n
Fig. Z49. To impart to it the latter property
and at the same time to confer on it
a high degree of sensibility, the needles are rendered a little, and
but a little, unequal in their directive force. The directive force
of the combination, being the difference of the directive forces of
the two needles, is therefore extremely small, and the system is
proportionately sensitive to the influence of the current.
354. Differential reometer. — In certain researches a differen-
tial reometer i8 found useful. In this apparatus two wires of
220
VOLTAIC ELECTRICITY.
exactly the same material and diameter are coiled round the
instrument, and two currents are made to pass in opposite direc-
tions upon them so as to exercise opposite deflecting forces on the
needle. The deviation of the needle in this case measures the
difference of the intensities of the two currents.
355. Great sensitiveness of these instruments illustrated.
— The extreme sensitiveness and extensive utility of these reoscopic
apparatus will be rendered apparent hereafter. Meanwhile it may
be observed that if the extremities p and n of the conducting wires
be dipped in acidulated water, a slight chemical action will take
place, which will produce a current by which the needle will be
visibly affected.
In all cases it is easy to determine the direction of the current
by the direction in which the north pole of the needle is deflected.
355a> [Pouillet's tangent galvanometer. — The instruments
above described are, in proportion to their sensibility to weak cur-
rents, incapable of indicating accurately the relative strengths of
powerful currents. Of the various instruments that have been de-
vised for the measurement of currents of high intensity, the simplest
and most generally applicable is the tange7it galvanometer of Pouillet.
The construction a*nd mode of action of this instrument will be
understood by reference to Jig. 249*7, where A B c D E represents a
ribbon or thick wire of cop-
per bent round so as to form
nearly a complete circle ;
in the centre of this circle
is a short magnet m, sus-
pended by a fibre of silk,
and attached to the upper
side of this is a light strip
of glass or wood, a 6, which
indicates, on a divided cir-
cle, the extent to which the
magnet is deflected. In
using the instrument, it is
placed so that the plane of
the circle BCD coincides,
as nearly as possible, with
the plane of the magnetic
meridian, and the current
•whose intensity is to be
measured is caused to cir-
culate round the circle by
connecting the extremities
A and E with the conductors Fis- *49 «•
TANGENT GALVANOMETER.
221
by which it is conveyed to and away from the apparatus. These
conductors are carried for some distance parallel to each other, and
as close together as convenient ; by this means the portion of the
current which is approaching the apparatus and the portion which
is leaving it are made to neutralise each other as to any effect they
might produce upon the magnet m. The current passing round the
circle BCD causes the magnet to be deflected to the right or the
left according to the direction in which it moves ; and, when the
magnet is short in comparison with the diameter of the circle, the
tangent of the angle through which it turns is proportional to the
intensity of the current. Hence the name of the instrument.
Let M M' (Jig- 2496.) be the magnetic
meridian ; A B the copper circle of the
galvanometer, as seen from above ; and
c D the suspended magnet (whose rela-
tive length is here exaggerated for the
sake of clearness). The effect of the
current circulating in A B is to cause the
magnet to deviate from the magnetic
meridian through the angle B o c = a.
In this position the forces which act
upon the magnet are in equilibrium.
These forces acting upon the pole c are :
1°, the horizontal component of the
earth's magnetism, acting in a line c E,
parallel to the magnetic meridian M M';
and 2°, the force exerted by the current,
which, according to what has been
already explained (2^1.) acts along
the line c F, perpendicular to the plane
of the current : c E and c F are therefore
perpendicular to each other. Let the directive force of the earth
upon the magnet be represented in amount by c E = t ; and the
force exerted by the current, a force which is proportional to its
intensity, by c F = i. Each of these forces can be resolved into
a component parallel to c D, the axis of the magnet, and a compo-
nent perpendicular thereto : namely, c E into b E and c b ; and c F
into a F and c a. The components b E and a F, acting parallel to
the needle, have no tendency to turn it either way about its centre;
therefore the only other two forces acting upon the pole, namely,
c b tending to diminish the angle of deflection a, and c a tending
to increase the deflection, must be equal to each other when the
needle is in equilibrium. But
c a = i . cos o and
c b — t . sin o,
222 VOLTAIC ELECTRICITY.
for the angles F c a and c E b are both, by construction, equal to a.
Therefore,
i . cos a = t . sin a,
sin a
< =
That is to say, the tangent of the angle through which the
magnet is deflected is proportional to the force exerted by the
current upon the pole c, and consequently to the intensity of the
current ; and, by analogous reasoning, the same would be found
to hold good for the force exerted upon the pole D.]
CHAP. XI.
PHOTOMAGNETISM AND DIAMAGNETISM.
356. Faraday's discovery. — About the year 1845 Dr. Faraday
made two beautiful discoveries, by one of which the phenomena of
magnetism have been placed in relation with those of light/ and by
the other the domain of magnetic power has been immensely en-
larged, by demonstrating its influence in various degrees over
almost all natural bodies, whatever be their physical state, whether
solid, liquid, or gaseous.
• 3 57. The photomaguetic phenomena, which have been deve-
loped by these remarkable researches, are briefly noticed in
Hand Book, " Optics," Chap. XII. We shall here, however, re-
sume the subject, and shall explain more fully the apparatus by
which the phenomena can be exhibited.
358. Apparatus for their exhibition. — Two rods of soft iron,
wrapped in the usual manner with covered wire, are mounted so
that their axes are horizontal and in the same direction as shown
in Jig. 250. An adjustment is provided, by means of which the
opposite poles F and E can, within certain limits, be moved to and
from each other. The axes of the two rods are perforated from
end to end, so that light can be transmitted without interrup-
tion from a to b. Any transparent body through which the light
is required to be transmitted for the purpose of experiment is
placed on a suitable stand rf, between E and r. At the extremity
THOTOMAGNETISH.
223
a of the axial perforation a polarising prism is placed, and at the
other extremity b an analysing prism is mounted so as to be capable
Fig. 250.
of being turned round the axis by an arm which carries an index
moving on a graduated circular plate as shown at j. By reference
to " Optics," Chap X., it will be seen that, by such a combination
of prisms, rays of light can be polarised and the direction of their
planes of polarisation determined. If the analysing prism b be
turned round its axis, the light which passes through it, supposing
it to be polarised, will be extinguished in two opposite positions of
the analysing prism, and will be seen with its full intensity in two
intermediate positions at right angles to these. The plane which
passes through the ray in the two latter positions is the plane of
polarisation.
It is shown in " Optics," Chap. XII., that a transparent medium
v/hich possesses the power of rotatory polarisation, will exert that
power in different degrees on the different component parts of
solar light; the planes of polarisation being turned more or less
from their original position, according as the light is more or less
refrangible. If a prism d of any transparent medium, having the
property of rotatory polarisation, be placed therefore between the
poles E and r, a polarised ray of compound solar light transmitted
through it will have its plane of polarisation changed in different
degrees by the prism d ; consequently the position in which the
analysing prism b would extinguish the different constituent rays
will be different. This circumstance will be attended with the
exhibition of a series of chromatic tints to an eye receiving the
224 VOLTAIC ELECTRICITY.
light at b. Thus, when the prism has that position in which the
index is at right angles to the plane of polarisation of the red
light that light will be extinguished, and the light received by the
eye will have the complementary tint. In like manner, when the
index is in the direction of the plane of polarisation of the blue
ray, the light transmitted will have the tint complementary to
blue, and so on.
These phenomena are purely optical, and have no reference to
the magnetic influence. We shall now see, however, how that
influence is capable of reproducing the same phenomena with
bodies which, in their natural state, have no rotatory polarisation.
For this purpose, after placing the body on which the experi-
ment is made, as described above at d, so that a ray of light trans-
mitted along the perforation of the soft iron rods shall pass through
it, a voltaic current is transmitted along the wire coiled upon the
rods, so as to render them magnetic. This is accomplished by the
apparatus shown in the figure in the following manner.
The current produced by a battery consisting of ten or twelve
pairs of Bunsen's arrangement, arriving by the wire B, is received
by the commutator G, from which it is transmitted, as indicated
by the arrow, to the wire coiled upon D, after passing round
which it goes along the wire g to the coils on c, after passing
which it issues along the wire h to the commutator, and thence
along the wire A to the negative pole of the battery.
By means of the commutator G the direction of the current
may be reversed at pleasure, so that it may be made to enter the
coils on c through the wire ^, to pass from c to D by the wire g,
and to issue from D to the commutator, and thence by B to
return to the battery.
By thus reversing the current the poles E and r can be made to
change their names at pleasure.
359. Photomagrnetic phenomena. — If a rod of flint glass, or,
better still, that particular sort of heavy glass used by Professor Fa-
raday, and described in " Optics " (305.), be placed at d, between the
poles E and r, and a polarised ray of homogeneous light be trans-
mitted through it, the direction of its plane of polarisation will be
determined by the analysing prism b. Let the index of that prism
be placed at right angles to the plane of polarisation, so that the
polarised ray will be extinguished. This being done, let the con-
nections of the conducting wires A and B with the battery be
established, so that the current may pass through the coils c and
D, and render the soft iron bars surrounding the ray magnetic.
The moment the current is thus re-established, the ray will be no
longer extinguished by the prism b in its actual position ; and to
PHOTOM AGNETISM. 2 2 5
extinguish it it will be necessary to turn the index, right or left,
through a certain angle.
If the current be reversed, the direction in which the index
must be turned to extinguish the ray must be also reversed.
Hence it appears that the current, or the magnetic virtue which
it imparts to the bars, exercises upon the ray of light, or upon
the transparent medium through which the ray passes, or upon
both of these, such an influence as to impart the power of rotatory
polarisation to the medium rf, and that this rotatory polarisation
is positive or negative, according to the position of the magnetic
poles E and F relatively to d.
The acquisition of this quality and its removal is absolutely
instantaneous. This is proved by the fact of the instantaneous
appearance and disappearance of the light at Z>, at the moment
when the connections forming the voltaic circuit are made and
broken.
360. Effects on polarised solar light. — If, instead of polarised
homogeneous light, a ray of polarised solar light be transmitted
through c?, ther light transmitted at &, while the current is esta-
blished, will not be extinguished in any position which can be
given to the index of the prism &, but a series of complementary
tints of coloured light will be transmitted as the index is moved
from one position to another. This is explained by the fact that
the rotatory power produced by the current, is different for the
different component parts of the solar light, the planes of pola-
risation of which being therefore turned through different angles,
they will be extinguished in different positions of the index ; and
when the index has such a position as will extinguish any one
ray, the complementary tint will be transmitted at b.
Since the original experiments made by Professor Faraday the
investigation has been pursued by M. Bertin, M. Pouillet, M. Ed-
mund Becquerel, and M. Matthiessen, from which it appears that,
besides the glass used by Faraday, many other substances, solid
and liquid, exhibit, in different degrees, like properties. Among
these the principal are the silicates of lead in gerieral, the flint
glass of commerce, rock salt, and common glass. And among
liquid substances, the bichloride of tin, the sulphuret of carbon,
water, olive oil and alcohol, and all aqueous and alcoholic solu-
tions.
361. Diamagnetic phenomena. — Dr. Faraday demonstrated,
at the epoch above mentioned, that a certain class of substances,
or rather bodies placed under certain physical conditions, without
being themselves magnetic, are repelled by sufficiently powerful
electro-magnets. To such substances he gave the name diamag-
226
VOLTAIC ELECTRICITY.
Fig.
netic, and the body of phenomena thus developed has accordingly
received the title of diamagnetism.
Bodies possess this remarkable property in all the three states,
solid, liquid, and gaseous.
The apparatus by which diamagnetic phenomena can be expe-
rimentally exhibited with greatest convenience and facility is that
which has been applied to the exhibition of the photomagnetic
effects, and which is represented in fig. 250.: to adapt it, how-
ever, to this purpose the poles E and r are so arranged that pieces
of soft iron of various forms, adapted to each class of experiments,
can be attached to them, so that these pieces, or their extremities,
become in fact the poles of the magnets.
362. Diamagrnetism of solids. — If two pieces of soft iron, s
and Q, conical in their form and rounded at the ends, be attached
to the poles, as shown in fig. 251., a small
ball of iron B or any other substance sus-
ceptible of magnetism, resting in contact with
them as shown in the figure, will adhere to
them with more or less force so long as the
current is transmitted through the coils, but
will be disengaged from them the moment the
current is suspended.
If a similar ball B of any diamagnetic sub-
stance, bismuth for example, be similarly suspended, it will be
repelled from the magnetic poles the moment the current is esta-
blished, and will continue to be so repelled so long as the current
continues to be transmitted. It will remain during such an in-
terval in the same manner as a pendulum would, if drawn from
the perpendicular and retained at the extremity of its arc of
vibration. The moment, however, the connections are broken,
and the current discontinued, the ball of
bismuth will fall down into contact with s
and Q as before.
If a small cube of copper m, be suspended
in the space between the magnetic poles, as
shown in fig. 252., and be made to revolve
rapidly by first twisting the thread by
which it is suspended and then letting it
untwist, its rotation will be suddenly
retarded the moment the poles s and Q are
rendered magnetic by the transmission of
the current, and the rotation will become
quicker the moment the connexions are
broken and the current discontinued.
If a small bar of any magnetic body, such as iron, be similarly
Fig. ajz.
DIAMAGNETISM. 227
suspended, as shown in^". 253., between the poles of the electro-
magnets, it will be brought to rest by their attraction in such a
position that its ends shall be presented
to the two poles, and consequently its
length is in the direction of the axes of
the magnets. This position Professor
Faraday has called the axial direction.
If a similar bar of bismuth or any
other diamagnetic body be similarly
Fig. 155. suspended, the position in which it will
be brought to rest by the repulsion of
the magnets, will be that in which its length is at right angles to
the axes of the magnets, a position to which Professor Faraday
gives the name of the equatorial direction.
Thus it appears that the influence of the magnets is to maintain
magnetic bodies in the axial, and diamagnetic bodies in the equa-
torial position.
363. Various diamagnetic bodies. — The number of diamag-
netic bodies is very considerable. Among the metals, bismuth is that
in which the property is more pronounced ; lead and zinc come
next, but their action is much more feeble. Among the metaloids
which manifest the property are phosphorus, selenium, and sul-
phur ; and among compound bodies water, alcohol, ether, spirit of
turpentine, most of the acids and saline solutions, wax, amber,
mother o' pearl, tortoise shell, quill, carbon, and many others.
Liquids are submitted to similar experiments by being enclosed
in small and very thin tubes of glass. When these tubes are sus-
pended as above described, they are found to assume the axial or
equatorial position, according as the liquid is magnetic or diamag-
netic.
364. Diamagnetism varies with the surrounding medium.
— Professor Faraday has shown that the properties of magnetism
and diamagnetism cannot be said to belong, in an absolute sense,
to all bodies, but that, on the contrary, the same body may be
magnetic or diamagnetic, according to the medium with which it is
surrounded ; and as that medium is changed, it will accordingly
assume alternately the axial or equatorial direction when sus-
pended between the magnetic poles. For example, if a weak solu-
tion of the protosulphate of iron, included in a thin glass tube,
be suspended between the magnetic poles, it will take the axial
direction ; if it be immersed in water when so suspended it will
still keep the axial direction ; but if immersed in a stronger solution
of the protosulphate of iron than that which is contained in the
tube, it will then take the equatorial direction, showing that it
Q i
228
Y^OLTAIC ELECTEICITF.
possesses the magnetic or diamagnetic property according to the
medium in which it is immersed.
365. Pluck er's apparatus.— In the prosecution of diamagnetic
researches M. Pliicker used an experimental apparatus somewhat
different in form from that shown in fig. 250., which was attended
Fig. z54.
with several advantages. This apparatus, which is represented in
fig. 254., consists of a large electro-magnet, similar to that shown
in Jig. 250., but having the legs vertical and the poles a and b con-
sequently not presented one to the other, but standing in the same
horizontal line. Upon a and b, as in the case of the apparatus
DIAMAGNETISM.
229
represented in jig. 250., polar pieces of soft iron of various forms,
according to the experiment to be performed, can be adapted.
These pieces, placed in various positions with relation to each
other, form a sort of horizontal magnetic area or field, in which
the bodies to be submitted to experiment are suspended by a
hook attached to a fine silver wire, having the properties of the
balance of torsion already described (61.). This magnetic stage
is covered and enclosed by a glass case, and when the hook is
not used for the measurement of torsional forces it is adapted
to support a very sensitive common balance, of which all the parts
are formed of gloss — the dishes being watch glasses.
When the dishes are filled with the liquid of which the mag-
netic or diamagnetic properties are sought, the equilibrium is esta-
blished before the current is transmitted, the dish containing the
liquid being suspended over the magnetic poles. Upon closing
the circuit the equilibrium no longer subsists, and the dish con-
taining the liquid is either attracted or repelled according as it
is magnetic or diamagnetic.
The coils surrounding the electro-magnet consist of several
distinct wires, two or more of which may be put in connection at
pleasure, so that the current may be transmitted upon them
without passing on th3 others. In this way the force of the
electro-magnet may be varied at will, while the intensity of the
current remains the same. The apparatus for making this ad-
justment is shown at n and n\ the commutator being at c.
366. The diamagrnetic properties of liquids can also be
exhibited in a remarkable manner by means of the apparatus
shown in jig. 250. For this purpose, pieces D and c of the form
shown in jig. 255. are attached to the poles, and the liquid under
Fig. 255.
experiment, contained in a watch glass, is placed upon them as
shown in the figure
230 VOLTAIC ELECTRICITY.
If a solution of chloride of iron be placed thus upon the arma-
tures D and c, as soon as the current is established the solution
will assume a convex form or two distinct convex forms, according
to the distance between the magnetic poles, as shown at A and B.
These forms will continue so long as the current is maintained ;
and the same forms will be assumed by all magnetic liquids.
The forms assumed by diamagnetic liquids, such as mercury,
will be the inverse of these.
This experiment can, however, be performed with still greater
convenience with the apparatus of M. Pliick er, shown in Jig. 254..
367. Diamag-netism of flame. — It was observed by M. Ban-
oalari that the flame of a candle placed between the poles of the
electro-magnet was repelled, as if blown by a current of air, while
the current was transmitted, as shown in Jig. 256. All flames
present the same phenomenon, but in different degrees. M. Quet
obtained such effects in a very decided manner by submitting the
electric light to the effects of the magnetic poles, as shown in
fig. 257.
Fig. 256. Fig. z57.
No satisfactory theory has yet been proposed to explain the
phenomena of diamagnetism. Various hypotheses have been
imagined, but none which has commanded any general assent.
Dr. Faraday ascribes the phenomena to induction, assuming that
in the diamagnetic body inductive currents are produced which
act by repulsion upon the voltaic currents to which, according to
the theory of Ampere, the magnetic virtue is due. MM. Edmund
Becquerel and Plucker have each proposed other hypotheses,
which suppose the diamagnetic bodies to be arrested by a magnetic
medium which exercises the power of repulsion.
THERMO-ELECTRICITY. 231
CHAP. XII.
THERMO-ELECTRICITY.
368. Disturbance of the thermal equilibrium of conductors
produces a disturbance of the electric equilibrium. — If a
piece of metal B, fig. 258., or
C - other conductor, be interposed
between two pieces, c, of a
different metal, the points of
contact being reduced to dif-
ferent temperatures, the na-
Fig> ls8> tural electricity at these points
will be decomposed, the posi-
tive fluid passing in one direction, and the negative fluid in the
other. If the extremities of the pieces c be connected by a wire,
a constant current will be established along such wire. The
intensity of this current will be invariable so long as the tempe-
ratures of the points of contact of n with c remain the same ; and
it will in general be greater, the greater the difference of these
temperatures. If the temperatures of the points of contact be
rendered equal, the current will cease. (See also 157.).
These facts may be verified by connecting the extremities of o
•with the wires of any reoscopic apparatus. The moment a dif-
ference of temperature is produced at the points of contact, the
needle of the reoscope will be deflected ; the deflection will in-
crease or diminish with every increase or diminution of the dif-
ference of the temperatures ; and if the temperatures be equalised,
the needle of the reoscope will return to its position of rest, no
deflection being produced.
369. Thermo-electric current. — A current thus produced is
called a thermo-electric current Those which are produced by the
ordinary voltaic arrangements are called for distinction hydro-elec-
tric currents, a liquid conductor always entering the combination.
370. Experimental illustration. — A convenient and simple
apparatus for the experimental illustration of a thermo-electric
current is represented in Jig. 259. A narrow strip of copper cd
is bent into a rectangular form, and soldered at both ends to a plate
of bismuth ee'. A magnetic needle a b moves freely on its pivot
within the rectangle. The apparatus is so placed, that its vertical
plane coincides with that of the magnetic meridian ; and the needle,
when undisturbed by the current, is at rest in the same direction.
Now, if a lamp /be applied to one end e of the plate of bismuth,
so as to raise its temperature above that of the other end, the
needle will be immediately deflected, and the deflection will in-
232 VOLTAIC ELECTRICITY.
crease as the difference of the temperatures of the ends of the plate
of bismuth is increased. If the end e of the bismuth be cooled to a
temperature below that of the surrounding atmosphere, the needle
will be deflected the other way, showing that the direction of the
current has been reversed. And by repeating the same experi-
ments with the other end e\ these results will be confirmed..
371* Conditions which determine the direction of the
current. — When the temperature of the end e of the bismuth is
more elevated than that of the end e', the north pole of the needle
is deflected to the left of a person standing at the end e, from which
it appears that the current flows round the rectangle in the di-
rection represented by the arrow.
If cold be applied to the end e, the needle will be deflected to
the right, showing that the direction of the current will be re-
versed, the positive fluid always flowing towards the warmer end
of the bismuth.
372. A constant difference of temperature produces a
constant current. — If means be taken to maintain the extremities
of the bismuth at a constant difference of temperature, the needle
will maintain a constant deflection. Thus, if one end of the
bismuth be immersed in boiling water and the other in melting
ice, so that their temperatures shall be constantly maintained at
212° and 32°, the deflection of the needle will be invariable.
If the temperature of the one be gradually lowered, and the other
gradually raised, the deflection of the needle will be gradually
diminished ; and when the temperatures are equalised, the needle
will resume its position in the magnetic meridian.
THERMO-ELECTRICITY.
233
373. Different metals have different thermo-electric ener-
gies. — This property, in virtue of which a derangement of the
electric equilibrium attends a derangement of the thermal equi-
librium, is common to all the metals, and, indeed, to conductors
generally ; but, like other physical properties, they are endowed
with it in very different degrees. Among the metals, bismuth and
antimony have the greatest thermo-electric energy, whether they
are placed in contact with each other, or with any other metal.
If a bar of either of these metals be placed with its extremities in
contact with the wires of a reometer, a deflection of the needle will
be produced by the mere warmth of the finger applied to one end
of the bar. If the finger be applied to both ends, the deflection will
be redressed, and the needle will return to the magnetic meridian.
It has been ascertained that if different parts of the same
mass of bismuth or antimony be raised to different temperatures,
the electric equilibrium will be disturbed, and currents will be
established in different directions through it, depending on the
relative temperatures. These currents are, however, much less
intense than in the case where the derangement of temperature is
produced at the points of contact or junction of different conductors.
3 74. Pouillet's thermo-electric apparatus. — M. Pouillet has
with great felicity availed himself of these properties of thermo-
electricity, to determine some important and interesting pro-
perties of currents. The apparatus constructed and applied by
him in these researches is represented in^. 261.
Fig. z6i
Two rods, A and B, of bismuth, each about sixteen inches in length and an
inch in thickness, are bent at the ends at right angles, and being supported
on vertical stands are so arranged that the ends c D and E F may be let down
234 VOLTAIC ELECTRICITY.
into cups. The cups c and E are filled with melting ice, and D and F with
boiling water, so that the ends c and E are kept at the constant temperature
of 32°, and the ends D and F at the constant temperature of 212°.
A differential reometer (354.) is placed at M. Two conducting circuits are
formed either of one or several wires, one commencing from F, and after passing
through the wire of the reometer M, returning to E; the other commencing
from D, and after passing through the wire of the reometer in a contrary
direction to the former, returning to c. The wires conducting the current
are soldered to the extremities c, D, E, F of the bismuth rods which are im-
mersed in the cups.
If the two currents thus transmitted, the one between F and E, and the
other between D and G, have equal intensities, the needle of the reometer M
will be undisturbed ; but if there be any difference of intensity, its quantity
and the wire on which the excess prevails will be indicated by the quantity
and direction of the deflection of the needle.
The successive wires along which the current passes are brought into
metallic contact by means of mercurial cups, a, b, c, d, &c., into which their
ends are immersed.
The circuits through which the current passes may be simple or compound.
If simple, they consist of wire of one uniform material and thickness. If
compound, they consist of two or more wires differing in material, thickness,
or length.
The wire composing a simple circuit is divided into two lengths, one ex-
tending from D or F to the cup e or d, where the current enters the convolu-
tions of the reometer, and the other extending from the cup b or/, where the
current issues from the reometer to c or E, where it returns to the thermo-
electric source. The wires composing a compound current may consist of a
succession of lengths, the current passing from one to another by means of
the metallic cups. Thus, as represented in the figure, the wires F c, c d, and
/ E, forming, with one wire of the reometer, one circuit, and the wires D e, b a,
and a c, forming with the other wire of the reometer the other circuit, may
differ from each other in material, in thickness, and in length.
The current^ pass, as indicated by the arrows, from the extremity of the
bismuth which has the higher temperature through the wires to the ex-
tremity which has the lower temperature.
375. Relation between the intensity of tne current and
the length and section of tne conducting1 wire. — If the two
circuits be simple and be composed of similar wires of equal
lengths, the intensity of the two currents will be found to be equal,
the needle of the reometer being undisturbed. But if the length
of the circuit be greater in the one than in the other, the inten-
sities will be unequal, that current which passes over the longest
wire having a less intensity in the exact proportion in which it has
a greater length.
If the section of the wire composing one circuit be greater than that of the
wire composing the other circuit, their lengths being equal, the current
carried by the wire of greater section will be more intense than the other in
exactly the proportion in which the section is greater.
If the wire composing one of two simple circuits have a length less than
that composing the other, and a section also less in the same proportion than
the section of the other, the currents passing over them will have the same
CONDUCTIVITY OF METALS. 235
intensity, for the excess of intensity due to the lesser length of the one is
compensated by the excess due to the greater section of the other.
In general, therefore, if I and I' express the intensities of the two currents
transmitted from D and F (fig. 261.) over two simple circuits of wire of the
same metal, whose sections are respectively s and s', and whose lengths are
L and L', we shall have : —
•t*tt-;S|
L L''
that is to say, the intensities are directly as the sections and inversely as the
lengths of the wire.
If two simple circuits be compared, consisting of wires of different metals
this proportion will no longer be maintained, because in that case wires of
equal length and equal section will no longer give the currents equal inten-
sities, because they will not have equal conducting powers. That circuit
which, being alike in other respects, is composed of the metal of greatest
conducting power, will give a current of proportionally greater intensity.
The relative intensities, therefore, of the currents carried by wires of different
metalg of equal length and thickness are the exponents of the relative con-
ducting powers of these metals.
In general, if c and c' express the conducting powers of the metals com-
posing two simple circuits, we shall have : —
i:i'::cx- : c' x— •
L L
376. [Conducting powers of metals. —The statements of
various experimenters respecting the relative conducting powers
of different metals often differ very considerably. This is to
be attributed in part to the imperfections of the methods em-
ployed, but also in great measure to the great relative influence
exerted upon the conducting powers of the metals by small im-
purities. An extensive series of experiments, in which great
care was taken to guard against this source of error, has been
made by Dr. Matthiessen, whose results are, therefore, pro-
bably the most trustworthy that have been yet obtained. The
following are the conducting powers found by him for several
metals, compared with that of silver taken as loo. In each
case, except where the contrary is stated, the temperature of the
wires is supposed to be the freezing point : —
Metal..
Silver
Copp
Gold
Cadn
Zinc
The method by which these determinations were made is a
modification of that described in the next paragraph.]
377. [Wneatstone's method of measuring: conducting
powers. — Another method of comparing the conducting powers
of different substances, much more accurately than it can be done
by that above described, has been proposed by Professor Wheat-
stone. The principle of this method may be thus stated. Let B
,
Conducting Powers.
Metali.
Conducting Pow«rt.
Iron ...
Lead
(atao-4°C.) 144
99 9
237
Platinum -
fat zo'7° C.) 10-5
1-61
236 VOLTAIC ELECTRICITY.
(Jig 26 1 a.) be a galvanic battery, the poles of which are connected
•with the angles Y and z of the irregular parallelogram u z v Y, and
G a delicate reometer connected with the angles u and v of the
parallelogram. The portions of the parallelogram formed by
thick black lines in the figure are made of copper wires, so thick
Fig. z6ia.
that they offer no perceptible resistance to the passage of the cur-
rent ; the irregular curves A, c and E represent portions of wire
whose conducting power is known ; and s represents the wire to be
examined. Now, by the laws which determine the passage of
currents along the several branches of a conductor when it
divides into two or more, it follows that if the conducting powers,
or resistances, of A, c, R, and s are such that
A : c : : s : R,
no current will circulate between the points u and v, and conse-
quently the reometer will not be affected. But if the resistances
of these four conductors stand in any other proportion to each
other, a current will pass through the reometer either from u to
v, or from v to u. But the conductor R being so constructed that
its length, and consequently its resistance (375.)* can be increased
or diminished at pleasure by a known amount, the proportion
A : c :: s : R
can always be obtained ; and hence, A, c and R being known it is
easy to calculate s. In practice it is most convenient, when it can
be done, to take A = c, in which case s = R.]
3770. [Tbe reostat.— An instrument whereby a resistance of
known amount, capable of being increased or diminished at will
by a known quantity, as the resistance R in (377.)? can be intro-
duced into the path of a current, is called a rsostat. Such instru-
KEOSTAT. 237
ments are constructed in various forms, one of the commonest of
which is represented in fig. 2616. This consists of two parallel
rollers, A and B, the former of brass, and the latter of wood, upon
which a piece of German-silver wire is wound in such a way that,
when the handle c is turned in one direction, it winds off B and
on to A, and when the handle is turned in the opposite direction, it
winds off A on to B. The current arrives at and leaves the appa-
ratus by wires connected with the binding screws D and E, of
which D is in electrical communication with the brass roller A,
and so with the end of the wire fixed to it, while B (similarly
situated at the other side of the apparatus, but not shown in the
figure), communicates with the end of the wire fixed to the
wooden roller B. Accordingly, when all the wire is wound upon the
Fig. z6i6.
roller B, the current arriving at one end by one of the binding screws
— say D — must traverse the whole length of the wire before it can
arrive at the other ; but if some of the wire is wound on to A, the
current will not need to traverse this portion, an easier passage
being made for it up to the point where the wire quits the roller
A by the metal of the roller itself. Thus, by winding more wire
upon A, we diminish the resistance which the current encounters in
its passage from D to E or from E to D ; and by winding more wire
upon B we increase the resistance. A simple measuring arrange-
ment shows what proportion of the whole length of the wire the
current has to traverse in any position of the apparatus.]
378. Equivalent simple circuit. — A simple circuit composed
of a wire of any proposed metal and of any proposed thickness
can always be assigned upon which the current would have the
same intensity as it has on any given compound circuit ; for by
increasing the length of such circuit the intensity of the current
may be indefinitely diminished, and by diminishing its length the
intensity may be indefinitely increased. A length may therefore
be always found which will give the current any required intensity.
The length of such a standard wire which would give the
23 8 VOLTAIC ELECTRICITY
current of a simple circuit the same intensity as that of a com-
pound circuit, is called the reduced length of the compound
circuit.
379. Ratio of intensities in two compound circuits. — It
is evident, therefore, that the intensities of the currents on two
compound circuits are in the inverse ratio of their reduced lengths,
for the wires composing such reduced lengths are supposed to be
of the same material and to have the same thickness.
380. Intensity of the current on a given conductor varies
with the thermo-electric energy of the source. — In all that
has been stated above, we have assumed that the source of thermo-
electric agency remains the same, and that the changes of in-
tensity of thte current are altogether due to the greater or less
facility with which it is allowed to pass along the conducting
wires from one pole of the thermo-electric source to the other.
But it is evident, that with the same conducting circuit, whether
it be simple or compound, the intensity of the current will vary
either with the degree of disturbance of the thermal equilibrium
of the system or with the thermo-electric energy of the substance
composing the system.
In the case already explained, the ends of the cylinders A and
B have been maintained at the fixed temperatures of 32° and
212°. If they had been maintained at any other fixed tempera-
tures, like phenomena would have been manifested ; with this
difference only, that with the same circuit the intensity of the
current would be different, since it would be increased if the
difference of the temperature of the extremities were increased,
and would be diminished if that difference were diminished.
In like manner, if, instead of bismuth, antimony, zinc, or any
other metal were used, the same circuit and the same tempera-
tures of the ends c and D or B and r would exhibit a current
of different intensity, such difference being due to the different
degree of thermo-electric agency with which the different metals
are endowed.
The relative thermo-electric agency of different sources of these
currents, whether it be due to a greater or less disturbance of the
thermal equilibrium, or to the peculiar properties of the substance
whose temperature is deranged, or, in fine, to both of these causes
combined, is in all cases proportional to the intensity of the
current which it produces in a wire of given material, length, and
thickness, or in general to the intensity of the current it transmits
through a given circuit.
The relative thermo-electric energy of two systems may be
ascertained by placing them as at A and B, Jig. 261., and con-
necting them by simple circuits of similar wire with the diffe-
THERMO-ELECTRICITY. 239
rential reometer. Let the lengths of the wires composing the two
circuits be so adjusted, that the currents passing upon them shall
have the same intensity. The thermo-electric energy of the two
systems will then be in the direct ratio of the lengths of the
circuits.
381. Thermo-electric piles. — The intensity of a thermo-
electric current may be augmented indefinitely, by combining
together a number of similar thermo-electric elements, in a
manner similar to that adopted in the formation of a common
voltaic battery. It is only necessary, in making such arrange-
ment, to dispose the elements so that the several partial currents
shall all flow in the same direction.
Such an arrangement is represented in fig. 262., where the (wo metals
(bismuth and copper, for example) composing each thermo-electric pair
are distinguished by the thin and thick bars. If the points of junction
marked i, 3, 5, &c. be raised to 212°, while the points 2, 4, 6, &c. are kept
at 32°, a current will flow from each of the points i, 3, 5, &c. towards the
points 2, 4, 6, &c. respectively, and these currents severally overlaying each
other, exactly as in the voltaic batteries, will form a current having the
sum of their intensities.
382. Thermo-electric pile of Nobili and Melloni. — Various
expedients have been suggested for the practical construction of
such thermo-electric piles, one of the most efficient of which is
that of MM. Nobili and Melloni.
This pile is composed of a series of thin plates of bismuth and antimony
bent at their extremities, so that when soldered together they have the
form and arrangement indicated in fig. 263. The spaces between the suc-
cessive plates are filled by pieces of pasteboard, by which the combination
acquires sufficient solidity, and the plates are retained in their position
without being pressed into contact with each other. The pile thus formed
is mounted in a frame as represented in fig. 264., and its poles are connected
with two pieces of metal by which the current may be transmitted to any
conductors destined to receive it. It will be perceived that all the points
of junction of the plates of bismuth and antimony, which are presented at the
same side of the frame, are alternate in their order, the ist, 3rd, 5th, &c.
being on one side, and the 2nd, 4th, 6th, &c. on the other. If, then, one
side be exposed to any source of heat or cold from which the other is re-
moved, a corresponding difference of temperature will be produced at the
alternate joints of the metal, and a current of proportionate intensity will
240
VOLTAIC ELECTRICITY,
flow between the poles o and p upon any conductor by which they may be
connected.
Fig.z6?.
Fig. 264.
It is necessary, in the practical construction of this apparatus, that the
metallic plates composing it should be all of the same length, so that when
combined the ends of the system where the metallic joints are collected
should form an even and plain surface, which it is usual to coat with lamp-
black, so as to augment its absorbing power, and at the same time to render
it more even and uniform.
The form of electric pile used by Melloni in his experiments on
radiant heat, has been already described in "Heat" (577-)> and
represented there in jig. 281. Another view of the apparatus,
differently arranged, is given in Jig. 265., where F andE are the
Fig. z6S.
screens, D the stage upon which the bodies under experiment are
placed, H the thermometric pile, c the galvanometer, and A and u
the polar wires of the pile.
ELECTRO-CHEMISTRY.
^jrlSl
Librar
CHAP. XIII.
.Or
ELECTRO-CHEMISTRY.
383. Decomposing power of a voltaic current — When a
voltaic current of sufficient intensity is made to pass through cer-
tain bodies consisting of constituents chemically combined, it is
found that decomposition is produced attended by peculiar cir-
cumstances and conditions. The compound is resolved into two
constituents, which appear to be transported in contrary di-
rections, one with and the other against the course of the current.
The former is disengaged at the place where the current leaves,
and the other at the place where it enters, the compound.
All compounds are not resolvable into their constituents by this
agency, and those which are, are not equally so ; some being re-
solved by a very feeble current, while others yield only to one of
extreme intensity.
384. Electrolytes and electrolysis. — Bodies which are capa-
ble of being decomposed by an electric current have been called
electrolytes, and decomposition thus produced has been denominated
electrolysis.
385. Liquids alone susceptible of electrolysis. — To render
electrolysis practicable, the molecules of the electrolyte must have
a perfect freedom of motion amongst each other. The electrolyte
must therefore be liquid. It may be reduced to this state either
by solution or fusion.
386. Faraday's electro-chemical nomenclature. — It has
been usual to apply the term poles either to the terminal elements
of the pile, or to the extremities of the wire or other conductor by
which the current passes from one end and enters the other.
These are not always identical with the points at which the current
enters and leaves an electrolyte. The same current may pass
successively through several electrolytes, and each will have its
point of entrance and exit ; but it is not considered that the same
current shall have more than two poles. These and other con-
siderations induced Dr. Faraday to propose a nomenclature for
the exposition of the phenomena of electrolysis, which has to some
extent obtained acceptation.
387. Positive and negative electrodes. — He proposed to
call the points at which the current enters and departs from the.
electrolyte, electrodes, from the Greek word 6065 (hodos), a path or
way. He proposed further to distinguish the points of entrance
242 VOLTAIC ELECTRICITY.
and departure by the terms Anode and Kathode, from the Greek
words &vo5os (anodos), the way up, and uddoSos (kathodes), the way
down.
388. Only partially accepted. — Dr. Faraday also gave the
name ions to the two constituents into which an electrolyte is
resolved by the current, from the Greek word <W (ion), going or
passing, their characteristic property being the tendency to pass
to the one or the other electrode. That which passes to the posi-
tive electrode, and which therefore moves against the current, he
called the Anion ; and that which passes to the negative electrode
and therefore moves with the current, he called the Ration.
These terms have not, however, obtained acceptation; Neither
have the terms " Anode " and " Kathode," positive and negative
electrode, or positive and negative pole, being almost universally
preferred.
The constituent of an electrolyte which moves with the current
is distinguished as the positive element, and that which moves
against it as the negative element. These terms are derived from
the hypothesis that the constituent which appears at the positive
electrode, and which moves, or seems to move, towards it after de-
composition, is attracted by it as a particle negatively electrified
would be ; while that which appears at the negative electrode is
attracted to it as would be a particle positively electrified.
389. Composition of water. — To render intelligible the pro-
cess of electrolysis, let us take the example of water, the first sub-
stance upon which the decomposing power of the pile was observed.
Water is a binary compound, whose simple constituents are the.
gases called oxygen and hydrogen. Nine grains weight of water
consist of eight grains of oxygen and one grain of hydrogen.
The specific gravity of oxygen being sixteen times that of
hydrogen, it follows that the volumes of these gases which com-
pose water are in the ratio of two to one ; so that a quantity of
water which contains as much oxygen as, in the gaseous state,
would have the volume of a cubic inch, contains as much hydrogen
as would, under the same pressure, have the volume of two cubic
inches.
The combination of these gases, so as to convert them into
water, is determined by passing the electric spark taken from a
common machine through a mixture of them. If eight parts by
weight of oxygen and one of hydrogen, or, what is the same, one
part by measure of oxygen and two of hydrogen, be introduced
into the same receiver, on passing through them the electric spark
an explosion will take place ; the gases will disappear, and the
receiver will be filled first with steam, which being condensed, will
be presented in the form of water. The weight of water con-
ELECTRO-CHEMISTRY. 243
tained in the receiver will be equal precisely to the sum of the
weights of the two gases.
These being premised, the phenomena attending the electrolysis
of water may be easily understood.
390. Electrolysis of water. — Let a glass tube, closed at one
end, be filled with water slightly acidulated, and, stopping the open
end, let it be inverted and immersed in similarly acidulated water
contained in any open vessel. The column in the tube will be
sustained there by the atmospheric pressure, as the mercurial
column is sustained in a barometric tube ; but in this case the tube
will remain completely filled, no vacant space appearing at the top,
the height of the column being considerably less than that which
would balance the atmospheric pressure. Let two platinum
wires be connected with the poles of a voltaic pile, and let their
extremities, being immersed in the vessel containing the tube, be
bent so as to be presented upwards in the tube without touching
each other. Immediately small bubbles of gas will be observed to
issue from the points of the wires, and to rise through the water
and collect in the top of the tube, and this will continue until the
entire tube is filled with gas, by the pressure of which the water
will be expelled from it. If the tube be now removed from the
vessel, and the gas be transferred to a receiver, so arranged that the
electric spark may be transmitted through it, on such transmission
the gas will be reconverted into water.
The gases, therefore, evolved at the points of the wires, which in
this case are the electrodes, are the constituents of water; and
since they cannot combine to form water, except in the definite
ratio of I to 2 by measure, they must have been evolved in that
exact proportion at the electrodes.
391. Explanation of this phenomenon by the electro-che-
mical hypothesis. — This phenomenon is explained by the sup-
position that the voltaic current exercises forces directed upon
each molecule of the water, by which the molecules of oxygen are
impelled or attracted towards the positive electrode, and therefore
against the current, and the molecules of hydrogen towards the
negative electrode, and therefore with the current. The electro-
chemical hypothesis is adopted by different parties in different
senses.
According to some, each molecule of oxygen is invested with
an atmosphere of negative, and each molecule of hydrogen with
an atmosphere of positive electricity, which are respectively in-
separable from them. When these gases are in their free and
uncombined state, these fluids are neutralised by equal doses of the
opposite fluids received from some external source, since other-
B 2
244 VOLTAIC ELECTRICITY.
wise they would have all the properties of electrified bodies,
which they are not observed to have. But when they enter into
combination, the molecule of oxygen dismisses the dose of posi-
tive electricity, and the molecule of hydrogen the dose of negative
electricity which previously neutralised their proper fluids ; and
these latter fluids then exercising their mutual attraction, cause
the two gaseous molecules to coalesce and to form a molecule of
water.
When decomposition takes place, a series of opposite effects
are educed. The molecule of oxygen after decomposition is
charged with its natural negative, and the molecule of hydrogen
with its natural positive fluid, and these molecules must borrow
from the decomposing agent or some other source, the doses of
the opposite fluids which are necessary to neutralise them. In
the present case, the molecule of oxygen is reduced to its natural
state by the positive fluid it receives at the positive electrode, and
the molecule of hydrogen by the negative fluid it receives at the
negative electrode.
The electro-chemical hypothesis is, however, differently under-
stood and differently stated by different scientific authorities. It
is considered by some that the decomposing forces in the case of
the voltaic current, are the attractions and repulsions which the
two opposite fluids developed at the electrodes exercise upon the
atmospheres of electric fluid, which are assumed in this theory to
surround and to be inseparable from the molecules of oxygen and
hydrogen which compose each molecule of water, the resultants of
these attractions and repulsions being two forces, one acting on
the oxygen and directed towards the positive electrode, and the
other acting on the hydrogen and directed towards the negative
electrode. Others, with Dr. Faraday, deny the existence of these
attractions, and regard the electrodes as mere paths by which the
current enters and leaves the electrolyte, and that the effect of
the current in passing through the electrolyte is to propel the
molecules of oxygen and hydrogen in contrary directions, the
latter in the direction of the current, and the former in the con-
trary direction ; and that this combined with the series of decom-
positions and recompositions imagined by Grotthus, which we
shall presently explain, supplies the most satisfactory exposition
of the phenomena.
Our limits, however, compel us to dismiss these speculations,
and confine our observations rather to the facts developed by
experimental research, using, nevertheless, the language derived
from the theory for the purposes of explanation.
392. Method of electrolysis which separates the consti-
tuents.— The process of electrolysis may be so conducted that
ELECTRO-CHEMISTRY.
24?
v
JFft
the constituent gases shall be developed and collected in separate
receivers.
The apparatus represented in fg. 266., contrived by Mitscherlich, is very
convenient for the exhibition of this and other elec-
trolytic phenomena. Two glass tubes o and h, about
half an inch in diameter, and 6 or 8 inches in length,
are closed at the top and open at the bottom, having
two short lateral tubes projecting from them, which
are stopped by corks, through which pass two plati-
num wires which terminate within the tubes in a
small brush of platinum wire, which may with advan-
tage be surrounded at the ends with spongy platinum.
The tubes o h, being uniformly cylindrical and con-
veniently graduated, are filled with acidulated water,
and immersed in a cistern of similarly acidulated
water g.
If the external extremities of the platinum wires be
connected by means of binding screws a and b, or by
mercurial cups with wires which proceed from the
Fig. z66.
poles of a voltaic arrangement, their internal extremities will become elec-
trodes, and electrolysis will commence. Oxygen gas will be evolved from
the positive, and hydrogen from the negative electrode, and these gases will
collect in the two tubes, the oxygen in the tube o containing the positive,
and the hydrogen in the tube h containing the negative electrode. The
graduated scales will indicate the relative measures of the two gases evolved,
and il will be observed that throughout the process the quantity of gas in the
tube h is double the quantity in the tube o. If the gases be removed from
the tubes to other receivers and submitted to chemical tests, one will be
found to be oxygen and the other hydrogen.
393. How are the constituents transferred to the elec-
trodes 1 — In the apparatus^. 266., the tubes containing the
electrodes are represented as being near together. The process
of electrolysis, however, will equally ensue when the cistern g is a
trough of considerable length, the tubes o and h being at its ex-
tremities. It appears, therefore, that a considerable extent of
liquid may intervene between the electrodes without arresting
the process of decomposition. The question then arises, where
does the decomposition take place ? At the positive electrode, or
at the negative electrode, or at what intermediate point ? If it take
place at the positive electrode, a constant current of hydrogen
must flow from that point through the liquid to the negative
electrode ; if at the negative electrode, a like current of oxygen
must flow from that point to the positive electrode ; and if at any
intermediate point, two currents must flow in contrary directions
from that point, one of oxygen to the positive, and one of hydrogen
to the negative electrode. But no trace of the existence of any
such currents has ever been found. Innumerable expedients
have been contrived to arrest the one or the other gas in its pro-
246 VOLTAIC ELECTRICITY.
gress to the electrode without success ; and therefore the strongest
physical evidence supports the position that neither of these con-
stituent gases does actually exist in the separate state at any part
of the electrolyte, except at the very electrodes themselves, at
which they are respectively evolved.
If this be assumed, then it will follow that the molecules of
oxygen and hydrogen evolved at the two electrodes, were not
previously the component parts of the same molecule of water.
The molecule of oxygen evolved at the positive electrode must
be supplied by a molecule of water contiguous to that electrode,
while the molecule of hydrogen simultaneously evolved at the
negative electrode must have been supplied by another molecule
of water contiguous to the latter electrode. What then becomes
of the molecule of hydrogen dismissed by the former, and the
molecule of oxygen dismissed by the latter ? Do they coalesce
and form a molecule of water ? But such a combination would
again involve the supposition of currents of gas passing through the
electrolyte, of the existence of which no trace has been observed.
394. Solution on the hypothesis of Grotthus. — The only
hypothesis which has been proposed presenting any satisfactory
explanation of the phenomena is that of Grotthus, in which a
series of decompositions and recompositions are supposed to take
place between the electrodes.
Let OH, O'H', O"H", &c., represent a series of molecules of water ranged
between the positive electrode p and the negative electrode s.
P. . . O H. . . O' H' . . . O" H" . . . 0"'H'" . . . 0'"'H"" . . . N.
When o H is decomposed and o is detached in a separate state at p, the
positive fluid inseparable from H, according to the electro -chemical hypo-
thesis, being no longer neutralised by an opposite fluid, attracts the negative
fluid of o7, and repels the positive fluid of H', and decomposing the molecule
of water O'H', the molecule o' coalesces with H, and forms a molecule of
water. In like manner, H' decomposes o'' H", and combines with o'' ; H'' de-
composes o*" H'", and combines with o'" ; and H'" decomposes o"" H"", and
combines with o"" ; and, in fine, H"" is disengaged at the negative electrode
N. Thus, as the series of decompositions and recompositions proceeds, the
molecules of oxygen are disengaged at the positive electrode p, and those of
hydrogen at the negative electrode N.
In this hypothesis it is further supposed, as already stated, that the
molecule of oxygen o, disengaged at the positive electrode p, receives from
that electrode a dose of positive electricity, which being equal in quantity
to its own proper negative electricity, neutralises it ; and, in like manner,
the molecule of hydrogen H"", disengaged at the negative electrode N,
receives from it a corresponding dose of negative electricity which neutralises
its own positive electricity. It is thus that the two gases, when liberated at
the electrodes, are in their natural and unelectrified state.
395. Effect of acid and salt on the electrolysis of water.
— In the electrolysis of water as described above, the acid held in
ELECTEO-CHEMISTRY. 247
solution undergoes no change. It produces, nevertheless, an im-
portant influence on the development of the phenomena. If the
electrodes be immersed in pure water, decomposition will only be
produced when the current is one of extraordinary intensity.
But if a quantity of sulphuric acid even so inconsiderable as one
per cent, be present, a current of much less intensity will effect
the electrolysis ; and by increasing the proportion of the acid
gradually from one to ten or fifteen per cent, the decomposition
will require a less and less intense current.
It appears, therefore, that the acid without being itself affected
by the current, renders the water more susceptible of decompo-
sition. It seems to lessen the affinity which binds the molecules
of oxygen and hydrogen, of which each molecule of water consists.
Various other acids and salts soluble in water produce the same
effect.
The electrolyte, properly speaking, is therefore in these cases
the water alone. The bath in which the electrodes are immersed,
and in which the phenomena of the electrolysis are developed,
may contain various substances in solution ; but so long as these
are not directly affected by the current, they must not be con-
sidered as forming any part of the electrolyte, although they not
only influence the phenomena as above stated, but are also involved
in important secondary phenomena, as will presently appear.
The process of the electrolysis of water has been presented here
in its most simple form, no other effect save the mere decompo-
sition of the electrolyte being educed. If, however, the platinum
electrodes which have no sensible affinity for the constituents of
water be replaced by electrodes composed of any metal having a
stronger affinity for oxygen, other phenomena will be developed.
The oxygen dismissed by the water at the positive electrode,
instead of being liberated, will immediately enter into combination
with the metal of the electrode, forming an oxide of that metal.
This oxide may adhere to the electrode, forming a crust upon it.
In that case, if the oxide be a conductor, it will itself become the
electrode. If it be not a conductor it will impede and finally
arrest the course of the current, and put an end to the electrolysis.
If it be soluble in water it will disappear from the electrode as fast
as it is formed, being dissolved by the water ; and in that case the
water will become a solution of the oxide, the strength of which
will be gradually increased as the process is continued.
If the water composing the bath hold an acid in solution, for
which the oxide thus formed at the positive electrode has an
affinity, the oxide will enter into combination with the acid, arid
will form a salt which will either be dissolved or precipitated, ac-
cording as it is soluble or not in the bath.
248 VOLTAIC ELECTRICITY.
While the oxygen disengaged from the water at the positive
electrode undergoes these various combinations, the hydrogen is
frequently liberated in the free state at the negative electrode,
and may be collected and measured. In such case it will always
be found that the quantity of the hydrogen developed at the
negative electrode, is the exact equivalent of the oxygen which
has entered into combination with the metal at the positive elec-
trode, and also that the quantity of the metal oxidated is exactly
that which corresponds with the quantities of the two gases which
are disengaged, and with the quantity of water which is decom-
posed.
396. Secondary action of the hydrogen at the negative
electrode. — In some cases the hydrogen is not developed in the
form of gas at the negative electrode, but in its place the pure
metal, which is the base of the oxide dissolved in the bath, is
deposited there. In such cases the phenomena become more
complicated, but nevertheless sufficiently evident. The hydro-
gen developed at the negative electrode, instead of being dis-
engaged in the free state, attracts the oxygen from the oxide, and
combining with it forms water, liberating at the same time the
metallic base of the oxide which is deposited on the negative
electrode.
Thus there is in such cases both a decomposition and a recom-
position of water. It is decomposed at the one electrode to pro-
duce the oxide, and recomposed at the other electrode to reduce
or decompose the same oxide.
397. Its action on bodies dissolved in the bath. — This effect
of the hydrogen developed at the negative electrode is not limited
to the oxide or salt produced by the action of the positive elec-
trode. It will equally apply to any metallic oxide or salt which
may be dissolved in the bath. Thus, while the oxygen may be
disengaged in a free state and collected in the gaseous form over
the positive electrode, the hydrogen developed at the negative
electrode may reduce and decompose any metallic salt or oxide,
which may have been previously dissolved in the bath.
398. Example of zinc and platinum electrodes in water. —
To render this more clear, let it be supposed that while the nega-
tive electrode is still platinum, the positive electrode is a plate of
zinc, a metal eminently susceptible of oxidation. In this case no
gas will appear at the zinc, but the protoxide of that metal will be
formed. This substance being insoluble in water will adhere to
the electrode if the bath contain pure water ; but if it be acid-
ulated, with sulphuric acid for example, the protoxide so soon as
it is formed will combine with the sulphuric acid, producing the
salt called the sulphate of zinc, or more strictly the sulphate of the
oxide of zinc. This being soluble, will be dissolved in .the bath.
ELECTRO-CHEMISTRY. 249
399. [Secondary effects of the current. — In all these cases
the observed results may be accounted for by supposing that the
direct action of the current is limited to the decomposition of
water, and that all the other phenomena are not directly de-
pendent upon the current at all, but result from the action of the
oxygen and hydrogen liberated from the water upon the substances
held in solution or upon the electrodes. But there is no reason to
suppose that such a view would truly represent the physical
process which takes place. On the contrary, when the current
acts upon a solution of a salt in water, or upon any other mixture
of electrolytes, it stands to reason that its action will not always be
confined to one particular constituent of the mixture, but will
take effect chiefly on the constituent most easily decomposed.
For instance, in the case of a solution of sulphate of copper, the
salt is decomposed in preference to the water ; but if we take a
solution of chloride of potassium, the water is decomposed in
preference to the salt.]
