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(Elarcntron
A TREATISE
ON
ELECTRICITY AND MAGNETISM
MAXWELL
VOL. I.
Uonfron
MACMILLAN AND CO.
PUBLISHERS TO THE UNIVERSITY OF
Clarendon press Series
A TREATISE
ON
ELECTRICITY AND MAGNETISM
BY
JAMES CLERK MAXWELL, M.A
LLD. EDIN., P.E.SS. LONDON AND EDINBURGH
HONORARY FELLOW OF TRINITY COLLEGE,
AND PROFESSOR OF EXPERIMENTAL PHYSICS
IN THE UNIVERSITY OF CAMBRIDGE
VOL. I
AT THE CLARENDON PRESS
1873
[All rights reserved]
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PREFACE.
THE fact that certain bodies, after being rubbed,
appear to attract other bodies, was known to the
ancients. In modern times, a great variety of other
phenomena have been observed, and have been found
to be related to these phenomena of attraction. They
have been classed under the name of Electric phe
nomena, amber, ?i\eKTpov, having been the substance
in which they were first described.
Other bodies, particularly the loadstone, and pieces
of iron and steel which have been subjected to certain
processes, have also been long known to exhibit phe
nomena of action at a distance. These phenomena,
with others related to them, were found to differ from
the electric phenomena, and have been classed under
the name of Magnetic phenomena, the loadstone, /zayi^?,
being found in the Thessalian Magnesia.
These two classes of phenomena have since been
found to be related to each other, and the relations
between the various phenomena of both classes, so
far as they are known, constitute the science of Elec-
tromagnetism.
In the following Treatise I propose to describe the
812245
Ti PREFACE.
most important of these phenomena, to shew how they
may be subjected to measurement, and to trace the
mathematical connexions of the quantities measured.
Having thus obtained the data for a mathematical
theory of electromagnetism, and having shewn how
this theory may be applied to the calculation of phe
nomena, I shall endeavour to place in as clear a light
as I can the relations between the mathematical form
of this theory and that of the fundamental science of
Dynamics, in order that we may be in some degree
prepared to determine the kind of dynamical pheno
mena among which we are to look for illustrations or
explanations of the electromagnetic phenomena.
In describing the phenomena, I shall select those
which most clearly illustrate the fundamental ideas of
the theory, omitting others, or reserving them till the
reader is more advanced.
The most important aspect of any phenomenon from
a mathematical point of view is that of a measurable
quantity. I shall therefore consider electrical pheno
mena chiefly with a view to their measurement, de
scribing the methods of measurement, and defining
the standards on which they depend.
In the application of mathematics to the calculation
of electrical quantities, I shall endeavour in the first
place to deduce the most general conclusions from the
data at our disposal, and in the next place to apply
the results to the simplest cases that can be chosen.
I shall avoid, as much as I can, those questions which,
though they have elicited the skill of mathematicians,
have not enlarged our knowledge of science.
PREFACE. vii
The internal relations of the different branches of
the science which we have to study are more numerous
and complex than those of any other science hitherto
developed. Its external relations, on the one hand to
dynamics, and on the other to heat, light, chemical
action, and the constitution of bodies, seem to indicate
the special importance of electrical science as an aid
to the interpretation of nature.
It appears to me, therefore, that the study of elec-
tromagnetism in all its extent has now become of the
first importance as a means of promoting the progress
of science.
The mathematical laws of the different classes of
phenomena have been to a great extent satisfactorily
made out.
The connexions between the different classes of phe
nomena have also been investigated, and the proba
bility of the rigorous exactness of the experimental
laws has been greatly strengthened by a more extended
knowledge of their relations to each other.
Finally, some progress has been made in the re
duction of electromagnet-ism to a dynamical science,
by shewing that no electromagnetic phenomenon is
contradictory to the supposition that it depends on
purely dynamical action.
What has been hitherto done, however, has by no
means exhausted the field of electrical research. It
has rather opened up that field, by pointing out sub
jects of enquiry, and furnishing us with means of
investigation.
It is hardly necessary to enlarge upon the beneficial
viii PREFACE.
results of magnetic research on navigation, and the
importance of a knowledge of the true direction of
the compass, and of the effect of the iron in a ship.
But the labours of those who have endeavoured to
render navigation more secure by means of magnetic
observations have at the same time greatly advanced
the progress of pure science.
Gauss, as a member of the German Magnetic Union,
brought his powerful intellect to bear on the theory
of magnetism, and on the methods of observing it,
and he not only added greatly to our knowledge of
the theory of attractions, but reconstructed the whole
of magnetic science as regards the instruments used,
the methods of observation, and the calculation of the
results, so that his memoirs on Terrestrial Magnetism
may be taken as models of physical research by all
those who are engaged in the measurement of any
of the forces in nature.
The important applications of electromagnetism to
telegraphy have also reacted on pure science by giving
a commercial value to accurate electrical measure
ments, and by affording to electricians the use of
apparatus on a scale which greatly transcends that
of any ordinary laboratory. The consequences of this
demand for electrical knowledge, and of these experi
mental opportunities for acquiring it, have been already
very great, both in stimulating the energies of ad
vanced electricians, and in diffusing among practical
men a degree of accurate knowledge which is likely
to conduce to the general scientific progress of the
whole engineering profession.
PREFACE. ix
There are several treatises in which electrical and
magnetic phenomena are described in a popular way.
These, however, are not what is wanted by those w r ho
have been brought face to face with quantities to be
measured, and whose minds do not rest satisfied with
lecture-room experiments.
There