400. [influence of concentration of the solution and size of
the electrodes. — In most cases, however, the decomposition is not
confined exclusively to either the water or the salt dissolved in it,
but affects both to a greater or less extent. The result, moreover,
depends not only on the nature of the salt, but also on the degree
of concentration of the solution, as well as on what is called the
density of the current, or the ratio of its intensity to the areat of
the electrodes. Thus, if a current is passed through a solution of
sulphate of copper, by means of comparatively small electrodes,
copper alone is usually separated at the negative electrode ; but if
the solution be made more dilute, or if the size of. the electrodes
be increased — the intensity of the current being kept the same as
before, and therefore its density being diminished — hydrogen will
be liberated at the negative electrode as well as copper, showing
that, under these circumstances, both the water and the sulphate
of copper are decomposed.]
401. — Electrolytic classification of the simple bodies. —
Attempts have been made to classify bodies according to the ten-
dencies they manifest to pass to the one or the other electrode, in
the process of electrolytic decomposition, those which evince the
strongest tendency to go to the positive electrode being considered
in the highest degree electro-negative, and those which show the
strongest tendency to go to the negative electrode in the highest
degree electro-positive. Although experimental research has not
yet supplied very extensive or accurate data for such a classi-
fication, the following proposed by Berzelius will be found useful,
as indicating in a general manner the electrical characters of a
large number of simple bodies, subject to such corrections and
modifications as further experiment and observation may suggest.
250 VOLTAIC ELECTRICITY.
402. I. Electro-negative bodies.
1. Oxygen. 8. Selenium. 15. Antimony.
2. Sulphur. 9. Arsenic. 16. Tellurium.
3. Nitrogen. lo. Chromium. 17. Columbiura .
4. Chlorine. II. Molydenum. 18. Titanium.
5. Iodine. 12. Tungsten. 19. Silicium.
6. Fluorine. Ij. Boron. 20. Osmium.
7. Phosphorus. 14. Carbon. zi. Hydrogen.
403. II. Electro-positive bodies.
1. Potassium. n. Zirconium. 21. Bismuth.
2. Sodium. 12. Manganese. 22. Uranium.
3. Lithium. 13. Zinc. 23. Copper.
4. Barium. 14. Cadmium. 24. Silver.
5. Strontium. 15. Iron. 25. Mercury.
6. Calcium. 16. Nickel. 26. Palladium.
7. Magnesium. 17. Cobalt. 27. Platinum.
8. Glucinium. 18. Cerium. 28. Rhodium.
9. Yttrium. 19. Lead. 29. Indium.
lo. Aluminium. 20. Tin. 30. Gold.
All the bodies named in the first series are supposed to be nega-
tive with relation to those in the second. Each of the bodies in
the first series is negative, and each of the bodies in the second
positive, with relation to those which follow.
The meaning is, that if an electrolyte composed of any two of
the bodies in the first list be submitted to the action of the cur-
rent, that which stands first in the list will go to the positive elec-
trode ; if an electrolyte composed of any body in "the first and
another in the second list be electrolysed, the former will go to the
positive electrode ; and, in fine, if an electrolyte composed of any
two of the bodies named in the second list be electrolysed, the first
named will go to the negative pole.
It has been objected that sulphur and nitrogen occupy too high
a place in the negative series, these bodies being less negative than
chlorine and fluorine, and that hydrogen ought rather to be placed
in the positive series.
404. The order of the series not certainly determined. —
It must be observed that the order of the simple bodies in these
series has not been determined in all cases by the direct obser-
vation of the phenomena of the electrolysis. It has been in many
cases only inferred from the analogies suggested by their chemical
relations.
405. Electrolytes which have compound constituents. —
When the constituents of an electrolyte are compound bodies, the
decomposition proceeds in the same manner as with those binary
compounds whose constituents are simple. Most of the salts which
have been submitted to experiment prove to be electrolytes, the
acid constituent appearing at the positive, and the base at the
negative electrode. Acids are therefore in general regarded as
electro -negative bodies analogous to " oxygen, and alkalies and
oxides as electro -positive bodies analogous to hydrogen.
ELECTRO-CHEMISTRY. 2 5 1
406. According: to Faraday, electrolytes whose consti-
tuents are simple can only be combined in a single pro-
portion. — It appears to result from the researches of Faraday,
that two simple bodies cannot combine in more than one pro-
portion so as to form an electrolyte.
When hydrochloric acid, whose constituents are chlorine and
hydrogen, is submitted to the current, electrolysis ensues, the
chlorine appearing at the positive and the hydrogen at the
negative electrode.
The protochlorides of the metals composed of the metallic base
and one equivalent of chlorine are also easily electrolysed, the
chlorine always appearing at the positive electrode ; but the
perchlorides of the same metals which contain two or more equi-
valents of chlorine are not susceptible of electrolysation.
In general, compounds which consist of two simple elements are
only electrolysable when their constituents are single equivalents.
Hence sulphuric acid which has three, and nitric acid which hag
five equivalents of oxygen, are neither of them susceptible of
electrolysation.
407. Apparent exceptions explained by secondary action.
— In the investigation of the chemical phenomena which attend
the transmission of the current through liquid compounds, results
will be occasionally observed which will at first seem incompatible
with this law. But in these cases the phenomena are invariably
the consequences, not of electrolysis, but of secondary action.
Thus, nitric acid submitted to the current is decomposed, losing
one equivalent of its oxygen, and reduced to nitrous acid. In
this case the real electrolyte is the water, which always exists in
more or less quantity in the acid. This water being decomposed,
the oxygen is delivered at the positive electrode, and the hydrogen
developed at the negative electrode attracts from the nitric acid
one equivalent of its oxygen, with which it combines and forms
water, reducing the nitric to nitrous acid.
Ammonia, which consists of one equivalent of nitrogen and
three of hydrogen, is not properly an electrolyte, though in solu-
tion it is decomposed by the secondary action of the current. In
this case, as in the former, the real electrolyte is the water in
which the ammonia is dissolved. Nitrogen, and not oxygen, is
disengaged at the positive electrode. The oxygen, which is the
primary result of the electrolysis of the water, attracts the hydro-
gen of the ammonia, with which it reproduces water and liberates
the nitrogen.
408. Secondary effects favoured by the nascent state of
the constituents : results of the researches of Becquerel
and Crosse. — It is a general law in chemistry that substances in
252 VOLTAIC ELECTRICITY.
the nascent state, that is, when just disengaged from compounds
with which they have been united, are in a condition most favour-
able for entering into combinations. This explains the great
facility with which the constituents of electrolytes combine with
the electrodes where even a feeble affinity prevails, and also the
various secondary effects. When oxygen is evolved against
copper, iron, or zinc, chlorine against gold, or sulphur against
silver at the electrode, oxides of copper, iron, or zinc, chloride of
gold, or sulphuret of silver, are readily formed. If the current
producing these changes be of very feeble intensity, so that the
new compounds are very slowly formed, so slowly as more to
resemble growth than strong chemical action, they will assume the
crystalline structure. In this manner Becquerel and Crosse have
succeeded in obtaining artificially mineral crystals, and exhibiting
on a small scale effects similar to those which are in progress on a
scale so vast in the mineral veins which pervade the crust of the
globe, and which, doubtless, result from feeble electric currents
established for countless centuries in its strata by the vicissitudes
of temperature and other physical causes.
409. The successive action of tne same current on dif-
ferent vessels of water. — If the same current be conducted
successively through a series of vessels containing acidulated
water, by connecting the water in each vessel with the water in
the succeeding vessel by platinum wires i, i', ix/, i'", &c., as
represented in jig. 267., the current will enter each vessel at the
extremity o, and will depart from it at the extremity h. The
water in each vessel will in this case constitute a separate electro-
lyte, and will be decomposed by the current. The ends o will be
all positive, and the ends h all negative electrodes. Oxygen will
be disengaged at all the ends o, and hydrogen at all the ends h ;
and if the gases disengaged be collected, the same quantity of
oxygen will be found to be disengaged at the ends o, and the same
quantity of hydrogen at the ends A, the volume of the latter being
double that of the former. The weight of the oxygen produced
will be eight times that of the hydrogen, and the weight of the
water decomposed will be nine times that of the hydrogen.
410. Tne same current has an uniform electrolytic
power. — Since it is ascertained by reometric instruments that
the same current has everywhere the same intensity, it follows
ELECTKO-CHEMISTRY, 253
that this constant intensity is attended with an electrolytic power
of corresponding uniformity. From this and other similar results
it is inferred that the quantity of electricity which passes in a
current is proportional to the quantity of a given electrolyte which
the current decomposes.
411. Voltameter of Faraday. — On this ground Faraday
gave the name of voltameter to an apparatus similar in principle
to that described in (392.), taking water as the standard electro-
lyte by which the quantity of electricity necessary to effect the
decomposition of any other electrolytes might be measured. Thus,
if it is found that a current which decomposes in a given time an
ounce of water, will in the same time decompose two ounces of
one electrolyte (A), and three ounces of another electrolyte (B), it
is inferred that the quantity of electricity necessary to decompose
a given weight of A is half that which would decompose an equal
weight of water, and that the quantity necessary to decompose a
given weight of B is a third of that which would decompose the
same weight of water, and, in fine, that the quantities of electricity
necessary to decompose equal weights of A and B are in the ratio
of 3 to 2.
412. Effect of the same current on different electrolytes —
Faraday's law. — If the series of vessels represented in Jig. 267.,
connected by metallic conductors i, i', &c., instead of containing
water, contain a series of different electrolytes, each electrolyte
will be decomposed exactly as it would be if it were the only
electrolyte through which the current passed.
Let us suppose that the first vessel of the series which the current enters
from p contains water, and that means are provided by which the quantities
jf oxygen and hydrogen liberated at o and h shall be indicated, and that in
like manner the quantities of the constituents of each of the other electro-
Ivtes disengaged at the respective electrodes can be determined. It will
then be found that for every grain weight of hydrogen liberated in the first
vessel, the number of grains weight of each* of the constituents of the several
electrolytes disengaged will be expressed by their respective chemical
equivalents.
Thus, if e, e', e", e'", &c. be the chemical equivalents of the several con-
stituents of the series of electrolytes, that of hydrogen being the unit, and
if ft express the number of .grains weight of hydrogen evolved in the volta-
meter tube over the first vessel in a given time, then the number of grains
weight of each of the constituents of the several electrolytes which shall be
evolved in the same time will be
e x h, e' x h, e" x h, e'" x h, &c., &c.
413. It comprises secondary results. — This remarkable
law extends not only to the direct results of electrolysis, but also
to all the secondary effects of the current.
Thus, it applies to the quantities of the several metallic electrodes which
VOLTAIC ELECTRICITY.
combine with the constituents which are the immediate results of the
electrolysis, and also to all combinations and decompositions which result
from the affinities which may exist between the results, primary or secondary,
of the electrolysis, and any foreign substances which the electrolyte may hold
in solution.
414. Practical example of its application. — As a practical
example of the application of this electro- chemical law, let us
suppose the first vessel which the current enters at P to contain
water, the next iodide of potassium, the succeeding one proto-
chloride of tin, the next hydrochloric acid, and the last sulphate
of soda. The current will severally decompose these, the oxygen,
iodine, chlorine, and acid appearing at the five positive electrodes,
and the hydrogen, potassium, tin, and soda at the five negative
electrodes. If the electrode against which the oxygen is evolved
be zinc, the oxide of zinc will result as a secondary product ; and
if the electrode against which the chlorine is evolved be gold, the
chloride of that metal will likewise be produced by secondary
action. The chemical equivalents of the several substances in-
volved in this process are as follows : —
Hvdrogen
Oxvgen -
\Vat<-r -
Iodine -
Potassium
Iodide of potassium
Chlorine ...
Tin
roo
800
9*00
- 126-30
: Jg$
• 35-47
Hydrochloric acid -
Sulphuric acid -
Soda ...
Sulphate of soda
Zinc ...
36-47
40-10
3i'30
71-40
31-30
Gold
Oxide of zinc -
Chloride of gold
19920
40-30
234-67
Protochlorideoftin
- 93^7
It will follow, therefore, from the general electrolytic law above
stated, that for every grain of hydrogen evolved at the negative
electrode in the first vessel, the following will be the quantities of
the chemical results produced in the several vessels : —
I. Oxygen evolved at positive
electrode .... 8*00
Water decomposed - - 9*00
Zinc oxidated ... 32-30
Oxide of zinc produced - 40*30
II. Iodine evolved at the positive
electrode .... 126*30
Potassium evolved at the ne-
gative electrode -. - - 39*26
loditle decomposed - - 165*56
III. Chlorine evolved at the posi-
tive electrode ... 35*47
Tin evolved at the negative
electrode - 57*90
Gold combined at positive
electrode ... - 199*20
Chloride of gold produced - 234*67
Protochloride of tin decom-
posed 93-37
IV. Chlorine evolved at positive
electrode .... 35*47
Hydrogen evolved at negatirs
electrode .... roo
Hydrochloric acid decom-
posed ----- 36-47
V. Sulphuric acid evolved at po-
sitive electrode - 40*10
Soda evolved at negative
electrode - ... 31-30
Sulphate decomposed - - 71*40
415. Sir H. Davy's experiments snowing: the transfer of
the constituents of electrolytes through intermediate solu-
tions. — If the series of vessels containing different electrolytes
be connected by liquid conductors by means of capillary siphons,
instead of the metallic conductors by which they are supposed to
ELECTRO-CHEMISTRY 255
be connected in the cases just described, phenomena are produced,
respecting which a remarkable discordance has arisen between the
highest scientific authorities.
From some of the early experiments of Sir H. Davy, confirmed
by those of Gautherot, Hjsinger, and Berzelius, it appeared that
the voltaic current was not only capable of decomposing various
classes of chemical compounds, but of transferring or decanting
their constituents successively through two or more vessels, to
bring them to the respective electrodes at which they are liberated.
Davy pushed this inquiry to its extreme limits, and by various
experiments, characterised by all that address for which he was
so remarkable, arrived at certain general results which we shall
now briefly state.
Let a series of cups
P 'jjjjfc-^ ABODE 1^5-^ N
be connected by capillary siphons, which may be conveniently formed ex
the fibres of asbestos or amianthus. Let any electrolyte, a solution of a
neutral salt for example, be placed in c, and let the other cups be filled
with distilled water. Let a plate of platinum connected with the positive
pole of a voltaic battery be immersed in the cup A, and a similar plate con-
nected with the negative pole be immersed in E. The voltaic current will
then enter the series of cups at A, and passing successively from cup to cup
through the siphons, will issue from them at E, as indicated by the arrows.
Let the water in the cups A, B, D, and E be tinged by the juice of red
cabbage, the property of which is to be rendered red by the presence of an
acid, and green by that of an alkali.
The current thus established will, according to Sir H. Daw, decompose
the salt in the cup c. The acid will be transported through the two siphons,
and the water. in B to the positive electrode in A. where it will be liberated,
and will enter into solution with the tinged water. At the same time the
alkali will pass through the two siphons, and the cup D to the negative
electrode, and will enter into solution with the water in i>.
The presence of the acid in A and of the alkali in E will be rendered
manifest by the red colour imparted to the contents of the former, and the
green to the latter.
416. While being- transferred they are deprived of their
chemical property. — Although to arrive at A and E respectively
the acid must pass through B and the alkali through E, their pre-
sence in these intermediate cups is not manifested by any change
of colour. It was therefore inferred by Sir H. Davy, that so long
as the constituents of the salt are under the immediate influence
of the current, they lose their usual properties, and only recover
them when dismissed at the electrodes by which they have been
respectively attracted.
If the direction of the current be reversed, so that it shall enter
at E and issue from A, the constituents of the salt will be trans-
ported back to the opposite ends of the series, the acid which had
256 VOLTAIC ELECTRICITY.
been deposited in A will be transferred successively through the
cups B, c, D, and the intermediate siphons to the cup E, and the
alkali in the contrary direction from E through D, c, B, and the
siphons to A. This will be manifested by the changes of colour
of the infusions. The liquid in A which had been reddened by
the acid, will first recover its original colour, and then become
green according as the ratio of the acid to the alkali in it is di-
minished ; and in like manner the infusion in E, which had been
rendered green by the alkali, will gradually recover its primitive
colour, and then become red as the proportion of the acid to the
alkali in it is augmented.
During these processes no change of colour will be observed in
the intermediate cups B and D.
The intermediate cups B and D being filled with various che-
mical solutions for which the constituents of the salt had strong
affinities, and with which under any ordinary circumstances they
would immediately enter into combination, these constituents
nevertheless invariably passed through the intermediate vessels
without producing any discoverable effect upon their contents'
Thus, sulphuric acid passed in this manner through solutions of
ammonia, lime, potash, and soda, without affecting them. In like
manner hydrochloric and nitric acids passed through concentrated
alkaline menstrua without any chemical effect. In a word, acids
and alkalis having the strongest mutual affinities, were thus reci-
procally made to pass each through the other without manifesting
any tendency to combination.
417. Exception in the case of producing- insoluble com-
pounds. — Strontia and baryta passed in the same way through
muriatic and nitric acids, and reciprocally these acids passed with
equal facility through solutions of strontia and baryta. But an
exception was encountered when it was attempted to transmit
strontia or baryta through a solution of sulphuric acid, or vice
versa. In this case the alkali was arrested in transitu by the acid,
or the acid by the alkali, and the salt resulting from their combi-
nation was precipitated in the intermediate cup. .
The exception therefore generalised, included those cases in
which bodies were attempted to be transmitted through menstrua
for which they have an affinity, and with which they would form
an insoluble compound.
418. This transfer denied by Faraday. — This transmission
of chemical substances through solutions with which they have
affinities by the voltaic current, those affinities being rendered
dormant by the influence of the current which appeared to be
established by the researches of Davy, published in 1807, and
wince that period received by the whole scientific world as an esta-
ELECTRO-CHEMISTRY. 2 97
Wished principle, has lately been affirmed by Dr. Faraday to be
founded in error. According to Faraday no such transfer of the
constituents of a body decomposed by the current can or does take
place. He maintains that in all cases of electrolysation it is an
absolutely indispensable condition that there be a continuous and
unbroken series of particles of the electrolyte between the two
electrodes at which its constituents are disengaged. Thus, when
water is decomposed, there must be a continuous line of water
between the positive electrode at which the oxygen is developed,
and the negative electrode at which the hydrogen is disengaged.
In like manner, when the sulphate of soda, or any other salt is
decomposed, there must be a continuous line of particles of the
salt between the positive electrode at which the acid appears, and
the negative electrode at which the alkali is deposited.
Dr. Faraday affirms, that in Davy's celebrated experiments, in
which the acid and alkaline constituents of the salt appear to be
drawn through intermediate cups, containing pure water or solu-
tions of substances foreign to the salt, the decomposition and
apparent transfer of the constituents of the salt could not have
commenced until, by capillary attraction, a portion of the salt had
passed over through the siphons, so that a continuous line of saline
particles was established between the electrodes. Dr. Faraday
admits such a transfer of the constituents, as may be explained by
the series of decompositions and recompositions involved in the
hypothesis of Grotthus.
419. Apparent transfer explained by him on Grotthus'
hypothesis. — It is also admitted by Dr. Faraday, that when pure
water intervenes between the metallic conductors proceeding from
the pile and the electrolyte, decomposition may ensue, but he
considers that in this case the true electrodes are not the extre-
mities of the metallic conductors, but the points where the pure
water ends and the electrolyte begins, and that accordingly in
such cases the constituents of the electrolyte will be disengaged,
not at the surfaces of the metallic conductors, but at the common
surfaces of the water and the electrolyte. As an example of this
he produces the following experiment. Let a solution of the
sulphate of magnesia be covered with pure water, care being
taken to avoid all admixture of the water with the saline solution.
Let a plate of platinum proceeding from the negative pole of a
battery be immersed in the water, at some distance from the
surface of the solution on which the water rests, and at the same
time let the solution be put in metallic communication with the
positive pole of the battery. The decomposition of the sulphate
will speedily commence, but the magnesia, instead of being de-
posited on the platinum plate immersed in the water, will appear
258 VOLTAIC ELECTRICITY.
at the common surface of the water and the solution. The water,
therefore, and not the platinum, is in this case the negative
electrode.
420. Faraday thinks that conduction and decomposition
are closely related. — Dr. Faraday maintains that the connec-
tion between conduction and decomposition, so far as relates to
liquids which are not metallic, is so constant that decomposition
may be regarded as the chief means by which the electric current
is transmitted through liquid compounds. Nevertheless, he admits,
that when the intensity of a current is too feeble to effect decom-
position, a quantity of electricity is transmitted sufficient to affect
the reoscope.
In accordance with these principles, Faraday affirms that water
which conducts the electric current in its liquid state, ceases to do
so when it is congealed, and then it also resists decomposition, and
in fine ceases to be an electrolyte. He holds that the same is true
of all electrolytes.
421. Maintains that non-metallic liquids only conduct
when capable of decomposition by the current. — The con-
nection between decomposition and conduction is further mani-
fested, according to Dr. Faraday, by the fact that liquids which do
not admit of electro-chemical decomposition, do not give passage
to the voltaic current. In short, that electrolytes are the only
liquid non-metallic conductors.
422. Faraday's doctrine not universally accepted — Pouil-
let's observations. — These views of Dr. Faraday have not yet
obtained general acceptation ; nor have the discoveries of Davy
of the transfer and decantation of the constituents of electrolytes
through solutions foreign to them, been yet admitted to be over-
thrown. Peschel and other German authorities, in full possession
of Faraday's views and the results of his experimental researches,
still continue to reproduce Davy's experiments, and to refer to
their results and consequences as established facts. Pouillet,
writing in 1847, and also in possession of Faraday's researches,
which he largely quotes, maintains nevertheless the transport of
the constituents under conditions more extraordinary still, and
more incompatible with Faraday's doctrine than any imagined by
Davy. In electro-chemical decomposition he says, — "There is at
once separation and transport. Numberless attempts have been
made to seize the molecule of water which is decomposed, or to
arrest en route the atoms of the constituent gases before their
arrival at the electrodes, but without success. For example, if
two cups of water, one containing the positive and the other the
negative wire of a battery, be connected by any conductor, sin-
gular phenomena will be observed. If the intermediate conductor
be metallic, decomposition will take place independently in both
ELECTRO-CHEMISTRY.
259
cups " (as already described), " but if the intermediate conductor
be the human body, as when a person dips a finger of one hand
into the water in one cup, and a finger of the other hand into the
other, the decomposition will sometimes proceed as in the case of
a metallic connection ; but more generally oxygen will be disen-
gaged at the wire which enters the positive cup, and hydrogen at
the wire which enters the negative cup, no gases appearing at the
fingers immersed in the one and the other. It would thus appear
that one or other of the constituent gases must pass through the
body of the operator, in order to arrive at the pole at which it is
disengaged. And even when the two cups are connected by a
piece of ice, the decomposition proceeds in the "same manner, one
or other gas appearing to pass through the ice, since they are dis-
engaged at the poles in the separate cups in the same manner." *
423. Davy's experiments repeated and confirmed by
Becquerel. — The experiments of Davy, in which the transfer of
the constituents of an electrolyte through water and through
solutions for which these constituents have affinities, was demon-
strated, have been repeated by Becquerel, who has obtained the
same results. The capillary siphons used by Becquerel were glass
tubes filled with moistened clay. He also found that the case in
which the constituent transferred would form an insoluble com-
pound with the matter forming the intermediate solution, forms an
exception to this principle of transfer ; but he observed that this
only happens when the intensity of the current is insufficient to de-
compose the compound thus formed in the intermediate solution.'!'
424.. The electrodes supposed to exercise different elec-
trolytic powers by Pouillet. — The question whether the
decomposing agency resides altogether at one or at the other elec-
trode, or is shared between them, has been recently investigated
by M. Pouillet.
Let three tubes of glass hav-
ing the form of the letter U,
Jig. 268., be prepared, each of
the vertical arms being about five
inches long, and half an inch in
diameter. Let the curved part
of the tubes connecting the legs
have a diameter of about the
twentieth of an inch when the
solutions used are good con-
ductors, but the same diameter
as the tubes themselves when the
E
Fig. 168.
• Pouillet, « Elements de Physique," ed. 1847, vol. i. p. 598.
f Becquerel. "Traite'de Physique," vol. ii. p. 330., ed. 1844.
s i
260 VOLTAIC ELECTRICITY,
conducting power is more imperfect. In this latter case, how-
ever, the results are less exact and satisfactory.
Let platinum wires E and E' proceeding from the poles of a
voltaic battery be plunged in the first and last tubes, and let the
intermediate tubes be connected by similar wires n' and i" i'".
Let acidulated water be poured into the tube EI, and the solu-
tions on which the relative effects of the two electrodes are to be
examined, into the other tubes 1 1" and i"' E'. After the electro-
lysis has been continued for a certain time, the quantity of the
solution decomposed in each leg may be ascertained by submitting
the contents of each leg to analysis. The quantity remaining un-
decomposed being thus ascertained and subtracted from the
original quantity, the remainder will be the quantity decomposed,
since the fluids are prevented from intermixing to any sensible
extent by the smallness of the connecting tube, and by being
nearly at the same level during the process. It may be assumed
that the decomposing agencies of the two electrodes, will be pro-
portional to the quantities of the solutions decomposed in the legs
in which they are respectively immersed.
425. Case in which the negative electrode alone acts. —
The current being first transmitted through a voltameter to indi-
cate the actual quantity of electricity transmitted, the tubes E j,
i' i'' and i'" E' were filled, the first with a solution of the chloride
of gold, the next with the chloride of copper, and the third with
the chloride of zinc. After the lapse of a certain interval the
contents of the tubes were severally examined, and it was found
that the solutions in the legs in which the positive electrodes were
immersed had suffered no decomposition. The quantities of the
chlorides contained in them respectively were undiminished,
while the chloride in each of the legs containing the negative
electrodes was diminished by exactly the quantity corresponding
to the metal deposited on .the negative wire, and the chlorine
transferred to the positive leg.
It was therefore inferred that in these cases the entire decom-
posing agency must be ascribed to the negative electrode.
The same results were obtained for the other metallic chlorides.
426. [This unequal action of the electrodes is only ap-
parent— These results nevertheless do not warrant the conclusion
drawn from them. They are due to a property possessed by the
current of carrying the electrolyte in one direction or the other
without decomposing it. Thus, in the decomposition of a solution
of sulphate of copper between copper electrodes, the solution
becomes more concentrated in contact with the positive, and more
dilute in contact with the negative electrode.]
247. liquid electrodes.— Series of electrolytes in imme-
ELECTRO-CHEMISTRY. 26 1
diate contact. — In general, the electrodes by which the current
enters and departs from an electrolyte, are solid and most fre-
quently metallic conductors. In an experiment already cited
(419.), Faraday has shown that water may become an electrode,
and Pouillet in some recent experiments has succeeded in gene-
ralising this result, and has shown not only that the current may
be transmitted to and received from an electrolyte' by liquid con-
ductors, but that a series of different electrolytes may become
mutual electrodes, the current passing immediately from one to
the other without any intermediate conductor, solid or liquid, and
that each of them shall be electrolysed. Thus, suppose that the
series of electrolytes are expressed by
aaf bb' cc' dd'
the current as indicated by the arrows entering A, and departing
from D, and being supposed to have sufficient intensity to effect
the electrolysis of all the solutions. Let the electro-negative con-
stituents be expressed by a, ft, c, rf, and the electro-positive by
a', ft', c', d'. It is evident that the points at which any two suc-
ceeding solutions touch, will be at the same time the negative
electrode of the first, and the positive electrode of the second, and
that, consequently, the positive constituent of the first and the-
negative constituent of the second will be disengaged at this point,
and being in the nascent state will be under the most favourable
conditions to combine in virtue of their affinities, and so to form
new compounds as secondary effects. Thus, the common surface
of A and B will be the negative electrode of A, and the positive
electrode of B, because it is at this surface that the current departs
from A and enters B, and accordingly the electro-positive consti-
tuent of of A, and the electro-negative constituent ft of B, will be
developed at this common surface, and if they have affinity, will
enter into combination.
428. Experimental illustra-
tion of this. — These principles
may be experimentally illustrated
and verified by placing the elec-
trolytic solutions in U-shaped
tubes T, T', T", as represented in
Jig. 269.
Let two electrolytic solutions A and
B be introduced into the first tube T,
so carefully as to prevent them from
intermixing, and let their common
surface be at o. In like manner let
262 VOLTAIC ELECTRICITY.
the solutions B and c be introduced into the tube T', and the solutions c
and D into the tube T", their common surfaces being at o' and o". Let the
legs of the tubes T and T', which contain the solution B, be connected by a
glass siphon containing the same solution, and the legs of the tubes T'
and T", containing the solution c, be similarly connected. Let the positive
wire of a battery be immersed in A, and the negative wire in D, the current
being sufficiently intense to electrolyse all the solutions.*
In this case o will be the positive electrode of B, and the negative elec-
trode of A ; o' the positive electrode of c, and the negative electrode of B ;
and o" the positive electrode of D, and the negative electrode of c.
If A be pure water, B the chloride of zinc, the water being decomposed,
oxygen will be disengaged at the positive wire, and hydrogen at the common
surface o. The chloride being also decomposed, the chlorine, its electro-
negative constituent, will be disengaged at o, where it will enter into com-
bination with the hydrogen, and form hydrochloric acid, the presence of
which may be ascertained by the usual tests. The oxide of zinc, the
electro-positive constituent of B, will be disengaged at o', and will form a
compound with the electro-negative constituent of c, and so on.
429. Electrolysis of the alkalis and earths. — The decom-
posing power of the voltaic current had not long been known
before it became, in the hands of Sir H. Davy and his successors,
the means of resolving the alkalis and earths, before that time
considered as simple bodies, into their constituents. This class of
bodies was shown to be oxidised metals. When submitted to
such conditions as enabled a strong voltaic current to pass through
them, oxygen was liberated at the positive electrode, and the
metallic base appeared at the negative electrode.
430. The series of new metals. — A new series of metals
was thus discovered, which received names derived from those of
the alkalis and earths of which they formed the bases. Thus, the
metallic base of potash was called potassium, that of soda, sodium,
that of lime, calcium, that of silica, silicium, and so on.
In many cases it is difficult to maintain those metals in their
simple state, owing to their strong affinity for oxygen. Thus
potassium, if exposed to- the atmosphere at common temperatures,
enters directly into combination with the air, and burns. When
it is desired to collect and preserve it in the metallic state it is
decomposed by the current in contact with mercury, with which it
enters into combination, forming an amalgam. It is afterwards
separated by distillation from the mercury, and preserved in the
metallic state under the oil of naphtha, in a glass tube hermetically
closed, the air being previously expelled.
43 1 . Schoenbein's experiments on the passivity of iron.
* This is not the experimental arrangement adopted by M. Pouillet. It
has occurred to me, as a method of exhibiting his principle under a more
general form and somewhat more clearly and satisfactorily than his ap-
paratus, in which the siphons s, s' have no place.
ELECTRO-CHEMISTRY. 263
— Among the effects of the voltaic current which have been not
satisfactorily or not at all explained, are those by which iron,
under certain conditions, is enabled to resist oxidation even when
exposed to agents of the greatest power ; such, for example,
as nitric acid. The most remarkable researches on this subject
are those of Schoenbein. In his experiments, the wires proceeding
from the poles of the battery were immersed in two mercurial
cups, which we shall call P and N. A bath of water B, acidulated
with about 8 per cent, of sulphuric acid, was then connected with
the cup N by a platinum wire. A piece of iron wire was placed
with one extremity in P, and the other in the bath B. No oxi-
dation was manifested at the end immersed in the bath, and no
hydrogen was evolved at the platinum wire. In fine, no elec-
trolysis took place.
Several circumstances were found to restore to the iron its
oxidable property, and to establish the electrolysis of the liquid
in the bath, but only for a short interval of a few seconds. These
circumstances were : — I. The contact for a moment of the pla-
tinum and iron wires in the bath. 2. The momentary suspen-
sion of the current by breaking the contact at any point of the
circuit. 3. The contact of any oxidable metal, such as zinc,
tin, copper, or silver, with the iron in the bath. 4. The momen-
tary diversion of a portion of the current, by connecting the cups
p and N by a copper wire, without breaking the connections of
the original circuit. 5. By agitating the end of the iron wire in
the bath.
If in connecting B and P by the iron wire the wire be first im-
mersed in B, oxidation will take place for some seconds after the
other end is immersed in p.
The intensity of the current diverted by connecting the cups
p and N by a copper wire, can be varied at pleasure by varying
the length and section of the connecting wire (375.)- When
such a derived current is established, several curious and inte-
resting phenomena are observed. When the derived current has
great intensity, no effect is produced upon the iron. Upon
gradually diminishing the intensity of the derived current, the
iron becomes active, that is, susceptible of oxidation. With a less
intensity it again becomes passive, and the oxidation ceases. As
the derived current is gradually reduced to that intensity at which
the iron becomes permanently passive, there are several successive
periods during which it is alternately active and passive, the
intervals between these periods being less and less. In the appa-
ratus of Schoenbein the iron became permanently active when the
copper wire conducting the derived current was half a line thick,
and from 6 inches to 1 6 feet long.
2t>4 VOLTAIC ELECTRICITY.
These effects are reproduced with all the oxacids, but are not
manifested either with the hydracids or the Haloid salts.
432. Other methods of rendering: iron passive. — Iron may
be rendered passive also by placing it as the positive electrode in
a solution of acetate of lead with a current of ordinary intensity.
The iron should be immersed in the solution for about half a
minute to a depth of about half an inch. A wire thus treated,
being washed clean, acquires the permanently passive property,
even though the part immersed in the solution has not been coated
with the peroxide of lead. And in this case the conditions above
stated, under which it recovers momentarily its active character,
become inoperative.
Iron thus galvanised acquires to a great degree the virtue of
platinum and the other highly negative metals, and for many pur-
poses may be substituted for them. Thus Schoenbein has con-
structed voltaic batteries of passive iron and zinc.
The iron wire used for telegraphic purposes is rendered passive
by this process.
433. Tree of Saturn. — The well known experiment of the
Tree of Saturn presents a remarkable example of the effect of a
feeble current of long continuance. A bundle of brass wires is
passed through a hole made longitudinally through the centre of a
bottle cork, and fitted tightly in it so as to diverge in a sort of
cone from the bottom of the cork. A plate of zinc is then tied
round the wires at the point where they diverge from the cork, so
as to be in contact with all the wires. The wires and cork are
then introduced into a glass flask containing a limpid solution of
the acetate of lead, and the top of the cork luted over to prevent
the admission of air. The zinc and brass thus immersed in the
solution form a voltaic pair, and a current passes through the
solution from tne zinc to the wire. The water of the solution is
slowly decomposed, the oxygen combining with the zinc, and the
hydrogen attracting the oxygen from the oxide of lead, and
reproducing water, while the metallic lead attaches itself to the
wires. The acetic acid, liberated by the secondary decomposition
of the acetate of lead, enters into combination with the oxide of
zinc, and produces the acetate of that metal, which passes into
solution in the water. The contents of the flask are gradually-
converted into a solution of the acetate of zinc, and the metallic lead,
the process being very slow, is crystallised in a variety of beautiful
forms upon the divergent brass wire. ,
434. Davy's method of preserving tne copper sheathing
of ships. — The method proposed by Sir H. Davy to preserve
from corrosion the copper sheathing of ships, depends on the long-
continued action of feeble currents. The copper is united with a
ELECTRO-CHEMISTRY. 2(35
mass of zinc, iron, or some more oxidable metal, so as to form a
voltaic combination. The sea water being a weak solution of salt,
a feeble permanent current is established between the more and
less oxidable metals, passing through the water from the former
to the latter, and causing its slow decomposition. The oxygen
combines with the protecting metal, and the hydrogen disengaged
on the copper, decomposes the salts held in solution in the sea
water, attracting their oxide constituents, such as lime, magnesia,
&c., which are deposited upon the copper in a rough crust. Upon
the coating thus formed collect marine vegetation, shells, and
other substances. Thus, while the copper sheathing is preserved
from corrosion, there arises the counteracting circumstance of an
appendage to the hull of the ship, which impedes its sailing qualities-
435. [Peculiar properties of electrolytic oxygen — Ozone.
Oxygen gas prepared by the electrolysis of dilute sulphuric acid,
possesses some properties which do not belong to pure oxygen
prepared by chemical processes, and which are due to the presence
in it of a small quantity of ozone. The most important of the
properties referred to are the peculiar smell of the gas, resembling
that developed by passing a succession of sparks from a common
electrical machine through the air, and its unusually active powers
of oxidation, as shown by its setting free iodine from a solution
of iodide of potassium, or decolourising a solution of indigo.
These latter effects are easily obtained by allowing the gas evolved
in a voltameter to bubble through a solution of iodide of potassium
or of indigo, respectively.
The quantity of ozone is greatest when the temperature of the
liquid in the voltameter is kept as low as possible. It is also in-
creased by the presence of substances which readily part with
oxygen, as chromic and permanganic acids, but its quantity is in
all cases very small in comparison with that of the oxygen which
accompanies it.]
436. [Nature of ozone. — For some time considerable doubt
existed as to the true chemical nature of ozone, some chemists
maintaining that it was a compound of hydrogen with more oxygen
than is required to convert it into water, while others declared
that it was nothing but a peculiar modification of oxygen. Recent
experiments seem to have proved pretty conclusively that the
latter opinion is correct.]
437. [Effect of ozone in lessening: the quantity of gas
evolved in a voltameter. — The formation of ozone introduces a
source of error into the results obtained, when the intensity of a
voltaic current is estimated from the quantity of oxygen and
hydrogen gases evolved by the decomposition of dilute sulphuric
acid in a voltameter. The hydrogen and ozonised oxygen being
liberated from platinum plates in close proximity, come in contact
266 VOLTAIC ELECTRICITY.
with each other, both within the liquid itself, and above its surface ;
the consequence is, that the ozone effects the oxidation of a portion
of the hydrogen, reconverting it into water, and thus lessening the
total volume of the gases evolved by three times the volume of the
oxygen which thus combines, under the form of ozone, with the
hydrogen.
This source of inaccuracy may be avoided to a considerable
extent by collecting the gases separately, and estimating the
strength of the current by the volume of hydrogen evolved. A
still better method is to place a solution of sulphate of copper or
of nitrate of silver, in the voltameter, instead of dilute sulphuric
acid, and to take the weight of copper or silver deposited on the
negative electrode as the measure of the strength of the current.]
438. [Polarisation o£ the electrodes. — It has been explained
in (175.) how secondary actions taking place between the liquid
in the battery and the metallic plates may lead to a diminution ki
the strength of the current. Perfectly analogous effects are often
produced in electrolytic cells, the substances which result from
the decomposition of the electrolyte sometimes forming a non-
conducting coating upon the electrodes, whereby the passage of
the current is prevented, and sometimes even tending to produce
a current in the opposite direction to that of the battery. This
latter phenomenon, known as the polarisation of the electrodes,
may be very well studied with a voltameter in which oxygen and
hydrogen are evolved from dilute sulphuric acid. It is caused by
the adherence of the gases to the two electrodes, the positive
electrode becoming as it were coated with oxygen and the negative
with hydrogen. The electrodes thus charged are precisely in the
condition of the platinum plates of a Grove's gas battery (i 74.), and
tend to produce a current in the opposite direction to that by
which they were charged, the negative electrode (which is charged
with hydrogen) acting as an electro-positive metal, and the
positive electrode (which is charged with oxygen) acting as an
electro-negative metal.
Removing the electrodes from the liquid and heating them, or
any other treatment which tends to remove the films of gas,
diminishes or destroys their polarisation.]
439. [Reverse currents due to polarisation of the elec-
trodes.— The actual production of a current in the opposite di-
rection to that of the battery can be easily shown by an arrange-
ment such as that represented in Jigm 269/1, where u represents a
Daniell's or Grove's battery of two or three cells ; v a voltameter,
with platinum electrodes ; G a rather delicate reometer, and R a
reotrope, whereby the connexion between the battery and the
voltameter can be broken, and a connexion established between
ELECTRO-CHEMISTRY.
267
the latter and the reometer almost at the same instant. For this
purpose, one of the electrodes of the voltameter is connected with
one pole (the positive pole, for instance) of the battery, and also
through the wire of the reometer, with the reotrope ; the other
electrode of the voltameter and the other (negative) pole of the
battery are likewise connected with the reotrope. This instru-
ment is so constructed that, when the piece of brass a, is in contact
with the spring c, the current of the battery passes through the
voltameter, but not through the reometer, but by turning the
handle so that the brass plate a comes into contact with the spring
&, the connexion between the battery and the voltameter is broken,
and connexion is made between the latter and the reometer, the
needle of which will be deflected so as to indicate a current tra-
versing the voltameter in an opposite direction to that produced
by the battery.
Fig. 069 a.
The effects exhibited by Ritter's secondary piles (208.) are like-
wise due to the similar polarisation of the plates composing them.]
440. [The chemical processes which take place in a voltaic
battery — are completely analogous to those which go on in an
electrolytic cell. In fact, each cell of the battery is a true elec-
trolytic cell ; the liquid contained in it undergoes electrolysis
exactly in the same manner as the liquid in a voltameter, its
electro-negative constituent appearing at the pole whereby the
negative current leaves the apparatus (the zinc pole), and the
electro-positive constituent appearing at the pole whereby the
positive current issues (the copper or platinum pole), and the
chemical action is propagated across the liquid in each case by a
268 VOLTAIC ELECTRICITY.
series of precisely similar interchanges between the atoms of
neighbouring molecules.]
441. [Amount of chemical action in the battery. — Not
merely do the processes which take place in the battery correspond
in kind with the chemical changes which the current produces in
any electrolyte through which it passes, but the amount of
chemical action which takes place in each cell of the battery is
precisely equivalent, in the absence of accidental disturbing
causes, to the chemical action produced by the current at any part
of its course outside the battery. For every equivalent of
hydrogen gas evolved by the current in a voltameter, or for every
equivalent of metal deposited in an electrolytic cell, one equi-
valent of zinc is dissolved in each cell of the battery, and one
equivalent of hydrogen is evolved at the negative plate, or in
the case of batteries with two liquids, such as Daniell's or Grove's,
an action which chemically corresponds to the evolution of one
equivalent of hydrogen takes place.]
442. [Advantage of using amalgamated zinc in the bat-
tery.— If a battery were constructed with ordinary commercial
zinc as the material of the positive plates, the quantity of zinc
dissolved would be found to be in excess of that required by the
law stated in the last paragraph. In fact, a piece of ordinary zinc
is rapidly dissolved by dilute sulphuric acid, without any apparent
electrical effect being produced. This is owing to the presence of
impurities, such as lead, carbon, &c. in the zinc : these impurities
being for the most part more electro-negative than zinc, cause the
formation of small local circuits, in which the pure zinc represents
the positive plate, and the particles of impurity at its surface
represent the negative plate j the connexion between them being
made, on the one hand through the acid, and on the other hand
through the body of the zinc plate. That such is the case is proved
by the fact, that chemically pure zinc does not dissolve by itself
in dilute sulphuric acid, and when used in conjunction with a
more electro-negative metal in a voltaic cell, it is dissolved only
so long as connexion between it and the other metal is maintained.
Precisely the same effect is produced with ordinary zinc, if its
surface is well amalgamated with mercury. Amalgamated zinc
does not dissolve in acid by itself, and, when used in the construc-
tion of a battery, it is not acted upon except when the circuit is
closed.]
CHAP. XIV.
ELECTRO-METALLURGY.
443. Origin of this art. — The decomposing power of the voltaic
current applied to solutions of the salts and oxides of metals has
ELECTRO-METALLURGY. 269
supplied various processes to the industrial arts, which may be
comprehended under the general denomination, Electro- metal-
lurgy.
444. The metallic constituent deposited on the negative
electrode. — If a current of sufficient intensity be transmitted
through a solution of a salt or oxide, having a metallic base, it
will be understood, from what has been already explained, that
while the oxygen or acid is developed at the positive electrode,
the metal will be evolved at the negative electrode.
445. Anybody may be used as the negative electrode.
The bodies used as electrodes must be superficially conductors,
since otherwise the current could not pass between them ; but
subject to this condition, they may be of any material or form. If
the body be metallic, its surface has necessarily the conducting
property. If it be formed of a material which is a non-conductor,
or an imperfect conductor, the power of conduction may be im-
parted to its surface by coating it with finely powdered black lead
and other similar expedients. This process is called metallising
the surface.
445. Use of a soluble positive electrode. — By the continu-
ance of the process of decomposition the solution will be rendered
gradually weaker, and the deposition of the metal would go on
more slowly. This inconvenience is remedied by using, as the
positive electrode, a plate of the same metal which is to be depo-
sited on the negative electrode. In this case the metal is dissolved
at the positive electrode as fast as it is deposited at the other, and
the solution is thus kept at a uniform strength.
447. Conditions which affect the state of the metal depo-
sited.— The state of the metal disengaged at the negative elec-
trode depends on the intensity of the current, the strength of the
solution, its acidity, and its temperature ; and the regulation of
these conditions in each particular case will require much prac-
tical skill on the part of the operator, since few general rules can
be given for his direction.
In the case, for example, of a solution of one of the salts of
copper, a feeble current will deposit on the electrode a coating of
copper so malleable that it may be cut with a knife. With a more
intense current the metal will become harder. As the intensity
of the current is gradually augmented, it becomes successively
brittle, granulous, crystalline, rough, pulverulent, and in fine
loses all cohesion, — practice alone will enable the operator to
observe the -conditions necessary to give the coating deposited on
the electrode the desired quality.
448. The deposit to be of uniform thickness. — It is in all
cases desirable, and in many indispensable, that the metallic
270 VOLTAIC ELECTRICITY.
coating deposited on the electrode shall have an uniform thickness.
To insure this, conditions should be established which will render
the action of the current on every part of the surface of the elec-
trode uniform, so that the same quantity of metal may be deposited
in the same time. Many precautions are necessary to attain this
object. Both electrodes should be connected at several points
with the conductors, which go to the poles of the battery, and they
should be presented to each other so that the intermediate spaces
should be as nearly as possible equal, since the intensities of the
currents between point and point vary with the distance. The
deposition of the metal is also much influenced by the form of the
body. It is in general more freely made on the salient and pro-
jecting parts, than in those which are sunk.
449. Means to prevent absorption of the solution by the
electrode. — If the body on which the metallic deposit is made
be one which is liable to absorb the solution, a coating of some
substance must be previously given to it which shall be impervious
to the solution.
450. Nonconducting- coating- used -where partial deposit
is required. — When a part only of a metallic or other conducting
body is desired to be coated with the metallic deposit, all the parts
immersed not intended to be so coated are protected by a coating
of wax, tallow, or other nonconductor.
451. Application of these principles to gilding-, silvering-,
due. — The most extensive and useful application of these prin-
ciples in the arts is the process of gilding and silvering articles
made of the baser metals. The article to be coated with gold
being previously made clean, is connected with the negative pole
of the battery, while a plate of gold is connected with its positive
pole. Both are then immersed in a bath consisting of a solution
of the chloride of gold and cyanide of potassium, in proportions
which vary with different gilders. Practice varies also as to the
temperature and the strength of the solution. The chloride is
decomposed, the metallic base being deposited as a coating on the
article connected with the negative pole, and the chlorine com-
bining with a corresponding portion of the gold connected with
the positive pole, and reproducing the chloride which is dissolved
in the bath as fast as it is decomposed, thus maintaining the
strength of the solution.
A coating of silver, copper, cobalt, nickel, and other metals is
deposited by similar processes.
4152. Cases in which the coating- is inadhesiye. — When
the article on which the coating is deposited is metallic, the coat-
ing will in some cases adhere with great tenacity. In others, the
result is less satisfactory ; as, for example, where gold is deposited
ELECTRO-METALLURGY. 271
on iron or steel. In such cases the difficulty may be surmounted
by first coating the article with a metal which will adhere to it,
and then depositing upon this the definite coating.
455. Application to gilding-, silvering-, or bronzing- objects
of art. — The extreme tenuity with which a metallic coating may
be deposited by such processes, supplies the means of imparting
to various objects of art the external appearance and qualities
of any proposed metal, without impairing in the slightest degree
their most delicate forms and lineaments. The most exquisitely
moulded statuette in plaster may thus acquire all the appearance
of having been executed in gold, silver, copper, or bronze, without
losing any of the artistic details on which its beauty depends.
454. Production of metallic moulds of articles. — If it be
desired to produce a metallic mould of any object, it is generally
necessary to mould it in separate pieces, which being afterwards
combined, a mould of the whole is obtained. That part intended
to be moulded is first rubbed with sweet oil, black lead, or some
other lubricant, which will prevent the metal deposited from
adhering to it, without separating the mould from the surface, in
BO sensible a degree as to prevent the perfect correspondence of
the mould with the original. All that part not intended to be
moulded is invested with wax or other material, to intercept the
solution. The object being then immersed, and the electrolysis
established, the metal will be deposited on the exposed surface.
When it has attained a sufficient thickness the object is with-
drawn from the solution, and the metallic deposit detached. It
will be found to exhibit, with the utmost possible precision, an
impression of the original. The same process being repeated for
each part of the object, and the partial moulds thus obtained being
combined, a metallic mould of the whole will be produced.
455. Production of objects in solid metal. — To reproduce
any object in metal it is only necessary to fill the mould of it,
obtained by the process above explained, with the solution of the
metal of which it is desired to form the object, the surface of the
mould being previously prepared, so as to prevent adhesion. The
solution is then put in connection with the positive pole of the
pile, while the mould is put in connection with the negative pole.
The metal is deposited on the mould, and when it has attained the
necessary thickness the mould is detached, and the object is ob-
tained.
In general, however, it is found more convenient to mould the
object to be reproduced in metal by the ordinary processes in
wax, plaster of paris, or fusible alloy. When moulds are made in
wax, plaster, or any nonconducting material, their inner surfaces
must be rubbed with black lead, to give them the conducting
272 VOLTAIC ELECTRICITY.
power. When the deposit is made of the necessary thickness, the
mould is broken off or otherwise detached.
Statues, statuettes, and bas-reliefs in plaster can thus be re-
produced in metal with the greatest facility and precision, at an
expense not much exceeding that of the metal of which they are
formed.
456. Reproduction of stereotypes and engraved plates. —
A mould in plaster of paris, wax, or gutta percha, being taken
from a wood engraving and a stereotype plate, a stereotype may
be obtained from the mould by the processes above described.
The pages now before the reader have been stereotyped by this
process.
Copper or steel engraved plates may be multiplied by like
methods. A mould is first taken, which exhibits the engraving in
relief. A metallic plate deposited upon this by the electrolytic
process will reproduce the engraved plate.
457. metallising textile fabrics. — The electro-metallurgic
processes have been extended by ingenious contrivances to other
substances besides metal. Thus a coating of metal may be de-
posited on cloth, lace, or other woven fabrics, by various ingenious
expedients, of which the following is an example : — On a plate of
copper attach smoothly a cloth of linen, cotton, or wool, and then
connect the plate with the negative pole of a voltaic battery, im-
merse it in a solution of the metal with which it is to be coated,
and connect a piece of the same metal with the positive pole ; de-
composition will then commence, and the molecules of metal, as
they are separated from the solution, must pass through the cloth
in advancing to the copper to which the cloth is attached. In
their passage through the cloth they are more or less arrested by
it. They insinuate themselves into its pores, and, in fine, form a
complete metallic cloth. Lace is metallised in this way by first
coating it with plumbago, and then subjecting it to the electro-
metallurgic process.
Quills, feathers, flowers, and other delicate fibrous substances
may be metallised in the same way. In the case of the most
delicate of these, the article is first dipped into a solution of phos-
phorus in sulphide of carbon, and is well wetted with the liquid.
It is then immersed in a solution of nitrate of silver. Phosphorus
has the property of reviving silver and gold from their solutions.
Consequently, the article is immediately coated with a very atte-
nuated film of the metal.
458. Glyphography. — If a thin stratum of wax or other soft
substance be spread upon a plate of metal, any subject or design
may be engraved upon the coating without more labour than
would be expended on a pencil drawing. When the engraving is
ELECTRO-METALLURGY. 273
thus made on the wax it is subjected to the electrotype process,
by which a sheet of copper or other metal is deposited upon it.
When this is detached it exhibits in relief the engraving, from
which impressions may be produced in the same manner as from a
wood engraving, to which it is altogether analogous.
459. Reproduction of dagruerreotypes. — One of the most
remarkable and unexpected applications of the electrotype process
is to daguerreotypes. The picture being taken upon the plate by
the usual process of daguerreotype, a small part of the back is
cleaned with sand paper, taking care not to allow the face of the
plate to be touched. A piece of wire is then soldered to the part
of the back thus prepared. The plate is then immersed in a solu-
tion of copper, and connected with the battery, the back being
protected by a coating of wax. After a deposit of sufficient depth
has been made upon the face of the plate, it is withdrawn from
the solution, and the plate of copper deposited being detached,
exhibits the picture with an expression softer and finer than the
original. By this process, when conducted with skill, several
copies may be taken from the same daguerreotype.
If the electrotype copy thus obtained be passed through a weak
solution of the cyanide of gold and potassium, in connection with a
weak battery, a beautiful golden tint will be imparted to the
picture, which serves to protect it from being tarnished.
460. Galvano-plastic apparatus. — Having thus explained,
generally, the principles upon which the galvano-plastic processes
are conducted, and the principal expedients by which they are
applied in the arts, we shall show the forms given in practice to
the apparatus by which the effects described above are produced.
One of the most simple forms consists of a cistern filled with a saturated
solution of the sulphate of copper. Two brass rods, communicating one
with the positive and the other with the negative pole of a voltaic battery,
are placed upon it, from one of which the mould, which has been previously
prepared, is suspended. A plate of pure copper being suspended from the
other rod and also immersed in the solution, the decomposition of the sul-
phate of copper commences the moment the current is established. Its
acid and oxygen constituents are attracted to the positive electrode, while
the pure copper is deposited on the negative electrode, which is in this case
the mould. Several moulds may be suspended from the same rod, and the
process will go on simultaneously with all of them. After the lapse of about
forty-eight hours, the moulds will be found covered with a solid and
compact stratum of copper, the adhesion of which to the mould will be
prevented by the means already explained.
The best moulds are those of gutta percha. To make them, the medal or
other object to be reproduced is first covered with plumbago, which will
prevent its adherence to the gutta percha. The gutta percha being then
softened by heating it in warm water, it is applied with a gentle pressure
T
274
VOLTAIC ELECTRICITY.
upon the object to be reproduced. After being left to cool and harden, it is
detached from the object, of which it will retain a perfect impression. The
gutta percha mould thus produced being coated with plumbago to give it the
conducting power, it is suspended in the solution, and connected with the
negative pole of the battery.
The plate of copper, which serves as the positive electrode, also main-
tains the solution at the point of saturation ; for the acid and oxygen, which
are disengaged in contact with it, enter into combination immediately with
the copper, producing the sulphate of that metal, which is dissolved in the
solution, replacing that which it has lost by decomposition.
461. Simple gralvano-plastic apparatus. — A form of appa-
ratus commonly used is represented in fig. 271., where A is a
Fig. 471.
brass rod, supported by hooks I, 2, 3, 4, on the edge of a large
cylindrical vessel of glass or porcelain. One of these hooks, 3,
supports a vertical rod a, on which there is a metallic ball pierced
horizontally, in which a conducting rod N is held by the tightening
screw b.
Supposing the deposit required is copper, the solution of the sulphate of
copper is poured into the vessel. In this vessel is immersed a smaller cylin-
drical vessel M N of unglazed porcelain filled with acidulated water, in which
a cylinder o of amalgamated zinc connected with N is plunged.
Let small bags s s, filled with crystals of the sulphate of copper, be sus-
pended upon the edge of the vessel and immersed in the solution, so that as
ELECTRO-METALLURGY.
275
the solution is weakened by decomposition, these crystals shall be dissolved
and restore its strength.
Let the objects p v T, &c., upon which the copper is to be deposited, be
now suspended upon the ring A by metallic rods : a complete voltaic combi-
nation Avill thus be formed, since the copper electrodes p v T, &c., will be
in metallic connection by the ring A, the rod a, and the conductor N,
with the zinc cylinder o; so that the whole will form a single pair on
Daniell's system (177.)- '1 his being done, the decomposition of the solution
will proceed, copper will be deposited upon PVT, &c., and the strength
of the solution will be restored by the dissolution of the copper crystals
in the bags s s.
462. Spenser's simple apparatus. — A bladder cover D,
Jig. 272., is tied upon one of the mouths of a cylindrical glass
vessel R, open at top and bottom, so as to form a diaphragm.
The metallic solution being poured into the cylindrical vessel c, R is plunged
in it with the end covered by the bladder downwards, and is then partially
tilled with acidulated water. This vessel is supported by a brass ring H H
resting on the edge of the vessel c, to which the conductors E and F are
attached, one K being connected with a disc of zinc A immersed in the
acidulated water in R, and the other with a similar disc of copper B immersed
in the solution in c.
This apparatus acts upon the same principle as that described above.
463. Fau's simple apparatus. — This does not differ much
from those above described.
The cylinder c,fig. 273., is filled with acidulated water ; a smaller cylinder
Fig. r/z. Fig. vjl-
B of zinc is immersed in it. In this latter cylinder B, a still smaller cylinder
of unglazed porcelain A is contained, and the latter is filled with the metallic
T 2
276 VOLTAIC ELECTRICITY.
solution. The conducting rod D D, in contact with the zinc B, by the rod R,
communicates with the object o to be metallised by means of the rods E F.
The sacks s s filled with crystals of the sulphate are immersed in the metallic
solution as before.
464. Brandely's simple apparatus. — In this apparatus the
metallic bath is contained in a large cistern of glazed earthenware
o, fg. 274.
Fig. 274.
A sack made of goldbeaters' skin serving as a diaphragm is nailed to the
edge of a long slit made in a beam of wood c c, which rests upon the edge
of the cistern o. This sack B B is filled with acidulated water, in which a
plate of zinc A is immersed. This zinc is connected by the metallic ribbon
p and the rod DD, and the hooks i, 2, 3, with the objects to be metallised,
which are suspended in the metallic bath contained in the cistern o. The
strength of this solution is maintained as before by bags of the salt s s
suspended in it. The action is in all respects similar to that of those already
described.
465. Compound galvano-plastic apparatus. — In the ar-
rangements above described, the metallic bath in which the process
is conducted constitutes a part of the voltaic apparatus.
In other arrangements, called the compound apparatus, the battery is
placed outside and apart from the metallic bath, and may be at any distance
from it, or even in another room. Such a compound apparatus is represented
in fig. 275., where B is the metallic bath, and B the pile. Two metallic rods
i and 2 communicate with the positive and negative poles of the pile. On
the negative rod 2 are suspended the objects to be metallised, and on the
positive rod I a plate A of the metal which is contained in the solution.
The circuit being closed, the metal decomposed in the solution by the
current is deposited upon the objects CD to be metallised, while a corre-
ELECTRO-TELEGRAPHY.
277
spending portion of the metal of the plate A combining with the acid enters
into the solution, and maintains its strength ; an object which is further
accomplished by the bags of crystals s s.
Fig. Z75.
In the simple apparatus the continued efficiency is more or less impeded,
by the transmission of the two liquid solutions by endosmose through the
porous diaphragm. This is avoided in the compound apparatus just de-
scribed, and others of similar arrangement.
CHAP. XV.
ELECTRO-TELEGRAPHY.
466. Common principle of all electric telegraphs. — Of all
the applications of electric agency to the uses of life, that which is
transcendently the most admirable in its effects, and the most
important in its consequences, is the electric telegraph. No force
of habit, however long continued, no degree of familiarity, can
efface the sense of wonder which the effects of this most marvellous
application of science excite.
The electric telegraph, whatever form it may assume, derives
its efficiency from the three following conditions : —
I . A power to develop the electric fluid continuously, and in
the necessary quantity.
278 VOLTAIC ELECTRICITY.
2. A power to convey it to any required distance without beir?
injuriously dissipated.
3. A power to cause it, after arriving at such distant point, to
make written or printed characters, or some sensible signs serving
the purpose of such characters.
The apparatus from which the moving power by which these
effects are produced is derived, is the voltaic pile. This is to the
electric telegraph what a boiler is to a steam engine. It is the
generator of the fluid by which the action of the machine is pro-
duced and maintained.
We have therefore first to explain how the electric fluid gene-
rated in the apparatus just explained, can be transmitted to a
distance without being wasted or dissipated in an injurious degree
en route.
If tubes or pipes could be constructed with sufficient facility
and cheapness, through which the subtle fluid could flow, and
which would be capable of confining it during its transit, this
object would be attained. As the galvanic battery is analogous
to the boiler, such tubes would be analogous in their form and
functions to the steam pipe of a steam engine.
467. Conducting -wires. — I'f a wire, coated with a noncon-
ducting substance capable of resisting the vicissitudes of weather,
were extended between any two distant points, one end of it being
attached to one of the extremities of a galvanic battery, a stream
of electricity would pass along the wire — provided the other end
of the wire were connected by a conductor with the other extremity
of the battery.
To fulfil this last condition, it. was usual, when the electric
telegraphs were first erected, to have a second wire extended from
the distant point back to the battery in which the electricity was
generated. But it was afterwards discovered that the earth itself
was the best, and by far the cheapest and most convenient,
conductor which could be used for this returning stream of
electricity.
Instead, therefore, of connecting the poles of the battery by a
second wire, they are connected respectively with the earth by
two independent wires, so that the returning current is first
transmitted to the earth, and through the earth to a corresponding
wire at the distant station, to which a telegraphic communication
is made.
This arrangement will be more readily understood by reference
to Jig. 276. If P be the point from which the current is trans-
mitted, it will pass along the wire p to a plate of metal, five or six
feet square, buried in the earth, from whence it will pass through
the earth, as indicated by the arrows, to another plate of metal »',
ELECTRO-TELEGRAPHY.
279
and from thence, by the wire ?<, to the negative pole N of the
battery.
In the arrangement, as here represented, the current is trans-
Fig. z76.
mitted through the wire and the earth from the positive to the
negative pole of the same battery. But the effects will be precisely
the same if P be imagined to represent the positive pole of a
battery at any one station, and N the negative pole of a different
battery at any other station, however distant ; provided only that
the negative pole of the former battery be connected with the
positive pole of the latter by a wire, or series of wires, or any
other continuous conductors.
It has not been found necessary in practice to wrap the wiret
with silk, or to case them with any other nonconductor. They
usually consist of iron, which is recommended at once by its
strength and cheapness, and are coated with zinc, the better to
resist oxidation, by the galvanic process.
The wires thus prepared are usually suspended on posts from
fifteen to thirty feet high, and at intervals of about sixty yards
(Jig. 277.), which is at the rate of about thirty to a mile.
To each of these poles are attached as many tubes or rollers of
porcelain or glass as there are wires to be supported. Each wire
passes through a tube, or is supported on a roller ; and the mate-
280
VOLTAIC ELECTRICITY.
rial of the tubes or rollers being among the most perfect of the
class of nonconducting substances, the escape of the electricity at
the point of contact is prevented.
Fig. 177-
468. Although the mode of carrying the conducting wires at a
certain elevation on supports above the ground has been the most
general mode of construction adopted on telegraphic lines, it has
been found in certain localities subject to difficulties and incon-
venience, and some projectors have considered that in all cases it
would be more advisable to carry the conducting wires under
ground.
This underground system has been adopted in the streets of
London, and of some other large towns. The English and Irish
Magnetic Telegraph Company have adopted it on a great extent
of their lines, which overspread the country. The European
Submarine Telegraph Company has also adopted it on the line
between London and Dover, which follows the course of the
old Dover mail-coach road by Gravesend, Ilochester, and Can-
terbury.
469. The methods adopted for the preservation and insulation
of these underground wires are various.
The wires proceeding from the central telegraph station in
London are wrapped with cotton thread, and coated with a
mixture of tar, resin, and grease. This coating forms a perfect
insulator. Nine of these wires are then packed in a half-inch
leaden pipe, and four or five such pipes are packed in an iron pipe
ELECTRO-TELEGRAPHY. 281
about three inches in diameter. These iron pipes are then laid
under the foot pavements, along the sides of the streets, and are
thus conducted to the terminal stations of the various railways,
where they are united to the lines of wire supported on posts
along the sides of the railways already described.
470. Provisions, called testing posts, are made at intervals of a
quarter of a mile along the streets, by which any failure or acci-
dental irregularity in the buried wires can be ascertained, and the
place of such defect always known within a quarter of a mile.
471. Telegraphic signs. — The current being by these means
transmitted instantaneously from any station to another, connected
with it by such conducting wires, it is necessary to select among
the many effects which it is capable of producing, such as may be
fitted for telegraphic signs.
There are a great variety of properties of the current which
supply means of accomplishing this. If it can be made to affect
any object in such a manner as to cause such object to produce
any effect sensible to the eye, the ear, or the touch, such effect
may be used as a sign ; and if it be capable of being varied, each
distinct variety of which it is susceptible may be adopted as a
distinct sign. Such signs may then be taken as signifying the
letters of the alphabet, the digits composing numbers, or such
single words as are of most frequent occurrence.
The rapidity and precision of the communication will depend on
the rate at which such signs can be produced in succession, and
on the certainty and accuracy with which their appearance at the
place of destination will follow the action of the producing cause
at the station from which the despatch is transmitted.
These preliminaries being understood, it remains to show what
effects of the electric current are available for this purpose.
These effects are: —
I. The power of the electric current to deflect a magnetic
needle from its position of rest.
II. The power of the current to impart temporary magnetism
to soft iron.
III. The power of the current to decompose certain chemical
solutions.
472. Signs made with the needle system. — Let us now
see how these three properties have been made instrumental to
the transmission of intelligence to a distance.
We have explained how a magnetic needle over which an elec-
tric current passes will be deflected to the right or to the left,
according to the direction given to the current. Now, it is always
easy to give the current the one direction or the other, or to
suspend it altogether, by merely changing the end of the galvanic
282
VOLTAIC ELECTRICITY.
trough with which the wires are connected, or by breaking the
contact.
A person, therefore, in London, having command over the end
of a wire which extends to Edinburgh, and is there connected
with a magnetic needle, in the manner already described, can
deflect that needle to the right or to the left at will.
Thus a single wire and a magnetic needle are capable of making
at least two signals.
By repeating the same signals a greater or less number of times,
and by variously combining them, signs may be multiplied ; but it
is found more convenient to provide two or more wires affecting
different needles, so as to vary the signs by combination, without
the delay attending repetition.
Such is, in general, the nature of the signals adopted in the
electric telegraphs in ordinary use in England, and in some other
parts of Europe.
It may aid the conception of the mode of operation and commu-
nication if we assimilate the apparatus to the dial of a clock with
its two hands. Let us suppose that a dial, instead of carrying
hands, carried two needles, and that their north poles, when
quiescent, both pointed to
•ABC
\\ \\\ •»&
D E F
v \v v\v
G H
X, Vfc
A
M N 0 P
/ // m in/
R S T
^ M ja
U V
** JJ
\V
X'Y
123456
twelve o'clock. When the
galvanic current is con-
ducted under either of them,
the north pole will turn
either to three o'clock or
to nine o'clock, according
to the direction given to the
current.
Now, it is easy to imagine
a person in London go-
verning the hands of such
a clock erected in Edin-
burgh, where their indica-
tions might be interpreted
according to a way pre-
viously agreed upon. Thus,
we may suppose that when
the needle No. I. turns to
nine, the letter A is ex-
pressed ; if it turn to three,
the letter B is expressed. If
the needle No. 2. turn to
nine o'clock the letter c is
Fig. i78.— THB SINGLE NF.EDLE TELEGRAPH, expressed ; if it turn to
,.-
z -- -r. --J -- - ----- -:•. ----- - I::'. : - -::--:
to ««^andX<x2. *» Arw^Ae letasr cii apcoKi: tf-54.2.
*e Uira"*to ••«, m* 5«. I. to ARC, tike letter •, »i M £•*.
AeMUb ax>JL ****** n J**m. 1m £* time*, im^nm?
284 VOLTAIC ELECTRICITY.
above, have been brought into extensive use in America, the
needle system being in no case adopted there.
The power of imparting temporary magnetism to soft iron by
the electric current, has been applied in the construction of tele-
graphs in a great variety of forms ; and indeed it may be stated
generally that there is no form of telegraph whatever, in which the
application of this property can be altogether dispensed with.
To explain the manner in which it is applied, let us suppose
the conducting wire at the station of transmission, London for
example, to be so arranged that its connection with the voltaic
battery may, with facility and promptitude, be established and
broken at the will of the agent who transmits the despatch. This
may be effected by means of a small lever acting like the key of
a pianoforte, which being depressed by the finger, transmits the
current. The current may thus be transmitted and suspended in
as rapid alternation as the succession of notes produced by the
action of the same key of a pianoforte.
At the station to which the despatch is transmitted, Edinburgh
for example, the conducting wire is coiled spirally round a piece
of soft iron, which has no magnetic attraction so long as the cur-
rent does not pass along the wire, but which acquires a powerful
magnetic virtue so long as the current passes. So instantaneously
does the current act upon the iron, that it may be made alter-
nately to acquire and lose the magnetic property several times in
a second.
Now let us suppose this soft iron to be placed under an iron
lever, like the key of a pianoforte, so that when the former has
acquired the magnetic property, it shall draw this key down as if
it were depressed by the finger, and when deprived of the mag-
netic property, it will cease to attract it, and allow it to recover
its position of rest. It is evident in this case that movements
would be impressed by the soft iron, rendered magnetic, on the
key at Edinburgh, simultaneous and exactly identical with the
movements impressed by the finger of the agent upon the key in
London. In fact, if the key in Edinburgh were the real key of a
pianoforte, the agent in London could strike the note and repeat
it as often and with such intervals as he might desire.
This lever at Edinburgh, which is worked by the agent in
London, may, by a variety of expedients, be made to act upon
other movable mechanism, so as to make visible signals, or to
produce sounds, to ring a bell or strike a hammer, or to trace
characters on paper by means of a pen or pencil, so as actually to
write the message, or to act upon common movable type so as to
print it. In fine, having once the power to produce a certain
mechanical effect at a distant station, the expedients are infinitely
ELECTRO-TELEGRAPHY. . 285
various by which such mechanical effect may be made subservient
to telegraphic purposes.
474. Morse's system. — The telegraph of Morse, extensively
used in the United States, affords an example of this. To com-
prehend its mode of operation, let us suppose the lever, on which
the temporary magnet acts, to govern the motion of a pencil or
style under which a ribbon of paper is moved, with a regulated
motion, by means of clockwork. When the current passes, the
style is pressed upon the paper, and when the current is suspended,
it is raised from it. If the current be maintained for an interval
more or less continued, the style will trace a line on the ribbon,
the length of which will be greater or less according to the dura-
tion of the current. If the current be maintained only for an
instant, the style will merely make a dot upon the ribbon. Lines,
therefore, of varying lengths, and dots separated by blank spaces,
will be traced upon the ribbon of paper as it passes under the
style, and the relative lengths of these lines, their combinations
with each other and with the dots, and the lengths of the blank
intervening spaces, are altogether under the control of the agent
who transmits the despatch.
[The following table shows the combinations of dots and lines
which have been agreed upon to represent the several letters of
the alphabet : —
B C D E F
K L M N 0
G H I
P Q
{ R S T U V W X Y Z]
A perspective view of the instrument, omitting the paper roller and
ribbon, is given va.fig. 280.
z. The wooden base upon which the instrument is screwed,
B. The brass base plate attached to the wooden base z.
A. The side frames supporting the mechanism.
A, h. Screws which secure the transverse bars connecting the side frames.
G. The key for winding up the drum containing the mainspring, or
supporting the weight, according as the mechanism is impelled by one or
the other power.
3, 4. Clock-work.
«. A lock or gauge to regulate the pressure of the rollers on the paper.
c. The pillar supporting the electro-magnet.
p. The adjusting screw passing into the pillar, c, projecting through the
armature, to enable the telegraphist to adjust the sound of the back stroke
of the armature at pleasure.
o. The spring bar, and
d. the screw to adjust the action of the pen lever.
D. The apparatus for adjusting the paper rollers.
286
VOLTAIC ELECTRICITY.
ELECTPtO-TELEGKAPHY. 287
475. Electro-chemical telegraphs. — The following descrip-
tion of the telegraph of Mr. Bain will convey some idea of the
general principle on which all forms of electro- chemical telegraphs
are based : —
Let a sheet of writing paper be wetted with a solution of prussiate of
potash, to which a little nitric and hydrochloric acid have been added. Let
a metallic desk be provided corresponding in magnitude with the sheet of
paper, and let this desk be put in communication with a galvanic battery so
as to form its negative pole. Let a piece of steel or copper wire forming a
pen be put in connection with the same battery so as to form its positive
pole. Let the sheet of moistened paper be now laid upon the metallic desk,
and let the steel or copper point which forms the positive pole of the battery
be brought into contact with it. The galvanic circuit being thus completed,
the current will be established, the solution with which the paper is wetted
will be decomposed at the point of contact, and a blue or brown spot will
appear. ]f the pen be now moved upon the paper, the continuous succession
of spots will form a blue or brown line, and the pen being moved in any
manner upon the paper, characters may be thus written upon it as it were in
blue or brown ink.
In this manner, any kind of writing may be inscribed upon the paper, and
there is no other limit to the celerity with which the characters may be
written, save the dexterity of the agent who moves the pen, and the suffi-
ciency of the current to produce the decomposition of the solution in the
time which the pen takes" to move over a given space of the paper.
The electro-chemical pen, the prepared paper, and the metallic desk being
understood, we shall now proceed to explain the manner in which a commu-
nication is written at the station where it arrives.
The metallic desk is a circular disc, about twenty inches in diameter. It
is fixed on a central axis, with which it is capable of revolving in its own
plane. An uniform movement of rotation is imparted to it by means of a
small roller, gently pressed against its under surface, and having sufficient
adhesion with it to cause the movement of the disc by the revolution of the
roller. This roller is itself kept in uniform revolution by means of a train
of wheelwork, deriving its motion either from a weight or main spring, and
regulated by a governor or fly. The rate at which the disc revolves may
be varied at the discretion of the superintendent, by shifting the position
of the roller towards the centre ; the nearer to the centre the roller is
placed the more rapid will be the motion of rotation. The moistened paper
being placed on this disc, we have a circular sheet kept in uniform revo-
lution.
The electro-chemical pen, already described, is placed on this paper at a
certain distance from its centre. This pen is supported by a pen-holder,
which is attached to a fine screw extending from the centre to the circum-
ference of the disc in the direction of one of its radii.
On this screw is fixed a small roller, which presses on the surface of the
disc, and has sufficient adhesion with it to receive from it a motion of
revolution. This roller causes the screw to move with a slow motion in a
direction from the centre to the circumference, carrying with it the electro-
chemical pen. We have thus two motions, the circular motion carrying the
moistened paper which passes under the pen, and the slow rectilinear motion
288 VOLTAIC ELECTRICITY.
of the pen itself directed from the centre to the circumference. By the
combination of these two motions, it is evident that the pen will trace upon
Fig. 281.
the paper a spiral curve, commencing at a certain distance from the centre,
and gradually extending towards the circumference. The intervals between
the successive coils of this spiral line will be determined by the relative
velocities of the circular disc, and of the electro-chemical pen. The relation
between these velocities may likewise be so regulated, that the coils of the
spiral may be as close together as is consistent with the distinctness of the
traces left upon the paper.
Now, let us suppose that the galvanic circuit is completed in the manner
customary with the electric telegraph, that is to say, the wire which termi-
nates at the point of the electro-chemical pen is carried from the station of
arrival to the station of departure, where it is connected with the galvanic
battery, and the returning current is formed in the usual way by the earth
itself. When the communication between the wire and the galvanic battery
at the station of departure is established, the current will pass through the
wire, will be transmitted from the point of the electro-chemical pen to the
moistened paper, and will, as already described, make a blue or brown line
on this paper. If the current were continuous and uninterrupted, this line
would be an unbroken spiral, such as has been already described ; but if the
current be interrupted at intervals, during each such interval the pen will
cease to decompose the solution, and no mark will be made on the paper.
If such interruption be frequent, the spiral, instead of being a continuous
line, will be a broken one, consisting of lines interrupted by blank spaces.
If the current be allowed to act only for an instant of time, there will be a
blue or brown dot upon the paper ; but if it be allowed to continue during
a long interval, there will be a line.
Now, if the intervals of the transmission and suspension of the current be
regulated by any agency in operation at the station of departure, lines and
ELECTRO^TELEGRAPHY. 289
dots corresponding precisely to these intervals -will be produced by the
electro-chemical pen on the paper, and will be continued regularly along
the spiral line already described. It will be evident, without further expla-
nation, that characters may thus be produced on the prepared papepr cor-
responding to those of the telegraphic alphabet already described, and thus
the language of the communication will be written in these conventional
symbols.
There is no other limit to the celerity with which a message may be thus
written, save the sufficiency of the current to effect the decomposition while
the pen passes over the paper, and the power of the agency used at the
station of departure to produce, in rapid succession, the proper intervals in
the transmission and suspension of the current.
But the prominent feature of this system is the extraordinary celerity of
which it is susceptible. In an experiment performed by M. Le Verrier and
myself before Committees of the Institute and the Legislative Assembly at
Paris, despatches were sent a thousand miles, at the rate of nearly zoooo
words an hour.*
47 5 a. [Retardation of the current in submarine tele-
graph wires. — Although, with moderate lengths of wire, elec-
trical effects appear to be manifested throughout the whole length,
the instant that both ends are connected with the battery, the
enormous lengths of wire employed for telegraphic purposes have
afforded an opportunity of ascertaining that the passage of the
current, though extremely rapid, is not instantaneous ; and, what
is more remarkable still, that the current does not attain its full
intensity at a distant point of the conductor, until some time after
it first arrives there. These effects are seen much more distinctly
in submarine or underground lines, than with land lines sus-
pended in the air. They were first accurately investigated by
Faraday in 1854, whose principal results we will briefly state.
The line experimented upon was a cable consisting of a cop-
per conducting wire I oo miles long and -^ of an inch in dia-
meter, insulated by a covering of gutta percha, £ of an inch
in thickness. The copper conductor had therefore a superficial
area of 8,300 square feet, and the external surface of the gutta
percha amounted to 30,000 square feet. During the experi-
ments, the cable was immersed in water, and three reometers
were connected with it at different points, one near each end,
and one near the middle, so as to indicate whatever currents
passed through it. One end of the cable being connected with
the ground, and the opposite end with one pole of a battery,
the other pole of which was also in connection with the ground,
the reometer nearest to the battery, which we may distinguish as
reometer A, was deflected almost instantly, then the reometer B at
the middle of the cable, and lastly, after two or three seconds, the
reometer c, placed near the further end ; and in all the reometers
* Lardner's " Electric Telegraph," § 9.
u
290 VOLTAIC ELECTRICITY.
the deflection gradually increased to a maximum, at which it re-
mained constant. On now breaking contact between the battery
and the cable, the needles of the three reometers came successively
to rest : first A, then B, and lastly, c. When contact was made
between the battery and the cable only for a moment, the needle
of A was deflected, and came back immediately afterwards to rest ;
then that of B did the same ; and afterwards that of c, showing that
a wave, as it were, of electricity, passed from one end of the wire
to the other.
Similar results have since been obtained with land lines, but
they are then much less marked.
The cause of these phenomena is the inductive action between
the electricity of the conducting wire, and the natural electricity
of the water which surrounds the gutta percha coating. The wire
when surrounded by water, but separated from it by the gutta
perclxa, maybe compared to the inner coating of a Leyden jar, the
water forming the outer coating. Hence the first portions of
electricity which enter it are neutralised by the opposite electricity
which collects at the outside of the gutta percha, and therefore a
much larger quantity must enter the cable, before any can pass'
out at the other end, than would be required if it was not sur-
rounded by water or any other conductor. In land lines, where
such an external conductor does not exist, the retardation of the
current by inductive action is, as we have already said, much less
perceptible.]
CHAP. XVI.
CALORIFIC, LUMINOUS, AND PHYSIOLOGICAL EFFECTS OF THE
VOLTAIC CURRENT.
476. [Conditions on which the production of heat by the
current depends. — When the poles of a voltaic battery are
joined by a simple metallic conductor, which does not pass near
to any other conductor or to a magnet or magnetisable substance,
none of the mechanical, electrical, magnetic, or chemical effects,
which have been described in previous chapters, can take place :
in this case, the only effect produced by the current outside the
battery is an elevation of the temperature of the conducting wire.
The quantity of heat which a given current is thus able to evolve
in any conductor in a given time, depends not only on the in-
tensity of the current itself, but also on the dimensions of the
conductor and on the conducting power of the substance of which
it is formed.
The exact influence of each of these conditions upon the pheno-
CALORIFIC EFFECTS. 291
menon was first ascertained by Mr. Joule, of Manchester, in
1841. He found that the quantity of heat evolved in a given
time is —
directly proportional to the square of the intensity of the current,
directly proportional to the length of the conductor,
inversely proportional to the sectional area of the conductor, and
inversely proportional to the conducting power of the material of
which the conductor is made.
If we represent the quantity of heat by W, the intensity of the
current by I, the length of the conductor by /, its section by .v,
and its specific conducting power bye, the relations just stated
may be expressed by the following simple mathematical for-
mula : — 7
W = I2_ .
te
But, since the resistance which the conductor opposes to the
passage of the current (and which we will denote by R ) is directly
proportional to the length of the conductor, and inversely pro-
portional to its section and conducting power, we have
and therefore the above expression for the heat evolved by the
current may be put in the following still simpler form : —
W=I*R;
which is equivalent to saying that the quantity of heat evolved by
the current in a conductor in a given time is proportional to the
square of the intensity of the current and to the resistance of the
conductor. Accordingly, if the intensity of the current is doubled,
the quantity of heat evolved will be quadrupled; if the intensity
is tripled, the quantity of heat will be increased nine-fold, and so
on, the resistance being supposed to remain always the same.
The same formula, taken in connection with Ohm's law of the
intensity of the current (219.), shows that in order to develope a
large quantity of heat in a long thin wire, offering a great resist-
ance, we must use several cells connected in series; while to
develope much heat in a short thick wire, offering little resistance,
we must use a single cell with very large plates, or several cells
connected abreast. Hence the efficacy of such arrangements as
Hare's deflagrator (199.)* consisting of a single pair of plates,
having a very large surface.
When the current of a battery of moderate power is sent
through a long thin wire, the resistance of the wire prevents the
current from attaining any great intensity, and accordingly the
wire is not very strongly heated ; but by gradually diminishing the
length of the wire, the resistance is diminished, consequently the
292 VOLTAIC ELECTRICITY.
intensity is increased, and as the heat evolved increases in the
duplicate ratio of the increase of the intensity, the temperature of
the wire will rise higher and higher as its length is shortened.]
477. Calorific effects. — The calorific power of a battery thus
depending on the intensity of the current produced by it, the
batteries constructed on the systems of Grove (179.) and Bunsen
(180.), in which platinum or carbon is combined with zinc, and
excited by two fluids, are the most efficient. With piles of
the latter kind, consisting of ten to twenty pairs, the development
of heat is so considerable that substances which resist the most
powerful blast furnaces are easily fused and burned. Extraordinary
effects are produced by this calorific agency. Metallic wire,
submerged in water, is rendered incandescent, and may be fused
either in vacuo or in an atmosphere of any gas, such as azote or
carbonic acid, which is not a supporter of combustion.
478. [Sources of tbe beat developed by tbe current. — It
has been proved by the experiments of Favre that the heat; de-
veloped by the galvanic current is entirely due to the chemical
action which takes place in the battery. If this same action goes.
on without producing a current, the heat generated is the same as
though a current were formed; the only difference is that it
appears at a different place. The effect of introducing a resistance
to the passage of the current at any part of the circuit, is to cause
an evolution of heat at that point, but not to increase the quantity
generated. This quantity remains always the same, in a battery of
given construction, for the same quantity of zinc dissolved. If the
poles are connected by a short thick wire, little or no heat is de-
veloped in the wire, but almost the whole appears in the battery
itself : if the connecting wire is thinner, some of the heat will be
evolved in it, and less will appear in the battery. The develop-
ment of heat attains a maximum in the wire, and a minimum in
the battery, when the resistance of the wire is equal to the internal
resistance of the battery : in this case, as much heat appears in the
wire as in the battery, and in no case is it possible to make the
quantity of heat evolved outside the battery exceed the quantity
evolved within it.
If, however, instead of the current being allowed to expend
itself entirely in generating heat, it is made to do work of any kind,
— such, for instance, as the mechanical work of giving motion to
an electro -magnetic engine, or the chemical work of decomposing
water — the total quantity of heat developed in the circuit is no
longer equal to what would result from the same kind and amount
of chemical action if it took place without producing a current.
Under such circumstances, the quantity of heat evolved is less
than that which corresponds to the chemical action that goes on in
the battery, by an amount proportional to the quantity of work
CALORIFIC EFFECTS. 293
clone. Thus, if the current is caused to drive an electro-magnetic
engine, the total heat of the circuit is found to be diminished by
precisely as much heat as would be generated by employing the
whole power of the electro-magnetic engine in overcoming fric-
tion. Or, if the current is employed to decompose water, the
heat which it would otherwise develope is lessened by as much as
would result from the recombination of the oxygen and hydrogen
set free.
Even in such cases as these, therefore, it must be observed that
the ultimate dynamical effect of the chemical action which takes
place in the battery remains the same in amount as though the only
result were the production of heat, notwithstanding that part of
it is manifested, at least for a time, under other forms.]
479. Experimental illustration of the conditions -which
affect the calorific power of a current. — If the poles of a
powerful battery be connected by an iron or platinum wire from
two to three feet in length, the metal will become incandescent.
If its length or thickness be diminished, it will fuse or burn. If
its length or thickness be increased, it will acquire first a darker
degree of incandescence, and then will be only heated without
being rendered luminous. The same current which will render
iron or platinum wire incandescent or fuse it, will only raise the
temperature of silver or copper wire of the same length and
thickness without rendering it incandescent. If, on the other
hand, the iron or platinum be replaced by tin or lead of much
greater length or thickness, these metals will be readily fus«d by
the same current.
These phenomena p.re explained by the different conductivity
of these different metals, silver and copper being among the best,
and lead and tin being among the worst metallic conductors of
electricity.
If two pointed pencils of thick platinum wire, being connected
with the poles of the battery, be presented point to point, so that
the current may pass between them, they will be fused at the
points and united, as though they were soldered together. This
effect will equally be produced under water.
4$0. Substances ignited and exploded by the current. —
Combustible or explosive substances, whether solid or liquid, maj
be ignited by the heat developed in transmitting a current through
them. Ether, alcohol, phosphorus, and gunpowder, present ex-
amples of this.
48 1 . Application of this in civil and military engineering.
— This property has been applied with great advantage in
engineering operations, for the purpose of springing mines, an
operation which may thus be effected with equal facility under
water. Experiments made by the Russian military engineers at
294 VOLTAIC ELECTRICITY.
St. Petersburg, and by the English at Chatham, have demonstrated
the advantage of this agency in military operations, more especially
in the springing of subaqueous mines.
In the course of the construction of the South Eastern Railway
it was required to detach enormous masses of the cliff near Dover,
which, by the direct application of human labour, could not have
been accomplished, save at an impracticable cost. Nine tons of
gunpowder, deposited in three charges, at from fifty to seventy
feet from the face of the cliff, were fired by a conducting wire,
connected with a powerful battery, placed at I ooo feet from the
mine. The explosion detached 600000 tons weight of chalk
from the cliff. It was proved that this might have been equally
effected at the distance of 3000 feet. (See also 302.)
482. Jacobi's experiments on conduction by water. —
Jacobi instituted a series of experiments, with a view to ascertain
how far water might be substituted for a metallic conductor for
telegraphic purposes. He first established (as Peschel states) a
conduction of this nature between Oranienbaum and an arm of
the Gulph of Finland, a distance of 5600 feet, one half through
water, and the other through an insulated copper wire, three
fourths of a line in diameter, which was carried over a dam, so
that the entire length of the connection was 11200 feet. The
electric current was excited by a Grove's battery of twenty-four
pairs, and a common voltaic pile of 150 six-inch plates. A zinc
plate of five square feet was sunk in the sea from one pole of the
battery, and at the opposite end of the connecting wire a similar
plate was sunk in a canal joining the sea. Charcoal points were
used for completing the circuit of the Grove's battery ; these, and
also a fine platinum wire, were made red hot, and these pheno-
mena appeared to be more intense than when copper wires were
used as conductors. In a later experiment he employed a similar
conduction, the distance in this case being 9030 feet, namely,
from the winter palace of the emperor, to the Fontanka near the
Obuchowski bridge. One of the conductors was a copper wire
carried underground, the other was the Neva itself, in which a
zinc plate of five square feet was sunk beneath the surface of the
river. At the other extremity a similar zinc plate was immersed
in a small pond, whose level was five or six feet above the Fon-
tanka, from which it was separated by a floodgate. The battery
consisted of twenty-five small Daniell's constant batteries, by
means of which, notwithstanding the great extent of water, all the
galvanic and magnetic phenomena were produced. At Lenz's
suggestion, a different species of conduction was tried between the
game stations. A connection was established with a point of the
iron roof of the winter palace, which was connected with the
ELECTRIC LIGHi'. 295
ground by means of conducting rods, and the current was carried
equally well along the moist earth.
483. Combustion of the metals. — If thin strips of metal or
common metallic leaf be placed in connection with the poles of a
battery, it will undergo combustion, the colour of the flame
varying with the metal, and in all cases displaying very striking
and brilliant effects. Gold thus burned gives a bluish-white light,
and produces a dark brown oxide. Silver burns with a bright
sea-green flame, and copper with a bluish-green flame, mingled
with red sparks, and emits a green smoke. Zinc burns with a
dazzling white light, tin with red sparks, and lead with a purple
flame. These phenomena are produced with increased splendour,
if the metal to be burned attached to one pole be brought into
contact with mercury connected with the other pole.
484. [Spark produced by the voltaic current.— Except with
batteries composed of an extraordinary number of cells, the tension
at the ends of the conductors is not sufficient to produce any per-
ceptible spark at the moment when the circuit is closed, but a
battery of very moderate power will exhibit a spark of more or
less intensity when the circuit is opened.
The spark on closing the circuit was obtained in a remarkable
manner by Mr. Gassiot, by means of a battery composed of 3,520
pairs of zinc and copper plates charged with rain water. When
the terminals of this battery were brought within -^th of an inch of
each other, a continuous stream of sparks passed between them
during a space of five weeks.
The spark produced on opening the circuit is greatly increased
in brilliance by causing the current to traverse, at some part of its
course, a helix of covered copper wire surrounding a core of soft
iron. This effect is due to the mutual inductive action exercised
upon each other by the several convolutions of the helix, whereby
a momentary induced current, in the same direction as that of the
battery, is produced when the circuit is opened.
The spark may also be generally obtained by the following
methods.]
Fasten a fine sewing-needle to the end of one of the wires, and
touch the other pole with the free end of the needle ; a starlike
red spark will be emitted. A continued stream of these sparks
may be obtained by connecting a small round or triangular file
with one pole, and presenting to it and removing from it with great
rapidity the point of a copper wire attached to the other pole.
485. The electric light. — Of all the luminous effects pro-
duced by the agency of electricity, by far the most splendid is the
light produced by the passage of the current, proceeding from a
powerful battery, between two pencils of hard charcoal presented
VOLTAIC ELECTRICITY.
Fig.zSj.
point to point. The charcoal being an imperfect conductor is
rendered incandescent by the current, and being infusible at any
temperature hitherto attained, the degree of splendour of which
its incandescence is susceptible has no other practical limit except
the power of the battery.
The charcoal best adapted for this experiment is deposited in
gas retorts at the part exposed to the greatest heat. This
is hardened and formed into pencil-shaped pointed cylinders,
from two to four inches in length, and
mounted as represented in ^g-. 283., where
p and n, the two metallic pencil holders,
are in metallic connection with the poles
of the pile, and so mounted that the char-
coal pencils fixed in them can at pleasure
be made to approach each other until their
points come into contact, or to recede from
each other to any necessary distance.
When they are brought into contact, the
current will pass between them, and the
charcoal will become intensely luminous.
When separated to a short distance, a
splendid flame will pass between them of
the form represented in fig. 284.. It will be observed that the
form of the flame is not symmetrical with relation
to the two poles, the part next the positive point
having the greatest diameter, and the diameter be-
coming gradually less in approaching the negative
point.
486. Incandescence of charcoal by the cur-
rent not combustion. — It would be a great error
to ascribe the light produced in charcoal pencils
to the combustion of that substance. None of the
consequences or effects of combustion attend the phenomena, no
carbonic acid is produced, nor does the charcoal undergo any
diminution of weight save a small amount due to mere me-
chanical causes. On the contrary, at the points where the calo-
rific action is most intense, it becomes more hard and dense. But
what negatives still more clearly the supposition of combustion is,
v;hat the incandescence is still more intense in a vacuum, or in any
of the gases that do not support combustion, than in the ordinary
atmosphere.
Peschel states that, instead of two charcoal pencils, he has laid
a piece of charcoal, or well burnt coke, upon the surface of mer-
cury, connected with one pole of the battery, while he has touched
it with a piece of platinum connected with the other pole. In this
Fig. z84.
ELECTRIC LIGHT.
297
manner he obtained a light whose splendour was intolerable to
che eye.
487. Electric lamps of Messrs. Foucault, Deleuil, and
Dubosc-Soleil. — M. Foucault first applied the electric light pro-
duced by charcoal pencils as a substitute for the lime light in the
gas microscope.
This apparatus, in the form in which it is now constructed by M. Dubosc
of Paris, is represented in fig. 285. M. Dubosc has applied to his photo-
Fig. 285.
electric microscrope a self-adjusting apparatus, by which the light is main-
tained with a nearly uniform brilliancy, notwithstanding the gradual waste
of the charcoal. This is accomplished by an electro-magnet, by which
the current is re-established, whenever it has a tendency to be suspended.
Photo-electric apparatus of MM. Deleuil. — This apparatus, which is repre-
sented \nfiy. 286., has a self-acting adjustment, and is of cheaper construction
than that of M. Dubosc. The negative charcoal pencil is supported by a
metallic rod which slides with friction in a suoport D, but being once regu-
298
VOLTAIC ELECTRICITY.
lated remains fixed. The positive pole is continually raised by the current
itself as the charcoal is wasted. This is accomplished by a regulating
TTJT
Fig. 286.
apparatus placed under the stage. A lever, attached at one end to a spiral
spring, is capable of oscillating through a very small angle on a centre,
being maintained at the other end between the points of two screws seen
under the stage in the figure, which limit its play. The lever is drawn
upwards by the spring, and in the contrary direction by the electro-magnet.
In fine, a small straight spring fixed at the extremity of the lever is pressed
upon small teeth ranged like those of a rack on the rod, which carries
the positive charcoal pencil, and transmits to this latter the motion of the
lever.
This being understood, so long as the current passes with its full intensity,
the electro-magnet attracting its armature, which is fixed to the lever,
one arm of the lever is raised, and the opposite arm is lowered, and conse-
quently the spring is drawn down, so that its upper extremity is lowered
from one tooth to another of the rack ; when, on the contrary, the distance
between the charcoal points being augmented, the current is enfeebled, the
electro-magnet being no longer capable of supporting the arm of the lever,
the end is drawn upwards by the spiral spring, and the small spring being
pressed against a tooth of the rack, drives it upwards and raises the pencil.
The charcoal points are, therefore, again brought into contiguity, and the
current is re-established.
I have had an apparatus of this kind in operation with great
PHYSIOLOGICAL EFFECTS. 299
efficiency for some years. It is worked by a battery on Bunsen's
principle, consisting of fifty pairs.
488. Method of applying the heat of the electric current
to the fusion of refractory bodies and the decomposition of
the alkalis. — This is accomplished by substituting for the charcoal
pencil, p, Jig. 283., a piece of charcoal in the form of a small cup,
as represented mfig- 287.
A small piece of the substance to be acted on is placed in the
charcoal cup s, and the electric flame is made to
play upon it by bringing it into proximity with the
pencil above it. In this way gold or platinum may
be fused, or even burned. If a small piece of soda
s or potash be placed in the cup s, its decomposition
will be effected by the flame, and small globules of
sodium or potassium will be pi^oduced in the cup,
Fig. 187. which will launch themselves towards the point of the
pencil, undergoing at the same time combustion, and
thus reproducing the alkali.
489. Physiological effects of the current. — This class of
effects is found to consist of three successive phases : first, when
the current first commences to pass through the members affected
by it ; secondly, during its continuance ; and, thirdly, at the
moment of its cessation. A sharp convulsive shock attends the-
first and last ; and the intermediate period is marked only by
slight and irregular quiverings of the muscles. The shock of a
voltaic battery has been said to be distinguished from that pro-
duced by a Leyden jar, inasmuch as the latter is felt less deeply,
affecting only our external organs, and being only instantaneous
in its duration ; while theformerpervades the system, propagating
itself through the whole course of the nerves which extend
between its points of admission and departure.
It appears that the physiological effect of the current depends
altogether on its intensity, and little or not at all upon its quantity.
This is proved by the fact, that the effect of a battery of small
plates is as great as one consisting of the same number of large
plates. A single pair, however extensive be its surface, produces
no sensible shock. To produce any sensible effect, from ten to
fifteen pairs are necessary. A battery of 50 to loo pairs gives a
pretty strong convulsive shock. If the hands, previously wetted
with salted water, grasp two handles, like those represented at p
and N, fig. 2IO., connected with such a battery, violent shuddering
of the fingers, arms, and chest will be produced ; and if there be
any sore or tender parts of the skin, a pricking or burning sensa-
tion will be produced there.
The voltaic shock may be transmitted through a chain of
300 VOLTAIC ELECTRICITY.
persons in the same manner as the electric shock, if their hands,
which are joined, be well moistened with salted or acidulated
water, to increase the conducting power of the skin.
As the strongest phases of the shock are the moments of the
commencement and cessation of the current, any expedient which
produces a rapid intermission of the current will augment its
physiological effect. This may be accomplished by various simple
mechanical expedients, by which the contact of the conductors
connecting the poles may be made and broken in rapid succession ;
but no means are so simple and effectual for the attainment of this
object as the contrivances for the production of the magneto-
electric current described in (295.), which, in fact, is exactly the
rapidly intermitting current here required.
490. Therapeutic agency of electricity. — Electric excitation
has been tried as a curative agent for various classes of maladies
from the date of the discovery of the Leyden jar. Soon after the
discovery of galvanism, Galvani himself proposed it as a therapeutic
agent ; but although a great number of scientific practitioners in
different countries have devoted themselves to the investigation
of its effects, there still remains much doubt, not only as to its
curative influence, but as to the classes of maladies to which it may
be with advantage applied, and even as to its mode of application.
It appears, however, to be generally admitted that voltaic elec-
tricity is much better fitted for medical purposes than common
electricity, and that of the different forms of voltaic electricity in-
termitting currents produced by induction are in general to be pre-
ferred to the immediate currents produced by the battery. It is
even maintained by practitioners who have more especially de-
voted themselves to the study of its effects, that different induced
currents have different therapeutic properties.
A current produced by the immediate induction of another
current proceeding directly from the voltaic battery is called an
induced current of the first order.
If an induced current of the first order be applied to produce,
by induction, another current in an independent wire, such cur-
rent is called an induced current of the second order.
It is maintained by practitioners that these two orders of in-
duced currents have different therapeutic effects, and that the
effects of both of them differ from those of a primary current.
Induced currents, however intense, having only a feeble chemical
action, it follows that when they are transmitted through the
organs, they do not produce there the effect of primary currents,
and consequently do not tend to produce the same disorgan-
isation. Dr. Duchenne, who has made numerous experiments
on the medical application of galvanic electricity, has ascertained
DUCHENNE'S APPARATUS. 301
that induced currents used to electrify the muscles of the face,
act but very feebly on the retina, while the primary current
proceeding from the battery acts so strongly on that organ as to
affect it dangerously, as the effects of practice have proved. The
same practitioner holds, that while the induced currents of the
first order produce strong muscular contractions, and are attended
with little effect on the cutaneous sensibility, induced currents of
the second order, on the contrary, exalt the cutaneous sensibility
to such a degree, that their application should be avoided in the
case of all patients whose skin is very irritable.
It appears to result from the experience of practitioners that
the use of voltaic electricity in therapeutics should be guided by a
profound knowledge of its physiological properties. Matteucci,
in his lectures on the physical phenomena of living bodies, recom-
mends that in the application of voltaic electricity a current oi
very feeble intensity should be first employed. He mentions the
case of a paralytic patient who was seized with strong tetanic
convulsions, in consequence of the application of a current pro-
duced only by a single pair. He recommends further, that in no
case shoul'd the voltaic action be prolonged beyond a moderate
interval, that the intermitting current should always be preferred
to the continued, and that after each series of twenty or thirty
shocks the operation should be suspended.
An infinite variety of apparatus have been contrived for the
therapeutic application of voltaic electricity. The following may
serve as examples of these : —
49 1 . Duchenne's electro-voltaic apparatus. — This apparatus
consists of a bobbin wrapped with coils of two wires, like that
already explained in (290.).
This bobbin is enclosed in a brass tube Q,fig. 288. The apparatus is fixed
upon a mahogany case containing two drawers. The first contains a compass
needle mounted as a reometer, and serving to measure the intensity of the
primary current. The second contains in a compact form a charcoal bat-
tery. The zinc element M has itself the form of the drawer, and contains a
solution of sea salt, and a rectangular piece u made of the charcoal of coke
well calcined and prepared in the same manner as for Bunsen's battery. In
the central part of the charcoal is a little cavity, in which a small quantity
of nitric acid is poured, which is immediately absorbed. Two ribbons of
copper proceeding from the poles of the battery are connected with the
buttons L, and N attached to the front of the drawer. The first of these L
is connected with the zinc end of the battery, and represents the negative
pole; and the second is connected with the charcoal end, and represents the
positive pole.
When the drawers are closed, the buttons L and N are put in connection
with two pieces connected with the arrangement combined within the
cylinder o. One of these pieces is movable, so that the circuit can be closed
and broken at pleasure.
302
VOLTAIC ELECTRICITY.
The induced current is produced only at. the moments when the primary
current commences and terminates. It is therefore necessary that the
Fig. z88.
latter current should be subject to continued intermission. In the present
apparatus, these intermissions may be rendered at pleasure more or less
rapid. To render them rapid, the current passes into a piece of soft iron A.
which oscillates very rapidly under the influence of a bundle of soft iron
wires placed in the axis of the bobbin, and temporarily magnetised by the
current. It is this piece A which, by its alternate motion to and fro, inter-
rupts and re-establishes the primary current, and by that means produces
the intermission of the induced current.
To produce a slow intermission of the current, the oscillating piece A is
rendered fixed by means of a little rod b ; and instead of making the current
pass through the piece A, it is made to pass through an elastic ribbon «, and
through the metal teeth of a wooden wheel with which that ribbon is con-
nected, and which appears in the figure above the needle of the galvano-
meter. By turning a handle provided for the purpose, but which is not
represented in the figure, the current is interrupted as often as the ribbon e
ceases to touch a tooth ; and as there are four teeth, there are four inter-
missions in each revolution, so that the operator, by turning a handle more
or less rapidly, can vary at will the rate of intermission, and, consequently,
the number of shocks imparted in a given time.
To transmit the shocks, the extremities of the wire conducting the induced
current are put in connection with two buttons E and F at the end of the
cylinder, and these buttons are themselves connected by means of two con-
ducting wires wrapped with silk, with two exciters having glass handles
o o. The operator holding them by the glass handles, and applying their
bases to the two parts of the body of the patient, between which he intends
DUCHENNE'S APPARATUS.
303
to transmit the shock, the desired effect is produced, its intensity being regu-
lated by turning the handle already mentioned.
A regulator is also provided by which the intensity of the current can be
varied at will. This consists of a copper cylinder which envelopes the
bobbin, and which can be drawn from it more or less, like a drawer, by the
aid of a graduated rod. The greatest intensity is produced when the
regulator is drawn out, so as to uncover the bobbin altogether, and the
minimum when it completely covers it. The effect of this cylindrical cover
is explained by the induced currents which are produced in its mass.
492. Duchenne's magneto-electric apparatus. — This appa-
ratus, represented in Jig. 289., acts upon the principle explained
Fig z89.
in (297.). The magnet M B has two arms connected at their pos-
terior extremities by an armature of soft iron. In front is another
armature x, also of soft iron, which turns upon a horizontal axis,
to which motion is imparted by the wheel and pinion A, and the
handle B.
Upon the two arms of the magnets a copper wire wrapped with silk is
coiled, destined to receive the inductive action of the magnets. Upon this
first wire a second F c is coiled, in which an induced current of the second
order is produced.
When a motion of rotation is imparted to the armature x, this piece, being
magnetised at each moment that it passes the poles of the magnets M s,
304 VOLTAIC ELECTRICITY.
exercises upon the distribution of magnetism in them an action which
produces in the first wire an induced current of the first order, and this wire,
reacting upon the second wire, produces in it an induced current of the
second order. These currents, however, may be separately developed by
means of pieces j and i, each of which is double, but one of which only is
shown in the figure. The current passes by them through the covered
helical wires to the exciters N N, which are similar to those already described
in the former apparatus.
The intermissions necessary for the production of the induced currents are
obtained by means of the commutator B, which is analogous to that already
described in the case of Clarke's magneto-electric apparatus,^, an., and
by means of a system of metallic pieces, o, L, Y, and T.
The intensity of the shocks is regulated by the button and screw v, which
serve to bring the magnets and the armature x nearer to or more distant
from each other ; but a more effectual regulator is supplied by two copper
cylinders G G, which envelope the bobbins, and, by means of the graduated
rod H, can be drawn off or on them to any desired extent. These have the
same effect as the similar envelope described in the former apparatus.
The therapeutic effects of these apparatus are reputed, among
French medical practitioners, to be beneficial in several classes of
maladies, and especially in paralytic cases.
493. Pulvermacber's galvanic chain. — This apparatus, which
is represented uifig. 290., consists of a series of small cylindrical
Fig. 19=.
rods of wood, upon which are rolled, one beside the other, with-
out contact, however, a wire of zinc and a wire of copper. One
of these rods with the wires rolled upon it is shown upon a larger
scale in fig. 29 1 .
At each of its ends the zinc wire erf, fig. 291., of the cylinder
A is jointed to the copper wire of the cylinder B by means of two
little rings of copper implanted in the wood. The zinc wire of the
cylinder B is then connected, in the same manner, with the copper
PULVERMACHER'S CHAIN. 305
wire of the third cylinder, and so on, so that the zinc of one
cylinder always forms, with the copper of the following cylinder, a
couple altogether analogous to the arrangement of the ordinary
galvanic pile.
The combination thus forming a sort of flexible chain is held by the
operator, as shown in jig. 290.,
and plunged in a vessel con-
taining vinegar and water.
The wooden rods, which are
very porous, imbibing the aci-
dulated liquid, assume the cha-
racter of the discs of cloth or
pasteboard in the original vol-
taic pile shown in fig. 129. ;
and the chemical action which
ensues between the zinc and
Fi?. 291. the acetic acid of the vinegar
produces a current, the inten-
sity of which is proportional to the number of pairs in the chain. Thus a
chain consisting of 120 pairs will impart a strong shock.
The interruption of the current is produced by two armatures H and N,
fig. 290., to which the two poles of the chain are attached. The armature N
serves only to establish more surely the contact with the hand; but the
armature M, besides this, serves to interrupt the current. For that purpose,
a piece of clockwork is contained within it, which imparts an oscillating
motion to a movable piece, so that the pole of the pile is alternately thrown
into and out of contact with the armature. The rapidity of the oscillations,
and, consequently, the number of shocks imparted in a given time, can be
varied within certain limits by means of a little regulator, which is ad-
justed by the hand. In fine, the clockwork is wound up by turning the
handle o,fg. 290.
494. Medical application of the voltaic shock. — The in-
fluence of the galvanic shock on the nervous system in certain
classes of malady has been tried with more or less success, and
apparatus have been contrived for its convenient application, both
generally and locally, to the system. The most convenient forms
of apparatus for this purpose are those which have been explained
in the preceding paragraphs, and which have derived great con-
venience and efficacy from the expedients by which the operator
is enabled to measure and regulate the intensity of the shock with
the greatest certainty and precision by surrounding the rim of
the electro-magnet with loose cylinders or globes of thin copper,
movable upon them in the manner above described, so as to
increase or diminish at will the force of the induced current.
495. Effects on bodies recently deprived of life. — This
class of phenomena is well known, and, indeed, was the origin of
the discovery of galvanism. Galvani's original experiment on the
limbs of a frog, already noticed (158.), has often been repeated.
3o6 VOLTAIC ELECTRICITY.
Bailey substituted for the legs of the frog those of the grass-
hopper, and obtained the same results.
Experiments made on the bodies of men and inferior animals
recently deprived of life have afforded remarkable results. Aldini
gave violent action in this way to the various members of a dead
body. The legs and feet were moved rapidly, the eyes opened
and closed, and the mouth, cheeks, and all the features of the face
were agitated by distortions. Dr. Ure connected one of the poles
of a battery with the supraorbital nerve of a man cut down after
hanging for an hour, and connected the other pole with the nerves
of the heel. On completing the circuit the muscles are described
to have been moved with a fearful activity, so that rage, anguish,
and despair, with horrid smiles, were successively expressed by
the countenance.
This agency has been used occasionally with success as an ex-
pedient for restoring suspended animation.
The bodies and members of inferior animals recently killed are
susceptible of the same influence, though in a less degree. The
current sent through the claw of a lobster recently torn from the
body, will cause its instant contraction.
496. Effect of tlie shock upon a leech. — If a half-crown
piece be laid upon a sheet of amalgamated zinc, a leech placed
upon the coin will betray no sense of a shock, until, by moving,
some part of it comes into contact with the zinc. The connection
being thus established, the leech will receive a shock, as will be
rendered manifest by the sudden recoil of the part which first
touches the zinc.
497. Excitation of the nerves of taste. — If a metallic plate,
connected with one pole of the battery, be applied to the end of
the tongue, and another wetted with salted water, and connected
with the other pole, be applied to any part of the face, the metal
on the tongue will excite a peculiar taste, acid or alkaline, ac-
cording as it is connected with the positive or negative pole. This
is explained by the decomposition of the saliva by the current.
498. Excitation of the nerves of sight. — If a metallic plate,
wetted with salted or acidulated water, be applied at or near the
eyelids, and another be applied at any other part of the person,
a peculiar flash or luminous appearance will be perceived the
moment the plates are put into connection with the poles of a
battery. The sensation will be reproduced, but with less in-
tensity, the moment the connection is broken. A like effect, but
less intense, is produced, when the current is transmitted through
the cheek and gums.
499. Excitation of the nerves of hearing. — If the wires
connected with the poles of a battery be placed in contact with
ELECTRIC FISHES.
307
the interior of the two ears, a slight shock will be felt in the head
at the moment when the connexion is made or broken, and a roar-
ing sound will be heard so long as the connexion is maintained.
500. [Development of electricity in the animal organism.
The chemical processes which go on in the voltaic battery having
been shown to be attended with the development of enormous
quantities of electricity, it is reasonable to expect that the count-
less and complex chemical changes which take place in the bodies
of animals must likewise give rise to electrical phenomena. Such
in fact is really found to be the case. Numerous isolated observa-
tions on the part of older investigators, but especially the elaborate
researches of Matteucci and Du Bois-Reymond, have demonstrated
the existence of electric currents in all parts of the body, and
more particularly in muscular and nervous tissue.
The current in the muscles, or muscular current, is found to obey
the following general laws : — the longitudinal section, natural or
artificial, of a muscle is positive in respect to its natural or arti-
ficial transverse section ; any point in the longitudinal section is
positive with respect to any other point at a greater distance than
itself from the middle of the section ; and any point in the trans-
verse section is positive with respect to any other point situated
nearer than itself to the centre of the section. The currents
existing between different points of the same section, longitudinal
or transverse, are however much feebler than those existing
between a longitudinal and a transverse section. After death
these currents disappear pari passu with the irritability of the
tissue, and cease with the onset of rigor mortis.
The current in the nerves, or nervous current, is subject to the
same laws as the muscular current, and, like the latter, its in-
tensity is in direct proportion to the irritability of the part.
When either of these tissues is thrown into action — i.e., when a
nerve is stimulated or a muscle contracts — the natural current
above described is diminished in intensity or even reversed in
direction.]
501. Electrical Fishes. — The most conspicuous example of
the development of electricity in the animal organisation is pre-
sented by certain species of fish. Of these electrical fishes there
are seven genera : —
i. Torpedo narke risso. 5. Silurus electricus.
i. „ unimaculata. 6. Tetraodon electricus.
j. „ marmorata. 7. Gymnotus electricus.
4. ,, galvanii.
502. Properties of the torpedo ; observations of Walsh.
According to the observations of Walsh, who first submitted this
animal to exact inquiry, the following are its effects : —
If the finger or the palm of the hand be applied to any part of
X 2
308 VOLTAIC ELECTRICITY.
the body of the animal out of the water, a shock will be felt similar
to that produced by a voltaic pile.
It; instead of applying the hand directly, a good conductor, such
as a rod of metal several feet in length, be interposed, the shock
will still be felt.
If nonconductors be interposed, the shock is not felt.
If the continuity of the interposed conductor be anywhere
broken, the shock is not felt.
The shock may be transmitted along a chain of several persons
with joined hands, but in this case the force of the shock is rapidly
diminished as the number of persons is increased. In this case
the first person of the chain should touch the torpedo on the belly,
and the last on the back.
When the animal is in the water, the shocks are less intense
than in the air.
It is evident that the development of electricity is produced by
a voluntary action of the animal. It often happens that in touch-
ing it no shock is felt. But when the observer irritates the animal,
shocks of increasing intensity are produced in very rapid succes-
sion. Walsh counted as many as fifty electrical discharges pro-
duced in this way in a minute.
503. Observations of Becquerel and Breschet. — In a series
of observations and experiments made on the torpedos of Chioggia
near Venice by MM. Becquerel and Breschet, it was ascertained
that when the back and belly were connected by the wires of a
sensitive reoscope, a current was indicated as passing from the
back to the belly. They also found that the animal could at will
transmit the current between any two points of its body.
504. Observations of Matteucci. — In a series of experiments
made on the torpedos of the Adriatic, M. Matteucci confirmed the
results obtained by MM. Becquerel and Breschet, and also suc-
ceeded in obtaining the spark from the current
passing between the back and belly.
505. The electric or gran. — In the several
species of fish endowed with this quality, the
organ in which the electric fluids are developed
differs in form, magnitude, position, and structure.
506. Tne torpedo,^. 292., is a flat, cartila-
ginous fish which resembles the common ray.
Its body is smooth, and has the form of a nearly
circular disc, the anterior border of which is
formed by two prolongations of the muscle which
are connected on each side with the pectoral fins,
and which have between these organs an oval
Fig. 291. space in which the electric apparatus is deposited.
THE TORPEDO.
3^9
This apparatus, which is shown in fig. 293., is composed of a
multitude of membranous prismatic tubes lying closely together,
and subdivided by horizontal partitions into small cells, like those
of a honeycomb, filled with mucous matter, and traversed by the
ramifications of several large trunks of the pneumogastric nerves.
Four or five hundred of these prisms are commonly counted in
each organ. Hunter in one case found 1182. They are nearly
at right angles to the surface of the skin, to which they are
strongly attached at the ends. When the structure of each of
these prisms is examined, they are found to consist of a multitude
of thin plates whose planes are perpendicular to the axis of the
prism, separated from each other by strata of mucous matter, and
forming a combination resembling the original galvanic pile.
Four bundles of nerves of considerable volume are distributed
Fig. 49$.
in the organ, and, according to Matteucci, the seat of the elec-
trical power is at their origin.
3io
VOLTAIC ELECTRICITY.
In fig. 293. A is the brain, B the spinal cord, c the eye and
optic nerve,v D the electric organs, E the pneumogastric nerves
ramifying through this organ, F the branch of these nerves con-
stituting the lateral nerve, and G the spinal nerve.
These organs develope electricity, which is identified in all its
physical properties with that of the electric or voltaic apparatus.
The torpedo, though less powerful than the gymnotus, is capable,
nevertheless, of rendering insensible the arms of those who
touch it.
It has been lately ascertained that the electric functions of these
organs have a close connection with the posterior lobe of the
brain, since by destroying this lobe or dividing the nerves which
proceed from it, the animal is deprived of the electric power.
Several species of the torpedo inhabit the seas that wash the
coast of Europe. They have been frequently found near the
shores of Vendee and Provence in France.
507. The Silurus electricus, fig. 294., another of these
species, which is found in the Nile and Senegal, has a length of
Fig. 494.
from twelve to sixteen inches. The seat of its electric power
seems to be a particular tissue situate between the skin and the
muscles of the sides, having the appearance of a foliated cellular
tissue. The Arabs give to this fish the name Raasch, an Arabic
word which signifies thunder.
508. Gymnotus electricus. —
One of the species which possess
this curious physical power is
the Gymnotus electricus, or elec-
tric eel, fig. 295. This species,
which inhabits Southern America,
closely resembles common eels,
wanting, however, the fins at the
end of the tail, and no scales being
visible upon its skin, which is co-
vered with a glutinous matter.
Its length is from six to seven
feet, and it is commonly met with
THE ELECTRIC EEL. 311
in the streams and ponds, which are found in various places in the
immense plains which overspread the valleys of the Cordilleras,
the banks of the Oronoco, &c. The electric shocks which the
animal is enabled ti give at will have an intensity sufficient to
paralyse not only men but horses. It uses this organ accordingly,
not only to defend itself from the attacks of its enemies, but to
kill at a distance the fishes on which it feeds, the water being a
sufficient conductor of electricity to transmit the shock. Its first
discharges are generally weak ; but when the animal is irritated
and roused, they become stronger, and at length acquire a terrible
intensity. When the animal has communicated a certain number
of these shocks, it becomes exhausted, and is forced to desist,
and it is not until after the lapse of a certain interval that it is
enabled to recommence. It would appear as though the electric
organ, like the scientific machine, when once completely dis-
charged, requires a continued action of the exciting power, which
in this case is a vital function of the animal, to recharge it.
Manner of capturing- them. — The natives of the coun-
tries which the animal inhabits, avail themselves of this temporary
suspension of its offensive power to capture it. Troops of wild
horses are driven into the reservoir in which the creature is
known to prevail ; immediately the horses are fiercely attacked,
receiving a rapid succession of intense electric shocks, by which
they are more or less stunned and paralysed, and not unfrequently~
killed ; but the assault has the effect of exhausting the electric
eels, and rendering them comparatively inoffensive, so that they
are easily captured, either by the net or harpoon.
Electric organs. — The apparatus by which the gymnotus
produces these electric shocks, is extended along the entire length
of the back to the tail, and consists of four longitudinal masses
composed of a great number of membranous folds, connected by
an infinite number of smaller membranes placed transversely to
them. The small prismatic cells formed by the combination of
these membranes are filled with gelatinous matter, and the whole
apparatus is supplied with large nerves.
BOOK THE THIRD.
MAGNETISM.
CHAPTER I.
DEFINITIONS AND PRIMARY PHENOMENA.
509. Natural magnets — loadstone. — Certain ferruginous mi-
neral ores are found in various countries, which being brought
into proximity with iron manifest an attraction for it. These are
called natural magnets, a term derived from Magnesia, a city of
Lydia, in Asia Minor, where the Greeks first discovered and ob-
served the properties of these minerals.
The natural magnet is also called the loadstone, or more pro-
perly lodestone, or leadstone, a name indicative of the guiding pro-
perty of the magnet, just as the polar star was called the lodestar.
The natural magnet is a compound consisting of one equivalent
of the protoxide and one of the sesquioxide of iron. This mineral
abounds in Sweden and Norway, where it is worked for the pro-
duction of the iron of commerce, yielding the best quality of that
metal known.
5 1 o. Artificial magnets. — The same property may be im-
parted to any mass of iron, having any desired magnitude or form,
by processes which will be explained hereafter. Such pieces of
iron having thus acquired these properties are called artificial
magnets; and it is with these chiefly that scientific experiments
are made, since they can be produced in unlimited quantity of
any desired form and magnitude, and having the magnetic virtue,
within practical limits, in any desired degree.
511. Neutral line or equator — poles. — This attractive
power is not diffused uniformly over every part of the surface.
It is found to exist in some parts with much greater force than in
ofhers, and on a magnet a certain line is found where it disappears.
This line divides the magnet into two parts or regions, in which
the attractive power prevails in varying degrees, its energy aug-
menting with the distance from the neutral line just mentioned.
This neutral line may be called the equator of the magnet.
MAGNETIC FORCE.
313
The two regions of attraction separated by the equator are
called the poles of the magnet.
Sometimes this term pole is applied to two points, which are the
centres of ail the magnetic attractions, in the same manner as the
centre of gravity is the centre of all the gravitating forces which
act upon the particles of a body.
512. Experimental illustration. — The neutral line and the
varying attraction of the parts of the surface of the magnet which
it separates may be manifested experimentally as follows. Let a
magnet, whether natural or artificial, be rolled in a mass of fine
iron filings. They will adhere to it, and will collect in two tufts
on its surface, separated by a space
upon which no filings will appear.
This effect, as exhibited by a na-
tural magnet of rough and irregular
form, is represented in Jig. 296.; and
as exhibited by an artificial magnet
in the form of a regular rod or cy-
linder whose length is considerable
as compared with its thickness, is re-
presented in. Jig. 297.; the equator
being represented by K Q, and the poles by A and u.
Fig. 197-
513. The distribution of the magnetic force may also be
illustrated as follows. Let a magnet, whether natural or artificial,
be placed under a plate of glass or a sheet of paper, and let iron
filings be scattered on the paper or glass over the magnet by
means of a sieve, the paper or glass being gently agitated so as to
give free motion to the particles. They will be observed to affect
a peculiar arrangement corresponding with and indicating the
neutral line or equator and the poles, as represented in Jig. 298.,
where EQ is the equator, and A and B the poles of the magnet.
514. The variation of magnetic force may be ascertained
by presenting different parts of the surface to a small ball of iron
suspended by a fibre of silk so as to form a pendulum. The
attraction of the surface will draw this ball out of the perpendi-
cular to an extent greater or less, according to the energy of the
3M
MAGNETISM.
attraction. If the equator of the magnet be presented to it, no
attraction will be manifested, and the force indicated will be aug-
mented according as the point presented to the pendulum is more
distant from the equator and nearer to the pole.
515. Curve of varying intensity. — This varying distribution
of the attractive force over the surface of a magnet may be repre-
sented by a curve whose distance from the magnet varies propor-
tionally to the intensity of this force. Thus if, in Jig. 299., E a be
D-
Fig. 299.
the equator and A and B the poles of the magnet, the curve E c D F
may be imagined to be drawn in such a manner that its distance
from the bar E B shall be everywhere proportional to the intensity
of the attractive force of the one pole, and a similar curve E C'D'F'
will in like manner be proportional to the varying attractions
of the several parts of the other pole. These curves necessarily
touch the magnet at the equator E Q, where the attraction is
nothing, and they recede from it more and more as their distance
from the equator increases.
MAGNETIC POLES. 3 1 5
5 1 6. Magnetic attraction and repulsion.— If two magnets,
so placed as to have free motion, be presented to each other,
they will exhibit either mutual attraction or mutual repulsion,
according to the parts of their surfaces which are brought into
proximity. Let E and E', fig. 300., be two magnets their poles
A E D
Fig. joo.
being respectively A B and A' B'. Let the two poles of each of
these be successively presented to the same pole of a third magnet.
It will be found that one will be attracted and the other repelled.
Thus, the poles A and A' will be both attracted, and the poles
B and B' will be both repelled by the pole of the third magnet, to
which they are successively presented.
517. Kike poles repel, and unlike attract. — The poles
A and A', which are both attracted, and the poles B and B', which
are both repelled by the same pole of a third magnet, are said to
be like poles ; and the poles A and B', and B and A', one of which
is attracted and the other repelled by the same pole of a third
magnet, are said to be unlike poles.
Thus the two poles of the same magnet are always unlike poles,
since one is always attracted, and the other repelled, by the same
pole of any magnet to which they are successively presented.
If two like poles of two magnets, such as A and A' or B and B',
be presented to each other, they will be mutually repelled ; and if
two unlike poles, as A and B' or B and A', be presented to each
other, they will be mutually attracted.
Thus it is a general law of magnetic force, that like poles
mutually repel and unlike poles mutually attract.
518. Experimental illustrations. — Let a magnetic needle,
p P', fig. 301., be supported on a centre.
Let one of the poles A of another magnet be presented to p ; it will either
attract or repel p, so that the magnet p p' will turn in the one direction or
the other. Suppose, for example, that it repels P ; let it then be similarly
presented to p', and it will be found to attract it. In this case A and p are
like, and A and p7 unlike poles, and, consequently, p and p' are also unlike
poles.
The experiment may be further varied by presenting successively to the
two poles P and p', the other pole of the magnet A ; in that case it will
>e found that it will repel p', and attract P.
Let a piece of iron, such as a key for example, be suspended by either pole
. 302., of a magnet. Let another magnet of similar form and equal
oe foi
Le
*'fy
3i6
MAGNETISM.
force be presented to the former, with its unlike pole A directed towards B,
and let it be moved so that A shall gradually approach B. The attraction
Fig. joi.
of B upon the key will be gradually diminished as the unlike pole approxi-
mates, and will at length become insufficient to support the key, which will
Fig. joz.
fall. In this case, the magnetic force of A counteracts that of B, and when
the two poles come together their attractions will be neutralised.
519. magnets arrange themselves mutually parallel witn
poles reversed. — If a magnet AB, fig. 300., be placed in a fixed
position on a horizontal plane, and another magnet be suspended
freely at its equator E' by a fibre of untwisted silk, the point of
suspension being brought so as to be vertical over the equator E
of the fixed magnet, the magnet suspended being thus free to
revolve round its equator E' in a horizontal plane, it will so re-
volve, and will oscillate until at length it comes to rest in a posi-
tion parallel to the fixed magnet A B ; the like poles, however,
being in contrary directions, that is to say, the pole A', which is
similar to A being over B, and the pole B', which is similar to B
being over A. This phenomenon follows obviously from what has
been just explained; for if the magnet A'B' be turned to any
MAGNETIC AXIS. 317
other direction, the arm E B attracting the unlike arm E' A', and
at the same time the arm E A attracting the unlike arm E' B', the
suspended magnet A' B' will be under the operation of forces called
a couple*, consisting of two equal and contrary forces whose
combined effect is to turn the magnet round E' as a centre.
When, however, the magnet A' B' ranges itself parallel to A B,
the like poles being in contrary directions, the forces exerted
balance each other, since the pole A attracts B' as much as the
pole B attracts A'.
Magnetic axis. — It has been already stated that certain
points within the two parts into which a magnet is divided by the
equator, which are the centres of magnetic force, are the magnetic
poles. A straight line joining these two points is called the
magnetic axis.
How ascertained experimentally. — If a magnet have a
symmetrical form, and the magnetic force be uniformly diffused
through it, its magnetic axis will coincide with the geometrical
axis of its figure. Thus, for example, if a cylindrical rod be
uniformly magnetised, its magnetic axis will be the axis of the
cylinder; but this regular position of the magnetic axis does not
always prevail, and as its direction is of considerable importance,
it is necessary that its position may in all cases be determined.
This may be done by the following expedient : —
Let the magnet, the direction of whose axis it is required to
ascertain, be suspended as already described, with its equator
exactly over that of a fixed magnet resting upon a horizontal
plane. The suspended magnet will then settle itself into such a
position that its magnetic axis will be parallel to the magnetic axis
of the fixed magnet which is under it. Its position when thus in
equilibrium being observed, let it be reversed in the stirrup, so
that without changing the position of its poles, its under side shall
be turned upwards, and vice versa. If after this change the direc-
tion of the bar remain unaltered, its magnetic axis will coincide
with its geometrical axis ; but if, as will generally happen, it take
a different direction after being reversed, then the true direction
of the magnetic axis will be intermediate between its directions
before and after reversion.
To render this more clear, let A B, fg. 303., be the geometrical
axis of a regularly shaped prismatic magnet, and let it be required
to discover the direction of its magnetic axis. Let a, b be the
poles, and the line M N passing through them therefore its mag-
netic axis.
If this magnet be reversed in the manner already described over
• "Mechanics," (155.;.
3i8
MAGNETISM.
a fixed magnet, its magnetic axis in the new position will coincide
with its direction in the first position, and the magnet when re-
versed will take the position represented by the dotted line, the
geometrical axis being in the direction A' B', intersecting its
former direction AB at o. The poles a, b
will coincide with their former position,
as will also the magnetic axis M N. It is
evident that the geometric axis o A will
form with the magnetic axis o a the same
angle as it forms with that axis in the
second position, that is to say, the angle
A o M will be equal to the angle A' o M ;
and, consequently, the magnetic axis M N
will bisect the angle A o A', formed by
the geometric axis of the magnet in its
second position.
520. Hypothesis of two fluids, bo-
real and austral. — These various phe-
nomena of attraction and repulsion, with
others which will presently be stated,
have been explained by different suppo-
sitions, one of which assumes that all
bodies susceptible of magnetism are per-
vaded by a subtle imponderable fluid,
which is compound, consisting of two
constituents called, for reasons which
will hereafter appear, the austral fluid
and the boreal fluid. Each of these is self-repulsive ; but they are
reciprocally attractive, that is to say, the austral fluid repels the
austral, and the boreal the boreal; but the austral and boreal
fluids reciprocally attract.
521. Natural or unmagrnetised state. — When a body per-
vaded by the compound fluid is in its natural state and not
magnetic, the two fluids are in combination, each molecule of the
one being combined with a molecule of the other ; consequently,
in such state, neither attraction or repulsion is exercised, inas-
much as whatever is attracted by one molecule is repelled by the
other.
522. Magnetised state. — When a body is magnetic, the fluid
which pervades it is decomposed, the austral being directed towards
one side of the equator, and the boreal towards the other. That
side of the equator towards which the austral fluid is directed is
the austral, and that towards which the boreal fluid is directed is
the boreal pole of the magnet.
If the austral poles of the two magnets be presented to each
COERCIVE FORCE. 319
other, they will mutually repel, in consequence of the mutual re-
pulsion of the fluids which are directed towards them; and the
same effect will take place if the boreal poles be presented to each
other. If the austral pole of the one magnet be presented to the
boreal pole of another, mutual attraction will take place, because
the austral and boreal fluids, though separately self-repulsive, are
reciprocally attractive.
It is in this manner that the hypothesis of two self- repulsive and
mutually attractive fluids supplies an explanation of the general
magnetic law, that like poles repel and unlike poles attract. It
must be observed that the attraction and repulsion in this hypo-
thesis are imputed not to the matter composing the magnetic body,
but to the hypothetical fluids by which this matter is supposed to
be pervaded.
523. Coercive force. — The force with which the opposite
fluids are combined in bodies susceptible of magnetism varies.
In some the combination is feeble, so that they are easily decom-
posed, and the body consequently easily magnetised. In others
they are more strongly combined, resisting decomposition, and
rendering magnetism more difficult.
The facility with which after decomposition they are recombined,
so as to restore the body to its natural or unmagnetised state, is
always proportionate to that with which they are decomposed.
This force, which resists decomposition and recomposition with
more or less intensity, is called the coercive force. It has great
intensity in highly tempered steel, which consequently, when
once magnetised, retains its magnetism ; and it is scarcely sensible
in soft iron, which, when magnetism is momentarily imparted to it,
loses the vivtue almost instantaneously.
It might be assumed hypothetically that all bodies whatever are
pervaded by the two magnetic fluids in a state of combination, and
that some are unsusceptible of magnetism only because no power
has been discovered sufficiently energetic to overcome their
coercive force, while those which are susceptible of magnetism,
and which retain the virtue once imparted to them, have a coercive
force sufficiently limited to allow of decomposition, but sufficiently
energetic to prevent spontaneous recomposition ; and that bodies
like soft iron, which are only susceptible of temporary magnetism,
have so little coercive force that, when removed from the influence
of the decomposing agent, the fluids are spontaneously recombined.
524. Magnetic substances. — The only substances in which
the magnetic fluid has been decomposed, and which are therefore
susceptible of magnetism, are iron, nickel, cobalt, chromium, and
manganese, the first being that in which the magnetic property is
manifested by the most striking phenomena.
320 MAGNETISM.
CHAP. II.
MAGNETISM BY INDUCTION.
525. Soft iron rendered temporarily magnetic. — If the
extremity of a bar of soft iron be presented to one of the poles of
a magnet, this bar will itself become immediately magnetic. It
will manifest a neutral line and two poles, that pole which is in
contact with the magnet being of a contrary name to the pole
which it touches. Thus, if A B, fig. 304., be the bar of soft iron
Fig. 304.
which is brought in contact with the boreal pole b of the magnet a 6,
then A will be the austral and B the boreal pole of the bar of soft
iron thus rendered magnetic by contact, and E will be its equator,
which however will not be in the middle of the bar, but nearer to
the point of contact. These effects are thus explained by the
hypothesis of two fluids.
The attraction of the boreal pole of the magnet a b acting upon
the magnetic fluid which pervades the bar AB, decomposes it,
attracting the austral fluid towards the point of contact A, and
repelling the boreal fluid towards B. The austral fluid accordingly
predominates at the end A, and the boreal at the end B, a neutral
line or equator E separating them.
This state of the bar A B can be rendered experimentally mani-
fest by any of the tests already explained. If it be rolled in iron
filings, they will attach themselves in two tufts separated by an
intermediate point which is free from them ; and if the test pen-
dulum (514.) be successively presented to different points of the
bar, the varying intensity of the attraction will be indicated.
If the bar A B be detached from the magnet, it will instantly
lose its magnetic virtue, the fluids which were decomposed and
separated will spontaneously recombine, and the bar will be re-
duced to its natural state, as may be proved by subjecting it
after separation to any of the tests already explained.
Thus is manifested the fact that the magnetism of soft iron has
no perceptible coercive force. The magnetic fluid is decomposed
by the contact of the pole of any magnet however feeble, and
when detached it is recomposed spontaneously and immediately.
526. This may be effected by proximity without contact.
MAGNETIC INDUCTION. 321
— If the bar A B be presented at a small distance from the pole b,
it will manifest magnetism in the same manner; and if it be
gradually removed from the pole, the magnetism it manifests will
dimmish in degree, until at length it wholly disappears.
If the end B instead of A be presented to Z>, the poles of the
temporary magnet will be reversed, B becoming the austral, and A
the boreal.
If a series of bars of soft iron A B, A'B', A"B", jig. 305., be
Fig. 305.
brought into successive contiguity so as to form a series without
absolute contact, the extremity A of the first being presented to
the boreal pole b of the fixed magnet, then each bar of the series
will be rendered magnetic. The attraction of the boreal fluid at
b will decompose the magnetic fluid of the bar A B, attracting the
austral fluid towards A, and repelling the boreal fluid towards B.
The boreal fluid thus driven towards B will produce a like decom-
position of the fluid in the second bar A'B', the austral fluid
being attracted towards A' and the boreal repelled towards B' ;
and like effects will be produced upon the next bar A" B'', and
so on.
If the bars be brought gradually closer together, the intensity
of the magnetism thus developed will be increased, and will con-
tinue to be increased until the bars are brought into contact.
527. Experimental illustration. — This may be rendered evi-
dent by the simple experiment shown in^. 306., where several
Fig. 306.
pieces of soft iron are in succession suspended one from another
to the pole of a magnetic bar.
528. Induction is the name given to this process, by which
magnetism is developed by magnetic action at a distance.
529. Magnets with poles reversed neutralise each other.
— If a second magnet of equal intensity with the first be laid
upon ab,Jig. 305., with its poles reversed, so that its austral pole
y
322 MAGNETISM.
shall coincide with b and its boreal with a, the bars A B, A'B',
A" B" magnetised by induction will instantly be reduced to their
natural state, and deprived of the magnetic influence. This is
easily explained. The attraction of the pole ft, which draws
towards it the austral and repels the boreal fluids of the bar A B,
is neutralised by the attraction and repulsion of the austral pole
of the second magnet laid upon it, which repels the austral fluid
of the bar A B with a force equal to that with which the boreal
fluid of the pole b attracts it, and attracts the boreal fluid with
as much force as that with which the pole b repels it. Thus
the attraction and repulsion of the two poles of the combined
magnets neutralise each other, and the fluids which were decom-
posed in the bar A B spontaneously recombine ; and the same
effects take place in the other bars.
All these effects may be rendered experimentally manifest by
submitting the bars A B, A' B', A" B'' to any of the tests already
explained.
530. A magnet broken at its equator produces two
mag-nets. — It might be supposed, from what has been stated,
that if a magnetic bar were divided at its equator, two magnets
would be produced, one having austral and the other boreal mag-
netism, so that one of them would attract an austral and repel a
boreal pole, while the other would produce the contrary attraction
and repulsion. This, however, is not found to be the case. If a
magnet be broken in two at its equator, two complete magnets
will result, having each an equator at or near its centre, and two
poles, austral and boreal ; and if these be again broken, other
magnets will be formed, each having an equator and two poles as
before ; and in the same manner, whatever be the number of parts,
and however minute they be, into which a magnet is divided, each
part will still be a complete magnet, with an equator and two
poles.
531. Decomposition of magnetic fluid is not attended by
its transfer between pole and pole. — It cannot, in a word, be
assumed that the boreal fluid passes to one, and the austral fluid to
the other side of the equator ; for if this were the case, the fracture
of the magnet at the equator would leave the two parts, one sur-
charged with austral and the other with boreal fluid, whereas by
what has been just stated it is apparent that after such division
both parts will possess both fluids.
532. The decomposition is therefore molecular. — Each
molecule of the magnet is invested by an atmosphere composed of
the two fluids, and the decomposition takes place in these atmo-
spheres, the boreal fluid passing to one side of the molecule, and
the austral fluid to the other. When a bar is magnetised, there-
EFFECTS OF INDUCTION. 323
fore, the material molecules which form it are invested with the
magnetic fluids, but the austral fluids are all presented towards
the austral pole, and the boreal fluids towards the boreal pole.
When the bar is not magnetic, but in its natural state, the two
fluids surrounding each molecule are diffused through each other
and combined, neither prevailing more at one side than the other.
533. The coercive force of iron varies with its molecular
structure. — The metal in different states of aggregation possesses
different degrees of coercive force. Soft iron, when pure, is con-
sidered to be divested altogether of coercive force, or at least it
possesses it in an insensible degree. In a more impure state, or
when modified in its molecular structure by pressure, percussion,
torsion, or other mechanical effects, it acquires more or less coer-
cive power, and accordingly resists the reception of magnetism,
and when magnetism has been imparted to it, retains it with a
proportional force. Steel has still more coercive force than iron,
and steel of different tempers manifests the coercive force in dif-
ferent degrees, that which possesses it in the highest degree being
the steel which is of the highest temper, and which possesses in
the greatest degree the qualities of hardness and brittleness.
534. Effect of induction on hard iron or steel. — If a bar of
hard iron or steel be placed with its end in contact with a magnet,
in the same manner as has been already described with respect to
soft iron, it will exhibit no magnetism; but if it be kept in con-
tact with the magnet for a considerable length of time, it will
gradually acquire the same magnetic properties as have been de-
scribed in respect to bars of soft iron, — with this difference, how-
ever, that having thus acquired them, it does not lose them when
detached from the magnet, as is the case with soft iron. Thus it
would appear, that it is not literally true that a bar of steel when
brought into contact with the pole of a magnet receives no mag-
netism, but rather that it receives magnetism in an insensible
degree ; for if continued contact impart sensible magnetism, it
must be admitted that contact for shorter intervals must impart
more or less magnetism, since it is by the accumulation of the
effects produced from moment to moment that the sensible mag-
netism manifested by continued contact is produced.
It appears, therefore, that the coercive energy of the bar of
steel resists the action of the magnet, so that while the pole of the
magnet accomplishes the decomposition of the magnetic fluid in a
bar of soft iron instantaneously, or at least in an indefinitely small
interval of time, it accomplishes in a bar of steel the same decom-
position, but only after a long protracted interval, the decompo-
sition proceeding by little and little, from moment to moment,
luring such interval.
3^4 MAGNETISM.
Various expedients, as will appear hereafter, have been con-
trived, by which the decomposition in the case of steel bars havin<r
a great coercive force is expedited. These consist generally in
moving the pole of the magnet successively over the various points
of the steel bar, upon which it is desired to produce the decom-
position, the motion being always made with the contact of the
same pole, and in the same direction. The pole is thus made to
act successively upon every part of the surface of the bar to be
magnetised, and being brought into closer contact with it acts
more energetically; whereas when applied to only one point, the
energy of its action upon other points is enfeebled by distance,
the intensity of the magnetic attraction diminishing, like that of
gravity, in the same proportion as the square of the distance
increases.
Since steel bars having once received the magnetic virtue in
this manner retain it for an indefinite time, artificial magnets
can be produced by these means of any required form and ma°--
nitude.
535- Forms of magnetic needles and bars. — Thus a mag-
netic needle generally receives
the form of a lozenge, as repre-
sented in j%. 307., having a co-
nical cup of agate at its centre,
which is supported upon a pivot
in such a manner as that the
needle is free to turn in a hori-
zontal plane, round the pivot as
a centre. In this case the weight
of the needle must be so regu-
307. lated as to be in equilibrium on
the pivot.
Bar magnets are pieces of steel in the form of cylinders or
prisms whose length is considerable compared with their depth
or thickness. In producing such magnets certain processes are
necessary, which will be explained hereafter.
536. Compound magnets consist of several bar magnets, equal
and similar in magnitude, being placed one upon the other with
their corresponding poles together.
537. Effects of neat on magnetism. — Since the elevation or
depression of temperature by producing dilatation and contraction
affects the molecular state of a body, it might be expected to
modify also its magnetic properties, and this is accordingly found
to be the case.
538. A red heat destroys the magnetism of iron. — The
elevation of temperature and the molecular dilatation consequent
MAGNETIC BODIES.
325
upon it destroys the coercive force, and allows the recombination of
the magnetic fluid. When after such change the magnet is allowed
to cool, it will continue divested of its magnetic qualities. These
effects may, however, be again imparted to it by the process
already mentioned.
5*39. Different magnetic bodies lose their magnetism at
different temperatures. — Thus the magnetism of nickel is effaced
when it is raised to the temperature of 660°, iron at a cherry red,
and cobalt at a temperature much more elevated.
540. Beat opposed to induction. — But not only does in-
creased temperature deprive permanent magnets of their mag-
netism, but it renders even soft iron unsusceptible of magnetism
by induction, for it is found that soft iron rendered incandescent
does not become magnetic, when brought into contact or conti-
guity with the pole of a magnet.
541. Induced magnetism may be rendered permanent by
hammering and other mechanical effects. — If a bar of soft
iron, when rendered magnetic by induction, be hammered, rolled,
or twisted, it will retain its magnetism. It would follow, there-
fore, that the change of molecular arrangement thus produced
confers upon it a coercive force which it had not previously.
542. Compounds of iron are differently susceptible of
magnetism according to the proportion of iron they contain.
Exceptions, however, to this are represented in the peroxide, the
persulphate, and some other compounds containing iron in small
proportion, in which the magnetic virtue is not at all present.
543. Compounds of other magnetic bodies are not sus-
ceptible. — Nickel, cobalt, chromium, and manganese are the only
simple bodies which, in common with iron, enjoy the magnetic
property, and this property completely disappears 'in most of the
chemical compounds of which they form a part. Magnetism, how-
ever, has been rendered manifest under a great variety of circum-
stances connected with the development of electricity which have
been already explained.
544. Consecutive points. — In the production of artificial
magnets, it frequently happens that a magnetic bar has more than
one equator, and consequently more than two poles. This fact
may be experimentally ascertained by exposing successively the
length of a bar to any of the tests already explained. Thus, if
presented to the test pendulum, it will be attracted with a con-
tinually decreasing force as it approaches each equator, and with
an increasing force as it recedes from it. If the bar be rolled in
iron filings, they will be attached to it in a succession of tufts sepa-
rated by spaces where none are attached, indicating the equators.
If it be placed under a glass plate or sheet of paper on which
326 MAGNETISM.
fine iron filings are sprinkled, they will arrange themselves ac-
cording to a series of concentric curves, as represented in
Jig- 308.
Fig. 308.
It is evident that the magnetic bar in this case is equivalent to a
succession of independent magnets placed pole to pole.
The equators in these cases are called consecutive points.
CHAP. in.
TERRESTRIAL MAGNETISM.
545. Analogy of the earth to a magnet. — If a small and
sensitive magnetic needle, suspended by a fibre of silk so as to be
free to assume any position, which the attractions that act upon it
may have a tendency to give to it, be carried over a magnetic bar
from end to end, it will assume in different positions different di-
rections, depending on the effect produced by the attractions and
repulsions exercised by the bar upon it.
Let a b,fig. 309., be such a needle, the thread of suspension oe being first
placed vertically over the equator E of the magnetic bar AB. The austral
magnetism of A E will attract the boreal magnetism of b e, and will repel the
austral magnetism of a e ; and in like manner the boreal magnetism of B E
will attract the austral magnetism of a e, and will repel, the boreal magnetism
of b e. These attractions and repulsions will moreover be respectively equal,
since the distance of ae and b e from B A and B E are equal. T.he needle a b
will therefore settle itself parallel to the bar A B, the pole a being directed to
B, and the pole b being directed to A.
If the suspending thread o e be removed towards A to p e, the attraction of
A upon b will become greater than the attraction of B upon a, because the
distance of A from b will be less than the distance of B from a ; and, for a
like reason, the repulsion of A upon a will be greater than the repulsion of B
upon b. The needle a b will therefore be affected as if the end b were heavier
TERRESTRIAL MAGNETISM.
327
thau a, and it will throw itself into the inclined position represented in the
ligure, the pole b inclining downwards.
V
M E
Fig. 309.
If it be carried still further towards A, the inequality of the attractions
and repulsions increasing in consequence of the greater inequality of the
distances of a and b from A and B, the inclination of b downwards will be
proportionally augmented, as represented at P'. In fine, when the thread of
suspension is moved to a point v" over the pole A, the needle will become
vertical, the pole b attracted by A pointing downwards. If the needle be
carried in like manner from E to B, like effects will be manifested, as repre-
sented in the figure, the pole a inclining downwards, arising from the same
causes.
A magnetic needle similarly suspended, carried over the surface
of the earth in the directions north and south, undergoes changes
of direction such as would be produced, on the principles ex-
plained above, if the globe were a magnet having its poles at
certain points, not far distant from its poles of rotation. To
render this experimentally evident, it will be necessary to be pro-
vided with two magnetic instruments, one mounted so that the
needle shall have a motion in a horizontal plane round a vertical
axis, and the other so that it shall have a motion in a vertical plane
round a horizontal axis.
546. The azimuth compass is an instrument consisting of a
magnetic bar or needle balanced on a vertical pivot, so as to be
capable of turning freely in a horizontal plane, the point of the
needle playing in a circle, of which its pivot is the centre. It is
variously mounted and designated, according to the circumstances
and purpose of its application. When used to indicate the relative
bearings or horizontal directions of distant objects, whether ter-
restrial or celestial, a graduated circle is placed under the needle
and concentric with it. The divisions of this circle indicate the
bearings of any distant object, in relation to the direction of the
needle, Jig. 310.
The most efficient form of azimuth or variation compass, as it is otherwise
called, is shown in fig. 310. The needle B B' is enclosed in a copper case with
a glass top, the rim of which supports a telescope FF*, which plays in &
3 28 MAGNETISM.
vertical circle so as to be capable of being directed to any celestial or ter-
restrial object. The frame can be turned round the centre of the box so
Fig. 310.
that any azimuth can be given to the telescope. The azimuth angle through
which the telescope is turned is indicated by the graduated circle surrounding
the compass. In fine, the inclination of the telescope to the horizon, or, what
is the same, the altitude of the object to which it is directed, is shown by the
graduated arc M.
Screws N N' are placed in the feet, by which the instrument is levelled ;
and a spirit level E is suspended upon the axis of the telescope by which
the instrument is adjusted.
AZIMUTH COMPASS.
329
By comparing the direction of any celestial object, whose real azimuth is
known, with the direction of the needle, its apparent azimuth will be found,
and the difference between the apparent and real azimuth is in that case the
variation of the compass.
The pivot in this form of compass is rendered vertical by means
of a plumb line or spirit level.
547. The azimuth compass used at sea has the pivot
supporting the needle fixed in the bottom of a cylindrical box,
closed at the top by a plate of glass, so as to protect it from
the air. The magnetic bar is attached to the under side of a
circular card, upon which is engraved a radiating diagram, di-
viding the circle into thirty-two parts called points. The compass
box is suspended so as to preserve its horizontal position un-
disturbed by the motion of the vessel, by means of two concentric
hoops called gimbals*, one a little less than and included within
the other. It is supported at two points upon the lesser hoop,
which are diametrically opposite, and this lesser hoop itself is
supported by two points upon the greater hoop, which are
also diametrically opposite, but at right angles to the former. By
these means the box, being at liberty to swing in two planes at K
right angles to each other, will maintain itself horizontal, and will
therefore keep the pivot supporting the needle vertical, whatever
be the changes of position of the vessel.
This arrangement is represented in fig. 311., a vertical section of the
compass box being given io.fig. 312.
The sides of the cylindrical box are b b', its bottom ff, and the glass
which covers it v. The magnetic bar or needle is supported on a vertical
pivot by means of a conical cup, and can be raised and lowered at pleasure
Fig. JH
by means of a screw w. The compass card is represented in section at rr'
fig. 31*., and the divisions upon it marked by radiating lines called the rose
are represented \nfig. 311.
• " Mechanics " (549. >
330 MAGNETISM.
Two narrow plates, p and //, are attached to the sides of the box so as to
be diametrically opposed. In p there is a narrow vertical slit. In p' there
is a wider vertical slit, along which is stretched vertically a thin wire. The
eye placed at o looks through the two slits, and turns the instrument round
its support until the object of observation is intersected by the vertical wire,
extended along the slit p'. Provisions are made in the instrument by
which the direction thus observed can be ascertained relatively to that of
the needle. The angle included between the direction of the observed
object, and that of the needle, is the bearing of the object relativelv to the
needle.
The compass box is suspended within the hoop e e', at two points z z'
diametrically opposed, and the hoop e e' is itself suspended within the fixed
hoop c c', at two points x x', also diametrically opposed, but at right angles
to 2 2'.
The ordinary mariner's compass enclosed in its case, called a binnacle, is
shown in fig. 313., where K is a plate of ground glass for the purpose of
Fig.jij.
admitting light to the instrument at night. A strong lamp with a
reflector is placed opposite this, by which the interior of the box is illu-
minated, and the light is reflected to a plate of talc, or other semi-transpa-
rent substance, on which the divisions of the compass are marked. A line
marked over the box coincides with the course of the vessel, and the helms-
man so regulates it that this line shall form an angle with the north pole of
the needle equal to that which the course of the vessel is required to have
with the meridian.
548. The dipping: needle,^?^. 314., consists of a magnetic needle
A B, supported and balanced on a horizontal axis, and playing
therefore in a vertical plane. The angles through which it turns
are indicated by a graduated circle D D, the centre of which
coincides with the axis of the needle, and the frame which sup-
ports it has an azimuthal motion round a vertical axis, which is
indicated and measured by the graduated horizontal circle P P.
MARINER'S COMPASS —DIPPING NEEDLE. 331
The instrument is adjusted by means of a spirit level, and regulating
screws Q Q inserted in the feet.
549. Analysis of magnetic phenomena of the earth. —
Supplied with these instruments, it will be easy to submit to
observation the magnetic phenomena manifested at different parts
of the earth.
Fig. 314.
If the azimuth compass be placed anywhere in the northern
hemisphere, at London for example, the needle will take a certain
position, forming an angle with the terrestrial meridian, and di-
recting one pole to a point a certain number of degrees west of the
north, and the other to a point a like number of degrees east of
the south. If it be turned aside from this direction, it will, when
liberated, oscillate on the one side and the other of this direction,
and soon come to rest in it.
Since an unmagnetised needle would rest indifferently in any
332 MAGNETISM.
direction, this preference of the magnetised needle for one par-
ticular direction, must be ascribed to magnetic force exerted by
the earth attracting one of the poles of the needle in one direction,
and the other pole in the opposite direction. That this is not the
casual attraction of unmagnetic ferruginous matter contained
within the earth, is proved by the fact that, if the direction of the
needle be reversed, it will, when liberated, make a pirouette upon
its pivot, and after some oscillations resume its former direction.
This remarkable property is reproduced in all parts of the earth,
on land and water, and equally on the summits of lofty mountains,
in the lowest valleys, and in the deepest mines.
550. The magnetic meridian is the direction thus assumed
by the horizontal needle in any given place.
The direction of a needle which would point due north and
south is the true meridian, or the terrestrial meridian of the place.
551. The declination or variation is the angle formed by the
magnetic meridian and the terrestrial meridian.
The declination is said to be eastern or western, according as the
pole of the needle, which is directed northwards, deviates to the
east or to the west of the terrestrial meridian.
552. Magnetic polarity of the earth. — To explain these
phenomena, therefore, the globe of the earth itself is considered as
a magnet, whose poles attract and repel the poles of the horizontal
needle, each pole of the earth attracting that of an unlike name,
and repelling that of a like name. If, therefore, the northern pole
of the earth be considered as that which is pervaded by boreal
magnetism, and the southern pole by austral magnetism, the
former will attract the austral and repel the boreal pole, and the
latter will attract the boreal and repel the austral pole of the
needle. Hence it will follow that the pole of the needle which is
directed northwards is the austral, and that which is directed
southwards is the boreal pole.
553. Variation of the dip. — It was shown in (545.) that when
a needle which is free to play in a vertical plane was carried over
a magnet, it rested in the horizontal position only when suspended
vertically over the equator of the magnet, and its austral and
boreal poles were inclined downwards, according as the needle
was suspended at the boreal or austral side of the equator, and
that this inclination was augmented as the distance from the
equator at which the needle was suspended was increased. Now
it remains to be seen whether any phenomenon analogous to this
is presented by the earth.
For this purpose let the dipping needle, fig. 314., be arranged with its
axis at right angles to the direction of the needle of the azimuth compass.
It will then be found, that in general the dipping needle will not rest in a
VARIATION AND DIP. 333
horizontal position, but will assume a direction inclined to the vertical line,
as represented in the figure, one pole being presented downwards, and the
other upwards. The angle which the lower arm of the needle makes with
the horizontal line is called the dip.
If this apparatus be carried in this hemisphere northwards, in the direction
in which a horizontal needle would point, the austral pole will be inclined
downwards, and the dip will continually increase ; but if it be carried
southwards, the dip will continually diminish. By continuing to transport
it southwards, the dip continually diminishing, a station will at length be
found where the needle will rest in the horizontal position. If it be carried
further southwards, the boreal pole will begin to turn downwards ; in other
words, the dip will be south instead of north, and as it is carried further
southwards, this dip will continue to increase.
If the needle be carried northwards, in this hemisphere the dip continually
augmenting, a station will at length be attained where the needle will
become vertical, the austral pole being presented downwards, and the boreal
pole upwards. In the same manner, in the southern hemisphere, if the
needle be carried southwards, a station will at length be attained where it
will become vertical, the boreal pole being presented downwards, and the
austral pole pointing to the zenith.
Complete analogy of the earth to a magnet. — By com-
paring these results with those which have been already described
in the case where the needle was carried successively over a
magnetic bar, the complete identity of the phenomena will be
apparent, and it will be evident that the earth and the needle
comport themselves in relation to each other exactly as do a small
and a great magnet, over which it might be carried, the point
where the needle is horizontal being over the magnetic equator,
and those two points where it is vertical being the magnetic
poles.
554. The magnetic equator. — The needle being brought
to that point where it rests horizontal, the magnetic equator
will be at right angles to its direction. By transporting it suc-
cessively in the one or the other direction thus indicated, the
successive points upon the earth's surface where the needle rests
horizontal, and where the dip is nothing, will be ascertained.
The line upon the earth drawn through these points is the
magnetic equator.
555. Its form and position not regular. — This line is not, as
might be expected, a great circle of the earth. It follows a course
crossing the terrestrial equator from south to north, on the west
coast of Africa, near the island of St. Thomas, at about 7° or 8°
long. E., in a direction intersecting the equator at an angle of
about 12° or 13°. It then passes across Africa towards Ceylon,
and intersects that island near the point of the Indian promontory.
It keeps a course from this of from 8° to 9° of N. lat. through the
Indian Archipelago, and then gradually declining towards the
334 MAGNETISM.
line again intersects it at a point in the Pacific Ocean in long.
170° W., the angle at which it intersects the line being more acute
than at the other point of intersection. It then follows a course
a few degrees south of the line, and striking the west coast of
South America near Lima, it crosses the South American conti-
nent, attaining the greatest south latitude near Bahia ; and then
again ascending towards the line, traverses the Atlantic and strikes
the coast of Africa, as already stated, near the island of St.
Thomas.
The magnetic equator, unlike the ecliptic, is not any regular
curve, but follows the course we have just indicated in a direction
slightly sinuous.
556. Variation of tlie dip groing north or south. — It has
been explained, that proceeding towards north or south, from
the magnetic equator, the needle dips on the one side or on the
other, the dip increasing with the distance from the magnetic
equator to which the needle is transported north or south.
557. The lines of equal dip, therefore, may be considered as
bearing the same relation to the magnetic equator which parallels
of latitude bear to the terrestrial equator, being arranged nearly
parallel to the former, though not in a manner so regular as in the
case of parallels of latitude.
558. Magnetic meridians. — If the horizontal needle be trans-
ported north or south, following a course indicated by its direc-
tion, it will be carried over a magnetic meridian. These magnetic
meridians, therefore, bear to the magnetic equator a relation
analogous to those which terrestrial meridians bear to the terres-
trial equator, but, like the lines of equal dip, they are much more
irregular.
559. Method of ascertaining: the declination of the
needles. — Astronomy supplies various methods of determining
in a given place the declination of the needle. It may be gene-
rally stated that this problem may be solved by
observing any object whose angular distance
from i the true north is otherwise known, and
comparing the direction of such object with the
direction of the needle. Let p, fig. 3 1 5., be the
place of observation ; let P N be the direction of
the true north, or, what is the same, the direction
of the terrestrial meridian ; and let P N' be the
direction of the magnetic needle, or, what is the
same, the magnetic meridian. The angle N p N
will then be the declination of the needle, being
the angle formed by the terrestrial and magnetic
meridians (551.)-
VARIATION OF DECLINATION. 335
Let o be any object seen on the horizon in the direction r o ;
the angle o p N is called the true azimuth of this object, and the
angle OPN' is culled its magnetic azimuth. This magnetic azimuth
may always be observed by means of an azimuth compass.
If, then, an object be selected whose true azimuth is otherwise
known, the declination of the needle may be determined by
taking the difference between the true and magnetic azimuths of
the object.
There are numerous celestial objects of which the azimuths are
either given in tables, or may be calculated by rules and formulae
supplied by astronomy; such, for example, as the sun and moon
at the moments they rise or set, or when they are at any proposed
or observed altitudes. By the aid of such objects, which are
visible occasionally at all places, the declination of the needle may
be found.
560. local declinations. — At different places upon the earth's
surface the needle has different declinations. In Europe its mean
declination is about 1 7°, increasing in going westward.
561. Agonic lines. — There are two lines on the earth's sur-
face which have been called agonic lines, upon which there is no
declination ; and where, therefore, the needle is directed along the
terrestrial meridian. One of these passes over the American and
the other over the Asiatic continent, and the former has con-
sequently been called the American and the latter the Asiatic
agonic. These lines run north and south, but do not follow the
course of meridians. It has been ascertained that their position is
not fixed, but is liable to sensible changes in considerable intervals
of time.
562. Variation of declination. — In proceeding in either
direction, east or west from these lines, the declination of the
needle gradually increases, and becomes a maximum at a certain
intermediate point between them. On the west of the Asiatic
agonic the declination is west, on the east it is east.
At present the declination in England is about 24° W. ; in
Boston in the U. States it is 5 £° W. Its mean value in Europe
is 17° W. At Bonn it is 20°, at Edinburgh 26°, Iceland 38°,
Greenland, 50°, Konigsberg, 13°, and St. Petersburg 6°.
The following table, however, will exhibit more distinctly the
variation of the declination in different parts of the globe. The
longitudes expressed in the first column are measured westward
from the meridian of Paris, and the declinations given in the
second column are those which are observed on the terrestrial
equator, those in the third column corresponding to the mean
latitude of 45°.
336
MAGNETISM.
Table of the Declinations of the Magnetic Needle in different
Longitudes, and in Lat.=o and Lat.=^.$°.
Longitudes West
Decli
lations.
Longitudes West
Decli
nations.
of the Meridian
of Paris.
Lat.=0.
Lat.=450.
of the Meridian
of Paris.
Lat.=0.
Lat.=45°.
O
IQ° W
22° W
190
rE
li° E
IO
10
10 W
16 W
z< W
26 W
200
ZIO
E
5 E
8 E
4 E
?°
II W
45 W
22O
3 E
2 E
40
4 W
14 W
ZJO
E
I E
£
!*
24 W
zo W
240
250
I W
0
70
i E
II W
200
E
3 E
80
10 E
3 W
270
E
4 E
9°
10 E
4 E
z8o
4 E
100
8 E
II E
290
4 E
HO
6 E
17 E
300
W
z E
IZO
S E
18 £
310
7 W
I W
I JO
140
5 E
6 E
19 E
19 E
320
330
ii W
13 W
5 W
10 W
150
6 E
19 E
340
17 W
14 W
1 60
7 E
19 E
350
18 \V
17 W
170
9 E
if E
360
19 W
22 W
180
10 E
14 E
563. Isog-onic lines are lines traced upon the globe at a point
at which the magnetic needle has the same declinations. These,
as well as the isoclinic lines, or lines of equal dip, are irregular in
their arrangement, and not very exactly ascertained.
564. Local dip. — The local variations of the dip are also im-
perfectly known. In Europe it ranges from 60° to 70°. In 1836
the dip observed at the undermentioned places was as follows : —
Pekin -
Rome .
Brussels
St. Petersburg
St. Helena
Rio de Janeiro
54° 49;
61° 42'
68° 32'
-,0 Of
14° 5^
13° 30'
565. The position of the magnetic polos, or the points
where the dip is 90°, is determined with considerable difficulty,
inasmuch as for a considerable distance round that point the dip
is nearly 90°. Hansteen considered that there were grounds for
supposing that there were two magnetic poles in each hemisphere.
One of these in the northern hemisphere he supposed to be west
of Hudson's Bay, in 80° lat. K, and 96° long. W. ; and the other
in Northern Asia, in 8l°lat. 1ST., and 1 1 6° long. E. The two
southern magnetic poles he supposed to be situate near the
southern pole. This supposition, however, appears to be at
present abandoned, and the observations of Gauss lead to the con-
clusion that there is but one magnetic pole in each hemisphere.
In the northern voyages made between 1829 and 1833, Sir
VARIATION OF TERRESTRIAL MAGNETISM. 337
James Ross found the dipping needle to stand vertical in the
neighbourhood of Hudson's Bay at 70° 5' 17" lat. N., and
96° 46' long. W. The dipping needle, according to the observa-
tions of Sir James Ross, was nowhere absolutely vertical, departing
from the vertical in all cases by a small angle, amounting generally
to one minute of a degree. This, however, might be ascribed to
the error of observation, or the imperfection of instruments
exposed to such a climate.
The existence of the magnetic pole, however, at or near the
point indicated, was proved by carrying round it at a certain
distance a horizontal needle, which always pointed to the spot in
whatever direction it was carried. Gauss has fixed the position
of the magnetic pole in the southern hemisphere, by theory, at
about 66° lat. S., and 146° long. E.
566. The magnetic poles are not therefore antipodal, like
the terrestrial poles ; or, in other words, they do not form the ex-
tremities of the same diameter of the globe : they «re not even on
the same meridian. If Gauss's statement be assumed to be cor-
rect, the southern magnetic pole is on a meridian 146° E. of the
meridian of Greenwich, and therefore 214° W. of that meridian;
whereas the northern magnetic pole is on a meridian 96° 46' W
The angle, therefore, between the two meridians passing through
the two poles will be about 1 17^°. It would follow, therefore, that
these points lie upon terrestrial meridians at an angle of 1 17^°
from each other, and that upon these they are at nearly eqiiiil
distances from the terrestrial poles ; the distance of the northern
magnetic pole from the northern terrestrial pole being nearly 20°,
and the distance of the southern magnetic pole from the southern
terrestrial pole being about 24°.
567. Periodical variations of terrestrial magnetism. — It
appears, from observations made at intervals of time more or less
distant for about two centuries back, that the magnetic condition
of the earth is subject to a periodical change ; but neither the
quantity nor the law of this change is exactly known. It was not
until recently that magnetic observations were conducted in such
a manner, as to supply the data necessary for the development of
the laws of magnetic variation, and they have not been yet con-
tinued a sufficient length of time to render these laws manifest.
Independently of observation, theory affords no means of ascer-
taining these laws, since it is not certainly known what are the
physical causes to which the magnetism of the earth must be
ascribed.
In the following table are given the declinations of the needle
observed at Paris between the years 1580 and 1835, and the dip
between the years 1671 and 1835.
z
MAGNETISM.
568. Table of Declinations observed at Paris.
Year.
Declination.
Year.
Declination.
1580
ii°3o'E
1817
22° I9' W
1618
8
1823
zz Z3
1663
1678
0
i 30 W
1824
I82S
zz Z3
ZZ Z2
1700
8 10
1827
ZZ ZO
1780
'9 55
1828
zz 5
1785
22
1829
ZZ IZ
1805
zz 5
iB^Z
zz 3
1813
zz 28
1835
zz 4
1814
1816
22 34
22 25
I85I
zo z<;
Table of the Dip observed at Paris.
Year.
Dip.
Year.
Dip.
1671
75°
1820
68^ 20.
1754
72 15'
1821
68 14
1776
72 25
I8Z2
68 II
I780
71 48
1823
68 8
I79I
1798
70 52
69 51
1825
1826
68 o
68 o
1806
1810
1814
1816
69 iz
68 50
68 36
68 40
1829
183?
1835
1841
67 41
67 40
67 Z4
67 9
1818
68 35
1851
66 39
1819
68 25
569. The intensity of terrestrial magnetism, like that of a
common magnet, may be estimated by the rate of vibration which
it produces in a magnetic needle submitted to its attraction. This
method of determining the intensity of magnetic force is in all
respects analogous to those, by which the intensity of the earth's
attraction is determined by a common pendulum.* The same needle
being exposed to a varying attraction, will vary its rate of vibra-
tion, the force which attracts it being proportional to the square
of the number of vibrations which it makes in a given time. Thus,
if at one place it makes ten vibrations per minute, and in another
only eight, the magnetic force which produces the first will be to
that which produces the second rate of vibration, as IOO to 64..
570. In this manner it has been found that the intensity of
terrestrial magnetism is least at the magnetic equator, and that it
increases gradually in approaching the poles.
571. Xsodynamic lines, are lines upon the earth where
the magnetic intensities are equal, and resemble in their general
arrangement, without however coinciding with them, the isoclinio
curves or magnetic parallels of equal dip.
572. Their near coincidence with isothermal lines.- It
•"Mechanics" (505.).
MAGNETIC INTENSITY. 339
has been found that there is so near a coincidence between the
isodynamic and the isothermal lines, that a strong presumption is
raised that terrestrial magnetism either arises from terrestrial
heat, .or that these phenomena have at least a common origin.
573. Equatorial and polar intensities. — It appears to
follow from the general result of observations made on the inten-
sity of terrestrial magnetism, that its intensity at the poles is to
its intensity at the equator nearly in the ratio of 3 to 2.
574. Effect of the terrestrial magnetism on soft iron. — If
anything were wanted to complete the demonstration that the
globe of the earth is a true magnet, it would be supplied by the
effects produced by it upon substances susceptible of magnetism,
but which are not yet magnetised. It has been already shown
that when a bar of soft iron is presented to the pole of a magnet
its natural magnetism is decomposed, the austral fluid being at-
tracted to one extremity, and the boreal fluid repelled to the
other, so that the bar of soft iron becomes magnetised, and con-
tinues so as long as it is exposed to the influence of the magnet.
Now, if a bar of soft iron be presented to the earth in the same
manner, precisely the same effects will ensue. Thus, if it be held
in the direction of the dipping needle, so that one of its ends shall
be presented in the direction of the magnetic attraction of the
earth, it will become magnetic, as may be proved by any of the
tests of magnetism already explained. Thus, if a sensitive needle
be presented to that end of the bar which in the northern hemi-
sphere is directed downwards, austral magnetism will be mani-
fested, the boreal pole of the needle being attracted, and the
austral pole repelled. If the needle be presented to the upper
end of the bar, contrary effects will be manifested; and if it be
presented to the middle of the bar, the neutral line or equator
will be indicated. If the bar be now inverted, the upper end
being presented downwards, and vice versa, still parallel to the
dipping needle, its poles will also be inverted, the lower, which
previously was boreal, being austral, and vice versa.
If the bar be held in any other direction, inclined obliquely to
the dipping needle, the same effects will be manifested, but in a
less degree, just as would be the case if similarly presented to an
artificial magnet ; and, in fine, if it be held at right angles to the
direction of the dipping needle, no magnetism whatever will be
developed in it.
575. Its effects on steel bars. — If the same experiments be
made with bars of hard iron or steel, no sensible magnetism will at
first be developed ; but if they be held for a considerable time in
the same position, they will at length become magnetic, as would
happen under like conditions with an artificial magnet. Iron and
34o MAGNETISM.
steel tools which are hung up in workshops in a vertical position
are found to become magnetic, an effect explained by this cause.
576. Diurnal variation of the needle. — Besides the changes
in the magnetic state of the earth, the periods of which are
measured by long intervals of time, there are more minute and
rapid changes, depending apparently upon the vicissitudes of the
seasons and the diurnal changes.
The magnitude of the diurnal variation depends upon the situation of the
place, the day, and the season, but is obviously connected with the function
of solar heat. At Paris it is observed that during the night the needle is
nearly stationary ; at sunrise it begins to move, its north pole turning
westwards, as if it were repelled by the influence of the sun. About noon,
or more generally between noon and three o'clock, its western variation
attains a maximum, and then it begins to move eastward, which movement
continues until some time between nine and eleven o'clock at night, when
the needle resumes the position it had when it commenced its western
motion in the morning.
The amplitude of this diurnal range of the needle is, according to Cassini's
observations, greatest during summer and least during winter. Its mean
amount for the months of April, May, June, July, August, and September is
stated at from 13 to 15 minutes; and for the months of October, November,
December, January, and March, at from 8 to 10 minutes. There are, how-
ever, occasionally, days upon which its range amounts to 25 minutes, and
others when it does not surpass 5 or 6 minutes. Cassini repeated his mag-
netic observations in the cellars constructed under the Paris observatory at
a depth of about a hundred feet below the surface, and therefore removed
from the immediate influence of the light and heat of the day. The ampli-
tude of the variations and all the peculiarities of the movement of the needle
here, were found to be precisely the same as at the surface.
In fmore northern latitudes, as, for example, in Denmark, Iceland, and
North America, the diurnal variations of the needle are in general more
considerable and less regular. It appears, also, that in these places the
needle is not stationary during the night, as in Paris, and that it is towards
evening that it attains its maximum westward deviation. On the contrary,
on going from the north towards the magnetic equator the diurnal variations
diminish, and cease altogether on arriving at this line. It appears, however,
according to the observations of Captain Duperrey, that the position of the
sun north or south of the terrestrial equator has a perceptible influence on
the oscillation of the needle.
On the south of the magnetic equator the diurnal variations are produced,
as might be expected, in a contrary manner; the northern pole of the
magnet turns to the east at the same hours that, in the northern hemisphere,
it turns to the west.
It has not yet been certainly ascertained whether in each hemisphere
these diurnal variations of the needle correspond in the places where the
eastern and western declinations also correspond.
The dip is also subject to certain diurnal variations, but much smaller in
their range than in the case of the horizontal needle.
As a general result of these observations it may be inferred, that if a
magnetic needle were suspended in such a manner as to be free to move in
any direction whatever, it would, during twenty-four hours, move round its
MAGNETISATION. 341
centre of suspension in such a manner as to describe a small cone, whoso
base would be an ellipse or some other curve more or less elongated, and
whose axis is the mean direction of the dipping needle.
577. Disturbances in the magnetic intensity. — The in-
tensity as well as the direction of the magnetic attraction of the
earth at a given place are subject to continual disturbances, in-
dependently of those more regular variations just mentioned.
These disturbances are in general connected with the electrical
state of the atmosphere, and are observed to accompany the phe-
nomena of the aurora borealis, earthquakes, volcanic eruptions,
sudden vicissitudes of temperature, storms, and other atmospheric
disturbances.
578. Influence of aurora borealis. — During the appearance
of the aurora borealis in high latitudes, a considerable deflection
of the needle is generally manifested, amounting often to several
degrees. So closely and necessarily is magnetic disturbance
connected with this atmospheric phenomenon, that practised ob-
servers can ascertain the existence of an aurora borealis by the
indications of the needle, when the phenomenon itself is not
visible.
CHAP. IV.
MAGNETISATION.
579. Magnetisation is founded upon the property of induction
(Ch. II.). When one of the poles of a magnet is presented to any
body which is susceptible of magnetism, it will have a tendency to
decompose the magnetic fluid, attracting one of its constituents
and repelling the other. If the coercive force by which the fluids
are combined be greater than the energy of the attraction of the
magnet, no decomposition will take place, and the body to which
it is presented will not be magnetised, but the coercive force with
which the fluids are united will be rendered more feeble, and
the body will be more susceptible of being magnetised than
before.
If, however, the energy of the magnetic be greater than the
coercive force, a decomposition will take place, more or less in
proportion as the force of the magnet exceeds in a greater or less
degree the coercive force.
580. Artificial mag-nets. — It has been already explained,
that pure soft iron is almost, if not altogether, divested of coercive
force, so that a bar of this substance is converted into a magnet
342 MAGNETISM.
instantaneously when the pole of a magnet is presented to it ; but
the absence of coercive force, which renders this conversion so
prompt, is equally efficacious in depriving the bar of its magnetism
the moment the magnet which produces this magnetism is removed.
Soft iron, therefore, is inapplicable when the object is to produce
permanent magnetism. The material best suited for this purpose
is steel, especially that which has a fine grain, a uniform structure,
and is free from flaws. It is necessary that it should have a cer-
tain degree of hardness, and that this should be uniform through
its entire mass. If the hardness be too great, it is difficult to im-
part to it the magnetic virtue ; if not great enough, it loses its
magnetism for want of sufficient coercive force. To render steel
bars best fitted for artificial magnets, it has been found advan-
tageous to confer upon them in the first instance the highest degree
of temper, and thus to render them as hard and brittle as glass,
and then to anneal them until they are brought to a straw or violet
colour.
581. Best form for bar magnets. — The intensity of artificial
magnets depends also, to some extent, upon their form and magni-
tude. It has been ascertained, that a bar magnet has the best
proportion when its thickness is about one fourth and its length
twenty times its breadth.
C82. Horse shoe mag-nets. — These magnets are shaped as re-
presented in Jig. 316. When magnets are con-
structed in this form, the distance between the
two poles ought not to be greater than the
thickness of the bar of which the magnet con-
sists. The surface of the steel forming both
bars, in horse shoe magnets, should be rendered
as even and as well polished as possible.
583. The methods of producing- artificial
magnets by friction commonly practised, are
called the method of single touch, and the method
of double touch.
584. Method of single touch. — The bar
A'B', fig. 317-, which is to be magnetised,' is
laid upon a block of wood L projecting at each
end a couple of inches.
Under the ends are placed the opposite poles A and B of two powerful
magnets, so as to be in close contact with the bar to be magnetised. The
influence of the pole A will be to attract the boreal fluid of the bar towards
the end B', and to repel the austral fluid towards the end A' ; and the effect
of the pole B will be similar, that is to say, to repel the boreal fluid towards
the end B', and to attract the austral towards the end A'. It is evident,
therefore, that if the coercive force of the magnetism of the bar A' B' be not
greater than the force of the magnets A and B, a decomposition will take
SINGLE AND DOUBLE 1OUCH. 343
plac e by simple contact, and the bar A' B' will be converted into a magnet,
Laving its austral pole at A' and its boreal pole at B' ; and, indeed, this will
B L. A
Fig. 317.
be accomplished even though the coercive force of the bar A' B' be consider-
able, if it be left a sufficient length of time under the influence of the
magnets A and B.
Hut without waiting for this, its magnetisation may be accomplished
immediately by the following process. Let two bar magnets a and b be
placed in contact with the bar A' B' to be magnetised, near its middle point,
but without touching each other, aM let them be inclined in opposite
directions to the bar A' B', at angles of about 30°, as represented in the
figure. Let the bar which is applied on the side B' have its austral pole, and
that which is applied on the side A' its boreal pole, in contact with the bar
A' B' ; and to prevent the contact of the two bars a and b, let a small piece of
wood, lead, copper, or ether substance not susceptible of magnetism, be placed
between them. Taking the two bars a and b, one in the right and the other
in the left hand, let them now be drawn in contrary directions, slowly and
uniformly along the bar A' B', from its middle to its extremities, and being
then raised from it, let them be again placed as before, near its middle point,
and drawn again uniformly and slowly to its extremities ; and let this
process be repeated until the bar A' B' has been magnetised.
It is evident that the action of the two magnetic poles a and b will be to
decompose the magnetic fluid of the bar A' B', and that in this they are
aided by the influence of the magnets A and B, which enfeeble, as has been
already shown, the coercive force.
This method is applicable with advantage to magnetise, in the
most complete and regular manner, compass needles, and bars
whose thickness does not exceed a quarter of an inch.
585. Method of double touch. — When the bars exceed this
thickness, this method is insufficient, and the method of double
touch is found more effectual.
The bars A and B, fig. 318., are placed as before, inclined at an angle
with each other, contrary poles being presented downwards. A small
block of wood L is placed between them, so as to keep the poles at a fixed
distance asunder, and they are maintained in their relative positions by
being attached to a block of wood. The bar a b to be magnetised is sup-
ported at the ends as before, by the contrary poles of two bar magnets. The
inclined bars being placed at the centre of the bar ab, they are moved to-
gether first to one extremity b, and then back along the length of the entire
bar to the other extremity a. They are then again drawn over the bar to 6,
and so backwards and forwards continuously until the bar is magnetised.
The operation is always terminated when the bars have passed over that
half of the bar ab opposite to that upon which the motion commenced.
Thus if the operation commenced by moving the united bars A B from the
344
MAGNETISM.
centre to the end 6, it will be terminated when they are moved from the
extremity a to the middle.
Fig. Ji8.
586. Inapplicable to compass needles and long: bars. —
By this method a greater quantity of magnetism is developed than
in the former, but it should never be employed for magnetising
compass needles or bars intended for delicate experiments, since
it almost always produces magnets with poles of unequal force,
and frequently gives them consecutive points (544.), especially
when the bars have considerable length.
587. Magnetic saturation. — Since the coercive force proper
to each body resists the recomposition of the magnetic fluids, it
follows that the quantity of magnetism which a bar or needle is
capable of retaining permanently, will be proportional to this
coercive force. If, by the continuance of the process of magne-
tisation and the influence of very powerful magnets, a greater
development of magnetism be produced than corresponds with the
coercive force, the fluids will be recomposed by the mutual attrac-
tion until the coercive force resists any further recomposition.
The tendency of the magnetic fluids to unite being then in equili-
brium with the coercive force, no further recomposition will take
place, and the bar will retain its magnetism undiminished. When
the bar is in this state, it is said to be magnetised to saturation.
It has been generally supposed that when bars are surcharged
.with magnetism they lose their surplus and fall suddenly to the
point of saturation, the recomposition of the fluids being in-
stantaneous. M. Pouillet, however, has shown that this recom-
position is gradual, and after magnetisation there is even in some
cases a reaction of the fluids, which is attended with an increase
instead of a diminution of magnetism. He observes that it happens
not unfrequently that the magnetism is not brought to permanent
equilibrium with the coercive force for several months.
588. Limit of magnetic force. — It must not be supposed
that by the continuance of the processes of magnetisation which
have been described above, an indefinite development of mag-
netism can be produced. When the resistance produced by the
coercive force to the decomposition of the fluids becomes equal to
EFFECTS OF TERRESTRIAL MAGNETISM. 345
the decomposing power of the magnetising bars, all further increase
of magnetism will cease.
It is remarkable that if a bar which has been magnetised to
saturation by magnets of a certain power be afterwards submitted
to the process of magnetisation by magnets of inferior power, it
will lose the excess of its magnetism and fall to the point of satura-
tion corresponding to the magnets of inferior power.
589. Influence of the temper of the bar on the coercive
force. — Let a bar of steel tempered at a bright red heat be
magnetised to saturation, and let its magnetic intensity be ascer-
tained by the vibration of a needle submitted to its attraction.
Let its temper be then brought by annealing to that of a straw
colour, and being again magnetised to saturation, let its magnetic
intensity be ascertained. In like manner, let its magnetic inten-
sities at each temper from the highest to the lowest be observed.
It will be found that the bars which have the highest temper have
the greatest coercive force, and therefore admit of the greatest
development of magnetism ; but even at the lowest tempers they
are still, when magnetised to saturation, susceptible of a consider-
able magnetic force.
Although highly tempered steel has this advantage of receiving
magnetism of great intensity, it is, on the other hand, subject to
the inconvenience of extreme brittleness, and consequent liability
to fracture. A slight reduction of temper causes but a small
diminution in its charge of magnetism, and renders it much less
liable to fracture.
590. Effects of terrestrial magnetism on bars. — It has
been already shown that the inductive power of terrestrial mag-
netism is capable of developing magnetism in iron bars, and, under
certain conditions, of either augmenting, diminishing, or even
obliterating the magnetic force of bars already magnetised. In
the preservation of artificial magnets, therefore, this influence must
be taken into account.
According to what has been explained, it appears that if a
magnetic bar be placed in the direction of the dipping needle in
this hemisphere, the earth's magnetism will have a tendency to
attract the austral magnetism downwards, and to repel the boreal
upwards. If, therefore, the austral pole of the bar be presented
downwards, this tendency will preserve or even augment the mag-
netic intensity of the bar. But if the magnet be in the inverted
position, having the boreal pole downwards, opposite effects will
ensue. The austral fluid being attracted downwards, and the
boreal driven upwards, a recombination of the fluids will take
place, which will be partial or complete according to the coercive
force of the bar. If the coercive force of the bar exceed the influ-
346 MAGNETISM.
ence of terrestrial magnetism, the effect will be only to diminish
the magnetic intensity of the bar ; but if not, the effect will be the
recomposition of the magnetic force and the reduction of the bar
to its natural state ; but if the bar be still held in the same posi-
tion, the continued effect of the terrestrial magnet will be again to
decompose the natural magnetism of the bar, driving the austral
fluid downwards and repelling the boreal upwards, and thus
reproducing the magnetism of the bar with reversed polarity.
591. Means of preserving: magnetic bars from these
effects by armatures or keepers. — When the magnetic bars to
be preserved are straight bars of equal length, they are laid
parallel to each other, their ends corresponding, but with poles
reversed, so that the austral pole of each shall be in juxtaposition
with the boreal pole of the other, as represented in Jig. 3 19.
A bar of soft iron, called the keeper or armature, is applied as represented
at K, in contact with the two opposite poles A
and B', and another similar bar K' in contact
with A' and B, so as to complete the parallelo-
gram. In this arrangement the action of the
A. poles A and B' upon the keeper K is to decom-
Fig. 319. poge j^s magnetism, driving the austral fluid
towards B' and the boreal fluid towards A. The boreal fluid of K exer-
cises a reciprocal attraction upon the austral fluid of A, and the austral
fluid of K exercises a corresponding attraction upon the boreal fluid of B'.
Like effects are produced by the keeper K' at the opposite poles A' and B.
In this manner the decomposition of the fluids in the two bars AB and A' B'
is maintained by the action of the keepers K and K'.
If the magnet have the horse shoe form, this object is obtained by a single
keeper, as represented in fig. 316. The keeper K is usually formed with a
round edge, so as to touch the magnet only in a line, and not in a surface,
as it would do if its edge were flat. It results from experience that a keeper
kept in contact in this manner for a certain length of time with a magnet,
augments the attractive force, and appears to feed, as it were, the magnetism,
592. Magnetism may be preserved by terrestrial induc-
tion.— Magnetic needles, suspended freely, so as to obey the
attraction of terrestrial magnetism, do not admit of being thus
protected by keepers; but neither do they require it, for the
austral pole of the needle being always directed towards the
boreal pole of the earth, and the boreal pole of the needle towards
the austral pole of the earth, the terrestrial magnet itself plays the
part of the keeper, continually attracting each fluid towards its
proper pole of the magnet, and thus maintaining its magnetic
intensity.
593. Compound mag-nets. — Compound magnets are formed
by the combination of several bar magnets of similar form and
equal magnitude, laid one upon another, their corresponding poles
being placed in juxtaposition.
COMPOUND MAGNETS.
347
A compound horse shoe magnet, such as that represented in fig. 316., is
formed in like manner of magnetised bars, superposed on each other, and
similar in form, their corresponding poles being placed in juxtaposition.
These bars, whether straight or in the horse shoe foim, are separately mag-
netised before being combined by the methods already explained.
In the case of the horse shoe magnet a ring is attached to the keeper, and
another to the top of the horse shoe,/#. 316., so that the magnet being sus-
pended from a fixed point, weights may be attached to the keeper tending to
separate it from the magnet. In this way horse shoe magnets often support
from ten to twenty times their own weight.
Compound magnets are sometimes constructed in the form of
straight bars : such an apparatus, consisting of twelve bars disposed
in three layers of four bars each, is shown in fig- 32°-
Fig. J20.
In making compound magnets each component bar is separately tempered
diid magnetised, the whole being afterwards combined by screws or bolts.
The total force of such a combination is always less than the sum of the
forces of its component magnets, owing to the mutual action of the magnets
on each other. This effect is, to some extent, mitigated by making the
lateral bars somewhat shorter than the central ones.
594. A natural magnet, mounted so as to develop its power by
the effect of induction, is shown in Jig.
321. A, B represent the positions of
its poles ; E and p are two masses of
soft iron, which adhere to it by virtue
of the magnetic force. By the effect
of these, the magnetism is augmented,
for the magnetism developed in E
and P decomposes by its reaction an
increased quantity of magnetism in A,
B, which again reacting on A, B, pro-
duces a further development of mag-
netic power, and so on. The keeper
o being of soft iron, increases this
reciprocal action.
595. Magnetised tracings on a
steel plate. — If the pole of a mag-
net be applied to a plate of steel of
about one tenth of an inch thick and
of any superficial magnitude, such as
a square foot, and be moved slowly
upon it, tracing any proposed figure,
Fig.
348 MAGNETISM.
the line traced upon the steel plate will be rendered magnetic, as
will be indicated by sprinkling steel filings upon the plate. They
will adhere to those points over which the magnet has been passed,
and will assume the form of the figure traced upon the plate.
596. The influence of neat upon magnetism, which was
noticed at a very early period in the progress of magnetic dis-
covery, has lately been the subject of a series of experimental
researches by M. Kupffer, from which it appears that a magnetic
bar when raised to a red heat does not lose its magnetism suddenly
at that temperature, but parts with it by slow degrees as its tem-
perature is raised. This curious fact was ascertained by testing
the magnetism of the bar, by the means explained in (569.), at
different temperatures, when it was found that at different degrees
of heat it produced different rates of oscillation of the test needle.
It was also ascertained that, in order to deprive a magnetic bar
of all its magnetism when raised to a given temperature, a certain
length of time was necessary. Thus a magnetic bar plunged in
boiling water, and retained there for ten minutes, lost only a por-
tion of its magnetism, and after being withdrawn and again
plunged in the water for some length of time, It lost an additional
portion of its attractive force; and by continuing in the same
manner its immersion for the same interval, its magnetic force was
gradually diminished, a part still, however, remaining after seven
or eight such immersions.
A magnetic bar, when raised to a red heat, not only loses its
magnetism, but it becomes as incapable of receiving magnetism
from any of the usual processes of magnetisation, as would be any
substance the most incapable of magnetism.
597. Astatic needle. — All magnets freely suspended being
subject to the influence of terrestrial magnetism, the effects pro-
duced upon them by other causes are necessarily compounded
with those of the earth. Thus, if a magnetic needle be exposed
to the influence of any physical agent, which, acting independently
upon it, would cause its north pole to be directed to the east, the
pole, being at the same time affected by the magnetism of the
earth, which acting alone upon it would cause it to be directed to the
north, will take the intermediate direction of the north-east. When,
in such cases, the exact effect of the earth's magnetism on the
direction of the needle is known, and the compound effect is ob-
served, the effect of the physical agent by which the needle is
disturbed may generally be eliminated and ascertained. It is,
nevertheless, often necessary to submit a magnetic needle to
experiments, which require that it should be rendered independent
of the directive influence of the earth's magnetism, and expedients
have accordingly been invented for accomplishing this. A needle
ASTATIC NEEDLE. 349
which is not affected by the earth's magnetism is called an astatic
needle.
A magnetic needle freely suspended over a fixed bar magnet
will have a tendency, as already explained, to take such a position
that its magnetic axis shall be parallel to that of the fixed magnet,
the poles being reversed. Now if the fixed magnet be placed
with its magnetic axis coinciding with the magnetic meridian, the
poles being reversed with relation to those of the earth, its direc-
tive influence on the needle will be exactly contrary to that of the
earth. While the earth has a tendency to turn the austral pole
of the needle to the north, the magnet has a tendency to turn it
to the south. If these tendencies be exactly equal, the needle
will totally lose its polarity, and will rest indifferently in any
direction in which it may be placed.
As the influence of the bar magnet on the needle increases as
its distance from it is diminished, and vice versa, it is evident that
it may always be placed at such a distance from it, that its direc-
tive force shall be exactly equal to that of the earth. In this case,
the needle will be rendered astatic.
A needle may also be rendered astatic by connecting with it a
second needle, having its magnetic axis parallel and its poles re-
versed, both needles having equal magnetic forces. The com-
pound needle thus formed being freely suspended, the directive
power of the earth on the one will be equal and contrary to its
directive power on the other, and it will consequently rest indif-
ferently in any direction.
It is in general, however, almost impracticable to ensure the
exact equality of the magnetism of two needles thus combined.
If one exceed the other, as is generally the case, the compound
will obey a feeble directive force equal to the difference of their
magnetism.
598. The law of magnetic attraction and repulsion is the
same as that of gravitation ; that is, these forces increase in the
same proportion as the square of the distance of the centre of
attraction or repulsion diminishes. This has been established by
experiments of two kinds, one of which is made upon the principle
of the pendulum, and the other by an instrument invented by
Coulomb, called the balance of torsion, which was applied with
great success to the measurement of various other physical forces.
To determine the law of magnetic attraction by the principle
of the pendulum, a magnetised needle properly suspended is first
put in a state of oscillation subject only to the earth's magnetism,
and the rate of its oscillation is observed. It is then submitted to
the combined effects of the attraction of a magnet and that of the
earth, and the rate of its vibration is again observed, from which
350 MAGNETISM.
the sum of the forces of the magnet and the earth is deduced.
The magnetic force of the earth, being computed from the first
observation, is then subtracted from the sum of the magnetic
forces of the earth and the magnet deduced from the second
observation, the remainder being the force exerted by the magnet.
This experiment being repeated in placing the magnet at different
distances from the needle, it is found that its force, whether at-
tractive or repulsive, varies inversely as the square of the distance.
599. The balance of torsion as applied to the measurement of
magnetic forces consists of a cage of glass, fig. 322., having a cover
Fig. 322.
which can be removed at will, in which two holes are made ; one
near the edge, in which is inserted the magnetic bar F G submitted
to experiment ; and the other in the centre, in which is inserted a
glass tube, through which an extremely fine silver wire passes, to
INDUCTION OF EARTH. 351
the lower end of which is suspended a magnetic needle A B : this
silver wire is rolled upon a horizontal pin at the top, which is
turned by a screw having a milled head, so that by rolling or un-
rolling the wire the needle A B may be raised or lowered.
The arrangement at the top of the glass tube by which the wire is sus-
pended consists of two pieces, one of which D turns in a hole made in the
centre of the other E. The piece r> is attached to the cylindrical piece
through which the wire passes, and by turning it round its centre the wire
supporting the needle AB is also turned. The head of the piece E is gra-
duated, and that of n carries upon it an index mark, which being brought
to the zero of the division on K, will afterwards show tlie angle through
which the piece D and the wire with it are turned.
Now let us suppose that the austral pole of the magnet o is brought down
to the graduated circle upon the base of the instrument, and that the austral
pole A of the suspended needle is brought near to it. The pole of the magnet
will then repel that of the needle, and the wire by which the needle is sus-
pended will suffer a torsion or twist in the direction in which the needle
turns. When the tendency of the wire to untwist itself shall be equal to the
repulsive force exerted by G upon A, the needle will rest. By turning the
head D the needle may then be moved, so that the pole A shall be brought
to any required distance from G, and the force of torsion of the wire will be
equal to the force of magnetic repulsion between o and A. But the force of
torsion is always proportional to the angle of torsion; that is, the angle
through which the head D has been turned from that position in which the
index upon it coincided with the zero of the scale upon E. This angle can,
of course, be read off, and the intensity of the repulsion corresponding to the
distance between G and A can be thus found.
In the same manner the intensity of the repulsion at any other distance,
greater or less between o and A, can be determined, and it will accordingly
be found that these intensities will be inversely as the square of the distance
between G and A.
To simplifV the explanation, we have omitted here the consideration of the
influence of the magnetism of the earth upon the needle. This, however, is
3;isily determined previously to the action of the magnet FG. Supposing
this magnet to be raised so as to leave the pole A under no other influence
than that of the earth, the amount of torsion necessary to retain the pole A
in a given position against the magnetism of the earth can be ascertained in
the manner explained above. The magnet F o being then lowered, the
torsion necessary to retain the pole A in the same position can be deter-
mined, and this latter torsion is that which will equilibrate with the repul-
sion between G and A.
600. The inductive force of the earth, considered as a mag-
net, will decompose the natural magnetism of all bodies which
have not sufficient coercive force to resist its influence. Such
bodies, when placed in the northern hemisphere, will be so affected
that the austral fluid will be attracted towards the boreal pole of
the earth, that is, in the direction of the lower pole of the dipping
needle, and the boreal fluid will be repulsed towards its upper
pole. All such bodies, therefore, will be rendered temporarily
magnetic, and will acquire a polarity corresponding in its direction
352 MAGNETISM.
to that of the dipping needle. If their coercive force be suffi-
ciently feeble, and their form be favourable to the development
of the magnetic effects, these effects can be rendered manifest by
presenting a compass needle to different parts of the body so
affected. If it be presented to the part corresponding with the
lower pole of the dipping needle in the northern hemisphere, the
austral pole of the compass needle will be attracted and the boreal
repelled ; and if it be presented to the region corresponding with
the upper pole of the dipping needle, effects the reverse of these
will be produced.
60 1. Experimental illustration. — Let a rod of soft iron be
suspended vertically at any part of the earth where the dip is nearly
90°, and it will be found that the bar will be rendered magnetic, the
lower end having the properties of an austral, and the upper end of a
boreal pole, as may be rendered manifest by presenting a magnetic
needle, freely suspended, to the one and the other, and the direction
of which will be immediately affected in accordance with the pro-
perties of these poles respectively.
That the polarity of the bar is not proper to it, but merely in-
duced upon it by the magnetism of the earth, may be demonstrated
by placing the bar first at right angles to the magnetic meridian,
so that both ends of it shall be similarly affected, when all mag-
netism will disappear, and the test needle, when presented to it,
will suffer no change of direction. But if its primitive position
be reversed, the end which was downwards and had austral po-
larity being presented upwards, it will te found not only to have
lost the austral, but to have acquired boreal polarity ; while the
lower end previously turned upwards, which possessed boreal
polarity, will now have the properties of austral polarity.
602. Thus it appears that all bodies having so feeble a coercive
force as to allow of any degree of decomposition of their natural
magnetism, will, in the northern hemisphere, acquire a polarity
in the direction of the dipping needle, the austral pole being
directed obliquely downwards ; and in the southern hemisphere,
the boreal pole being similarly directed, and the obliquity of such
polarity following the direction of the dipping needle, will decrease,
as the place of observation is nearer to the magnetic equator, the
line upon which the dipping needle is horizontal.
603. The temporary magnetism becomes permanent
under the influence of a great variety of effects, mechanical, phy-
sical, and chemical, which have a tendency to augment the co-
ercive force of the body while it possesses magnetic polarity.
Thus if a bar of soft iron when suspended vertically, as described
above, and therefore rendered magnetic by the earth, be submitted
to percussion or hammering at either end, it will acquire a certain
COMPENSATORS 353
coercive force which will resist the recomposition of the magnetic
fluids, and the bar will accordingly retain a certain degree of its
polarity after it h;is been removed from the vertical position.
In like manner, if a bundle of straight pieces of soft iron wire,
ten or twelve inches in length, being suspended vertically, and
therefore rendered magnetic, be twisted so as to form a sort of
wire rope, the whole mass will retain its polarity when removed
from the vertical position, the torsion conferring upon it a coercive
force sufficient to resist the recomposition of the fluids.
In the same manner various chemical effects, such as oxidation,
thermal changes, and other physical incidents, are capable of so
affecting the coercive force as to cause the temporary magnetism
produced by terrestrial induction to become permanent.
604. These circumstances explain various effects which are well
known, such as the magnetisation of iron tools and implements
suspended in workshops ; and to the same cause may most pro-
bably be ascribed the production of natural magnets. The sub-
stances of which these are composed, at former epochs in the
history of the earth were probably in such a state of aggregation
as to deprive them of so much of their coercive force, that the
earth conferred upon them temporary magnetism, which at a
posterior epoch was rendered permanent by a change in their
aggregation, which increased the coercive force.
605. Compensators for ships' compasses are expedients by
which the errors of the compass needle produced by the attractions
and repulsions of such magnetic substances as may be contained in
the vessel are neutralised or corrected.
The errors of the compass needle must proceed from one or
more of three causes : —
i°. From the inductive influence of the needle itself upon
bodies composed of iron around it, and the reciprocal action of
the bodies thus magnetised by induction upon the needle. This
cause of disturbance, which can never be very intense, can always
be neutralised by removing all substances susceptible of magnetism
to such a distance from the compass needle as to render the effects
of such induction insensible.
2°. The needle may be disturbed by the permanent magnetism
of masses of iron, which either enter into the construction of the
vessel, or form part of its armament or cargo. This cause of dis-
turbance being permanent in its character, so long as the structure
of the vessel, its armament, and cargo remain unchanged, can,
when once detected, be always allowed for, so that the error of the
compass may be corrected.
If the influence of terrestrial magnetism upon the vessel be
supposed to cease or to be neutralised, the compass needJe would
354 MAGNETISM.
be affected by no other influence than that of the magnetism of the
vessel and its contents ; and in obedience to that influence, it
would assume a certain determinate direction, making a definite
angle with the keel of the vessel ; and it would retain this position
relatively to the keel, however the direction of the keel itself might
be changed. Thus, if the vessel were made to revolve horizontally
round a vertical line through its centre, the compass needle would
revolve with it without suffering any change of direction relatively
to the keel.
Now let us suppose the vessel to have that position in which the
direction given to the needle by the magnetism of the vessel shall
coincide with the magnetic meridian. In that case, since the
magnetism of the vessel and the magnetism of the earth give the
needle the same direction, there will be no deviation. But if the
vessel be then made to revolve horizontally round its centre, the
line of direction of its magnetic influence will revolve, making a
constantly varying angle with the magnetic meridian. The mag-
netism of the vessel would therefore cause the needle to deviate
from the magnetic meridian, through a gradually increasing angle,
on that side towards which the line of direction of the influence of
the vessel turns. This deviation would increase to a certain limit ;
after which it would again decrease, and the needle would return
to the magnetic meridian, when the vessel would have made half a
revolution, after which it would deviate to the other side of the
magnetic meridian, would attain a certain limit, after which it
would again return in the other direction, and again coincide with
the magnetic meridian, when the vessel would have completed its
revolution.
If, therefore, the vessel be thus made to revolve horizontally
round its centre, and the arc through which the needle oscillates
on the one side and the other be observed, the line which bisects
this arc will be the direction which would be given to the com-
pass needle by the magnetism of the vessel acting upon it, inde-
pendently of the magnetism of the earth ; and this deviation being
known, the correction necessary for the magnetism of the vessel
would be obtained, since the line of direction of the magnetic
meridian will in all cases be that of the bisecting line.
606. Barlow's compensator. — 3°. The third and most diffi-
cult cause of error of ships' compasses is due to the temporary
magnetism impressed upon the masses of iron contained in the
vessel by the inductive action of the earth. This is the more
difficult to determine and correct, inasmuch as its effects are not
only much greater than those proceeding from the other causes,
but are subject to incessant variation, according to the position
which the vessel assumes with relation to the direction of the
COMPENSATORS. 355
earth's magnetism. When the vessel is made to turn as above
described, horizontally, round its centre, the bodies it contains,
which are susceptible of magnetism, suffer a varying action, ac-
cording to the various positions they assume relatively to the
direction of the earth's magnetism. But in making one complete
revolution, they assume every possible variety of position, and
receive from the earth's magnetism every possible variety of
effect.
Let us suppose, then, the vessel placed within a few hundred yards of the
shore, and two observers to be stationed one at the compass in the vessel, and
the other with a compass on the shore, being provided with instruments by
which the relative directions of the two needles to those of the line joining
the two observers can be accurately observed. Now if the magnetism of the
vessel exerted no disturbing action, the direction of the two needles would be
parallel, since the direction of the earth's magnetism will be sensibly the
same at two places so near each other. But it will be found, on the con-
trary, that the needle on the vessel will deviate from parallelism with the
needle on the shore by a certain angle, and this angle can be measured by
the combined observations at the two stations, and when measured the error
or deviation of the needle in that particular position of the vessel will be
known. The direction of the keel of the vessel being then changed, the
deviation corresponding to its new position will be found in the same man-
ner ; and the vessel being thus gradually made to revolve round its centre,
the deviation of the needle from the magnetic meridian corresponding to the
direction of the keel at each observation will be determined, and its devia-
tions for all intermediate directions may be computed by the method of
interpolation.
This being done, the ship's compass is brought on shore and placed upon
a wooden pillar, capable of being turned round its vertical axis. In the side
of this pillar a number of holes placed vertically one under another are made,
into which a copper rod can be inserted, carrying at its extremity two
circular discs of iron, about a foot in diameter, and having such a thickness
as would weigh 3 Ibs. per square foot. These plates of iron will produce a
disturbing effect upon the compass needle at the top of the wooden pillar,
similar in kind to that produced by the vessel ; and this disturbance may
be made to vary in degree by transferring the copper rod, carrying the iron
discs from hole to hole in the wooden pillar, so as to vary its distance from the
compass needle. By a series of trials such a position may be given to it
that, when the wooden pillar is made to turn through one complete revolution,
the compass needle shall make precisely the same series of deviations as that
which it makes upon the deck during one complete revolution of the vessel.
Now let us suppose that the compass thus supported with the iron discs,
adjusted as here stated, is transported on board the vessel, it is evident that
the disturbing effect which produces the deviation of the needle will be
doubled, since the needle is at once affected by the induced magnetism of the
vessel, and by that of the iron discs. To determine, therefore, the deviation
of the needle at any moment, it is only necessary to observe its direction, first,
when the copper rod with the discs is inserted in the pillar ; and, secondly,
when it is not so inserted. The difference between the two directions will
then be the amount of the deviation.
356
BOOK THE FOURTH.
ACOUSTICS.
CHAPTER I.
THEORY OF UNDULATIONS.
607. A vast mass of discoveries produced by the labour of
modern inquirers in several branches of physics, and more espe-
cially in those where the phenomena of sound, heat, light, and the
other imponderable agents are investigated, have conferred upon
the physical theory of undulations much interest and importance.
608. Undulations in general. — When a mass of matter,
whatever be its form or conditions, being in a state of stable
equilibrium, is disturbed, either collectively or in the internal
arrangement of its constituent parts, by any external force which
operates upon it for a moment, it will have a tendency to return
to the state from which it was disturbed, and will so return, pro-
vided the disturbing force have not permanently deranged its struc-
ture. After it has returned to the position of equilibrium, h will
have a tendency, by reason of its inertia, to depart from such
position again, and to make an excursion in a contrary direction,
and so continually to pass on the one side and the other of this
position, with an alternate motion more or less rapid, until, at
length, by the resistance of the medium in which it is placed, and
other causes, it is gradually brought to rest, and settles finally in
its previous position of stable equilibrium.
Alternate motions, thus produced and continued, are variously
expressed by the terms vibrations, oscillations, waves, or undulations,
according to the state and form of the body in which they take
place, and to the character of the motions.
One of the most familiar and generally known examples of this
class of motion has already been noticed in the case of the pen-
dulum. There the oscillation is produced by the alternate dis-
placement of the entire mass of the body, which partakes in the
common motion of vibration.
609. Formation of a wave, — It does not always follow,
UNDULATIONS. 357
however, that the particles of the vibrating body thus share in a
common motion. If an elastic string be extended between two
iixed points, and be drawn laterally from its position of rest by a
force applied at its middle point, it will return to that position of
rest and pass beyond it, and will thus alternately oscillate on the
one side and on the other of such a position. In this case the
oscillatory motion bears a close analogy to that of the pendulum,
as will be more fully noticed hereafter.
Let A K,jig. 323., be a flexible cord attached to a fixed point at
B, and held by the hand at A. If this cord be jerked smartly once
or twice up and down by the hand at A, it will immediately change
its form, and an apparent movement will be produced, passing
from the end A towards the end B, similar to that of waves upon
water. The first effect of the motion will be to cause the cord
to assume the curved form A s o, rising above the position of equi-
librium. This will be succeeded by a corresponding curved form
o s' P, depressed to the same extent below the position of equili-
brium. If the cord be jerked but once, then the point o.will
appear to advance towards B, the elevation A s o following it, and
the depression of o s' P preceding it, so that the appearances pro-
duced successively will be those represented in Jigs. 323, 324,
325, 326-
The curve A s o s' P is called a wave.
The curve A s o, which rises above the position of equilibrium,
is called the elevation of the wave, s being the summit or point of
greatest elevation.
The curve o s' P is called the depression of the wave, the point
s' being that of greatest depression.
The distance s Q of the highest point above the position of equi-
librium is called the height of the wave ; and in like manner the
distance s' Q' of the lowest point of the depression below the posi-
tion of equilibrium is called the depth of the wave.
The distance A P between the beginning of the elevation and the
end of the depression is called the length of the wave; the distance
A o the length of the elevation, and o P that of the depression.
It is found that such a wave, on arriving at the extremity B, as
represented inj%-. 326., will return from B to A, as represented in
fig*> 327> 328, 329, 330., in the same manner exactly as it had
advanced from A to B.
Having thus returned to A, it will begin another movement
towards B, and so proceed and return as before.
6lO. Waves progressive and stationary. — A wave which
thus moves in some certain direction, is called a progressive
undulation.
Let a cord be extended between two fixed points, A and B,
358
ACOUSTICS.
fig. 331., and let it be divided into any number of equal parts,
three for example, at c and D. Let the points c and D be tern-
Fig. 319.
porarily fixed, and let the three parts of the cord be drawn from
c ^ their position of rest in con-
A / \/'""~ ~"'"\/^ Ny B trai7 directions, so that the
cord will assume the undu-
lating form represented in the
figure. If the parts of the
Fig. 331-
cord be simultaneously discharged, each part will vibrate between
the fixed points c and D, the adjacent vibrations being always in
contrary directions.
Now let the points c and D be liberated. No change will then
take place in the vibratory motion of the cord, and it will there-
fore alternately throw itself into the positions represented in the
figure by the continuous line and the dotted line. But as it con-
APPARATUS OF AUGUST. 359
tinues to vibrate, the parts c and D, although free, will be
stationary, and waves will be formed, whose elevation and de-
pression will be alternately above and below the lines joining the
points A, c, D, and B.
Such an undulation not having any progressive motion, is ac-
cordingly called a stationary undulation.
The points c and D of the wave, which never change their
position, are called nodal points.
This species of undulation may be considered to be produced by
the alternate elevation and depression of the several parts of the
cord above and below its position of equilibrium.
As the circumstances attending, and the laws which govern, the
vibrations or undulations of bodies vary with the state in which
they are found, according as they are solid, liquid, or gaseous, it
will be convenient to consider such eiFects as exhibited in these
states severally.
611. Vibrations of cords and membranes. — Solid bodies
exhibit the phenomena of vibration in various forms and degrees,
according to their figure and to the degree of their elasticity.
Cords and wires have their elasticity developed by tension. The
same may be said of bodies which have considerable superficial
extent with little thickness, such as thin membranes like paper or
parchment. When these are stretched tight and struck, they will
vibrate on the one side and on the other of their position of equi-
librium, in the same manner as a stretched cord.
Elastic substances, whatever be their form, are susceptible of
vibration, the manner and degree of this varying in an infinite
variety of ways, according to the form of the body, and to the
manner in which the force disturbing this form and producing the
vibration is applied.
612. Apparatus of August. — Those solids whose breadth or
thickness is very small in proportion to their length,
such as thin rods, cords, or wires, are susceptible of
three kinds of vibration, which have been deno-
minated the transverse, the longitudinal, and the
torsional.
An apparatus to exhibit these elfects experi-
mentally, contrived by Professor August, is repre-
sented in Jig. 332. This apparatus consists of a
piece of brass wire formed into a spiral, one end of
which is attached to a frame from which it is sus-
pended, and the other end supports a weight by
which it is strained. The transverse vibrations are
Fig. jjz. produced by fixing the lower end of the wire by
means of the movable clamp represented in the
3 6o
ACOUSTICS.
figure. The wire is then drawn aside from its position of equi-
librium and suddenly let go, after which it vibrates on the one
:side and on the other of this position.
To show the longitudinal vibrations, the weight suspended from
the wire is drawn downwards by the hand, the wire yielding in
consequence of its spiral form. When the weight is disengaged,
the wire draws it up, the spiral elasticity being greater than the
weight. The weight, however, rises in this case above the position
of equilibrium, then falling returns to it ; but in consequence of
its inertia descends below it, and thus alternately rises above and
falls below this position, until at length it comes to rest.
The torsional vibrations are shown by turning the weight round
its vertical diameter. When so turned and let go, it will turn
back again until it attains its position of equilibrium ; but by
reason of its inertia it will continue to turn beyond that position
until stopped by the resistance of the wire, when it will return,
and thus alternately twist round in the one direction and in the
other, until it comes to rest.
613. Elastic string-s. — Of the various forms of solid bodies
susceptible of vibration, that which is attended with the greatest
inteiest and importance is an extended cord ; inasmuch as it not
only produces the phenomena in such a manner and form as to
render the laws which govern them more easily ascertained, but
also constitutes the principle of an extensive class of musical in-
struments, and is therefore of high importance in the theory of
musical sounds.
Let A -R,Jig. 333-» be such an extended string. If it be drawn
aside at its middle point c from its position of equilibrium, so as
to be bent into the form A D B, and then disengaged, it will in
virtue of its elasticity return to the position A c B ; the point D
approaching c with an accelerated motion, exactly in the same
manner as the ball of a pendulum approaches the centre point of
its vibration. Having arrived at the position A c B, the string in
consequence of its inertia will be carried beyond that position,
and will arrive at a position A D' B on the other side of A c B,
nearly at the same distance as A D B was. The motion of the
middle point c from c to D' is gradually retarded, until it entirely
LAWS OF VIBRATION. 361
ceases at D', precisely similar to the motion of the ball of a pen-
dulum in ascending from the middle point to the extreme limit oi
its vibration. All these observations will be equally applicable to
any other point of the string, such as c, which oscillates in like
manner between the points d and d'. All the circumstances
which were explained in the case of the pendulum, and which
showed that the oscillations, whether made through longer or
shorter arcs, were made in the same time, are equally applicable
to this case of a vibrating string. Thus, the force which impels
any point, such as D, towards the line A B, increases as the distance
of D from the line A B increases. Therefore, the greater the
extent of the excursion which the string has to make, the greater
in proportion will be the force which will impel it ; and con-
sequently, the time of vibration will be the same, although the
amplitude of the vibrations be greater. It is, therefore, the
general property of all extended strings, when put in vibration,
that they will oscillate on either side of their position of rest in
equal times, whether the amplitude of the vibrations is great or
small. It follows from this, that the time of oscillation will be
the same during the continuance of the vibration of the same
string, although the amplitude of the oscillations it performs be
continually diminished.
These observations, with the necessary qualifications, are appli-
cable to all vibrating bodies. In all cases, the force tending to
bring them back to the position of equilibrium is great, in pro-
portion to the extent of their departure from it ; and, consequently,
the time of oscillating on either side of their position of equilibrium
will be the same, although the amplitude of each oscillation is
variable.
614. Their laws. — The following Jaws which govern the
vibration of strings have been demonstrated by theory and verified
by experiment.
Let N express the number of vibrations per second which, the string makes.
Let L express the length of the string.
Let s express the force with which the string is stretched.
Let i> express the diameter of the string.
I. The number N will be inversely proportional to i., other
things being: the same. — That is to say, the number of vibra-
tions made by a string per second will be increased in the same
proportion as the length of the string is diminished, and vice versa,
the tension of the string and its thickness remaining the same.
II. The number N varies in the proportion of the square
root of s, other things being the same. — That is to say, the
number of vibrations performed by a string per second will be
increased in proportion to the square root of the force which
362 ACOUSTICS.
stretches the string. If the string be extended by a fourfold
force, the number of vibrations which it performs per second will
be doubled ; if it be extended by a ninefold force, the number of
vibrations it performs per second will be increased in a threefold
proportion, and so on.
III. The number of vibrations performed per second is in
the inverse proportion of the diameter of the string-, other
things being: the same. — That is to say, if two strings composed
of the same material be stretched with the same force, one having
double the diameter of the other, the latter will perform twice as
many vibrations per second as the former.
The three preceding rules may be expressed in combination by the follow-
ing formula : —
K = «x^
LD
m which a is a number depending on the quality of the material of the string
and which -will vary in the formula if two different strings be compared to-
gether.
It follows, from this formula, that
The constant number a, therefore, is found by dividing the product of the
numbers expressing the vibrations per second, the length of the string, and
its thickness by the square root of that which expresses the force by which
the string is extended.
The manner in which the preceding laws may be verified by experiment
will be explained hereafter.
The constant number a will depend upon the physical properties of the
material of which the string is composed. It will, therefore, be the same for
all strings of the same material and structure, but will differ when strings of
a different material or different structure are compared together.
615. Elastic plate. — If an elastic rod, being fixed at one end
and free at the other (fig. 334.)* be drawn aside from its position
of equilibrium and let go, it will pass into a state of vibration, and
its vibrations will be isochronous, for the reasons which have been
explained in a general manner. With rods of the same material
and structure the rate of vibration will depend on the length and
thickness, but will be independent of the breadth.
With the same length the number of vibrations per second will
be proportional to the thickness.
With the same thickness the number will be inversely as the
square of the length.
Chaldni verified these laws by experiments made on thin bars.
More recently, however, M. Baudrimont showed, by experiments
made on plates of glass, zinc, copper, rock crystal, and wood, that
NODAL POINTS.
363
Fig. 334.
the results ceased to be in ac-
cordance with the law in certain
cases, especially when the thickness
exceeds 4 or 5 twelfths of an inch.
It must also be understood that
these laws are only applicable so
long as there are no nodal points.
6 1 6. Elastic wires. — The vi-
brations produced by elastic wires
fixed at one end are not, like the
vibrations of a common pendulum,
generally made in the same plane ;
in other words, the free extremity
of the wire does not describe a
circular arc between its extreme
positions. It appears to be im-
pressed with, at the same time, two
vibratory motions in planes at right
angles to each other, and moves in
a curve produced by the compo-
sition of these motions. These
effects are rendered experimentally
apparent in a beautiful manner, by the following expedient. Let
several elastic steel wires, knitting needles, for example, be fixed at
one end in a vice or in a board, and let small balls of polished steel,
capable of reflecting light intensely, be attached to the vibrating
ends. Each of these small polished balls will reflect to the eye a
brilliant point, and when they are set in motion this brilliant point
will produce a continued line of light, in the same manner and
upon the same principle on which the end of a lighted stick made
rapidly to revolve appears one continued circle of light.* Now,
when the needles are put into a state of vibration, the brilliant-
points will appear to describe a complicated curve, exhibited to the
eye by an unbroken line of light reflected from the polished ball.
617. Nodal points. — Elastic rods are susceptible of the sta-
tionary undulations already described, as well as strings. The
nodal points in the one and the other can be ascertained experi-
mentally by placing the vibrating string or wire in a horizontal
position, and suspending upon it light rings of paper. They will
be thrown off so long as they rest upon any part of the string or
wire except the node ; but when they come to a node, they will
remain there unmoved, although the vibration of the string or
wire may continue.
This experiment may be easily performed upon a string stretched
* Optics (373.).
364 ACOUSTICS.
in a horizontal position. If such a string be taken between the
fingers at two points, each distant by one fourth of its length from
the two extremities, and being drawn aside in opposite directions,
be disengaged, it will vibrate with a stationary undulation, the
nodal point being in the centre, and each half of the string vibrat-
ing independently of the other. If a light paper ring be suspended
on such a string at the middle point, it will remain unmoved ; but
if drawn aside from the middle point, it will be thrown off and
agitated until it returns to that point, where it will again remain
at rest.
6 1 8. CTodal lines. — A solid, in the form of a thin elastic plate,
made to vibrate, will also be susceptible of stationary undulations,
and will have a regular series of nodal points. Such a plate may
be considered as consisting of a series of rods or wires, placed in
contact and connected together, and the series of their nodal points
will form upon the plate a series of
b jj nodal lines.
To render these nodal lines expe-
a rimentally apparent, it is only neces-
sary to spread upon the plate a thin
coating of fine sand ; when the plate
is put into vibration, the sand will
be thrown from the vibrating points,
and will collect upon the nodal lines,
Fig. 335. and affect an arrangement of which
an example is given inj%-. 335. This
will be more fully explained hereafter when we treat of sound.
619. Undulation of liquids. — Circular -waves. — If a vessel
containing a liquid remain at rest, the liquid being subject to no
external disturbance, the surface will form a uniform level plane.
Now, if a depression be made at any point of this surface by
dropping in a pebble, or by immersing the end of a rod, and sud-
denly withdrawing it, a series of circular waves will immediately
be formed round the point, as a centre, where such depression is
made, and each such wave will expand in a progressively increasing
circle, wave following wave until they encounter the bounding
sides of the vessel.
620. Apparent progressive motion of waves an illusion. —
In this phenomenon a curious deception is produced. When we
perceive the waves thus apparently advancing, one following
another, we are irresistibly impressed with the notion that the
fluid itself is advancing in the same direction ; we consider that
the same wave is composed of the same water, and that the enth-e
surface of the liquid is in progressive motion. A little reflection,
however, on the consequences of such a supposition will prove
WAVES 365
chat it is unfounded. The ship which floats on the waves of the
sea is not carried forward with them ; they pass beneath her in
lifting her on their summits, and in letting her sink into the abyss
between them. Observe a sea-fowl floating on the water, and the
same effect will be seen. If, however, the water itself partook of
the motion of the waves, the ship and the fowl would each be
carried forward with a motion in common with the liquid. Once
on the summit of a wave, there they would constantly remain ; or
if once in the depression between two waves, they would like-
wise continue there, one wave always preceding and the other
following them.
It is evident, therefore, that the impression produced, that the
water is in progressive motion, is an illusion. But, it may be
asked, to what then does the progressive motion belong ? That
such a progressive motion does take place in something, we have
proof from the evidence of sight ; and that no progressive motion
takes place in the liquid we have still more unquestionable evi-
dence. To what, then, does the motion belong ? We answer, to
the form of the surface, and not the liquid composing it.
To render intelligible the manner in which the waves upon a
liquid are produced, let A BCD, Jig. 336.,
A I M II j] ° ji> be a vessel containing a liquid whose surface
Ji_E when at rest is LL. Let us imagine a-
\ :@pL siphon MNO inserted in this vessel, filled
with water to the same level as the vessel.
It is evident that the water included within
f=yj^ the siphon will hold the same position pre-
cisely as the water of the vessel which the
siphon displaces. If we suppose a piston
inserted in the leg M N to press down the
water from the level I,L to the depth D', the water in the leg N o
will rise to the height E. If the piston be suddenly withdrawn,
the water in the leg M N will again rise, and the water in the leg
N o will fall, the surfaces D' and E will return to the common level
L L, but they will not remain there, for, in consequence of the
inertia, the ascending motion of the column i> and the descending
motion of the column E will be continued, so that the surface D'
will rise above L, L, and the surface E will fall below it, and having
attained a certain limit, they will again return respectively to the
level L L, and oscillate above and below it until, by friction and
atmospheric resistance, they are brought to rest at the common
level L L.
Now if we imagine the siphon to be withdrawn, so that the
water which occupies its place may be affected by the same
pressure at D', the same oscillation will take place ; but, at the
366
ACOUSTICS.
same time, the lateral pressure which is obstructed by the sides of
the siphon will cause other oscillations, by the combination of
which the phenomenon of a wave will be produced.
Let A B c D, fig. 337., be an undulation produced on the surface of a
liquid. This undulation will appear to have a progressive motion from A
towards x.
Fig. 337-
Let us suppose that in the interval of one second the summit of the wave
R is transferred to b'. Now let us consider with what motion the particles
forming the surface of the water are affected during this interval.
The particle at B descends vertically to 6, while the particle B' ascends
vertically to b'. The several particles of the wave in the first position
between B and c descend in the vertical lines represented by dotted lines in
the figure to the several points of the surface between b and c. At the same
time, the several points of the surface of the wave in its first position between
c and B' rise in vertical lines, and form the surface of the wave in its second
position between c and b'.
In like manner, the particles of the wave in the first position between B'
and c' rise in vertical lines, and form the surface of the wave in its new
positions between b' and c'.
In the same manner, during the same interval the particles of liquid
forming the surface B A descend in vertical lines and form the surface b a.
Thus it appears that in the interval of one second the particles of water
forming the surface ABC fall in vertical lines, and those forming the surface
c B' c' rise in vertical lines, and at the end of a second the series of particles
form the surface a be b' c'.
In this manner, in the interval of one second, not only the crest
of the wave is transferred from B to b', but all the parts which
form its profile are transferred to corresponding points holding
the same relative position to the new summit V. Thus we see
that the form of the wave has a progressive motion, while the
particles of water composing its surface have a vertical motion
either upwards cr downwards, as the case may be.
621. Stationary waves. — Hence it appears that each of the
particles composing the surface of a liquid is affected by an alter-
nate vertical motion. This motion, however, not being simul-
taneous but successive, an effect will be produced on the surface
which will be attended with the form of a wave, and such wave
STATIONARY WAVE. 367
will be progressive. The alternate vertical motion by which the
particles of the liquid are affected will, however, sometimes take
place under such conditions as to produce, not a progressive, but
a stationary undulation. This would be the case if all the par-
ticles composing the surface were simultaneously moved upwards
and downwards in the same direction, their spaces varying in
magnitude according to their distance from a fixed point.
To explain this, let us suppose the particles of the surface of a liquid
between the points a e,/i(7. 338., to be simultaneously moved in vertical lines
Fig. 338.
upwards, the centre particle c being raised through a greater space than the
particles contiguous to it on either side. The heights to which the other
succeeding particles are raised will be continually diminishing, so that at
the end of a second the particles of liquid which, when at rest, formed the
surface a e, will form the curved surface abode.
In like manner, suppose the particles of the surface e i to be depressed in
vertical lines, corresponding exactly with those through which the particles
a e were elevated. Then the particles which originally formed the surface
e i would form the curved surface efg h t, and they would become the de-
pression of a wave. Thus the elevation of the wave would be a b c d e, and
its depression efg h i.
Having attained this form, the particles of the surface abode would fall
in vertical lines to their primitive level, and having attained that point,
would descend below it; while the particles e,f, g, h, i, would rise to their
primitive level, and having attained that position, would continue to rise
above it. In fine, the particles which originally formed the surface of the
undulation a 6 cdefg h i would ultimately form the surface a b' c' d' e'f g' h' i
represented by the dotted line.
Having attained this form, the particles would again return
to their primitive level, and would pass beyond it, and so on
alternately.
In this case, therefore, there would be an undulation, but not
a progressive one. The nodal points would be e, i, n, r, and these
points during the undulation would not be moved ; they would
neither sink nor rise, the undulatory motion affecting only those
between them.
This phenomenon of a stationary undulation produced on the
surface of a liquid may easily be explained, by two systems of
progressive undulation meeting each other under certain con-
ditions, and producing at the points we have here called nodal
j68 ACOUSTICS.
points the phenomenon of interference, which we shall presently
explain.
Stationary undulations may be produced on a surface of liquid
confined in a straight channel by exciting a succession of waves,
separated by equal intervals, moving against the end or side of
the channel, and reflected from it. The reflected waves, combined
with the direct waves, will produce the effect here described.
It may also be produced by exciting waves in a circle from its
central point. These waves being reflected from the circular sur-
face, will produce another series, which, combined with the former
would be attended, with the effect of a stationary undulation.
622. Depth of waves. — When a system of waves is produced
upon the surface of a liquid by any disturbing force, a question
arises to what depth in the liquid this disturbance of equilibrium
extends. It is possible to suppose a stratum of the liquid at any
supposed depth below which the vertical arrangement would not
be continued. Such a stratum may be regarded as the bottom of
the agitated part of the fluid.
The Messrs. Weber, to whose experimental inquiries, in this
department of physics, science is much indebted, have ascer-
tained that the equilibrium of the liquid is not disturbed to a
greater depth than about three hundred and fifty times the alti-
tude of the wave.
623. Reflection of waves. — If a series of progressive waves
impinge against any solid surface, they will be reflected, and
will return along the surface of the fluid as if they emanated from
a centre equally distant on the other side of the obstructing
surface.
To explain this, it is necessary to consider that when any part
of a wave encounters the obstructing surface, its progress is re-
tarded, and the particles composing it will oscillate vertically in
contact with the surface, exactly as they would oscillate if they
had at this point been first disturbed. They will therefore, at
this point, become the centre of a new system of waves, which
will be propagated around it, but which will form only semi-
circles, since the centre of undulation will be against the ob-
structing surface, which will, as it were, cut off half of each
circular undulation. As the several points of the wave meet the
obstructing surface in succession, other series of semicircular
waves will be formed, and we shall see that by the combination of
these various systems of semicircular waves, a single wave will be
formed, the centre of which will be a point just so far on the
other side of the obstructing surface, as the original centre was on
the side of the fluid.
Let c,fg. 339., be the original centre of undulation, and let
REFLECTION OF WAVES.
369
a wave w w issuing from it move towards the obstructing surface
A B. The first part of this wave which will meet the obstructing
surface will be the point v, which moves along the line c M per-
pendicular to it. After this, the other points of the wave on the
one side and on the other will successively strike it.
Let us take the moment at which the surface is struck at the
points B and A equally distant from the middle point M by two
parts of the wave. All the intermediate points between B and A
will have been previously struck ; and if the wave had not been
intercepted by the obstructing surface, it would at the moment
at which it strikes the points B and A have had the form of the
circular arc A o B, having the original point c as its centre.
But as the successive points of the wave strike the surface AB,
they will, according to what has been explained, each become the
centre of a new wave which will have a semicircular form ; and to
ascertain the magnitude of such wave at the moment the original
wave strikes the points A and B, it is only necessary to ascertain
the distance through which each semicircular wave will expand,
in the interval between the moment at which the vertex of the
original wave strikes the point M, and the moment at which the
two extremities of the wave strike the points A and B. It is
370 ACOUSTICS.
evident that if the wave had not been interrupted at M, its vertex
would have been moved on to o ; and as the new wave reflected
from M will have the same velocity, it follows that at the moment
the original wave would have arrived at o, the reflected wave will
have expanded through a semicircle whose radius is M o. There-
fore, if we take the point M as a centre, and a line equal to M o as
a radius, and describe a semicircle, this semicircle will be the
position of the new wave formed with M as a centre, at the moment
that the extremities of the original wave struck the points
A and B.
In like manner, it may be shown that if p be the position, which
the point of the original wave which struck N would have attained
had it not been interrupted, the distance through which the semi-
circular wave having N as a centre would have expanded in the
same time will be determined by describing a semicircle with N as
a centre, and N p as a radius. In the same manner it may be
shown that the forms of all the semicircular waves, produced \\ith
the points N of the obstructing surface between A and B as centres,
will be determined by taking the several parts of the radii c P,
which lie beyond the obstructing surface as radii, and the points N
where they cross the obstructing surface as centres. This has
been accordingly done in the diagram, by which it will be per-
ceived that the space to the left of the obstructing surface is inter-
sected by the numerous semicircular waves which have been formed.
But it appears also that the series of points where they intersect
each other most closely is that of a circular arc A o' B, having for
its centre the point c', whose distance behind the surface M is
equal to the distance of the centre c before it, so that c M shall be
equal to c' M. The effect will be, that a circular wave A o' B will
be formed, the intersection of the semicircles within this being so
inconsiderable as to be imperceptible. This wave A o' B will
accordingly expand from the surface A B towards c on the left in
the same manner as the wave A o B would have expanded on the
right towards c', if it had not been interrupted by the obstructing
surface.
If any radius of the original wave, such as CP, and the corre-
sponding radius CP' of the reflected wave be also drawn, these
two radii will evidently make equal angles with the line CMC/
which is perpendicular to the obstructing surface ; and conse-
quently, if from the point N a line N Q be drawn parallel to c M,
and therefore perpendicular to AB, the lines CN and NR will form
equal angles with it.
624. law of reflection. — The angle CNQ is called the angle
of incidence of the wave, and the angle Q N R is called the angle of
reflection ; and hence it is established as a general law, that in the
WAVES FROM THE FOCI OF AN ELLIPSE. 371
reflection of waves from any obstructing surface, the angle of in-
cidence is equal to the angle of reflection, — a law which has al-
ready been shown to prevail when a perfectly elastic body is
reflected by a perfectly hard surface.
When a wave strikes a curved surface, it will be reflected from
it in a different direction, according to the point of the surface at
which it is incident. It will be reflected from such point in the
same direction as it would be if it struck a plane which coincides
with the curved surface at this point.
625. Waves propagated from the foci of an ellipse.—
There are two species of
curves, which in those
branches of physics which
involve the principles of
undulation are attended
with consequences of
considerable importance.
These figures are the
ellipse and the parabola.
Fig. 340. represents an
ellipse : A B is its major
axis, and c D its minor
axis ; F r' are two points
upon its major axis called
its foci, which have the
following property. If
lines be drawn from the foci to any point p in the ellipse, these
lines will form equal angles with the ellipse at P, and their lengths
taken together will be equal to the major axis AB.
A remarkable consequence of this property follows, relative to
undulations having for their centres one or other of the foci. If a
series of progressive circular waves, propagated from the focus F
as a centre, strike the surface, they will be reflected from the
surface at angles equal to those at which they strike it, because,
by the law which has been already established, the angles of re-
flection will be equal to the angles of incidence. If, then, we
suppose several waves of the same system diverging from the focus
F, to strike successively the elliptical surface at the point p, they
will be reflected in the direction p F' towards the other focus. But
as all the points of the same wave move with the same velocity,
they will describe equal spaces in the same time. Let the points
ppp upon the lines PF' be those at which the points of the wave
will arrive simultaneously. It then follows, that the lines FP and
vp will, taken together, be equal, being in each case the spaces
described in the same time by different points of the same wave.
372
ACOUSTICS.
If, then, these equal lengths ?pp be taken from the lengths FPF',
which are also equal to each other, as has been already explained,
the remainders F' 'p will necessarily be equal ; therefore the points
p will lie at equal distances from F', and will therefore form a
circle round F' as a centre.
Hence it follows, that each circular wave which expands round
F will, after it has been reflected from the surface of the ellipse,
form another circular wave round F' as a centre.
626. Waves propagated from the focus of a parabola. —
The curve called a parabola is
represented in Jig. 341. The
point v is its vertex, and the
line VM is its axis.
A certain point F upon the
axis near the vertex, called the
focus, has the following property.
Let lines be drawn from this
point F to any points such as P
in the curve ; and let other lines
be drawn from the points p se-
verally parallel to the axis VM,
meeting lines ww' drawn per-
pendicular to the axis, and ter-
minated in the curve. The lines
FP and pp will be inclined at
equal angles to the curve at the
points P, and the sum of their
lengths will be everywhere the
same ; that is, if the length of
the line FP be added to the
Fig. 341. length of the line PJO, the same
sum will be obtained whichever
of the points p may be taken ; and this will be the case whatever
line w w' be drawn perpendicular to v M.
It follows from this property, that if the focus of a parabola be
the centre of a system of progressive waves, these waves, after
striking the surface, will be reflected so as to form a series of
parallel straight waves in the direction of the lines ww', and
moving from F towards M.
This may be demonstrated in precisely the same manner &s it
has been proved in the case of the ellipse that the reflected waves
form a circle round the focus T'; for the lines FP and pp, Jig. 341.,
forming equal angles with the curve, will necessarily correspond
with the direction of the incident and reflected waves, and the
sum of these lines being the same wherever the point P may be
WAVES FROM THE FOCUS OF A PARABOLA. 373
situated, the several points of the same wave striking different
points of the parabola will arrive together at the line w w', inas-
much as they move with the same velocity, and have equal spaces
to move over.
On the other hand, it follows, by precisely similar reasoning,
that if a series of parallel straight waves at right angles to v M,
moving from M towards v, should strike the parabolic surface,
their reflections would form a series of circular waves of which
the focus F would be the centre.
If two parabolas, A v B and A' v' B', fig. 342., face each other so
Fig. 341.
as to have their axes coincident and their concavities in opposite
directions, a system of progressive circular waves issuing from one
focus F, will be followed by a corresponding system, having for
the centre the other focus F'. The waves which diverge from F,
after striking on the surface A v B, will be converted into a series
of straight parallel waves moving at right angles to vv', and
towards v'. These will strike the surface A' v' B', and after being
reflected from it will form another series of circular waves, having
the other focus F' as their common centre.
A circular arc, if its extent be not great compared with the
length of its radius, may be considered as practically coinciding
with a parabolic surface whose focus is at the middle point of the
radius cf the circular surface.
For example, let AB, fig. 343., be a circular arc, whose centre
is c, and whose middle point is v.
Let F be the middle point of the
radius c v. Then A B may be
considered as so nearly coin-
ciding with a parabola whose
focus is F, and whose vertex is
v, that it will possess all the
Fig- 343-
properties ascribed to the parabola; and consequently spherical
surfaces, provided their extent be small compared with their
374 ACOUSTICS.
diameters, will have all the properties here ascribed to parabolic
surfaces.
627. Experimental illustration. — All these effects have been
beautifully verified by experiment by means of expedients con-
trived by the Messrs. Weber, whose arrangements, nevertheless,
for this object admit of still further simplification.
1. Let a trough of convenient magnitude be partially filled with mercury,
so as to present a surface of that fluid of sufficient extent. Let a piece of
writing paper be formed into a funnel, with an extremely small opening at
the point, so as to allow a minute stream of mercury to flow from it. Let a
piece of sheet iron, having a perfectly plane surface, be now immersed
vertically in the mercury, and let a small stream descend from the funnel at
any point upon the surface of the mercury in the vessel. A series of pro-
gressive circular waves will be produced around the point where the mercury
falls, which will spread around it. This will strike the plane surface of the
sheet iron, at d will be reflected from it, forming another series of circular
waves, whose centre will be a point equally distant on the other side of the
sheet iron, as already described.
2. Let a piece of sheet iron be bent into the form of an ellipse, such as
that represented at fig, 340. ; and let the position of the foci be indicated by
a small wire index attached to it. Let this be immersed in the mercury in
the trough ; and let the funnel be brought directly over the point of the
index which marks the position of one of the foci. When the mercury is
allowed to fall, a series of circular waves will be produced round that focus,
and, striking on the surface of the iron, will be reflected from it, forming
another series of circular waves, of which the other focus is the centre, as
already expressed.
3. Let a piece of sheet iron be bent into the form of a parabola, as repre-
sented iny?<7. 341., the position of the focus being, as before, marked by an
index. If this be immersed in the mercury, and the stream be let fall from
the funnel placed at the point of the index, a series of circular waves will be
produced around the focus, which, after being reflected from the parabolic
surface, will be converted into a series of parallel straight waves at right
angles to its axis, as already explained.
4. Let two pieces of sheet iron formed into parabolic surfaces, with indices
showing the foci, be immersed in the mercury in such a position that their
axes shall be in the same direction, and their concavities facing each other.
From the funnel let fall a stream upon one focus F,fg. 342. Circular waves
will be formed which, after reflection from the adjacent parabola, will become
parallel waves, and after a second reflection from the opposite parabola will
again become circular waves with the other focus as a centre.
5. If pieces of sheet iron be bent into the form of small circular arcs whose
length is small compared with their radius, the same effects will be produced
as those which were produced by parabolic surfaces.
628. Interference. — When two waves which proceed from
different centres encounter each other, effects ensue which are of
considerable importance in those branches of physics whose theory
is founded upon the principles of undulation.
I. If the elevation of one wave coincides with the elevation of
another, and the depressions also coincide, a wave would be pro-
INTERFERENCE. 375
duced, the height of whose elevation, and the depth of whose de-
pression, will be equal to the sum of the heights and depths of the
elevation and depression of the two waves which are thus, as it
were, superposed.
II. If, however, the elevation of one wave coincide with the
depression of the other, and vice versa, then the effect will be a
wave whose elevation will be equal to the difference of the eleva-
tions, and whose depression will be the difference of the depressions
of the two waves which thus meet.
III. If, in the former case, the heights and depressions of the
waves superposed be equal, the resulting wave will have double
the height of the elevation, and double the depth of the depres-
sion.
IV. If the heights and depressions be equal in the second case,
the two waves will mutually destroy each other, and no undulation
will take place at the point' in question ; for the difference of
elevations and the difference of depressions being nothing, there
will be neither elevation nor depression.
In fact, in this latter case, the depression of each wave is filled
up by the elevation of the other.
This phenomenon, involving the effacement of an undulation by
the circumstance of two waves meeting in the manner described,
is called in the theory of undulation an interference, and is at-
tended with remarkable consequences in several branches of
physics.
629. Experimental illustration. — The two systems of waves
formed by an elliptical surface, and propagated, one directly
around one of the foci, and the other formed by reflection around
the other, exhibit, in a very beautiful manner, the phenomena not
only of reflection, as has been already explained, but also of inter-
ference, as has been shown with remarkable elegance by the
Messrs. Weber already referred to. These phenomena are re-
presented in fig. 344., where a and b are the two foci. The
strongly marked circles indicate the elevation of the waves formed
around each focus, and the more lightly traced circles indicate
their depression. The points where the strongly marked circles
intersect the more faintly marked circles, being points where an
elevation coincides with a depression, are consequently points of
interference, according to what has been just explained. The
series of these points form lines of interference, which are marked
in the diagram by dotted lines, and which, as will be seen, have
the forms of ellipses and parabolas round the same foci.
630. Inflection of waves. — If a series of waves encounter a
solid surface in which there is an opening through which the
waves may be admitted, the series will be continued inside the
376
ACOUSTICS.
opening, and without interruption ; but other series of progressive
waves having a circular form will be generated, having the edge
of the opening as their centres.
Fig. 344.
Let MN, jig. 345. j represent such a surface, having an opening
whose edges are A and B, and
let c be a centre from which a
series of progressive circular
waves is propagated. These
waves, entering at the opening
A B, will continue their course
uninterrupted, forming the cir-
cular arcs D E. But around A
and B as centres, systems of
progressive circular waves will
be formed which will unite with
the waves D E, completing them
by circular arcs D F and E r,
meeting the obstructing surface
on the outside ; but these cir-
cular waves will also be formed
throughout the remainder of
their extent, as indicated in the
figure, on both sides of the ob-
structing surface, and inter-
secting the original system of
waves propagated from the
AERIAL UNDULATIONS. 377
centre c. They will also form, with these, series of points of
interference according to the principles already explained.
The effects here described as produced by the edges of an
opening through which a series of waves is transmitted are called
inflection, and they form an important feature in several branches of
physics whose theory is based upon the principles of undulation.
63 1. The undulations produced upon a large scale in the oceans,
lakes, rivers, and other large collections of water upon the surface
of the globe, are attended with important effects on the economy
of nature. Without these the ocean would be soon rendered
putrid by the mass of organised matter which would be mingled
with it, and which would chiefly float at its surface.
The principal physical cause which produces these undulations,
where they take place on a moderate scale, is the motion of the
atmosphere, but on a large scale they are produced by the com-
bined effects of the attraction of the sun and moon exerted upon
the surface of the ocean. The immense undulations excited by
these attractions produce the phenomena of the tides which are
explained in our Handbook of Astronomy.
632. Undulation of air and gases. — If any portion of the
atmosphere, or any other elastic fluid diffused through space, be
suddenly compressed and immediately relieved from the com-
pressing force, it will expand in virtue of its elasticity, and, like
all other similar examples already given, will, after its expansion,
exceed its former volume to a certain limited extent, after which
it will again contract, and thus oscillate alternately on the one
side and on the other of its position of repose.
We may consider this effect to be produced upon a small
sphere of air having any proposed radius, as, for example, an
inch.
Let us suppose that it is suddenly compressed, so as to form a
sphere of half an inch in radius, and being relieved from the com-
pressing force it expands again, and surpassing its former dimen-
sions, swells into a sphere of an inch and a half. It will again
contract and return to the magnitude of a sphere, with a radius
somewhat greater than half an inch, and will again expand, and so
oscillate, forming alternately spheres with radii less and greater
than an inch, until at length the oscillation ceases, and it resumes
permanently its original dimensions. These oscillations will not
be confined to the single sphere of air in which they commenced ;
the circumambient air will necessarily follow the contracting
sphere when first compressed, so that a spherical shell of air which
lies outside the sphere will expand, and become less dense than in
its state of equilibrium.
When the central sphere again expands, this external spherical
3/8 ACOUSTICS.
shell will contract, and will become more dense than in its state of
equilibrium. This shell will act in a similar manner upon another
spherical shell outside it, and this upon another outside it, and so
forth.
If then we suppose a number of successive spheres surrounding
the point of original compression, we shall have a series of alter-
nate spherical shells of air, which will be condensed and expanded
in a greater degree than when in a state of repose. This con-
densation and expansion thus spreading spherically round the
original centre of disturbance, is in all respects analogous to a
series of circular waves forming round the central point upon the
surface of a liquid, the elevation of the wave in the case of the
liquid corresponding to the condensation in the case of the gas,
and the depression of the wave corresponding to the expansion of
the gas.
633. Propagation of waves through an elastic fluid.— We
will limit our observations in the first instance to a single series of
particles of air, expanding in a straight line from the centre of
disturbance A, fig. 346., towards T. Let s A represent the space
through which the disturbing force acts, and let us imagine this air
suddenly pressed from s to A by some solid surface moving against
it, and let us suppose that this motion from s to A is made in a
second Now, if air were a body devoid of elasticity, and like a
BBB
Fig. 346.
perfectly rigid rod, the effect of this motion of the solid surface
from s to A would be to push the remote extremity T through a
space to the right corresponding with and equal to s A.
But such an effect does not take place, first, because air is highly
elastic, and has a tendency to yield to the force exerted by the
solid surface upon it, which moves from s to A ; and secondly,
because to transmit any effect from A to a remote point, such as T,
would require a much greater interval of time than that which
elapses during the movement of the surface from s to A. The
effect, therefore, of the compression in the interval of time which
elapses during the motion from s to A, is to displace the particles
of air which lie at a certain definite distance to the right of A.
Let the distance, for example, be AB. All the particles, there-
fore, of air which lie in succession from A to B will be affected
more or less by the compression, and will consequently be brought
into closer contiguity with each other ; but they will not be
PROPAGATION OF WAVES. 379
equally compressed, because to enable the series of particles of air
lying between A and B to assume a uniform density requires a
longer time than elapses during the motion of the solid surface
from s to A. At the instant, therefore, of the arrival of the com-
pressing surface at A, the line of particles between A and B will be
at different distances from each other ; and it is proved, by mathe-
matical principles, that the point where they are most closely
compressed is the middle point m, between A and B, and there-
fore, departing from this middle point wz, in either direction, they
are less and less compressed.
The condition, therefore, of the air between A and B is as
follows. Its density gradually increases from A to /n, and gradually
decreases from m to B. Now, it is also proved that the effect of
the elastic force of the air is such that, at the next moment of time
after the arrival of the compressing surface at A, the state of
varying compression which has been just described as prevailing
between A and B will prevail between another point in advance of
A, such as A', and a point Br equally in advance of B, and the point
of the greatest compression will, in like manner, have advanced to
w', at the same distance to the right of m. In short, the condi-
tions of the air between A' and B' will be in all respects similar to
its condition the previous moment between A and B ; and in like
manner, in the next moment, the same condition will prevail be-
tween the particles A" and B" to the right of A' and B'. Now, it
must be observed that as this state of varying density prevails
from left to right, the air behind it, in which it formerly prevailed,
resumes its primitive condition. In a word, the state of varying
density which has been described as prevailing between A and B
at the moment the compressing surface arrived at A will, in the
succeeding moments, advance from left to right towards T, and
will so advance at a uniform rate ; the distance between the points
A B, A' B', and A" B", &c. always remaining the same.
634. Aerial undulations. — This interval between the points
A and B is called a wave or undulation, from its analogy, not only
in form, but in its progressive motion, to the waves formed on the
surface of liquids, already described ; the difference being, that in
the one case the centre of the wave is the point of greatest eleva-
tion of the surface of the liquid, and in the other case it is the
point of greatest condensation or compression of the particles of
the air. The distance between A and B, or between A' and B', or
between A" and B'', which always remains the same as the wave
progresses, is called the length of the wave.
In what precedes we have supposed the compressing surface to
advance from s to A, and to produce a compression of the air in
380 ACOUSTICS.
advance of it. Let us now suppose this surface to be at A, the air
contiguous to it having its natural density.
If the surface proceed contrariwise from A to s, the air which was
contiguous to it at A will rush after it in virtue of its elasticity, so
that the air to the right of A will be disturbed and rendered less
dense than previously. An effect will be produced, in fine, pre-
cisely contrary to that which was produced when the surface
advanced from s to A ; the consequence of which will be that a
change will be made upon the air between A and B exactly the
reverse of that which was previously made, that is to say, the
middle point m will be that at which the rarefaction will be
greatest, and the density will increase gradually, proceeding from
the point m in either direction towards the points A and B.
The same observations as to the progressive motion will be
applicable as before, only that the centre of the progression m,
instead of being the point of greatest, will be the point of least
density.
635. Waves condensed and rarefied. — The space A B is also
in this case denominated a wave or undulation. But these two
species of waves are distinguished one from the other by being
denominated, the former a condensed wave, and the latter a rarefied
wave. Now, let it be supposed that the compressing surface
moves alternately backwards and forwards between s and A,
making its excursions in equal times. The two series of waves, as
already defined, will be produced in succession. While the con-
densed wave moves from s towards T, the rarefied wave immedi-
ately follows it, and in the same manner this rarefied wave will be
followed by another condensed wave, produced by the next oscil-
lation, and so on.
The analogy of tnese phenomena to the progressive undulations
on the surface of a liquid, as already described, is obvious and
striking.
What has been here described with reference to a single line of
particles extending from the centre of disturbance A in a parti-
cular direction, is equally applicable to every line diverging in
every conceivable direction around such centre, and hence it
follows that the succession of condensed and rarefied waves will
be propagated round the centre, each wave forming a spherical
surface, which is continually progressive and uniformly enlarges,
the wave moving from the common centre with a uniform mo-
tion.
636. Velocity and force of aerial waves. — The velocity
with which such undulations are propagated through the atmo-
sphere depends on, and varies with, the elasticity of the fluid.
The degree of compression of the wave, which corresponds to the
SOUND. 381
height of a wave in the case of liquids, depends on the energy of
the disturbing force. All 'the effects which have been described
in the case of waves formed upon the surface of a liquid are
reproduced, under analogous conditions, in the case of undulations
propagated through the atmosphere.
637. Interference of aerial waves. — Thus, if two series of
waves coincide as to their points of greatest and least condensa-
tion, a series will be formed whose greatest condensation and
rarefaction is determined by the sum of points, as prevailing in the
separate undulations ; and if the two series are so arranged that the
points of greatest condensation of the one coincide with the
greatest rarefaction of the other, and vice versa, the series will
have condensations and rarefactions determined by the difference
of each of the separate series ; and, in fine, if in this latter case
the condensations and rarefactions be equal, the undulations will
mutually efface each other, and the phenomena of interference,
already described as to liquids, will be reproduced.
As the undulations produced in the air are spread over spherical
surfaces having the centre of disturbance as a common centre, the
magnitude of these surfaces will be in the ratio of the squares of
their radii, or, what is the same, of the squares of their distances
from the point of central disturbance ; and, as the intensity of
the wave is diminished in proportion to the space over which it is
diffused, it follows that the effects or energy of these waves will
diminish as the squares of their distances from the centre of
propagation increases.
CHAP. II.
PRODUCTION AND PROPAGATION OF SOUND.
638. Sound is the sensation produced in the organs of hearing
when they are affected by undulations transmitted to them through
the atmosphere. These undulations are subject to an infinite
variety of physical conditions, and each variety is followed by a
different sensation.
The atmospheric undulations which thus produce the sensation
of sound, are themselves excited usually by the vibration of some
elastic bodies, whose condition of equilibrium is momentarily dis-
turbed, and which impart to the air in contact with them undula-
tions which correspond with and are determined by such vibration.
The vibrating bodies which thus impart undulation to the air
ACOUSTICS.
are called sounding or sonorous bodies ; and the air is said to be a
propagator or conductor of sound, and' is sometimes called a soni-
ferous medium.
The sounding body does
not, however, invariably act
in a direct manner upon the
air which conveys the undu-
lation to the organ of hearing.
It often happens that the vi-
brations of the sounding body
are first imparted to other
bodies susceptible of vibra-
tion, and after passing through
a succession of these, the un-
dulation is finally imparted to
the air, which is invariably the
last medium in the series, and
that from which the organ of
hearing receives it.
639. That the presence of
air or other conducting me-
dium is indispensable for the
production of sound, is proved
by the following experiment.
Let a small apparatus {Jig.
347.) called an alarum, con-
sisting of a bell a, which is
struck by a hammer &, moved
by clockwork, be placed under
the receiver of an air pump, through the top of which a rod slides,
air-tight, the end of the rod being connected with a detent which
governs the motion of the clockwork connected with the hammer.
This rod can, by a handle placed outside the receiver, be made to
disengage the detent, so as to make the bell ring whenever it is
desired.
This arrangement being made, and the alarum being placed
within the receiver, upon a soft cushion of wool e, so as to prevent
the vibration from being communicated to the pump plate, let
the receiver be exhausted in the usual way. When the air has
been withdrawn, let the bell be made to ring by means of the
sliding rod. No sound will be heard, although the percussion of
the tongue upon the bell, and the vibration of the bell itself are
visible. Now if a little air be admitted into the receiver, a faint
sound will begin to be heard, and this sound will become gradually
louder in proportion as the air is gradually readmitted.
Fig. 347-
SOUND PROGRESSIVE. 383
In this case the vibrations which directly act upon the ear are
not those of the air contained in the receiver. These latter act
upon the receiver itself and the pump plate, producing in them
sympathetic vibration ; and those vibrations impart vibrations to
the external air which are transmitted to the ear.
If in the preceding experiment a cushion had not been inter-
posed between the alarum and the pump plate, the sound of the
bell would have been audible, notwithstanding the absence of air
from the receiver. The vibration in this case would have been
propagated, first from the bell to the pump plate and to the bodies
in contact with it, and thence to the external air.
Another more simple method of performing this experiment is
shown in jig. 348 A bell is suspended within a glass globe, in
the neck of which there is a stopcock. The air
being exhausted from this globe by a syringe or
by the air pump, the sound of the bell will be
inaudible, and will become audible and gradually
louder by admitting the air by slow degrees.
Persons shut up in a close room are sensible
of sounds produced at a distance outside such
room ; and they may be equally sensible of
these, even though the windows and doors should
be absolutely air-tight. In such case the undu-
lations of the external air produce sympathetic
vibration on the windows, doors, or walls by
Fig. J48. which the hearers are enclosed, and then produce
corresponding vibrations in the air within the
room by which the organs of hearing are immediately affected.
640. Sound progressive. — It has been shown that the pro-
pagation of undulations through the atmosphere is progressive \
and if it be admitted that such undulations are the agencies by
which the sense of hearing is affected, it will follow that an interval
of time, more or less, must elapse between the vibration of the
sounding body and the perception of the sound by a hearer, and
that such interval will be proportionate to the distance of the
hearer from the sounding body, and to the velocity with which
sound is propagated through the intervening medium. But this
progressive propagation of sound can also be directly proved by
experiment
Let a series of observers, A, B, c, D, &c., be placed in a line, at
distances of about 1000 feet asunder, and let a pistol be discharged
at P, about 1000 feet from the first observer.
This observer will see the flash of the pisto) p.bout one second
3H ACOUSTICS.
before he hears the report. The observer B will hear the report
one second after it has been heard by A, and about two seconds
after he sees the flash. In the same manner, the third observer at
c will hear the report one second after it has been heard by the
observer at B, and two seconds after it has been heard by the ob-
server at A, and three seconds after he perceives the flash. In the
same way, the fourth observer at D will hear the report one second
later than it was heard by the third observer at c, and three
seconds later than it was heard by the observer at A, and four
seconds after he perceives the flash.
Now it must be observed, that at the moment the report is
heard by the second observer at B, it has ceased to be audible to
the first observer at A ; and when it is heard by the third observer
at c, it has ceased to be heard by the second observer at B, and so
forth. It follows, therefore, from this, that sound passes through
the air, not instantaneously, but progressively, and at a uniform
rate.
641. Breadth of sonorous waves. — As the sensation of sound
is produced by the wave of air impinging on the tympanum of the
ear, exactly as the momentum of a wave of the sea would strike
the shore, it follows that the interval between the production of
sound and its sensation, is the time which such a wave would take
to pass through the air from the sounding body to the ear ; and
since these waves are propagated through the air in regular suc-
cession, one following another without overlaying each other, as in
the case of waves upon a liquid, the breadth of a wave may always
be determined if we take the number of vibrations which the
sounding body makes in a second, and the velocity with which the
sound passes through the air. If, for example, it be known that
in a second a musical string makes 500 vibrations, and that the
sound of this string takes a second to reach the ear of a person at
a distance of 1000 feet, there are 500 waves in the distance of
I ooo feet, and consequently each wave measures two feet.
The velocity of the sound, therefore, and the rate of vibration,
are always sufficient data by which the length of a sonorous wave
can be computed.
642. [Distinction between musical sounds and ordinary
sounds. — In physics, every sound which is produced by a succes-'
sion of similar vibrations, following each other at equal inter-
vals of time so short that th? vibrations are not perceived as
separate, is called a musical sound. Such sounds, however, are not
necessarily agreeable, or musical in the popular sense. Noises, on
the other hand, are produced by vibrations following each other at
irregular intervals.]
Sounds are distinguished from each other by their pitch or tone,
in virtue of which they are high or low ; by their intensity, in
VELOCITY OF SOUND. 385
virtue of which they are loud or soft ; and by their quality, or the
property which enables us to distinguish between different instru-
ments or voices, when all sound the same note.
64.3. Pitch. — The pitch or tone of a sound is grave or acute.
In the former case it is low, and in the latter high, in the musical
scale. It will be shown hereafter that the physical condition
which determines this property of sound is the rate of vibration of
the sounding body.
The more rapid the vibrations are, the more acute will be the
sound. A bass note is produced by vibrations much less rapid
than a note in the treble. But it will also be shown that the
length of the sonorous waves depends on the rate of vibration of
the body which produces it : the slower the rate of vibration, the
longer will be the wave, and the more grave the tone.
All vibrations which are performed at the same rate produce
waves of equal length and sounds of the same pitch.
644. Zioudness. — The intensity of a sound, or its degree of
loudness, depends on the force with which the vibrations of the
sounding body are made, and consequently upon the degree of
condensation produced at the middle of the sonorous wave. Waves
of equal length, but having different degrees of condensation at
their centres, will produce notes of the same pitch, but of diffe-
rent degrees of loudness, in proportion to such degrees of conden-
sation.
645. Quality. — If we hear the same musical note produced in an
adjacent room successively upon a flute, a clarionet, and a hautboy,
we shall, without the least hesitation, distinguish the one instrument
from the other. [The property of sound, which enables us thus to
recognise individual instruments, is called its quality, or, in French,
its timbre. It depends upon the kind of vibration produced by the
instrument. Thus the soft mellow tone of a tuning-fork or a
stopped diapason organ pipe is produced by simple vibrations, like
those of a pendulum ; the more piercing character of the notes of a
horn or violin results from the vibrations produced by these in-
struments being of a more complex form.]
646. In the same medium, all sounds have the same ve-
locity. — That this is the case, is manifest from the absence of all
confusion in the effects of music, at whatever distance it may be
heard. If the different notes simultaneously produced by the
various instruments of an orchestra moved with different velocities
through the air, they would be heard by a distant auditor at dif-
ferent moments, the consequence of which would be, that a musical
performance would, to the auditors, save those in immediate prox-
imity with the performers, produce the most intolerable confusion
and cacophony ; for different notes produced simultaneously, and
cc
386 ACOUSTICS.
which, when heard together, form harmony, would at a distance
be heard in succession ; and sounds produced in succession would
be heard as if produced together, according to the different velo-
cities with which each note would pass through the air.
647. [Velocity. — The velocity of sound depends upon the ratio
which the elasticity of the medium by which it is propagated bears
to its density. Its velocity, therefore, through the air varies
with changes of temperature.
The experimental methods which have been adopted to ascer-
tain the velocity of sound are similar in principle to those which
have been briefly noticed by Avay of illustration. The most ac-
curate experiments which have been made with this object are
unquestionably those executed in. Holland by Moll and Van Beek
in June, 1823. The observations were made by discharging
cannon simultaneously on two hills, at a distance of 57,840 feet,
and noting the time that elapsed after the explosion at each station
before the report was heard at the other. The result of these
experiments, as calculated with great care by Dr. H. "W. Schroder
van der Kolk, gives 332-77 metres or 1 09 r8 feet as the distance
through which sound travels in one second, when the temperature
of the air is 32° Fahr.
It results from the mathematical theory of the propagation of
sound in air that, in order to get the velocity for any other tempe-
rature £°, expressed in degrees of Fahrenheit's thermometer, the
above value must be multiplied by \/i-f- -002036 (t — 32). Hence
at 62° F., which is about the mean temperature of the air in
London, the velocity of sound is 1 124! feet, or nearly 375 yards
per second. Changes of barometric pressure have no influence on
the velocity of sound.]
648. Distance measured by sound. — The production of
sound is in many cases attended with the evolution of light, as, for
example, in firearms and explosions generally, and in the case of
atmospheric electricity. In these cases, by noting the interval
between the flash and the report, and multiplying the number of
seconds in each interval by the number of feet per second in the
velocity of Kound, the distance can be ascertained with great pre-
cision. Thus, if a flash of lightning be seen ten seconds before
the thunder which attends it is heard, and the atmosphere be in
such condition that the velocity of sound is 1125 feet per second,
it is evident that the distance of the cloud in which the electricity
is evolved must be 1 1,250 feet.
Among the numerous discoveries bequeathed to the world by
Xewton, was a calculation, by theory, of the velocity with which
sound was propagated through the air. This calculation, based
INTENSITY OF SOUND. 387
upon the elasticity and density of the air, gave as a result about
one sixth less than that .which resulted from experiments.
This discrepancy remained without satisfactory explanation
until it was solved by Laplace, who showed that it arose from the
fact that Newton had neglected to take into account, in his com-
putation, the effects of the heat developed and absorbed by the
alternate compression and rarefaction of the air produced in the
amorous undulations. Laplace, taking account of these, gave a
formula for the velocity of sound which corresponds in its results
exactly with experiment.
649. All gases and vapours conduct sound. — As all elastic
fluids are, in common with air, susceptible of undulation, they are
equally capable of transmitting sound.
This may be rendered experimentally evident by the following
means. Let the alarum be placed under the receiver of an air
pump, as already described, and let the receiver be exhausted.
If, instead of introducing atmospheric air into the receiver, we
introduce any other elastic fluid, the sound of the alarum will be-
come gradually audible, according to the quantity of such fluid
which is introduced under the receiver. If a drop of any liquid
which is easily evaporated be introduced, the atmosphere of vapour
which is thus produced will also render the alarum audible.
650. The same sounding body Avill produce a louder or lower
sound, according as the density of the air which surrounds it is in-
creased or diminished. In the experiment already explained, in
which the alarum was placed under an exhausted receiver, the
sound increased in loudness as more and more air was admitted
within the receiver. If the alarum had been placed under a
condenser, and highly compressed air collected round it, the sound
would be still further increased.
When persons descend to any considerable depth in a diving
bell, the atmosphere around them is compressed by the weight of
the column of water above them. In such circumstances, a whisper
is almost as loud as the common voice in the open air, and when
one speaks with the ordinary force it produces an effect so loud as
to be painful.
On the summit of lofty mountains, where the barometric column
falls to one half its usual elevation, and where therefore the air is
highly rarefied, sounds are greatly diminished in intensity. Per-
sons who ascend in balloons find it necessary to speak with much
greater exertion, and, as would be said, louder, in order to render
themselves audible. When Saussure ascended Mont Blanc, he
found that the report of a pistol was not louder than a common
cracker.
651. Effect of atmospheric agitation on sound. — Violent
cc*
388 ACOUSTICS.
winds and other atmospheric agitations affect the transmission of
sound. When a strong wind blows from the hearer towards the
sounding body, a sound often ceases to be heard which would be
distinctly audible in a calm. A tranquil and frosty atmosphere
placed over a smooth and level surface is favourable to the trans-
mission of sound. Lieutenant Foster held a conversation with
a person on the opposite side of the harbour of Port Bowen, in the
third polar expedition of Sir Edward Parry, the distance between
the speakers being more than a mile.
It is said that the sound of the cannon at the battle of Waterloo
was heard at Dover, and that the cannon in naval engagements in
the Channel have been heard in the centre of England.
652. Liquids are also capable of propagating sound. Divers
can render themselves audible at the surface of the water ; and
stones or other objects struck together at the bottom produce a
sound audible at the surface.
It appears from the experiments of M. Colladon, made at Ge-
neva, that sounds are transmitted through water to great distances
with greater force than through air. A blow struck under the
water of the Lake of Geneva was distinctly heard across the whole
breadth of the lake, a distance of nine miles.
Solid bodies, such as walls or buildings interposed between the
sounding body and the hearer, diminish the loudness of the sound,
but do not obstruct it when the sound is made in air ; but it ap-
pears from the experiments of M. Colladon, that the interposition
of such obstacles almost destroys the transmission of sound in
water.
653. Sounds which destroy each other. — When two series
of sonorous undulations propagated from different sounding bodies
intersect each other, the phenomena of interference explained in
the theory of undulation are produced, and an ear placed at such
a point of interference will not be affected by any sense of sound,
so long as the two sounding bodies continue to vibrate ; but the
moment the vibration of either of the two is discontinued, the
other will become audible. Thus, it appears that two sounds
reaching the ear together, instead of producing, as might be ex-
pected, a louder sound than either would prod uce alone, may alto-
gether destroy each other and produce silence.
This phenomenon is precisely analogous to the case of two series
of waves formed upon the surface of the same liquid, at a point
where the elevation of a wave of one series coincides with the
depression of a wave of the other.
If two sounding bodies were placed in the foci of an ellipse, as
represented in jig. 340., an ear placed on any of the lines of in-
terference there indicated would be conscious of no sound ; but
PROPAGATION OF SOUND. 389
the moment that either of the'two sounding bodies became silent,
the other would be heard ; or if the ear of the listener were re-
moved to a position midway between two lines of interference,
then both sounds would be heard simultaneously, and combined
would be louder than either alone.
654. Experimental illustration. — This phenomenon of inter-
ference may be produced in a striking manner by means of the
common tuning fork, used to regulate the pitch of musical instru-
ments.
Let A and B, fig. 349., be two cylindrical glass vessels, held at
right angles to each other, and let the tuning fork, after it has
been put in vibration, be held in the middle
of the angle formed by their mouths. Al-
though, under such circumstances, the vi-
bration of the tuning fork will be imparted
to the columns of air included within the
two cylinders, no sound will be heard ; but
if either cylinder be removed, the sound
Fig. 349! wiM be distinctly audible in the other. In
this case, the silence produced by the com-
bined sounds is the consequence of interference.
Another example of this phenomenon may be produced by the
tuning fork itself. If this instrument, after being put into vibra-
tion, be held at a great distance from the ear, and slowly turned
round its axis, a position of the prongs will be found at which the
sound will become inaudible. This position will correspond to the
points of interference of the two systems of undulation propagated
from the two prongs.
655. Examples. — Solids which possess elasticity have likewise
the power of propagating sound. If the end of a beam composed
of any solid possessing elasticity be lightly scratched or rubbed,
the sound will be distinct to an ear placed at the other end, al-
though the same sound would not be audible to the ear of the
person who produces it, and who is contiguous to the place of its
origin.
The earth itself conducts sound, so as to render it sensible to
the ear when the air fails to do so. It is well known, that the
approach of a troop of horse can be heard at a distance by putting
the ear to the ground. In volcanic countries, it is said that the
rumbling noise which is usually the prognostic of an eruption is
first heard by the beasts of the field, because their ears are gene-
rally near the ground, and they then by their agitation and alarm
give warning to the inhabitants of the approaching catastrophe.
Savage tribes practise this method of ascertaining the approach
of persons from a great distance.
39o
ACOUSTICS.
656. Velocity of sound in different media. — The velocity
with which sound is propagated through different media varies
with their different physical conditions.
In the following table are given the velocities with which sound
is propagated through the several liquid and solid bodies therein
named.
TABLE.
Velocities of Sound in Liquids.
Liquid.
Temperature
(F.)
Velocity per
second (.Feet.)
JT
*
7^
%
n\
11,
47IS
4770
5123
5004
5230
5479
6495
3805
3803
3978
Sea- water (artificial) -------
Solution of chloride of sodium (36-9 per cent.) -
„ „ Sulphateofsocla (13-35 per cenO -
,, „ Carbonate of soda (20 7 per cent.) -
„ Nitrate of soda (37-5 per cent.)
,, „ Chloride of calcium (765 per cent.)
Velocities of Sound in Solid Bodies.
Substance
Velocity
(the Telocity in air
being =1.)
8-057
9-683
11-167
8 in
15-108
15-108
Zinc (distilled) - - - 1
Copper (annea e )
Iron (annealed) - _-_--.--
Steel (anneaed)
657. Effects of elasticity of air. — The velocity with which
sound is transmitted through the air varies with its elasticity ;
and where different strata are rendered differently elastic by the
unequal radiation of heat, the agency of electricity, or other
causes, the transmission of sound will be irregular. In passing
from stratum to stratum differing in elasticity, the speed with
which sound is propagated is not only varied, but the force of the
intensity of the undulations is diminished by the combined effects
of reflection and interference, so that the sound, on reaching the
ear, after passing through such varying media, is often very much
diminished.
The fact, that distant sounds are more distinctly heard by night
than by day, may be in part accounted for by this circumstance,
CHLADNTS EXPERIMENTS. 391
the strata of the atmosphere being during the day exposed to
vicissitudes of temperature more varying than during the night.
658. Blot's experiment. — The relative velocities of sound, as
transmitted by air and by metal, .are illustrated by the following
remarkable experiment of Biot : — A bell was suspended at the
centre of the mouth of a metal tube 3000 feet long, and a ring of
metal was at the same time placed close to the metal forming the
mouth of the tube, so that when the ring was sounded its vibra-
tions might affect the metal of the tube ; and when the bell was
sounded, its vibrations might affect only the air included within
the tube. A hammer was so adapted as to strike the ring and the
bell simultaneously. When this was done, an ear placed at the
remote end of the tube heard the sound of the ring, and after a
considerable interval heard the sound of the bell.
659. Chladni's experiments. — The solids composing the
body of an animal are capable of transmitting the sonorous undu-
lations to the organ of hearing, even though the air surrounding
that organ be excluded from communicating with the origin of the
sound.
Chladni showed that two persons stopping their ears could con-
verse with each other by holding the same stick between their teeth,
or by resting their teeth upon the same solid. The same effect was
produced when the stick was pressed against the breast or the
throat, and other parts of the body.
If a person speak, directing his mouth into a vessel composed of
any vibratory substance, such as glass or porcelain, the other
stopping his ears, and touching such vessel with a stick held
between his teeth, he will hear the words spoken.
The same effect will take place with vessels composed of metal
or wood.
If two persons hold between their teeth the same thread, stop-
ping their ears, they would hear each other speak, provided the
thread be stretched tight.
660. Xioudness dependent on distance. — In has been shown
that while the pitch of a sound depends upon the length of the
sonorous wave, or, what is the same, the number of waves which
strike the ear per second, the loudness depends on the degree of
condensation or rarefaction produced in each such wave ; but the
loudness is also dependent on the distance of the hearer from the
sounding body ; and therefore, when it is stated that it is propor-
tional to the condensation and rarefaction of the sonorous waves,
the estimate must be understood to be applied to sounds heard at
the same distance from their origin
In explaining the general theory of undulations, it has been
shown that as the undulation spreads round the centre from which
392
ACOUSTICS.
it emanates, its intensity diminishes as the square of the distance
is augmented ; and this general principle consequently becomes
applicable to sonorous undulations; and, therefore, when other
things are the same, the intensity or loudness of the sound di-
minishes in the same proportion as the square of the distance
of the hearer from the sounding body is augmented. Thus in a
theatre, if the linear dimensions be doubled, other arrangements
being the same, the loudness of the performers' voices, as heard
at any part of its circumference, will be diminished in a fourfold
proportion.
CHAP. III.
PHYSICAL THEORY OF MUSIC.
66 1. Tbe monocbord. — Of the various forms of apparatus
which have been contrived for the production of musical sounds
with a view to the experimental illustration of their theory, those
which are best adapted for this purpose are those which, under
various denominations, consist of strings submitted to tension
over a sounding board. An instrument of this form, consisting
of a single string, and called a monochord or sonometer, is repre-
sented in Jig. 350. It consists of a string of catgut or wire
attached to a fixed point, carried over a pulley, and stretched by
MUSICAL SCALE 393
a known weight. Under the string is a hollow box or sounding
board, to the frame of which the pulley is attached. The string
rests upon two bridges, one of which is fixed, and the other can
be moved with a sliding motion to or from, so as to vary at
pleasure the length of the part of the string included between the
two bridges.
A divided scale is placed under them, so that the length of the
vibrating part of the string may be regulated at pleasure. By
varying the weight, the tension of the string may be increased or
diminished in any desired proportion. This may be accomplished
with facility by circular weights which are provided for the pur-
pose, and which may be slipped upon the stem of the weight.
By means of this apparatus, the relation between the various
notes of the musical scale and the rate of vibration by which they
are respectively produced, have been ascertained.
662. Its application to determine the rates of vibrations
of musical notes. — It has been shown that the rate of vibration
of a string such as that of the monochord is inversely as its length,
other things being the same. Thus, if its length be halved, its
rate of vibration is doubled ; if its length be diminished or in-
creased in a threefold proportion, its rate of vibration will be
increased or diminished in the same proportion ; and so forth.
Let the bridges be placed at a distance from each other as.
great as the apparatus admits, and let the weight which stretches
the string be so adjusted, that the note produced by vibrating the
string shall correspond with any proposed note of the musical
scale ; such, for example, as fa==. =: , the low c of the treble
J -o-
clef. This being done, let the movable bridge be moved towards
the fixed bridge, continually sounding the string until it produces
the octave above the note first sounded, that is, until it produces
the middle c fe^EE of the treble.
If the length of the string be now ascertained by reference to
the scale of the monochord, it will be found to be precisely one
half its original length.
663. A double rate of vibration produces an octave. —
Hence it follows, that the same string will sound an octave higher
if the length is halved. But it has already been shown that the
rate of vibration will be doubled when the length of the string is
halved. Hence it follows, that two sounds, one of which is an
octave higher than the other, will be produced by vibrations,
the rate of which will be in the proportion of 2 to 1 ; and,
consequently, the length of the undulation producing the lower
394 ACOUSTICS.
note will be double that of the undulation producing the higher
note.
664. Rates of vibration for other intervals. — If, instead of
moving the bridge to the point necessary to produce the octave
to the fundamental note c, it be moved to such positions that the
string shall produce the successive notes of the scale between
it and its octave, the lengths of the string being noted by re-
ference to the scale, it will be found that they will be respec-
tively those which are inscribed below the annexed scale under
the notes severally. The length of the string producing the
fundamental note c is assumed to be I, the fractions expressing,
with reference to this length, the lengths which are found to
produce the successive notes of the scale severally.
Let the seven successive notes of the gamut be expressed as
follows : —
xit re mi fa sol la si ut
CDEFGAB C
if f I I I A i
The names given by continental writers to these seven notes are those
written beneath them in the upper line— ut, re, mi, fa, sol, la, si, ut; but
those by which they are most generally known in England are the letters of
the alphabet inscribed in the lower line, the fundamental note being c, and
the succeeding ones designated by the letters inscribed beneath them.
Let us suppose, then, that the monochord produces this fundamental
note c, and that the movable bridge be then advanced towards the fixed
bridge so as to shorten the string until it produces the note D. It will be
found that its length will be reduced £th, and that, consequently, the length
necessary to produce the note D will be |ths of that which produces the note
C. Let the bridge be now advanced until the string sound the note E ; its
length will then be |ths of that which produces the fundamental note. In
the same manner, being further shortened, let it produce the note F; its
length will be |ths of its original length. In the same manner, the lengths
of the string corresponding to each of the successive notes of the gamut, will
be found to be expressed by the fractions which are written in the above
diagram under the notes severally.
But since the number of vibrations per second is, by the principles already
established, in the inverse ratio of the length of the string, it follows, that
if the number of vibrations per second corresponding to the fundamental
note c be expressed by i, the number of vibrations per second corresponding
to the other notes successively will be as follows : —
ut re mi fa sol la si ut
CDEFGABC
1 f i i i \?
The meaning of which is, that in producing the note D, nine vibrations will
be made in the same time that eight are made by the note c. In like
HARMONY. 395
manner, when the note E is sounded, five of its vibrations correspond to four
of c, four vibrations of F correspond to three of c, three vibrations of o
correspond to two of c, five vibrations of A correspond to three of c, fifteen
vibrations of B correspond to eight of c, and, in fine, two vibrations of the
octave c correspond to one of the fundamental c.
The relative numbers corresponding to the notes of one octave being
known, those of the octaves higher or lower in the musical scale can be easily
calculated.
It appears from what has been already proved that the note which is an
octave higher than the fundamental note is produced by a rate of vibration
twice as rapid : and this principle would equally apply to any other note.
"We shall, therefore, always find the rate of vibration of a note which is an
octave above a given note by multiplying the rate of vibration of the given
note by 2 ; and, consequently, to find the rate of vibration of a note an
octave lower, it will only be necessary to divide the rate of vibration of the
given note by 2. If, therefore, it be desired to find the rate of vibration of
the series of notes continued upwards beyond the series given in the preceding
diagram, it will only be necessary to multiply the numbers in the preceding
series by 2.
665. Physical cause of harmony. — If these results be com-
pared with the effect produced upon the ear by the combination
of these musical notes sounded in pairs, we shall discover the
physical cause of those agreeable sensations denominated harmony,
and the opposite sensations denominated discord.
The most perfect harmony is that of the octave, which is so-
complete as to be nearly equivalent to unison. Now the fun-
damental note c produced simultaneously with its octave is
attended by two series of vibrations, of which two of the octave
correspond to one of the fundamental note. It follows, therefore,
that the commencement of every alternate vibration of the upper
note coincides with the commencement of a vibration of the
lower.
Next to the octave, the most agreeable harmony is that of the
fifth, which is produced when the fundamental note c is sounded
simultaneously with o. Now it appears by the preceding results
that three vibrations of G are simultaneous with two of c. It
follows, therefore, that every third vibration of G commences
simultaneously with every second vibration of c. The coincident
vibrations, therefore, are marked by the commencement of every
second vibration of the fundamental c, whereas, in the octave, a
coincidence takes place at the commencement of every vibration.
The coincidences, therefore, are more frequent in the octave
than in the fifth, in the proportion of I to 2.
The next harmony to that of the fifth is the fourth, which is
produced when the fundamental note c is sounded simultaneously
with F. Now it appears from the preceding results that four
vibrations of F are simultaneous with three of the fundamental
396
ACOUSTICS.
note, and, consequently, that there is a coincident vibration at the
commencement of every third vibration of the fundamental note.
The coincident vibrations are, therefore, less frequent than in the
fifth in the proportion of 3 to 2 ; and less frequent than in the
octave in the proportion of 3 to I .
The harmony which comes next in order to the fourth is that
of the third, produced when the fundamental note c is sounded
simultaneously with E. Now it appears from the preceding re-
sults that five vibrations of E .are made simultaneously with four
of c ; and that, consequently, there is a coincidence at every
fourth vibration of the fundamental note. The coincidences,
therefore, in this case are less frequent than in the fourth, in the
ratio of 3 to 4, less frequent than in the fifth in the proportion of
2 to 4, and less frequent than in the octave in the proportion of
I to 4.
Scale exhibiting the Effect of Binary Combinations of the Fundamental
Note with a Series of Three Octaves continued severally upwards and
downwards.
. ( . o -«->,•*=»• _~
^EEEHg"
33'H Id 33 ICTIQ ICZJia
10 11 12 13 14 15 16 17 18 19 20 21 22
H H H H H
The figures which are placed over each combination express the
number of vibrations which in each case take place simultaneously,
and the name of the interval, as it is technically called in music,
is written under the lower line. Thus, the interval between the
fundamental note c and the note B is a seventh ; and the figures
above indicate that fifteen vibrations of B are made in the same time
as eight vibrations of c. In the same way, the interval between c
and r in the treble is called an eleventh ; and the figures indicate
that eight vibrations of r are made while three of c take place.
666. Physical cause of the harmonics of the harp or
violin. — On inspecting the numbers which in the preceding scale
indicate the relative rates of vibration of these pairs of musical
sounds, it will be observed that there are certain combinations in
which a complete number of vibrations of the upper note are
made in the time of a single vibration of the lower note. These
are distinguished by the letter H written under the interval. The
first is the octave, in which two vibrations of the upper note cor-
respond to one of the lower; the second is the twelfth, in which
SENSIBILITY OF THE EAR. 397
three vibrations of the upper note correspond to one of the lower ;
the third is the fifteenth, in which four vibrations of the upper
note correspond to one of the lower ; the fifth is the nineteenth,
in which six vibrations of the upper correspond to one vibration
of the lower ; and, in fine, the seventh is the twenty-second, in
which eight vibrations of the upper correspond to one vibration of
the lower.
These combinations (which possess other and important pro-
perties) are called harmonics.
One of the most remarkable properties of the harmonics is, that
if the fundamental note be produced by sounding the open string,
a practised ear will detect in the sound mingled with the funda-
mental, the several harmonics to it, and more especially those
which are in nearest accord with the fundamental note. Thus
the octaves will be produced ; but these are so nearly in unison
witfy the fundamental note that the ear cannot distinguish them.
The twelfth, or that which has three vibrations for one of the
fundamental note, is distinctly perceptible to common ears. The
more practised can distinguish the seventeenth, or that which
vibrates five times more rapidly than the octave ; and some pre-
tend to be able to distinguish the vibrations of the nineteenth,
which vibrates six times for one of the fundamental note.
667. Experimental verification by Sauveur. — These phe-
nomena have been explained and verified in a satisfactory manner
by Sauveur, who showed that when a string is put into vibration
it undergoes subordinate vibrations, which take place in its aliquot
parts. Thus, if an edge touch the string gently, when in vibration,
at its middle point, as represented i&jig* 351., each half will con-
tinue to vibrate independently.
Fig. 351.
If the edge be in like manner applied at one third of the length,
the vibration will still continue, each third part vibrating inde-
pendently of the other ; and in fine, the condition of the entire
string when left to vibrate freely, is represented in fig. 352.,
where the subordinate vibrations produced in the aliquot parts of
the string are represented.
668. limit of the musical sensibility of tbe ear. — Since
39« ACOUSTICS.
the pitch of a musical note depends on the number of vibrations
produced per second, it follows that whenever two notes are pro-
duced by a different number of vibrations per second, they will
have a corresponding musical difference. Now a question arises
as to the limits of the power of the ear to distinguish minute dif-
ferences of this kind. For example, it may be asked whether two
musical notes produced by vibrations differing from each other
by only one in a million, that is to say, if, while one string make a
million of vibrations, another string shall make a million and one,
is the ear capable of perceiving that one note is more acute than
the other ? It is certain that no ear could discover such a
difference, although it is equally certain that such a difference
would exist. The question then is, what is the limit of sensibility
of the ear.
If two strings of the same wire were extended by equal weights
on the monochord, and the movable bridges brought to coincide,
so that the strings would be of precisely equal length, then it is
certain that when struck they would produce the same note,
since all the conditions affecting the vibration of the string would
be identical. Now, if one of the bridges be moved slowly, so as
gradually to lengthen the vibrating part of the string, the limit
may be found at which the ear will begin to be sensible of the
dissonance of the notes. The point thus determined may fix the
limit of the sensibility of the ear.
The comparative lengths of the two strings in such a case
would indicate the different rates of vibration of which the ear is
sensible.
Sensibility of practised organists. — The result of such an
experiment would of course be different for different ears, ac-
cording to their natural sensibility, and to the effects of cultivation
in improving their musical perception. Practised organists are
able to distinguish between notes which differ in their vibrations
to the extent of one in eighty.
Thus, if a string of the monochord have 20 inches between the
bridges, and the other 2o£ inches, their rates of vibration being
then in the proportion of 80 to 8 1, the difference would be dis-
tinguishable. Such an interval between two musical sounds is
called a comma.
But when the difference of the rates of vibration are much less
than this, they cannot be distinguished by the ear. The notes on
common square pianos are each produced by two strings, and on
grand pianos by three strings struck simultaneously by the same
hammer. In tuning the instrument, these strings are tuned
separately, until they are brought as nearly to the same pitch as
the ear can determine. When struck together however, a slight
SIRENE. 399
dissonance will in general be perceptible, which is adjusted by
tuning one or the other until the sounds are brought into unison.
Since, however, such unison is only determined by the ear,
and since the sensibility of that organ is limited, it follows that
the unison thus obtained can never be perfect otherwise than by
chance.
669. Methods of determining the absolute number of
vibrations producing- musical notes. — We have hitherto
noticed only the relative rates of vibration of different musical
notes. If the absolute number of vibrations per second, cor-
responding to any one note of the scale, were known, the absolute
number of vibrations of all others could be computed. Thus, the
note which is an octave higher than the note proposed, would be
produced by double the number of vibrations per second ; a note
one fifth above it would be produced by a number of vibrations
per second found by multiplying the given number by 3 divided
by 2, and so on. In a word, the number of vibrations per second
necessary to produce any given note would be found by multi-
plying the number of vibrations per second necessary to produce
the fundamental note by the fractions given in (664.) corre-
sponding to the proposed note.
670. The Sirene. — An instrument of great ingenuity and
beauty, called the Sirene, has been supplied by the invention of
M. Cagniard de la Tour, for the purpose cf ascertaining the whole
number of vibrations which correspond to any proposed musical
sound.
A tube of about four inches in diameter, represented at ff',fig. 353., to
which wind can be supplied by means of a bellows or otherwise through a
pipe y y', is terminated in a smooth circular plate v »', stopping its end. In
this plate, and near its edge, a number of small holes are pierced very close
together, and disposed in a circular form, as represented in fig. 354., the per-
forations being made, not perpendicular to the plate, but in an oblique
direction through it. Another plate of equal magnitude u u, and having a
circle of holes precisely similar, is fixed upon this so as to be capable of
revolving with any required velocity round its centre. As it revolves, the
holes in the upper plate u u' correspond in certain positions with the holes
in the lower plate v v' ; but in intermediate positions, the holes in the lower
plate not corresponding with those in the other plate, the exit of the air
from the tube//' is stopped. If, then, we suppose the upper of these two
plates to revolve upon the lower, a current of air being supplied to the tube
//' through yy', the air will escape where the holes in the superior plate
correspond in position with those in the lower plate, but in intermediate
positions it will be intercepted. The effect will be, that when the superior
plate moves with a uniform velocity, there will be a series of puffs of wind
allowed to escape from the holes of the inferior plate through those of the
superior plate in uniform succession with equal intervals of time between
them. This succession of puffs will produce undulations in the air sur-
rounding the instrument, and when their velocity is sufficiently increased
400
ACOUSTICS.
these undulations will produce a sound. If the motion be uniform, this
tiound will be maintained at a uniform pitch ; but as the motion of the plate
Fig. J54-
is increased, the pitch will become more elevated; and, in short, such a
velocity may be given to the superior plate as to make the instrument pro-
duce a sound of any desired pitch, acute or grave.
A small apparatus is connected with the superior plate, by which its
revolutions are counted and indicated. This
apparatus consists of a spindle x,fig. 353., which
carries upon it a worm or endless screw, which
drives the teeth of a small wheel r, connected
by pinions and wheelwork with another wheel c.
These wheels govern the motion of hands upon
small dials d d', fig. 355. These hands being
brought to their respective zeros at the com-
1 1
Fig. 355-
mencement of the experiment, their position at the end of any known in-
terval will indicate the number of puffs of air which have escaped from the
holes of the revolving plate M «' in the interval, and will consequently
determine the number of undulations of the air which correspond to the
sound produced.
A perspective view of this instrument is shown in fig. 356.
Experiments. — Various series of interesting experiments have
been performed with this instrument by its inventor, which have
shown that it not only indicates the pitch of the note produced
but also that the quality of the sound has a relation to the thick-
ness of the revolving plate, and of the fixed plate over which it
turns, and with the space between the holes pierced in these plates.
These conditions, however, have not been investigated with suffi-
cient precision to supply any general principles. M. Cagniard de
la Tour thinks, nevertheless, that when the interval between the
SAVART'S APPARATUS.
401
holes pierced in the plates is
very small, the sound approaches
to that of the human voice, and
when they are very considerable
it approaches to that of a trumpet.
671. Savart's apparatus. —
Another instrument for the ex-
perimental determination of the
number of vibrations corre-
sponding to a note of any pro-
posed pitch is due to M. Savart,
whose experimental investigations
have thrown so much light upon
the physics of sound.
This apparatus, which is represented
mfig. 357., consists of a frame a a con-
F'8- 356 structed in a very solid manner, sup-
porting a large wheel I connected, by an endless band x, with a small grooved
wheel fixed upon the axis of another large wheel d', which is formed into teeth
at its edge. These teeth strike successively a
piece of card or other thin elastic plate presented
to them, and fixed upon the frame a a, as repre-
sented in Jig. 358. The successive impulses given
to the card produce corresponding undulations in
the air, the effect of which is a musical sound.
The number of undulations per second thus
produced in the air will correspond with the
Fig- *&• number of teeth of the wheel d' which pass the edge
of the card in a second. Now, if the number of turns per second given to the
primary wheel b be known, the relative magnitudes of this wheel and the small
402 ACOUSTICS.
wheel attached to the axis of d', will determine the number of revolutions per
second given to the wheel d', and, consequently, the number of teeth of the
latter, which, in a second, will strike the edge of the card. In this way, un-
dulations of the air can be produced at the rate of 25000 per second.
Since by the stroke of each tooth of the wheel d', the card is
made to move first downwards and then upwards, or vice versa,
it is clear from what has been explained that, for each tooth ot
the wheel d' which passes the card, a condensed and a rarefied
wave of air will be produced.
In the sound, therefore, which results there will be as many
double vibrations, that is to say, undulations, including each a
condensed and rarefied wave, as there are teeth of the wheel d'
which pass the card; and to ascertain the number of such double
vibrations corresponding to any note, it will be only necessary to
observe the number of teeth of the wheel dr which passes the card
when the sound produced by the instrument is brought into
unison with the proposed note.
672. The absolute rates of vibration of musical notes
ascertained. — By accurate experiment, made both with the
Sirene and with the instrument of M. Savart, it has been found
that the A of the treble clef or fc "~ Is produced by imparting
undulations to the air at the rate of 880 single vibrations, or
440 double vibrations, per second. By single vibration is here
to be understood condensed waves only, or rarefied waves only ;
and by double vibration, the combination of a condensed and
rarefied wave. It is more usual to count the vibrations, taking
the latter, or the double vibration, as the unit, and we shall
therefore here adopt this nomenclature ; and it may therefore be
stated, in this sense, that the A of the diapason, the note usually
produced by the sounding fork for determining the pitch of
musical instruments, is produced by imparting to the air 440
undulations per second.
It must be stated, however, that some slight departure from
this standard prevails in different established orchestras. Thus, it
was estimated in 1822 that this note in the under-mentioned
orchestras, was produced by the number of vibrations per second
exhibited below : —
Orchestra of Berlin Opera .... 4J7'3*
„ Academic de la Musique, Paris - - 4Ji'J4
„ Opera Comique, Paris - - -42.761
„ Italian Opera, Paris - 424 14
In 1859, the pitch of the same note had risen at the Grand
Opera and the Italian Opera of Paris to 448 vibrations per second,
and to prevent further change, a ministerial decree dated Feb. 1 6,
1859, fixed the pitch of fe-~rr~ in future at 437*5 vibrations
TUNING FORK.
4°3
The number of vibrations corresponding to all the other notes of the
musical scale may be computed by the result here obtained, combined with
the relative numbers of vibrations given in (664. ). Thus, if it be desired to
determine the number of vibrations per second corresponding to the funda-
mental note *}'• 7, " , it will be only necessary to divide 440, the number of
vibrations of the note fe~'" - , by the fraction ^, or what is the same, to
divide it by 10, and multiply the quotient by 3. The number of vibrations,
therefore, per second which will produce the note szii^ii will be 44x3
=132.
673. Tuning fork. — To determine the pitch at which instru-
ments should be tuned, and to be enabled, as it were, to transport
a given pitch from place to place, an instrument called a tuning
fork or diapason has been contrived. This instrument is an
clastic steel bar, bent into the form of a fork, and mounted upon
a handle. If either of its prongs be smartly struck upon any
hard surface, they will both begin to vibrate, and if held near
the ear, will produce the perception of a musical note ; and so
long as the fork remains unaltered, this note will be always the
same. It may be also put in vibration by drawing up between
the prongs any bar thicker than the space between them, as
shown in the figure. The sound will be rendered more audible
if the handle of the fork, while in vibration, be pressed upon any
sonorous body such as a board or thin box.
In its original construction, the
fork is regulated so as to produce
a particular note, usually fercrrr
When tuning forks are required, hav-
ing somewhat a higher or lower pitch,
it has been generally found necessary
to provide a separate fork for each pitch,
Bjr an ingenious contrivance, however,
Mr. Daniel Klein, of the establishment
of Mr. Erard, at Paris, has found means
to vary within the necessary limits the
pitch of the same fork. He accomplishes
this by means of a small brass clamp,
which slides upon one of the prongs, as
shown in fig. 359., and which can be fixed
in its position by means of a clamping
screw: by varying the place of this
upon the prong, the pitch of the fork
Fig* 359- can be raised and lowered. Marks are
engraved upon the prong, showing the
D D 2
404 ACOUSTICS.
position which the clamp must have, so as to correspond with the pitch
adopted by each of the principal orchestras.
674. Range of musical sensibility of the ear. — On a
seven octave pianoforte the highest note in the treble is three
octaves above fe— f* .:. and the lowest note in the bass is four
octaves below it. The number of complete vibrations corre-
sponding to the former must be, therefore,
440x2x2x2 = 3520;
and the number of vibrations per second corresponding to the
latter is
440 440
~l_ T. IT i _ — -j
2X2X2X2~~l6 '*'
Now, since all ordinary ears are capable of appreciating the
musical sounds contained between these limits, it is clear that the
range of perception of the human ear is greater than that of such
an instrument, and that, consequently, this organ is capable of
distinguishing sounds produced by vibrations varying from 27 to
3520 per second.
675. [From experiments made with the apparatus represented
in Jig. 357., but with the substitution for the toothed wheel d', of
a simple bar of iron or wood, which, when it revolved, passed
between two plates of wood so as very nearly to touch them, as
shown in jig. 358., Savart concluded that the ear was capable of
perceiving vibrations as slow as at the rate of only 7 or 8 in a
second, as a continuous musical sound. But there can be no
doubt that in these experiments the tone which was continuously
heard was due to secondary vibrations, twice, or perhaps three
times, as rapid as those directly produced by the revolving bar, and
of the nature of the harmonic tones already mentioned in (666.)-
Probably about 1 6 vibrations in a second is the smallest number
which is capable of producing the impression of a continuous
sound. And the lower E of the pianoforte, two octaves below
E^EiHE* a n(>te produced by 41 £ vibrations in a second, and the
lowest employed in orchestral music, being the deepest tone of the
double-bass fiddle, is probably the lowest note of which the ear can
distinctly recognise the musical value. A smaller number of vibra-
tions produces a continuous droning, but not a sound which in the
ordinary sense can be called musical, as any one may convince
himself by striking the lowest notes of a 7-octave pianoforte.]
676. [By means of the revolving toothed wheel (fig. 357.)
LENGTH OF MUSICAL WAVES. 405
Savart found that musical sounds produced by 24,000 complete
undulations in a second could be distinctly recognised; and
Despretz, by means of small tuning-forks, has produced the tone
corresponding to 38,016 undulations per second. But such very
high tones are in the highest degree unpleasant. Hence we may
conclude that though the ear can perceive sounds throughout a
range of about II octaves, from 16 to 38,000 vibrations, the tones
which are available for musical purposes lie within a range of
about 7 octaves, from 40 to 4000 vibrations.]
677. length of the waves corresponding: to musical
notes. — It has been already shown, that by the combination of
the velocity of sound with the rate of undulation, the length of
the sonorous waves corresponding to any given note can be deter-
mined.
Thus, if we know that 440 undulations of the note fe~ Q— strike the ear
in a second, and also that the velocity with which this undulation passes
through the air is at the rate of 1125 feet per second, we may conclude that
in 1125 feet there are 440 complete undulations; consequently, that the
length of each such undulation is
440
By a like calculation, the length of the sonorous waves corresponding to
all the musical notes can be determined.
To find the length of the sonorous waves corresponding to the highest
and lowest notes of a seven octave pianoforte, we are to consider that the
highest note has been shown to be produced by 3520 vibrations per second ;
the length of each vibration will, therefore, be
^ = 0-32.
3520
The number of vibrations corresponding to the lowest note is 27-5 ; the
length, therefore, of the sonorous undulation will be
^ = 40-91 feet
To find the length of the vibrations corresponding to the gravest note
produced in Savart's experiments, we must divide 11*5 by 7; the quotient
will be 1607 feet, which is the length of the undulation required.
678. Application of the Sirene to count the rate at which
the wings of insects move. — The buzzing and humming noises
produced by winged insects are not, as might be supposed, vocal
sounds. They result from sonorous undulations imparted to the
air by the flapping of their wings. This may be rendered evident
by observing, that the noise always ceases when the insect alights
on any object.
The Sirene has been ingeniously applied for the purpose of as-
406 ACOUSTICS
certaining the rate at which the wings of such creatures flap. The
instrument being brought into unison with the sound produced
by the insect indicates, as in the case of any other musical sound,
the rate of vibration. In this way it has been ascertained that
the wings of a gnat flap at the rate of 1 5000 times per second.
The pitch of the note produced by this insect in the act of flying
is, therefore, more than two octaves above the highest note of a
seven octave pianoforte
CHAP. IV.
VIBRATIONS OF RODS AND PLATES.
6/9. Vibration of rods. — Among the numerous results of the
labours of contemporary philosophers, some of the most beautiful
and interesting are those which have attended the experimental
researches of Savart, made with a view to determine the pheno-
mena of the vibration of sonorous bodies, some of which we have
already briefly adverted to. Although these researches are too
complicated, and the reasoning and hypotheses raised upon them
are not sufficiently elementary to be introduced with any detail
into this volume, there are nevertheless some sufficiently simple to
admit of brief exposition, and so interesting that their omission,
even in the most elementary treatise, would be unpardonable.
The vibration of thin rods, whether they have the form of a
cylinder or a prism, or that of a narrow thin plate, may be con-
sidered as made transversely or longitudinally. If they are made
transversely, that is to say, at right angles to the length, they will
be governed by nearly the same principles as those which have
been already explained as applicable to elastic strings.
680. Let us suppose a glass tube, about seven f£et long, and
from an inch to an inch and a half in diameter, to be suspended in
equilibrium at its middle point. Let one half of it be rubbed
upon its surface, in the direction of its length, with a piece of damp
cloth. The friction will excite longitudinal vibration, that, with
a little practice, may be made to produce a musical sound, which
will be more or less acute, according to the force and rapidity of
the friction.
It will be found that the several sounds which will be suc-
cessively produced by thus increasing the force of the friction,
will correspond with the harmonics already explained in (666.) ;
that is to say, the rate of vibration of the lowest of these tones
MARLOYE'S HARP.
407
being expressed by I, that of the next above it will be expressed
by 2, and will therefore be the octave ; the next will be expressed
by 3, and will therefore be the twelfth ; and the next by 4, which
will therefore be the fifteenth.
If the same experiment be performed with long rods of any
form, and of any material whatever, the same result will be
noticed. When rods of wood are used, instead of a moistened
cloth, a cloth coated with resin may be employed. It is found
that rods, composed of the same material, will always emit the
same notes, provided they are of the same length, whatever be
their depth, thickness, or form, provided only that their length be
considerable compared with their other dimensions.
68 1. Marloye's harp. — This instrument, represented in jig.
360., consists of twenty thin deal cylindrical rods of decreasing
length, and so regulated
that the notes they pro-
duce shall be those of the
musical scale, the half notes
being distinguished by co-
loured rods like the black
keys of a pianoforte.
The rods are sounded by
pressing them between the
finger and thumb, previously
rubbed with powdered rosin,
and drawing the fingers lon-
gitudinally upon them. An
effect is produced having some
resemblance to that of the Pan •
dean pipes.
682. Nodal points. —
Were it possible to render
visible the state of vibra-
tion of each point of the
surface of these rods, it
would be found that the
degree of vibration would
vary from point to point,
and that at certain points
distributed over the sur-
face of these rods there
would be no vibration.
These nodal points, as they
have been called, are distributed according to certain lines sur-
rounding the rods.
4o8 ACOUSTICS.
But it is evident that motions so minute and so rapid as these
vibrations, cannot be rendered directly evident to the senses.
683. The following ingenious method of feeling the surface
while in vibration, and ascertaining the position of the nodal lines,
was practised with signal success by Savart. A light ring of paper
was formed, having a diameter considerably greater than that of
the tube or rod. This ring was suspended on the tube, as repre-
sented in Jig. 361.
The tube, which we shall suppose here, as before, to be formed of glass
and of the same dimensions as already explained, being suspended on its
Fig.j6i.
central point, and piit in vibration, as already described, by friction pro-
duced upon that half of the tube on which the ring is not suspended, it
will be found that the vibration of the tube will give the ring a jumping
motion which will throw it aside, and cause it to move to the right or left,
as the case may be, until it shall arrive at a point where it shall remain at
rest, its motion as it approaches this point being gradually diminished. At
this point it is evident that there is no vibration, and it is, consequently, a
nodal point.
Let this point be marked upon the glass with ink, and let the tube be
then turned a little round on its axis, so as to bring the point thus marked
a little aside from the highest position which it held when the ring rested
upon it. Let the tube be now again put in vibration, so as to produce the
same note as before. The ring will be again moved, and will find another
point of rest.
Let this point be marked as before, and let the tube be again turned, and
let the same process be repeated, so that a third nodal point shall1 be deter-
mined. By continuing this process, a succession of nodal points will be
found following each other round the tube, and thus a nodal line will be
determined.
This process may be continued until the entire course of the nodal line
shall be discovered.
Experiments conducted in this way have led to the discovery
that the nodal lines surrounding the tube have a sort of spiral or
screw-like form, represented in Jig. 361. The course is not that
of a regular helix, since it forms, at different points of the surface
of the tube, different angles with its axis, whereas a regular helix
will at every point form the same angle ; but this variation of the
inclination of the nodal line to the axis is not irregular, but under-
goes a succession of changes which are constantly repeated, so that
each revolution of the nodal line is a repetition in form of the
last.
If the ring be now suspended on the other half of the tube, a
similar nodal curve is formed, which is not, however a continuation
NODAL CURVES. 409
01 the former. The two spirals seem to have a common origin at
the end, and to proceed from that point, either in the same or
contrary directions, towards the other end of the tube.
684. Savart examined also the position of the nodal line on the
inner surface of the tube, by spreading upon it grains of sand, or
a small bit of cork. These were put in motion in the same manner
as the ring of paper by the vibration, and were brought to rest on
arriving at a nodal point. A series of nodal lines similar to the
exterior system was discovered.
When the friction is increased so as to make the tube sound the
harmonics to the fundamental note, the spirals formed by the nodal
line are reversed two, three, or four times, according-to the order
of the harmonic produced.
685. In the case of prismatic rods or flat laminae, the nodal
curves are still spirals, but more irregular and complicated than
in the case of tubes or cylinders.
The vibrations of thin plates were produced and examined by
the following expedients: — An apparatus was provided, repre-
sented in^g-. 362. A small piece of metal a, having a form slightly
conical, is fixed in the bottom
of a frame, and at its upper
surface a piece of cork, or
buffalo skin, is fixed to inter-
cept vibration. A corre-
sponding cylinder is moved
Fig. 36z. vertically, directly above it,
by a screw, which plays in
the frame 5, and which is also covered at its extremity with a piece
of cork.
When the screw is turned, the two extremities can be brought
into contact, so as to press between them with any desired force
any plate which may be interposed.
An elastic plate, the vibration of which it is desired to observe,
is inserted between them, and held compressed at any desired
point by turning the screw. The plate thus held can be put in
vibration by means of a violin bow, which being drawn upon its
edge, clear musical sounds may be produced, and brought into
unison with those of a pianoforte, or other musical instrument.
To ascertain the state of vibration of the different points of the
surface of the plate, sand or other light dust is spread upon it, to
which motion is imparted by the vibrating points. Those points
which are at rest, and which are therefore nodal points, impart no
motion to the grains of sand which lie upon them, and those which
are upon the vibrating points are successively thrown aside, until
410
ACOUSTICS.
they reach the lines of repose or nodal lines, where at length they
settle themselves.
When a musical sound of a uniform pitch has, therefore, been
continued for any length of time, the disposition of the grains of
sand upon the plate will indicate the position and direction of the
nodal lines.
686. lateral vibrations of rods or plates. — An.easy expe-
rimental method of determining the laws which govern these, is
indicated in fig. 363 The rod or plate being held at one end by
a vice, the length of the rod may be
varied at pleasure.
687. When experiments of this
kind were multiplied to some ex-
tent, it became apparent that the
nodal lines assumed such varied and
complicated forms that it was diffi-
cult to delineate them with accuracy
by the common methods of drawing.
An ingenious expedient suggested
itself to Savart, by which facsimiles
of all these figures were obtained.
Instead of sand, he used litmus
mixed with gum, dried, reduced to
a fine powder, and passed through
a sieve, so as to obtain grains of
equal and suitable magnitude. This
coloured and hygrometric powder
he spread upon the vibrating plates,
and when it had assumed the form
of the nodal lines, he applied to the
plates with gentle pressure damp
paper, to which the coloured powder adhered, and which, there-
fore, gave an exact impression of the form of the nodal lines.
In this manner he was enabled to feel, as it were, the state of
vibration of the different parts of the plate, and to ascertain with
precision the lines of no vibration, or the nodal lines, which sepa-
rated from each other those parts of the plate which vibrated
independently.
In this way many hundred experiments were made, and exact
diagrams obtained representing the condition of the vibrating
plates.
688. One of the consequences which most obviously followed
from these experiments was, that the nodal lines became more and
more multiplied the more acute the sound was which the plate
produced. This consequence was one which might have been
Fig. 363.
EXPERIMENTS OF SAVART.
anticipated from the analogy of the nodal lines of the plate to
the nodal points of the elastic string. It has been already shown,
that with a single nodal point in the middle of the string, the oc-
tave to the fundamental note is produced ; that when two nodal
points divide the string into three equal parts, the twelfth is pro-
duced ; that when three nodal points divide the string i.nto four
equal parts, the fifteenth is produced, and so on. What the sub-
divisions of the string are to the notes produced by its vibrations,
the subdivisions of the surface of the vibrating plate by the nodal
lines, are to the note which it produces ; and it was consequently
natural to expect, that the higher the note produced, the more
multiplied would be the divisions of the plate.
X
as
A
bic
1
m
H±]
J.-44-
m
M
m
^\*.+. --•/ .
m
Fig. 364.
689. Curious forms of the nodal lines. — But a circumstance
attending these divisions not less curious than their number was
4I2
ACOUSTICS.
their form, for which no analogy existed in the vibration of strings.
It would be impossible here to give any definite notion of the
infinite variety of which these nodal figures are susceptible ; they
change not only with the pitch of the note produced, but also with
the form and material of the plate, and the position of the point
at which.it is held in the instrument, represented in fg. 362. It
will not, however, be without interest to give an example of the
variety of figures presented by the nodal lines produced upon
the same square plate. These are represented in the series of
figures 364.
Similar experiments, made on circular plates, showed that the
nodal lines distributed themselves either in the direction of the
diameter, dividing the circle into a number of equal parts, or in
circular forms, more or less regular, having the centre of the plate
365.
at their common centre, or, in fine, in both of these combined. In
the annexed series of figures 365. are represented some of the
varieties of form thus obtained.
CHAP. V.
VIBRATIONS OF FLUIDS.
690. Fluids, whether in the liquid or gaseous state, have been
hitherto considered mprelv as conductors of sound, their sonorous
VIBRATIONS OF FLUIDS. 413
undulations having been derived from the vibratory impulses of
solid bodies acting upon them.
Fluids themselves, however, are capable of originating their own
undulations, and consequently must be considered not merely as
conductors of sound, but likewise as sonorous bodies.
If the Sirene of Cagniard de la Tour, already described, be sub-
merged in water, and made to act as it has been described already
to act in air, the pulsations of the water will produce a sound. In
this case, the origin of the sound is the action of the liquid upon
itself. The successive movements of the liquid through the holes
in the circular plate of the Sirene are the origin of the sonorous
undulations which are transmitted through the liquid.
Sounds produced by communication. — It is well known
that vocal sounds are increased in loudness and force when they
are produced at the mouth of any cavity of sufficient extent, depth,
and proper form. In that case the vibrations imparted by the
vocal organs to the air contiguous to the mouth are propagated to
the air in the cavity, the vibrations thus communicated increasing
in a very remarkable manner the loudness of the sound.
Vitruvius relates that in the ancient theatres, which were of
vast magnitude, this expedient was adopted to give increased force
to the voice of the actor, round whom hollow vessels were dis-
posed in the decorations of the scene, so as to elude the notice of
the audience, which, by the communicated vibrations of the con-
tained air, rendered the voice of the actor distinctly audible in the
remotest parts of the theatre.
In modern opera houses, the stage itself, when mounted with a
flat scene at the back, has this effect, and in certain parts of the
house the audience can hear the voice of the prompter almost as
distinctly as the notes of the artist. The prompter's seat is roofed
with a sort of arched hood, from the surface of which the sounds
he produces are reflected to the flat scene at the back of the stage,
from which they are again reflected to those parts of the house
where they are heard. The practical proof of the truth of this
explanation will be found in the fact, that the prompter imme-
diately ceases to be heard when the flat scene is withdrawn, and
the entire depth of the stage thrown open.
To reduce the phenomena of communicated vibrations to more
regularity, Savart contrived the apparatus shown in fig. 366.,
consisting of two cylinders sliding one within another, like the
tubes of a telescope, one of which is open at both ends, and the
other only at one end. By drawing the closed cylinder in and
out, the depth of the open cylinder can be varied at pleasure. The
cylinders are mounted upon a cradle or hinge joint upon the
summit of a vertical pillar fixed in a bar, which slides horizontally
4i 4 ACOUSTICS.
in its base. A vase made of bell- metal is mounted on a vertical
pillar, at a height corresponding with that of the mouth of the
>m
- Fig. 366.
cylinder, so that the latter can be moved to or from the vase at
pleasure, and can be inclined so that the mouth shall be more or
less obliquely presented to the vase.
If the vase be put in vibration, either by the blow of a hammer
or by drawing over its edge the bow of a violin, a musical sound
will be produced, which, being communicated to the air in the
cylinder, will impart vibration to it. But to render this fully
effective, it is necessary to vary the length of the cylinder by
drawing the closed cylinder in and out, until it has that length
which corresponds to the note produced by the vase.
69 1 . Wind instruments. — Innumerable examples might be
found of sonorous undulations produced by air upon air. The
Sirene itself, which has been already explained, forms an example
of this, and at the same time indicates the manner in which the
pulsations are imparted to the air. All wind instruments what-
ever are also examples of this. The air, by the impulses of which
the sonorous undulations are produced, proceeds either from a
bellows, as in the case of organs, or from the lungs, as in the case
of ordinary wind instruments. The pitch of the sound produced
depends partly upon the manner of imparting the first movement
to the air, and partly on varying the length of the tube containing
the column of air to which the first impulse is given.
When the tube has a length which is considerable in proportion
to its diameter, and is open at both ends, the gravest note which
it is capable of producing is determined by a sonorous undulation
of twice its own length. By varying the embouchure, and other-
wise managing the action of the air on entering the tube, notes
may be produced which are harmonics to the fundamental note
corresponding to the length of the tube.
ORGAN PIPES. 415
When these harmonics are produced, nodal points will be
formed in the column of air included in the tube ; and if the tube
were divided, and capable of being detached half-way between two
such points, the removal of a part of the tube would not alter the
pitch of the note produced.
In wind instruments in which various notes are produced by
the opening and closing of holes in their sides by means of the
fingers or keys, there is a virtual variation in the length of the
sounding part of the tube, which determines the pitch of the
various notes produced. In some cases, the length of the tube is
varied, not by apertures opened and closed at will, but by an
actual change of length in the tube itself. Examples of this are
presented in some brass instruments, and more particularly in the
trombone.
692. Although the length of the column of the air included in
the tube of a wind instrument alone determines the pitch of the
note, its quality depends in a striking and important manner upon
the material of which the tube is composed.
693. It is well known that organ builders find that the quality
of tone is so materially connected with the quality of the material
composing the tube, that a very slight change in the alloy com-
posing a metal tube would produce a total change in the quality
of the tone produced. The excellence of an organ depends in a
great degree upon the skill with which the material of the tubes,
whether wood or metal, is selected.
694. Organ pipes. — The general principles explained in the
preceding paragraphs are illustrated in a striking manner by the
effects of organ pipes. These are of two sorts, called mouth pipes
and reed pipes.
A mouth pipe consists of a. foot, which is a hollow cone receiving the wind
by which the pipe is sounded, from an air chest, in which the air is com-
pressed by a bellows. To this foot is attached the body of the pipe, which is
either square or round, the length always having a considerable proportion
to its diameter. At the place where the body of the pipe is connected with
the foot, there is an arrangement by which the quality of the sound produced
by the pipe is determined.
This arrangement consists of an oblique opening a' (fig. 367.) leading from
the foot c' by which the air enters, immediately above which is a lateral
opening in the body of the pipe bounded by an edge 6', against which the
air escaping from a' strikes. The edges a' and I' are called the lips, a' being
distinguished as the lower and b' the upper A front view of the pipe,
showing the upper lip 6, the lower lip a, and the foot c, is shown in
fig- 368.
Organ pipes are generally either square or circular in their transverse
section ; the wooden pipes being square and the metal circular. A section
of a square pipe is given \nfig. 369., and front and side views of a circular
pipe are given \nfigs. 370, 371.
4i6
ACOUSTICS
In Jig. 372. the embouchure is so formed that the upper lip 6 is movable,
so that the effects of varying the magnitude of the opening can be ascertained
experimentally.
I
Fig. 367. Fig. 368. Fig. 369. Fig. 370. Fig. 371. Fig. 371.
The air entering through c rushes through the mouth, where it encounters
the edge of the upper lip b, which partially obstructs it. The part which
passes up the pipe produces a momentary compression of the column of air
within the pipe against which the increased elasticity reacts, and this goes
on producing in the whole length of the pipe an alternate compression and
expansion, from which results a specific sound.
The pitch of the pipe is ascertained experimentally by the
bellows and air chest, shown in Jig. 373, The bellows is worked
by means of a pedal, the air being driven up to the air chest
through the pipe. When it is desired to trv a pipe, the foot of
the pipe is inserted in one of the holes; and when the corre-
sponding key is pressed down, the valve being opened and air
admitted to the pipe, the note is produced.
The quality or timbre of the note produced will vary with the
form of the lips and magnitude of the mouth. Thus the mouth
represented in Jig. 369. is different from that shown in^o-. 368.
[The pitch of the note sounded by an organ pipe depends
chiefly upon its length, but is influenced in a secondary degree by its
diameter and other circumstances. If the pipe is open at the top,
the vibrations of the column of air within it will take place so that
no condensation or rarefaction will be produced either there or
at the mouth, for it is obvious that the free communication with
ORGAN PIPES 417
the atmosphere which exists at these points must prevent such
effects taking place. At these points, therefore, the air will move
backwards and forwards, but will not suffer any considerable
change of density. So long as thuse conditions are fulfilled, the
coluCin of air may vibrate in any manner. The simplest mode
of vibration possible is when the air rushes backwards and for-
wards simultaneously from the two ends of the pipe towards the
middle and away from it. In this way a single node, being a point
where the air has no motion, but is alternately condensed and
rarefied, is established at the middle of the pipe. Vibrations of
this kind produce the so-called fundamental tone of the pipe, the
lowest which it is capable of sounding. The length of the com-
plete wave corresponding to the fundamental tone is twice the
length of the pipe. The next simplest mode of vibration is when
there are two nodes in the column of air within the pipe — one at
one-quarter of the distance from the mouth to the top, and
another at three-quarters of the distance. In this case the note
produced is the octave above the fundamental, and the wave-
length is halt' as great as that corresponding to the fundamental
note. The next simplest mode of vibration sounds the twelfth
above the fundamental, with a wave-length one-third as great
as the latter. In this case there are three nodes within the pipe,
at one-sixth, three-sixths, and five-sixths of its length. The
next set of nodes are four in number, at one-eighth, three-eighths,
five-eighths and seven-eighths, and the note produced is the
fifteenth, or double octave, and so on.
It will be seen that the column of air in the pipe is thus able
to subdivide itself precisely in the same way as a stretched string
(667.), and the tones resulting from the vibrations of the subdi-
visions are the higher harmonic tones of the fundamental (666.).
Practically the fundamental tone is produced almost by itself
when the pipe is sounded by blowing very gently. On blowing
more strongly, the higher harmonics become perceptible one after
the other in order, and in a long narrow pipe may even almost
entirely obliterate the fundamental tone. Generally, however, in
the pipes actually employed in the organ, the fundamental tone
predominates, though accompanied to some extent by the first two
or three higher tones.
In an organ pipe closed at the top, the conditions to be fulfilled
in the vibrations of the air are that there should be a node at the
top, and free motion of the air at the bottom. This state of things
would be obtained if we had the means of putting a solid partition
across an open pipe, sounding its fundamental note, at the place
of its central node. We should thus convert the open pipe
into a closed one of half its length, but should not alter its tone.
Hence the fundamental note of a pipe stopped at the top is an
£ £
41 8 ACOUSTICS.
octave below that of an open pipe of the same length, and the
length of the corresponding sound-wave is four times the length
of the pipe.
The harmonic tones produced by the subdivision of the column
of air in a stopped pipe are the twelfth above the fundamental, or
the third harmonic, then the seventeenth above the fundamental,
or the fifth harmonic ; next the seventh harmonic, and so on
through the series of tones the numbers of whose vibrations are
multiples of that of the fundamental tone by odd numbers.
The tones producible from a four-feet open pipe, or a two-
feet, stopped pipe, are accordingly those noted below, the funda-
mental tone of both pipes being tenor C, making 132 vibrations in
a second.
Harmonics') Fundamental. 7 8 9 I0
ofa4-ft. \ i 23456
open plpe. j C CGCJE r
Harmonics } C •> *> G E B-flat. D &c.
of a 2-ft. V i 3 5 _ _
stopped pipe- ) Fundamental.
The vibrations producing each tone are multiples of those pro-
ducing the fundamental tone by the numbers placed above and
below the names of the respective notes.]
695. Reed pipes. — A reed is, in general, a thin oblong plate
of some vibratory material, attached to an opening in such a man-
ner that a current of air can pass into the opening, grazing, as it
passes, the edges of the reed.
Let g,fig. 374-> represent, for example, an oblong plate of zinc or copper
about an eighth of an inch in thickness, along the centre of which an oblong
aperture is cut. At one end e of this aperture a thin and very elastic plate
of metal efia fastened, which nearly but not altogether covers the aperture.
Air rushing through the space around the edges of ef, will cause it to
vibrate, and this vibration will be imparted to the air in contact with it.
This is the most simple form of reed, and the sound may be produced with
it by merely applying the plate g to the lips and forcing the breath through
the opening.
The reed commonly used in organ pipes depends upon the same principle,
but is otherwise arranged. The parts are shown in Jig. 375., consisting of
two tubes d and c joined end to end, and separated by a piece a, which stops
the passage between them. The reed b passes under this piece a. This part
of the pipe is represented in detail in Jig. 376., where the oblong opening
covered by the reed a, and the sliding piece b connected with the rod e, by
which the length of the reed can be regulated at pleasure, are shown. The
reed covers in this case an oblong opening in a prismatic metal tube supposed
to be closed at its lower end. The opening establishes a communication
between the two tubes placed above and below th$ stopper. The reed in its
natural position very nearly closes the oblong opening; that is to say, it fits
it, so that when pushed in or drawn out, it grazes with its three free edges
HEED PIPES.
419
the borders of the opening ; when it is put in vibration, therefore, it opens
and closes the aperture alternately.
Fig. 374.
•"
In certain pipes there are reeds somewhat differently constructed,
which give a particular quality to the note. One of these is re-
presented in figs. 377, 378, 379., and it differs from the former
Fig. 376.
Fig. 377.
Fig. J78-
Fig. 379-
inasmuch as the reed does not pass through the aperture, but presses
upon its edges.
The mouth pieces of bassoons, hautboys, clarionets, &c., are
only different forms of the application of the reed. In these cases,
the pressure of the lips determines the length of the vibrating
part of the reed, just as the piece I does in fig. 376., and in
fg- 378.
420 ACOUSTICS.
696. The compass of an organ is usually expressed and deter-
mined by the length of its longest pipes, or those which produce
its lowest notes. Among the existing instruments of this class,
the most celebrated is that of Haarlem, built in 1 748 by Christian
Muller; its height is 103 feet, and its breadth 50 feet. The great
organ has 1 6 stops : the upper one I 5 ; and the quire organ 1 4 ;
and there are I 5 stops connected with the pedals. It includes
5000 pipes ; each pair of bellows is 9 feet long and 5 feet broad.
697. Among the largest English organs are tho.-e in York
]\ I inster, Birmingham Town Hall, and Christchurch, London. The
York organ has 24 stops in the great organ, and 10 in the quire
organ. The pedal organ has IO stops; two octaves, varying from
32 feet to 8 feet; 32 feet open diapason in metal, wood, and
trumpet. There are in the organ 4089 pipes, in 50 ranks.
The Birmingham organ contains the following stops : three open
diapasons to 16 feet c ; double and stop diapason ; two principals
of metal and two of wood; a twelfth and two fifteenths of metal,
and one of wood. A reed fifteenth, 4 feet ; posaun, 1 6 feet ;
trumpet, 1 6 feet; clarion, 8 feet; sesquialtra, 4 ranks ; mixture,
4 ranks; two octaves of German pedals, 32 feet metal; open dia-
pason to 8 feet c; 32 feet wood, ditto; 2 octaves of pedal trum-
pets. 1 6 feet to 8 feet c.
[The largest organ in existence is that in St. George's Hall,
Liverpool, built by Willis from plans by Samuel Wesley. It
contains 100 sounding stops, and 8000 pipes, varying in length
from 32 feet to three-eighths of an inch, and producing sounds
which are ten octaves apart.]
698. The sound produced by a jet of hydrogen, directed in a
glass tube, forms a remarkable example of the
manner in which the sonorous undulations of air
would be produced by movements originating in air
itself.
This apparatus consists of a small glass vessel in which
hydrogen is generated in the usual way, by the action of
acid on zinc or iron. A funnel and stopcock A, Jig. 380, are
provided, by which the supply of the acid may be renewed.
A pipe proceeds from the centre of the top of the vessel fur-
nished with a stopcock c, in which a small tube is inserted
terminating in a very small aperture, from which a fine jet of
the gas escapes when the stopcock is opened, and a sufficient
pressure produced by the accumulation of gas within the
vessel. The jet proceeding from t in this manner being in-
flamed, a glass tube of considerable length, and having a
diameter of about two inches is held over it, so that the jet is
made to burn at some distance above the lower end of the
Fig 380. tube. A musical sound will thus proceed from the air within
the tube, the pitch of which will depend upon the length of
the tube.
ECHOES. 421
[This effect is due to a rapid succession of small explosions,
produced by the mixture of atmospheric air and hydrogen,
whereby the air in the tube is thrown into a state of vibration.
The combustion of the hydrogen can be easily seen to take place
in successive bursts, by viewing the reflection of the flame in a
looking-glass which it held in the hand and turned rapidly back-
wards and forwards. The appearance then is that of a string
of luminous beads, instead of an unbroken line of light such as
would be produced by a flame burning continuously.]
699. Echoes. — It has been already shown, that when undula-
tions propagated through a fluid encounter a solid surface, they
will be reflected from it, and will proceed as though *they had
originally moved from a different centre of undulation.
Now, if this take place with the sonorous waves of air, such
waves encountefing the air will produce the same effect as if they
proceeded, not from the sounding body which originally produced
them, but from a sounding body placed at that centre from which
the waves thus reflected move. Upon these principles echoes are
explained.
If a body, placed at a certain distance from the hearer, produce
a sound, this sound would be heard first by means of the sonorous
undulations which produced it proceeding directly and uninter-
ruptedly from the sonorous body to the hearer, and afterwards by
sonorous undulations which, after striking on reflecting surfaces,
return to the ear. The repetition of the sound thus produced is
called an echo.
To produce an echo it will be necessary, therefore, that there
shall be a sufficient magnitude of reflecting surface, so placed with
respect to the ear, that the waves of sound reflected from it shall
arrive at the ear at the same moment, and that their combined
effect shall be sufficiently energetic to affect the organ in a sensible
manner.
If, for example, the sounding body be placed in a focus F of an ellipse, as
represented \nfig. 381., the hearer being at the other focus F', the sound will
be first heard by the effect of the undulations, -which are produced directly
along the Kne F F*, from one focus to the other. But it will be heard a little
6 later by the effect of the waves, which,
diverging from the sounding body at F,
strike upon the elliptic surface, and are
reflected to the other focus F', where the
hearer is placed. The interval which
elapses between the sound and the echo
in this case will be the time which sound
takes to move through the difference be-
J-'ii-T- 381. tween the direct distance F F', and the
sum of the two distances at any point in the ellipse from the foci F F'. It
has been already explained that the sum of these two distances is always
the same wherever the point of reflection may be, 'being equal to the major
422 ACOUSTICS
axis of the ellipse. It is for this reason that all the reflected rays of sound
from every part of the ellipse will meet the ear placed at F' at the same
moment, since they will take the same time to move over the same distance.
If the reflected surface were not elliptical, or if, being elliptical, the hearer
were not placed at the focus F', then the sum of the distances of the different
points of the reflecting surface from the ear would be different, and the
reflected rays of sound arriving from different points of the surface, would
reach the ear at different moments of time. In this case, each ray of sound
would be too feeble to produce sensation, or a confused effect would be
produced.
It is not necessary that the elliptic surface reflecting the sound should be
complete. If different portions of the reflecting surface, a, b, c, d, e, f,
fig. 381., be so placed that they would form part of the same ellipse, they
will still reflect the rays of the sound to the other focus of the ellipse; and
if they are so numerous or extensive as to reflect rays of sound to the ear in
sufficient quantity to affect the sense, an echo will be heard.
700. Tf surfaces lie in such a position round the points F and F',
that these points shall be at the same time the foci of different
ellipses, one greater than the other, a succession of echoes will
ensue, the sounds reflected from the greater elliptic surface
arriving at the ear later than those reflected from the lesser. The
interval between the successive echoes in such a case would be the
time which the sound takes to move over a space equal to the
difference between the major axes of the ellipses.
If a person who utters a sound stand in the centre s of a circle,
fig. 382., the circumference of which is either wholly or partly
composed of surfaces, such as a, J, c,
d, e, which reflect sound, he will hear
the echo of his own voice ; as in this
case the sonorous undulation, which
proceeds from the speaker encounter-
ing the reflecting surfaces in a direc-
tion perpendicular to them, will be
reflected by them back to the speaker,
as represented by the arrows, and will
reach his ear after an interval cor-
responding to that which sound re-
quires to move over twice the radius
oi the circle. If the speaker in such a case be surrounded by sur-
faces composing either wholly or partly two or more circles, of
which he is the common centre, then he will hear a succession of
echoes of his own voice, the interval between them corresponding
to the time which sound would take to move over twice the differ-
ence between the successive radii of the circles.
If a speaker stand at s, Jig. 383., midway between two parallel
walls A and B, these walls may be considered as forming part of a
circle of which he is the centre, and they will reflect to his ear the
ECHOES. 423
sounds of his own voice, producing an echo. In this case the posi-
tion of the speaker s being equally distant from A and B, the sounds
reflected from these surfaces will return to his ear simultaneously,
and produce a single perception. But a part of the undulation
reflected from B, not intercepted by the speaker at s, will arrive at
A, and will be reflected from A and again arrive at s, where it will
affect the ear. The same may be said of the sounds reflected from
A, which, proceeding to B, will be again reflected to s ; and as the
distances moved over by the sounds thus twice reflected are equal,
they will arrive simultaneously at s, and will then produce a
second echo. This second echo, therefore, will proceed from the
successive reflections of the sound by the two walls A and B, and
the interval between it and the first echo will be ihe time which
sound takes to move over twice the distance s A, or the whole
distance between the two walls.
Thus, i£ the two surfaces A and B were distant from each other
1125 feet, then the interval between the utterance of the sound
and the first echo would be one second, and the same interval
would take place between the successive echoes.
If the speaker, however, be placed at. a point s, fig. 384., which
is not midway between the two walls A and n, the echo proceeding
Fig. J84.
from the first reflection by the wall A will be heard before the echo
which proceeds from the reflection by the wall B, and in this case
a single reflection from each wall will produce two echoes.
If we suppose a second reflection from each wall to take place,
two echoes will be again produced. So that with two reflections
from each wall four echoes will be heard; and in general the
number of echoes which will be heard will be double the number
of reflections.
701. It may be asked, why the number of reflections, in such
case, should have any limit? The answer is, that the reflected
waves are always more feeble than the direct waves; and that
consequently intensity, or loudness, is lost by each reflection, until
at length the waves become so feeble as to be incapable of affect-
ing the ear. A speaker can articulate so as to be distinctly
audible at the average rate of four syllables per second. 1£
424 ACOUSTICS.
therefore, the reflecting surface be at the distance of 1125 feet,
the echo of his own voice will be perceived by him at the end of
two seconds after each syllable is uttered; and since, in two
seconds, he can utter eight syllables, it follows that he can hear,
successively, the echo of these eight syllables ; if he continue to
speak, the sounds he utters will be confused with those of the
echo.
The more distant the reflecting surfaces are, the greater will
be the number of syllables which can be rendered audible by
the ear.
It is not necessary that the surface producing an echo should
be either hard or polished. It is often observed at sea, that an
echo proceeds from the surface of the clouds. The sails of a
distant ship have been found also to return verv distinct echoes.
702. Remarkable cases of multiplied echoes. — Numerous
examples are recorded of multiplied repetitions of sound by echoes.
An echo is produced near Verdun by the walls of two towers,
which repeats twelve or thirteen times the same word. At Ader-
nach, in Bohemia, there is an echo which repeats seven syllables
three times distinctly. At Lurleyfels, on the Rhine, there is an
echo which repeats seventeen times. The echo of the Capo di
Bove, as well as that of the Metelli of Rome, was celebrated
among the ancients. It is matter of tradition that the latter was
capable of repeating the first line of the -ZEneid, which contains
fifteen syllables, eight times distinctly. An echo in the Villa Si-
monetta, near Milan, is said to repeat a loud sound thirty times
audibly. An echo in a building at Pavia is said to have answered
a question by repeating its last syllable thirty times.
703. Whispering galleries are formed by smooth walls having
a continuous curved form. The mouth of the speaker is pre-
sented at one point of the wall, and the ear of the hearer at an-
other and distant point. In this case the sound is successively
reflected from one point of the wall to another until it reaches
the ear.
704. Speaking tubes, by which words spoken in one place are
rendered audible at another distant place, depend on the same
principle. The rays of sound proceeding from the mouth at one
end of the tube, instead of diverging, and being scattered through
the surrounding atmosphere, are confined within the tube, being
successively reflected from its sides, as represented in jig. 385.;
so that a much greater number of rays of sound reach the ear at
the remote end, than could have reached it if they had proceeded
without reflection.
Speaking tubes, constructed on this principle, are used in large
buildings where numerous persons are employed, to save the time
SPEAKING TRUMPET.
425
which would be necessary in dispatching messages from one part
of the building to another. A speaking tube is sometimes used on
shipboard, being carried from the captain's cabin to the topmast.
Fig. 385-
A like effect is produced by the shafts of mines, walls, and chim-
neys, as well as by pipes used to convey heated air or water.
705. The speaking trumpet is another example of the practical
application of this principle. A longitudinal section of this instru-
ment is represented in
fig, 386. The force
of the trumpet is such,
that the rays of sound
which diverge from
the mouth of the
speaker are reflected
rig. 386. . parallel to the axis of
the instrument. The_
trumpet being directed to any point, a collection of parallel rays
of sound moves towards such point, and they reach the ear in much
greater number than would the diverging rays which would pro-
ceed from a speaker without such instrument.
A speaking trumpet as used on board ship is represented in
.fig- 387-
Fig. 387.
706. A hearing trumpet, represented in -fig. 388., is, in form
and application, the reverse of the speaking trum-
pet, but in principle the same. The rays of sound
proceeding from a speaker more or less distant,
enter the hearing trumpet nearly parallel ; and the
form of the inner surface of such instrument is such
that, after one or more reflections, they are made
to converge upon the tympanum of the ear.
Fig. 388. jf a soun(jing foody be placed in the focus of a
425 ACOUSTICS.
parabola formed of any material capable of reflecting sound, the
rays which issue from it will, after reflection, proceed in a direction
parallel to the axis of the parabola. This will be apparent from
what has been explained in (626.) ; and if, on the other hand,
rays parallel to the axis strike on such a surface, they will be
reflected converging towards the focus. Hence it appears that a
parabola, in the focus of which the mouth of the speaker is placed,
would be a good form for a speaking trumpet.
If a watch be placed in the focus of a parabolic surface, such as
a metallic speculum of that form, an ear placed in the direction
of its axis will distinctly hear the ticking, though at a considerable
distance; but if the parabolic reflector be removed, the ticking
will be no longer heard.
CHAP. VI.
THE EAR.
707. Theory of the organ not understood. — The form and
structure of the eye is so evidently adapted to the physical pro-
perties of light, and the purpose for which each of its parts is
adapted can be so clearly demonstrated, that it might naturally
be expected that a similar conformity could be shown to prevail
between the form and structure of the ear, and the physical pro-
perties of sound. With the exception, nevertheless, of one or two
exterior arrangements in the organ of hearing, the peculiar and
complicated form and structure of its internal parts have not hitherto
been shown by any satisfactory or conclusive reasoning to have
any relation to the principles of acoustics. In treating, therefore,
of the ear considered merely as a branch of applied physics, little
more remains than to describe its parts as anatomists have de-
monstrated them, indicating the obvious relation which the ex-
terior and more simple parts have to the laws of acoustics.
708. Description of the ear. — The ear consists of three dis-
tinct parts differing altogether each from the other in their form.
They are denominated by anatomists the external ear, the middle
ear, and the internal ear, being placed in that order, proceeding
inwards from the external and visible part of the organ.
709. The external ear. — The part of the external ear which
is visible outside the skull, behind the joint of the lower jaw
(fig. 389.), is called the pinna or auricle.
710. Concha. — The several parts of the auricle marked in the
THE EXTERNAL EAR.
427
Fig. J89.
figure by the numbers I, 2, 3, &c., are
distinguished by specific names in anatomy.
With the exception, however, of the cavity
7, called the concha, none of these parts
can be considered as having any important
acoustic properties. The depression 2,
called the fossa of the helix, and the sur-
rounding cartilage I, called the helix, may
possibly have some slight effect in reflecting
the rays of sound towards the concha 7,
and thence into the interior of the ear.
If such, however, were the purpose, it
would be much more effectually answered
by giving to this part of the organ a form
more closely resembling that of the wide
end of a trumpet. As the external ear is
actually constructed, the only part which perfectly answers this
purpose is the concha.
711. External meatus. — Proceeding inwards from the concha,
the remainder of the external ear is a tube something more than
an inch long, the diameter of which becomes rapidly smaller from
the concha inwards ; its calibre, however, is least about the middle
of its length, being slightly augmented between that point and its
connection with the middle ear. Its section is everywhere ellip-
tical, but in the external half the greater diameter of the ellipse
is vertical, and in the internal, horizontal. This tube does not
proceed straight onwards, but is twisted so that the distance from
the concha to the point where it enters the middle ear is less than
the total length of the tube. The external part of the tube is-
cartilaginous like the external ear, but its internal part is bony ;
the bony surface, however, being lined by a prolongation of the
skin of the auricle.
712. membrane of tympanum. — The internal extremity of
this tube is inserted in an opening leading into the middle ear,
which is inclined to the axis of the tube at an angle of about 45°.
Over this opening, which is slightly oval, an elastic membrane
called the membrane of the tympanum is tightly stretched like
parchment on the head of a drum.
In fig. 390. the several parts of the ear are shown divested of
the surrounding bony matter; and to render their arrangement
more distinct, they are exhibited upon an enlarged scale. The
concha, with the tube leading inwards from it marked a, terminates
at the inner end, as already stated, in the tense membrane of the
tympanum placed obliquely to the axis of the tube. The resem-
blance of this tube with the concha to the speaking or hearing
428
ACOUSTICS.
trumpet is evident, and the physical purposes which it fulfils are
obviously the same, being those of collecting and conducting the
Fig. 390.
>onorous undulations to the membrane of the tympanum, which
will vibrate sympathetically with them.
713. Tfce middle ear is a cavity surrounded by walls of bone,
which, however, are removed in Jig. 390 , to render visible its
internal structure. An opening corresponding to the membrane
of the tympanum is made in the external wall, and the external
part of the inner ear shown in the figure is part of its inner wall.
The inner and outer walls of this cavity are very close together ;
but the cavity measures, vertically as well as horizontally, about
half an inch, so that it may be regarded as resembling the
sounding board of a musical instrument, composed of two flat
surfaces, placed close and nearly parallel to each other, the super-
ficial extent of which is considerable compared with their distance
asunder.
THE MIDDLE EAR. 429
7 1 4. Eustachian tube. — This cavity is kept constantly filled
with air, which enters it through a tube b, called the eustachian
tube, which opens into the pharynx, forming part of the respiratory
passages behind the mouth. Without such a, means of keeping
the cavity supplied with air, having a pressure always equal to
that of the atmosphere, one or other of two injuries must ensue:
either the air in the cavity, having a temperature considerably
above that of the external air, would acquire a proportionally in-
creased pressure, which would either rupture the membrane of the
tympanum, or give it undue tension ; but if this did not take
place, the air confined in the cavity would be gradually absorbed
by its walls, and would consequently be rarified, in which case the
pressure of the external atmosphere, being greater than that of
the air in the cavity, would force the membrane of the tympanum
inward, and would ultimately rupture it. By means of the eusta-
chian tube, however, a permanent equilibrium is maintained be-
tween the air in the cavity and the external air, just as is the case
in a drum, or in the sounding board of a musical instrument,
where apertures are always provided to form a free communication
with the external air.
The middle ear is sometimes called the tympanum or drum,
but sometimes these terms are applied to what we have above
called the membrane of the tympanum, and in that case the cavity
included between the walls of the middle ear is called the tym-
panic cavity. .
715. Fenestrce ovalis and rotunda. — In the inner wall of
this cavity there are two principal foramina, a greater and a
lesser ; the former being called, from its oval shape, the fenestra
ovalis, and the latter the fenestra rotunda ; the former is shown at
/, in Jig. 390., and the latter at o. Over both of these elastic
membranes are tightly stretched, as the membrane of the tym-
panum is over the inner end of the external meatus.
716. Auricular bones. — Between the membrane of the tym-
panum and the membrane of the fenestra ovalis there is a chain,
consisting of three small bones articulated together, and moved
by muscles having their origin in the bones which form the walls
of the cavity. These three bones are shown in Jig. 390., at d, e,
and/. The first d is called, from its form, the malleus, or hammer ;
the end of its handle is attached to the membrane of the tympanum
near its centre ; its head, which is round, is inserted in a corre-
sponding cavity of the second bone e, called the incus, or anvil ;
and the smaller end projecting from this, articulated with the
third bone/, called the stapes, or stirrup, from the obvious ana-
logy of its form. The base of this stirrup corresponds in magni-
tude and form with the fenestra ovalis, in which it is inserted,
.130 ACOUSTICS.
keeping, as it would appear, the membrane which covers that
aperture in a certain state of tension upon it. The handle or
the malleus being firmly attached to the centre of the membrane
of the tympanum, draws that membrane inwards, so as to render
it more or less convex, or rather conical, towards the tympanic
cavity.
The muscles which act upon these small bones are supposed to
have the property of giving greater or less tension to the two
membranes which they connect, so as to render them more or less
sensitive to the sonorous undulations propagated through the
external ear. When the sounds are loud the muscles render the
membranes less sensitive, and when they are low they render them
more so. According to this supposition, when we listen attentively
to low sounds, we not only concentrate the attention of the mind
upon them, but we also act upon the nerves which govern the
muscles inserted in the chain of auricular bones, and thereby in-
crease the sensitiveness of the organ.
It must be observed, however, that this is a mere hypothesis, no
such action of these bones and muscles having been established as
a matter of fact.
717. The use of the auricular bones is supposed to be the trans-
mission of the pulsations imparted by the sonorous undulations
from the membrane of the tympanum to the membrane of the
fenestra ovalis. It has been ascertained, however, that if the
membrane of the tympanum were altogether destroyed, the sense
of hearing would still remain, though it would not be so perfect.
It must therefore be inferred that the auricular bones are not the
only means of transmitting the sonorous undulations to the in-
ternal ear, the air contained in the middle ear being itself sufficient
for that purpose.
It cannot be doubted that the membrane which covers the
fenestra rotunda has some share in producing the sensation of
sound ; and if so, the chain of bones can have no effect upon it,
the undulations being merely propagated to it by the air contained
in the middle ear.
718. Tlie internal ear. — We now come to consider the in-
ternal ear, which is, in fact, the true and only organ of the sense
of audition, the external and middle ears being merely accessories
by which the sonorous undulations are propagated to the fluids
included in the cavities of the internal ear.
The internal ear is a most curious and, as it must be acknow-
ledged, a most unintelligible organ, also called, from its compli-
cated structure, the labyrinth. Its channels and cavities are
curved and excavated in the hardest mass of bone found in the
whole body, called the petrous or bony part of the skull. It is
THE INTERNAL EAR. 431
shown \n fig. 390., as if all the surrounding mass of bone except
that which forms the immediate surfaces of the cavities were cut
away.
719. Vestibule. — It will be seen that this labyrinth consists
of three distinct parts : a middle chamber, called the vestibule, in
the exterior wall of which the fenestra ovalisyis formed, and into
the internal wall of which the auditory nerve n is admitted.
720. Semicircular canals. — At the posterior and upper part
of the vestibule are three curved tubular cavities, called the
semicircular canals, and distinguished by anatomists as the an-
terior, posterior, and superior semicircular canals, according to
their relative positions.
721. Cochlea. — On the interior and anterior side of the vesti-
bule, near the fenestra rotunda, is a cavity formed like a spiral
tube, called, from its resemblance to the cavity within the shell of
a snail, the cochlea, the Latin word for that animal. The semi-
circular canals, and the cochlea, have severally free communication
with the vestibule.
722. The auditory nerve. — The auditory nerve arrives at the
bony wall of the internal ear, through a passage called by ana-
tomists the internal auditory meatus. Before entering the fora-
mina provided for its admission into the internal ear, it separates
into two principal branches, one of which is directed to the vesti •
bule and the other to the cochlea, which are thence called
respectively, the vestibular and cochlear nerves.
723. The membranous canals. — Within the three semicir-
cular canals are included flexible membranous pipes of the same
form, called the membranous canals. These pipes include within
them the branches of the auditory nerve, which pass through the
semicircular canals, and they are distended by a specific liquid
called endolymph in which the nervous fibres are bathed. The
bony canals around these membranous canals are filled with another
liquid called perilymph, which also fills the cavities of the vestibule
and the cochlea. It appears, therefore, that all the cavities of the
internal ear are filled with liquid, and it must, accordingly, be by
this liquid that the sonorous undulations are propagated to the
fibres of the auditory nerves. The liquid being incompressible,
the pulsations imparted either by the auricular chain of bones, or
by the air included in the cavity of the middle ear, or by both of
these, to the membranes which cover the fenestra ovalis and the
fenestra rotunda, are received by the liquid perilymph within
these membranes, and propagated by it and the endolymph to the
various fibres of the auditory nerve.
This arrangement will be rendered more clearly intelligible by
reference to fig. 391., which is a perspective magnified view of the
432
ACOUSTICS.
labyrinth, — the canals, vestibule, and cochlea bein^ laid open so
as to display their interior.
Fig. 391.
724. The lamina spiralis. — The spiral tube of which the
cochlea is formed makes 2*- revolutions round its geometrical axis,
and it is everywhere divided through its centre by a thin plate
called the lamina spiralis, upon the surface of which the fibres of
the cochlear nerve are spread. The internal structure of the
cochlea will be rendered more intelligible by reference to Jig- 392.,
where I represents the central bone round which the spiral winds,
and 2 the lamina spiralis, which follows the course of the spiral
canal.
A section of the cochlea made by a plane passing through its
axis, showing the course and distribution of the nervous fibres, is
given in Jig. 393., where I is the principal auditory nerve, 2 the
nerves in the lamina spiralis, 3 the central nerve of the cochlea,
and 4 the vestibular nerve.
To render still more apparent the distribution of the cochlear
branch of the nerve upon the lamina spiralis, a perspective view
TEE LAMINA SP1RALIS.
433
of this lamina with the nervous fibres spread upon it, divested of
the surrounding part of the cochlea, is given in fig. 394.*
Tig- 39* • Fi6' 393-
The form and magnitude of the external ears of many species
of animals is more favourable for auscultation than the human ear.
Fig. 394
It will be evident, for example, that all ears formed like those of
the horse are better adapted for the collection of the sonorous un-
dulations.
* This figure is reproduced by permission of the author and publisher
from the original, made from a preparation by Professor Sappey, of Paris,
and published in his Descriptive Anatomy.
? a
434 ACOUSTICS.
725. THeory of tbe tympanum. — The physical theory of
the tympanum, though much better understood than that of the
internal parts of the organ, is still but imperfectly comprehended
It is evident that one at least of its purposes is to propagate the
sonorous undulations of the external air to the membranes of the
internal ear ; and it is probable that it may also have some effect.
not yet fully understood, in modifying the force of the vibrations'.
It has been demonstrated by Savart that a membrane tightly
extended over an opening, as parchment is on a tambourine or
drum head, will be thrown into vibration by a sound produced
near it. If fine sand be sprinkled upon a drum head, it will be
agitated and thrown into various forms by a sound produced near
it, the particles jumping upwards as if they were repelled by the
parchment. But no such effect will be produced if a piece of
card or board be laid upon the same opening, unless a sound of
extreme loudness be produced.
It will also be found that the susceptibility of such a membrane
to enter into vibration will vary according to its tension. It may,
therefore, be inferred that the membrane of the tympanum will
be thrown into vibrations by the sonorous pulsations of the ex-
ternal air. These vibrations will be imparted more or less to all
objects with which the tympanum is connected, and so much the
more so as these objects are more vibratory, and as the tympanum
itself is rendered more vibratory by its tension. Thus all the
masses of bone surrounding the middle ear, the labyrinth, and the
auditory nerve, will be thrown into vibration.
It is evident also that the membranes extended over the
fenestrae of the labyrinth, will be thrown into vibration by the
pulsations of the air included in the middle ear.
However useful the membrane of the tympanum and the auri-
cular bones, which are connected with the fenestra ovalis, may be,
they are not indispensable to the exercise of the sense of hearing.
When the membrane of the tympanum has been ruptured, the
air included in the middle ear communicating freely with the
external ear, the pulsations of the external air are propagated to
the membranes of the labyrinth, without other modification than
such as they may receive from the concha and the auditory
canal.
But «even if the auditory canal were closed, the pulsations of*
the external air would be propagated with more or less effect
the air in the middle ear, through the pharynx and the eustachii
tube.
726. But of all parts of the organs of sense, that which h
most completely resisted all attempts at explanation upon physic
principles is the structure of the labyrinth. Why its complicat
THEORY OF THE TYMPANUM. 435
cavities should have the peculiar form and disposition given to
them has not been explained.
727. Organ of hearing- in birds. — Although the sense of
hearing may exist in the absence of some of these parts, its effi-
ciency will be impaired ; and we find accord-
ingly, as we descend in the scale of organisation,
that these parts disappear one by one in animals
which are less and less elevated in the series.
With birds, for example, the auricle is alto-
gether wanting, and the external ear is reduced
to the auditory meatus. The cochlea also loses
its spiral form, and the tapering tube is straight
instead of being coiled round a cone, and is
proportionally shorter than with superior ani-
mals, as will appear by the outline of the bony
labyrinth of the barn owl shown in jig. 395.,
where 2 is the vestibule, and 3 the cochlea divested of the spiral
form.
728. Reptiles. — In reptiles generally the external auditory
meatus is wanting, and the ear commences with the membrane of
the tympanum, which is its exterior part. The structure of the
tympanic cavity is also simplified.
729. Pishes. — In most species of fishes both the external and
middle ears are wanting, and the organ is reduced to the labyrinth,
which consists of a membranous vestibule surmounted by three
semicircular canals, having below it a little sack, which appears to
supply the place of the cochlea. The auricular apparatus is placed
in the lateral part of the great cavity of the skull.
730. Lower species. — In descending still lower in the scale of
organisation, all traces of the semicircular canals and the cochlea
are effaced, and the organ is reduced to a membranous vestibule,
consisting of a little sack filled with a liquid, in which the last
fibres of the acoustic nerve are diffused. Such a vestibule seems
to be an essential element of the ear, never being absent so long
as that organ has any existence.
731. Cochlear branch the true auditory nerve. — The ex-
perimental researches of M. Flourens have led to the conclusion that
the cochlear branch of the nerve is the only part which is abso-
lutely essential to the sense of hearing ; the parts which traverse
the semicircular canals, and are diffused through the vestibule,
being merely accessory. That eminent physiologist showed, by
a numerous course of experiments on mammifers and birds, that
the removal of the vestibular nerves, and those of the membranous
canals, never destroyed the sense of hearing; but that, on the
other hand, the removal of the cochlear branch invariably pro-
436 ACOUSTICS.
duced absolute deafness, even though the vestibular and other
branches of the nerve remained unimpaired.
It was inferred from these remarkable experiments that the
nervous cord, which passes into the internal ear from the internal
meatus, is not a single nerve, but consists of two, one of which
only, being that which passes into the cochlea, is the true auditory
nerve, and that the other branches have functions connected with
the movements of the body, which are detailed at considerable
length in M. Flourens's experiments.*
* " Recherches Experimentales sur les Proprietes et les Formations du
Systfeme Nerveux dans les Animaux Vertdbres," par M. P. Flonrens, ch.
xxvii. xxviii. xxix. Paris 1841.
INDEX.
NOTE.— This Index refers to the numbers of the paragraphs, and not to the pages.
A.
Aerial undulations, 634.
Aerial waves, force and velocity of, 636 ;
interference of, 637.
Agonic lines, 561.
Alphabet, telegraphic, 474.
Amalgamated zinc, advantages of using,
442.
Ami fire's apparatus for exhibiting the ef-
fects of the earth's magnetic on vertical
currents, 316; astatic current* lormed by
this apparatus, 319; shows the effect of
terrestrial magnetism on a helical current,
32i ; illustrates the dip of a current, 323 ;
apparatus for supporting movable cur-
rents, 229.
Ampere's method of exhibiting the revo-
lution of a current round a magnet, 247 ;
reotrope lor reversing the voltaic current,
225 ; theory of magnetism, 34S-349
Animal organism, development of elec-
tricity in the, 500.
Ani >n, 388.
Anode, 387.
Arago, researches of, 304.
Armstrong's hydro-electrical machine, 44.
Astatic needle, 597.
Atmosphere, a nonconductor, 18, 19.
Atmospheric agitation, effect of, on sound,
Attraction and repulsion of electrified
bodies, I ; how explained on the hyp«-
tluMs of two electric fluids, 8 ; laws of,
77> 9J> &c~
Attraction and repulsion of voltaic cur-
rents, 325, &c. ; of magnets. 516.
Augu.-t (Professor), his apparatus for ob-
serving the vibration of strings, 612.
Aurora borealis, influence of, 578.
Auroral light, experimental imitations of,
130-
Azimuth compass, 546, 547.
Babhage, researches of, 304.
bagration's battery, 182.
Bar magnets, best forms for, 581.
Barlow's compensator, 606.
Battery, electrical, 74, 75.
Battery, voltaic, Bagration's, 182 ; Bec-
querel's, 183; Bum-en's, 180; Cruik-
shank's, 189; Daniell's, 177; Grove's,
179; Grove's gas ditto, 174; Miinch's,
191 ; Wheatstone's, 181 ; Wollaston's,
190.
Becijuerel, his battery, 183; his researches,
408 ; repeats and confirms Davy's ex-
periments, 423 ; his observations, with
those of Breschet, 503.
Blot's experiments on the velocity of sound
in iron, 658.
Boreal and austral fluids, hypothesis of,
520.
Brush-discharge, 125.
Bunsen's voltaic battery, 180.
C.
Cascade, charging by, 73.
Cavendish, his electric barometer, 135.
Charcoal, method of applying its heat to
the fusion of refractory bodies, and the
decomposition of the alkalies, 488.
Chemical action of frictional electricity,
150, 151 ; development of electricity by,
102, 163 ; in voltaic cell, relation of, to
decomposing power of the current, 440,
441.
Chemical theory of voltaic action, 166-175.
Children's great plate battery, 198.
Chladni, his experiments on the conduction
of sound by solid bodies, 659.
Circulating currents, 248.
Clarke's magneto-electric machines, 297.
Classification of bodies according to their
electromotive property, 161 ; of positive
and negative substances, 10.
Cleavage, electricity devel->ped by, 156.
Collecting and condensing plates, 55
Common electricity, inductive action of,
produces polarity, 277.
Compass, azimuth, 546, 547.
('on pi-nsator* for ships' compasses, 605.
Condenser, electric, 51, 54 56; principle of
its action, 50.
Condensing electroscope, 64.
438
INDEX.
Conducting power for electricity, h<">w af-
fected by temperature, 24; of different i
metals, ij6 ; ho* measured, 377.
Conduction in liquids, 420, 421.
Conductors of e'.ectric machine, 39; of
voltaic battery, 194.
Conductors and non-conductors, 12, 23 ;
table of, 13.
Conductors, electric, imperfect ones rup-
tured by strong electric discharges, ici ;
discontinuous ones produce luminous
effects, 127, 136.
Constant batteries, 175, &c.
Contact hypothesis of Volta, 160.
Contact-breaker, use of, 294.
Cords and membranes, vibrations of, 6ll.
Coulomb's electroscope, 61 ; his investi-
gation of electric forces, 77.
CouronD* des lasses, 188.
Cro<se'8 researches, results of, 408.
Cruiksh.mk's arrangement -of the voltaic
pile, 189.
Currents, electrical. 164; their direction,
165 ; laws of their intensity, 217, &c. ;
reciprocal effects of rectilinear currents,
325 ; action of a spiral or helical current on
a rectilinear current. 326; mutual action
of diverging or converging rectilinear cur-
rents, 327 ; experimental illustration ofthe
same, 328; mutual action of rectilinear cur-
rents which are not in the same plane. 329;
mutual action of different parts of the
same current, 330 : action of an indefinite
rectilinear current on one finite and
rectilinear at right angles to it. 332; case
in which the indefinite current is circular,
333; experimental verification of these
principles, 334; way of determining in
general the action of an indefinite rpcti-
linear current on a finite rectilinear one,
335 ; experimental verification of the
same, 336 ; effect of a straight kidefinite
current on a system of diverging or con-
verging currents, 337; experimental il-
lustration ot this action, 338; consequences
deducible from this action, 339; action
of an indefinite straight current on a
circulating one, 340; case in which the
indefinite straight current is perpendicu-
lar to the plane of the circulating cur-
rent, 341 ; case in which the straight
current is oblique to the plane of the
circulating current, 342 ; reciprocal effects
of curvilinear currents, 343 ; their mutual
effects in general, 344
Currents, circular, 255, 333 ; curvilinear,
343 ; finite, 332 ; helical, 326 , indefinite,
332; molecular, 349 i thermo-electric,
Cuthbertson's condenser, 56 ; discharging
electrometer, 71.
D.
Dan Jell's battery, 177 ; chemical theory of,
781.
Davy's experiments, showing the transfer
of the constituents of electrolytes through
intermediate solutions, 415; his method
of preserving the copper sheath'ng of
ships, 434; his voltaic pile, 196; his dis-
covery of the compound nature of the
alkalis and earths, 429.
Declination, magnetic, 551 ; how measured,
559 ; local and periodic variations of, 560-
563
Deflagrator, Hare's, 199 : Stratingh's, ZOD.
Delarive's floating battery, 228, 255, 271,
318.
Del uc's pile, 204.
Density of electric currents, 400.
Diamagnetism, 361-367
Dip. magnetic, 553 ; local variations of, 556-
564.
Dipping needle, 548-
Dischargers and discharsing rod 3.47 49
Disruptive effects of electric discharge, 101-
103.
Dissimi lated electricity, 52.
Dry piles, 203
Duchenn-'s electro-voltaic apparatus, 491 ;
his magneto-electric apparatus, 492.
E.
Ear, the, its theory not understood, 707 :
description of 708; external, 709 ; con-
clia, 710 i external meatus, 711 ; mem-
brane of tympanum. 712 ; middle ea- .
713; eustaehian tube. 714; fenestra;
ov;ilis and rotunda, 7*5 ; auricular bones
716; internal ear, 718; vestibule, 719 ,
semicircular canals, 720: cochlea, 72';
auditory nerve, 722 ; membranous cana's,
723; la'mina spiralis, 724: limit of the
car's music «1 sensibility, 668, 674-676.
Earth ; why it is called the common rese*--
voir, 27; the analogy of. to a magnet, 545 ;
analysis of the magnetic phenomena of,
549; direction of its magnetic attraction,
305 ; effect of its magnetism on a vertical
current which turns round on a vertical
axis, 312-314; inductive force ofthe earth,
600.
Echoes, 699-702.
Elastic plate, 615 : strings, 613-614.
Elasticity of air, effects of. 657.
E'ectric barometer, Cavendish's, 135.
Electric battery, 74-75-
Electric fluid, sense in which this term is
to be understood, 4; hyrothesis of one
electric fluM, 5 ; hypothesis of two elec-
tric fluids, 6
Electric forces investigated by Coulomb,
Electric lamps of Messrs. Foucault, Deleuil,
and Dubsoc-Soleil, 487.
Electric light, 485 ; attempt to explain it,
— thermal hypothesis, 137; hypothesis of
decomposition and reromposition, 138:
above the barometric column, 134 ; strati-
.fication of, 3cxs.
Electric mortars, T2O.
Electric pistol, 116.
Electric shock explained, 140; secondary,
141 ; methods of limiting and regulating
it by a jar, 144.
Electric spark, 124 ; cracking noise at-
tending it, 139.
Electric telegraphs, common principle of
all, 466; conducting wires, 467; methods
for preserving and insulating them, 469 ;
testing posts, 470; telegraphic signs,
471 ; signs made with the needle system,
472-474 ; telegraphs operating by an elec-
tro-magnet, 473; Morse's system, 474;
electro-chemical telegraphs, 475.
Electrical bells. 104; blowpipe, 92; fishes,
501 j orrery, 91 ; see saw, no.
CONTENTS.
XVll
BOOI
Magn
CHAPTER I.
DEFINITIONS AND PRIMARY PHENOMENA.
fleet. Page
jog. Natural magnets— loadstone - Jiz
Jio. Artificial magnets ... it>.
;n. N'eutral line or equator - - ib.
512. Experimental illustration - - 313
;ij. The disiribution of the magnetic
C III.
stism,
Sect. Page
542. Compounds of iron are differently
susceptible of magnetism - - 325
543. Compounds of other magnetic
bodies are not susceptible . - ib.
544. Consecutive points ... ib.
CHAP. III.
TERRESTRIAL MAGNETISM.
545. Analogy of the earth to a magnet 326
540. The azimuth compass ... 327
547. The azimuth compass used at sea 329
548. The dipping needle - - - 330
549. Analysis of magnetic phenomena
of the earth- - - - - 371
550. The magnetic meridian - - 332
551. The declination or variation - ib.
552. Magnetic polarity of the earth - ib.
553. Variation of the dip ... it).
Complete analogy of the earth to a
',14. The variation of magnetic force ib.
,•15. Curve of varying intensity - - 314
<;i6. Magnetic attraction and repulsion 315
\i"j. Like poles repel, and unlike at-
518. Experimental illustrations - - .ib.
519. Magnets arrange themselves mu-
tually parallel with poles re-
Magnetic axis - - - - 317
Hrw ascertained experimentally ib.
;20. Hypothesis of two fluids, boreal
and austral ----- 318
;n. Natural or unmagnetised state - ib.
512.. Magnetised state - - - - ib.
523. Coercive force .... 319
524. Magnetic substances ... if,.
CHAP. II.
MAGNETISM BY INDUCTION.
525. Soft iron rendered temporarily
554. The magnetic equator ... it,.
555. Its form and position not regular ib.
556. Variation of the dip going north
557. The lines of equal dip - - - »&.
558. Magnetic meridians - - - ib.
559. Method of ascertaining the declina-
tion of the needles ... ib.
560. Local declinations- - - - 335
562. Variation of declination - - !>.
565. Isogonic lines - ... 356
526. This may be effected by proximity
527. Experimental illustration - - 321
565. The position of the magnetic
529. Magnets with poles reversed neu-
tralise each other - - ib.
$30. A magnet broken at its equator
produces two magnets - - 32Z
S3i. Decomposition of magnetic fluid
is not attended by its transfer
between pole and pole - - ib.
532. The decomposition is, therefore,
566. The magnetic poles are not, there-
fore, antipodal - - - - ?37
567. Periodical variations of terrestrial
magnetism - - - - - ib.
568. Table of declinations observed at
569. The intensity of terrestrial mag-
571. Isodynamic lines - - - - ib.
c~>t Their near coincidence with iso-
thermal lines .... ib.
573. Equatorial and polar intensities - 339
574. Effect of the terrestrial magnetism
on soft iron ----- ib.
575. Its effect on steel bars - ib.
576. Diurnal variation of the needle - 340
577. Disturbances in the magnetic
533. The coercive forre of iron varies
with its molecular structure 4 - 323
534. Effect of induction on hard iron
535. Forms of magnetic needles and
536. Compound magnets ... if,.
537. Effects of heat on magnetism - ib.
538. A red heat destroys the magnetism
of iron - - - - - ib.
539. Different magnetic bodies losetheir
magnetism at different tempera,
tures --.--- 325
540. Heat opposed to induction - - ib.
541. Induced magnetism may be ren-
d«-red permanent by hammering
and other mechanical eff. cts - ib.
578. Influence of aurora borealis - ib.
CHAP. IV.
MAGNETISATION.
<-q. Magnetisation - - - - 341
s.-so. Artificial magnets .... ib.
XV111
CONTENTS.
Sect. Page
581. Best form for bar magnets - - J4Z
581. Horse shoe magnets - ib.
583. The methods of producing arti-
ficial magnets by friction - - ib.
584. Method of single touch ... ib.
585. Method of double touch - -343
586. Inapplicable to compass needles
and long bars - ... 344
587. Magnetic saturation ... ib.
588. Limit of magnetic force - - ib.
589. Influence of the temper of the bar
on the coercive force - 345
590. Effects of terrestrial magnetism on
bars ib.
591. Means of preserving magnetic
bars from these effects - - 346
Sect. I'age
591. Magnetism may be preserved by
terrestrial induction ... •}*$
593. Compound magnets - . . ,y,.
595. Magnetised tracings on a steel
596.
plate
. The influence of heat upon mag-
netism
597. Astatic needle .... ,//.
598 The law of magnetic attraction 34(5
599. The bnlance of t rsion - - -350
600 The inductive force of the earth 351
6or Experimental illustration . - \fi
603. The temporary magnetism be-
comes permanent ... #.
605. Compensators for ships' com-
passes - - - - - - 353
6c6. Barlow's compensator - 354
BOOK IV.
Acoustics
CHAPTER I.
THEORY OF UNDULATIONS.
Sect. Page
607. A vast mass of discoveries - - 356
608. Undulations in general - - it>.
609. Formation of a wave - ib.
610. Waves, progressive and stationary 357
611. Vibrations of cords and mem-
branes ---.-.- 359
6iZ. Apparatus of August - ib.
613. Elastic strings , - - 360
614. Their laws - - - - - 361
615. Elastic plate . 36z
616. Elastic wires - - - - - 363
617. Nodal points - - - - - ib.
618. Nodal lines ----- 364
619. Undulation of liquids— Circular
waves - - - - - ib.
610. Apparent progressive motion of
waves an il u-ion ... ih.
621. Stationary waves - 366
6zz. Depth of waves - 368
613 Reflection of waves ... ih.
62,4. Law of reflection - 370
615. Waves propagated from the foci
of an ell p*e - - - 371
6z6. .Waves propagated from the focus
of a parabola ... - 371
617. Experimental illustration - - 374
618 Interference - - - - - ib.
6zg. Experimental illustration - - 375
f.;c. Inflection of waves - ib.
63--. Undulation of air and gases - 377
633. Propagation of wave through an
e astic fluid - - - - 378
634. Aerial undulations ... 379
635. Waves condensed and rarefied - 380
r^6. Velocity and force of aerial waves ib.
637. Interference of aerial waves - - 381
CHAP. II.
PRODVCTIO.V AND PROPAGATION OF
Sect. rase
638. Sound --...- 381
640. Sound progressive - - - 383
641. Breadth of sonorous waves - -384
64z. Distinction between musical
sounds and ordinary sounds - ib.
643. Pitch - 385
644. Loudness - - - - - ib.
645. Quality ------ ib.
646. In the" same medium, all sounds
have the same velocity - - ih.
647. Velocity 386
648. Distance measured by sound • ib.
649. Allgasesandvapoursconduct sound 387
651. Effect of atmospheric agi.ation on
sound ------ ib.
653. Sounds which destroy each other 388
654. Experimental illustration - 389
655 Examples - - ib.
656. Velocity of sound in different
media" - .... 390
I 657. Effects of elasticity of air - - ib,
I 658. Biot's experiment - - 391
659. Chladui's experiments - - ib.
660. Louduess dependent on distance - ib.
&/7JV CHAP. III.
PHYSICAL THKOKY OF MUSIC.
6Yn. The monochord - - - - 39Z
66z. Its application to determine the
rates of vibration of musical
notes ----_- 393
CONTENTS.
Sect. Page
66}. A double rate of vibration pro-
duces an octave - - - 393
664. Rates of vibration for other in-
tervals ... - - 394
665. PhyMcal cause of harmony - - 395
666. Physical cause of the harmonics
of the harp or violin - 396
667. Experimental veritication by
Sauveiir - 397
668. Limit of musical sensibility of the
car - - - - - - ib.
Sensibility of practised organists 398
669. Methods of determining the abso-
lute number of vibrations produ-
cing musical notes - - - 399
670. The Sirene zb.
Experiments - 400
671. Savart's apparatus - 401
671. The absolute rates of vibration of
musical notes ascertained - 401
673. Tuning fork - - - - 403
674. Kange of musical sensibility of the
ear ...-.- 404
677. Length of the waves correspond-
ing to musical notes - - - 405
678. Application of the Sitene to count
the rate at which the wings of
insects move - ib.
CHAP. IV.
VIBRATIONS OP RODS AND PLATES.
679. Vibration of rods - ... 406
68 1. Marloye's harp .... 407
68i. Nodal points - ib.
686 Lateral vibrations of rods or
plates ----- 410
689. Curious forms of the nodal lines - 411
CHAP. V.
VIBRATIONS OF FLUIDS.
Sect. Page
690. Fluids 412
Sounds produced by communica-
tion ...... 413
691. Wind instruments ... 414
694. Organ pipes - - - - 415
695 Heed pipes ----- 418
699. Echoes ----- 42,1
702. Remarkable cases of multiplied
echoes - - 424
CHAP. VI.
707. Theory of the organ not under-
stood >- -
708 Description of the ear
709. The external ear -
710. Concha -
711. External meatus -
7iz. Membrane of tympanum
713. The middle ear -
714. Eustachian tube -
715. Fenestroe ovalis and rotunda
716. Auricular bones - - - -
718. The internal ear -
719. Vestibule -
720. Semicircular canals -
711. Cochlea -
722. The auditory nerve -
723. The membranous canals
724. The lamina spiralis - - -
725. Theory of the tympanum
727. Organ'of hearing in birds
728. Reptiles -
729. Fishes ......
730 Lower species -
731. Cochlear branch the true auditory
nerve .
ib.
4*8
429
ib.
ib.
430
ib.
ib.
ib.
432
434
4^
ib.
ib.
ib.
INDEX.
439
Electrical machines, their different parts,
37-39 : '''"""ion cylindrical. 40; N.iii e'»,
41 ; common plat'1, known as V.ui Ma-
nun's, 42 ; Kamsden's plate, 45 ; Arm-
strong's hydro-electrical, 44
Electricity, ctymnlogv of the word, i ; po.
sitive and negative, 2 ; its nature, 3 ; de-
veloped by various bodies when submitted
to friction, a; both kinds alwavs pro-
duced simultaneously, io« : method < f
producing it by glass and silk with amal-
gam, II ; passes hy preference on the
best conductors. 28; action of, at a dis-
tance, 29 , dissimulated or latent, 52 ;
free, 53 ; distribution of, on conductors,
80 87 ; mechani) al effects of, 93 ; current
of, passim: over a conductor raises its
temperature, in ; effect of, on fulminat-
ing silver, 115; velocity of, 230; its
th-rapeutic agency, 490.
Electrics and BOD -electrics, 16.
Electrified body, its action on a noncon-
ductor not electrified, 94 ; its action on a
nonconductor charged w-th like elec-
tricity, 95 ; on a nonconductor charged
with opposite electricity, 96; on a con-
ductor not electrified, 97 j on a conductor
charged with like electricity, 98; upon a
conductor charged with opposite elec-
tricity, 99.
V.ltctro-chemical series, 171.
Electro-chemical t»'lecraphs, 475.
l-'lectro-chemistry, 383, c\c.
Elect ro-cbemioal theory, phenomena
which supply its basis, "150; hypothesis,
391.
Electrodes, 227 ; positive and negative,
387 ; negative, secondary action of hy-
drogen ar, 396; of zinc and platinum, in
water, 398; supposed inequality in their
(it-composing power, 424-42.6 ; liquid, 4:7;
polarisation of, 438; reverse currents
due to polarisation of, 439 ; negative, any
body i ay be u-ed for, 445 ; soluble post-
live use of, 446.
Electrolysis, liquids alone susceptible of,
385 ; method of, which separates the
constituents of water, 392; secondary
effects of, 396, 400, 407, 408.
fc/lectrolytes, series ol, in immediate con-
tact, 427; which have compound con-
stituents, 405.
Electrolytic classification of simple bodies,
401-404.
Electro-magnets, formation of powerful,
2<Jl ; conditions which determine their
force, 282 ; of the Faculty of Sciences at
Paris, 283 ; their form in general, 284.
Electro-magnetic pow*>r applied as a sono-
meter, 488 ; as a mechanical agent in M.
Froment's workshop, 286.
Electro-magnetism, 232.
Electrometer, Lane's discharging, 70;
Cuthb^rtson's discharging, 71 , Harris's
circular, 74.
Electro-metallnrgfc apparatus, Spenser's,
462 ; Fau's, 463 : Brandely's, 464.
Electro-metallurgy, origin of, 443.
Electro-motive force, 161.
Electro-motive series, 161.
Electro-negat ve bodies, 402.
Electrophorus, 57.
Electro-positive bodies, 403.
Electroscopes, 58; pith ball, 59; needle,
60; Coulomb's, 6i; gold leaf, 63 ; con-
den.-ing, 64.
Equator, magnetic, 554, 555.
F.
Faraday, his experiments on the superficial
distribution of electricity, 83 ; on the
chemical effects of frictio'nal electricity,
151 ; on electricity produced by friction
as compared with that produced by
chemical action, 161 ; on magneto-in-
duction in revolving discs, 304 ; his dis-
covery of phofomagnetism, 356 ; of dia-
magnetism, 361; his voltameter, 411;
law of electrolysis, 412 ; experiments on
the retardation of tlie current in sub-
marine telegraph lines, 475«.
Favre on the sources ot the heat produced
by the current, 478.
Fishes, electric organ of, 505.
Fluids, vibrations of, 690.
Froinent, M., electro-motive machines con-
structed by him, 287.
Fundamental and harmonic tones, 666; of
organ-pipes, 694.
G.
Galvani, his discoveries, 158 ; his theory,
Galvanic (battery, current, &c.), see Voltaic
(battery, current,&c.).
Galvanism, discovery of, 158.
Galvanometer, Pouillet's tangent, 355; see
also Reomeler.
Gas-battery, Grove's, 174.
Gas>iot, his experiments of the spark pro-
duced at the moment of closing the voltaic
circuit, 484.
Glyphography, 458.
Gold leaf electroscope, 63.
Grotthus on the electrolysis of water, 394.
Grove's battery, 179.
Gunpowder exploded by electricity, 119.
Gymnotus electricus — manner of capturing
them— their electric organs, 508.
H.
Hare's deflagrator, 199.
Harmonic tones of harp or violin, 666 ; of
open and stopped organ-pipes, 694.
Harmony, physical cause of, 665.
Harris's circular electrometer, 72.
He iring trumpet, 706.
Heat, developed by frictional electricity,
1II-I2I ; by voltaic electricity, 476 481 ;
elecf icity produced by, 157, 368, &c.
Heat, effects of, on magnetism, 537, 596
Kent, opposed to induction, 540.
Helical currents, magnetic properties of,
268; their poles det'rmimd, 268; ex-
perimental illustrations of the same, 269-
adaptation of, to Ampdre'sandDela'rive's
apparatus, 471 ; their action on a mag-
netic needle, 474 ; magnetic induction of,
474 ; polarity produced by them, 275.
Helical pile of the Faculty of Sciences at
Paris, 192.
Helices, right and left handed, 266.
HerschePs researches, 304.
440
INDEX.
Horse-shoe magnets, 582.
Hydrogen, sounds producible by burning
jet of, 698.
I.
Induction, electro-static, 29—36 ; electro-
dynamic, by currents, 289-290; by mag-
nets, 291-293.
Inductive action, sudden effects of, 35.
Inductive effects of the successive convolu-
tions of the same helix, 303.
Inductive shoe k of the human bod}-, 35.
Insulating stools, 15, 46.
Insulators, 14, 22.
Intensity of electric currents, 218, &c.,
Interference of undulations, 637 ; of sound,
653.
Ions, 388.
Iron, method of rendering it passive, 4jz ;
its coercive force varies with its molecu-
lar structure, 533 ; its magnetism de-
stroyed by red heat, 538 ; effect of in-
duction on, 534 : compounds of, dif-
ferently susceptible of magnetism, 542.
Isoclinic lines. 563.
Isodynamic lines, 571 ; their near coinci-
dence with isothermal lines, 572.
Isogonic lines, 563.
J.
Jacobi's experiments on conduction by
water, 482.
Jar, Leyden. 67 ; principle of its action, 65-
66; position of the charge in, 68; im-
proved form of, 69 ; charged by cascade,
Joufe, laws of the development of heat by
the current discovered by, 476.
K.
Kathode, 387.
Ration, 388.
Kinnersley's thermometer, 121.
Lane's discharging electrometer, 70.
Leyden jar, 65-69.
Lichtenberg's figures, 132.
Light, conditions under which it is pro-
: duced by an electric current, 123 ; elec-
tric, 485.
Liquids, voltaic a tion between, 173 ; essen-
tial to the production of permanent cur-
rents, 172, 175; electric conduction in,
420, 421.
Liquids, undulation of, 619.
Local circuits, 442
Loudness of sound, 644; how affected by
distance, 660.
M.
Magnet, action of rectilinear currents on,
231-248; rotation of, round a current,
242-245 ; action of circulating currents
on, 251, &c.
Magnetic attraction, direction of the earth's,
305.
Magnetic attraction and repulsion, law of,
598.
Magnetic bars, method of preserving them,
591.
Magnetic bodies, different ones lose their
magnetism at different temperatures,
539-
Magnetic fluid, decomposition of, not at-
tended by its transfer between pole and
' pole, 531.
Magnetic induction, momentary current by,
289-291.
Magnetic intensity, disturbances in, 577.
Magnetic meiidians 558 ; needles, action of
electric discharge upon, 152-154; method
of ascertaining the declination of. 559;
table of their declinations in differenl
longitudes, 562.
Magnetic poles, 511 ; of the earth, 565, 566.
Magnetic poles, force exerted by a recti-
linear current upon, 237.
Magnetic saturation, 587.
Magnetisation, 579.
Magnetism, its effect on vertical and cir-
cular currents, shown by Ampere's ap-
paratus, 316310; Ampere's theory of,
345*349 » magnetism, induced. may be ren-
dered permanent l>y hammering, kc , 541;
periodical variations < f terrestrial mag-
netism, 567; effect of terrestrial magnet-
ism on sott iron, 574; on hard iron or
steel bars, 575, 590.
Magneto-electric apparatus, 491; medical
use of, 296.
Magnets, natural, 509: artificial, 510; ar-
range themselves mutually parallel with
poles reversed, 519; with poles reversed
neutralise ea< h other, 519 ; one broken at
equator produce two magnets, 530;
compound, 536; artificial methods of
producing them, 580-585; compound,
59*
Ma iners' compass, 547.
Matteucci's apparatus for exhibiting cur-
rents produced by induction, 298.
Melloni's thermo-electric pile, 382.
Meridian, magnetic. 550. 558.
Metals, the series of new, 430 ; ignition of,
by electricity, 114 ; have different thermo-
electric energies, 373 ; conducting powers
of, 376.
Metallising textile fabrics 457.
Monochord, 661 ; its application to deter-
mine the rates of vibrations of musical
notes, 662.
Morse's system of telegraphs, 474.
Miinch's voltaic battery, 191.
Muscular current, 500.
Musical notes, relative numbers of vibra-
tions producing them, 663, 664; wave-
lengths corresponding to, 677.
Musical sounds denned, 642.
N.
Nairne's cylinder electrical machine, 41.
Napoleon's voltaic pile, 197.
Needle, conditions on which it
netised positively and negatively, 278.
INDEX.
441
Nervous current, 500.
Neutral line or equator (in magnets), 511.
Nohili's reometer, 355 ; his thermo-electric
pile, 38z.
Nodal lines, 618, 683; curious forms of,
685
Nod.il points, 617, 68z; in organ pipes,
Nollet and Watson (Dr.), their experi-
ments, 149.
O.
Ohm's law, ..
Organs, rf mai kable, 696.
Organ pipes, 694, 695.
Oxygen, peculiar properties of electrolytic,
4?S-
Ozone, 435-437-
P.
Phosphorescent effect of electric spark,
I]Ii
Photomagnetic phenomena, 357-359.
Photomagi'etism an i diamagnetism, 356.
Pile, voltaic, invention of, 184; general
principle of. 185 ; earliest form of, 187.
Piles, dry, zoj ; Deluc's, ZO4 ; Kilter's
secondary, zo8, 459 ; Zamboni's, zo5-
Pitch of musical smncis, 642, 643; varia-
tions of, &7Z ; ran^e of, employed in
miiMc, 676 ; of lowest and highest audible
no'es, 675. 676.
Pith l>;ills, explanation of effects produced
by them, 8; u^e of string which suspends
them, zo; curious effect of their repul-
sion, 106
PI ticker's dhmagnetic apparatus, 365.
Pnhl's reotrope, zz6.
Points, effects of, in facilitating the passage
• of electricity, 86, gin.
Polarisation of electiodes, 438. 439.
Polarisation of li^ht, rotation of plfine of,
caused by magnetic lorce, 359.
Poles, positive and negative, 186.
Positive and negative electricities, z, 5, 6 ;
circumstances which favour th>- develop-
ment of one or the other, 9 ; always pro-
duced together, lOrt.
Positive and negative ftibstances, 10.
Pouillet. his apparatus for exhibiting the
effects of the earth's magnetiMn on verti-
cal currents. 314 ; its application to show
the eff'Ct of terrestrial magnetism on a
horizont 'I current. 315 ; his galvanometer,
355« ; his thermo-electric apparatus. 374 ;
his observations on Faraday's doctrine,
that electrolytes are the only liquid non-
metallic conductors. 4zz.
Pressure, electricity produced by, 156.
Pulvermacher's galvanic chain, 493.
Pyro-electricity, 157.
Q.
Quadrant electrometer, 6z.
Quality of sounds, 641, 645.
R.
Reduced length of a roltaic circuit, 378.
Reed-pipes, 695.
Reometers, 350-353 ; differential, 354.
Reoscopes, 350 ; way of constructing them,
35*-
Rt-ostat, 377^.
Reotropes, 225-226
Ifesidual charge, 76^.
Resistance of conductors, 215,476; internal
and external, zzo.
Retardation of current in submarine tele-
graph wires, 475*1.
Ritter's second iry piles, 208, 439.
Rubber of electric machines, 33.
S.
Saturn, tree of, 433.
Savart's apparatus for the experimental
determination of thenumber of vibrations
corresponding to a note of any proposed
pitch, 671.
Savary's magnetical experiments, vjg.
Schcenhein, on the passivity of iron, 431.
Secondary piles, zo8.
Silurus electricus, the, 507.
Sirdne, the, 670; its application to count
the rate at which the wings of insects
move. 678.
Simple bodies, electrolytic classification of,
401.
Shock, electric, 140; secondary, 35, 141.
Smee's battery, 176.
Sound, 638 ; progressive. 640 ; musical and
ordinary, 64Z ; distance measured by it,
648 ; conducted bv all gases and vapours,
649; those which destroy each other,
653 ; velocity of, in air, 647; in different
media, 656.
Sources of electricity, 155-157.
Spark, electric, 124 ; its duration, 124/1 ; in
rarefied air, izg-no, 134-135.
Spark, voltaic, 484.
Speaking tubes, 704 ; trumpet, 705.
Spiral currents, 254, z6z-z6j. lot).
Stratham's apparatus for exploding
powder by induced currents, 3oz.
gun-
T.
Tangent-galvanometer. 355^.
Telegraph, electric, 466-475rt.
Telegraphic alphabet, 474.
Telegraphic signals, retardation of, in sub-
marine wires, 475«.
Thermo-electric current, conditions which
determine its direction, 371 ; relation be-
tween its intensity and the length and
section of the conducting wire, 375.
Thermo-electric piles, 38i-j8z.
Thermo-Hectricity, 157, 368, &c.
Timbre, 645.
Torpedo, properties of, 501-506.
Torsion, balance of, 599.
Transfer of constituents of electrolytes.
415. &c.
Tuning-fork, 673.
Tympanum, theory of, 7Z5.
U.
Undulations, in general, 608 ; of air and
gases, 631.
442
INDEX.
v.
Van Marum's common plate electrical
machine, 42.
Variation of the compass, 551.
\Vlocity of electric-ty, 230 ; of sound,
646 647, 656.
Vibration, double rate of, produces an
octave, 663 ; of musical notes, their
abs.iluf rates of, ascertained, 669; of
rods. 679.
Vital fluid, 159
Volta, his contact theory, 160 ; his funda-
mental experiment, 160; his invention of
the pile, 184 ; his first pile, 187 ; his
counmne des tasses, 188.
Voltaic batteries, various forms of, 176-183,
187-193.
Voltaic cell, analogy of, to an electrolytic
cell, 440-441.
Voltaic current, formation of, 164; direc-
tion of, 165 ; chemical changes ac-
companying its production, 166.
Voltaic currents, law of their intensity, 217
&c. ; sewing needles attractel by them,
273 ; their inductive effect upon a magnet,
273 ; they render soft iron magnetic, 273 ;
decomposing power of, 383 ; effe.-tof tne
same, on different electrolytes, 411; Fara-
day's law, 412; spark produced by them,
484 ; substances ignited and exploded by
th-m, 480.
Voltaic jeux de bague, 206.
Voltameter, 411 ; error introduced into its
indications by the formation of ozone.
437-
W.
Walsh, his observations on the torpedo,
502.
Water, a conductor, 21 ; composition of,
389; constituents of, how transferred to
the electrodes, 393 ; effect of aciii and
salt on the electrolysis of, 395 ; elec-
trolysis of, 390.
Waves, formation of, 609 ; progressive and
stationary, 610 ; ai parent progressive
motion of, an illusion, 620 ; depth of,
612; reflection of, 623 ; propagated from
the foci of an ellipse, 625; from the
focus of a parabola, 626 ; propagation of,
through an elastic fluid, 633 ; sonorous,
breadth of, 641.
Wheatstone's voltaic battery, 181 ; his
method of measuring the conducting
power of metal*. 377.
Whimpering galleries, 703.
Wind instruments, 691.
Well i-ton, his arrangement of the voltaic
pile, 190.
Zamboni's voltaic pile, 205.
Zinc, amalgamation of, 442.
LONDON: FEINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
UNIVERSITY OF CALIFORNIA LIBRARY,
BERKELEY
THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW
Books not returned on time are subject to a fine of
50c per volume after the third day overdue, increasing
to $1.00 per volume after the sixth day. Books not in
demand may be renewed if application is made before
expiration of loan period.
50m
YB 09872
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11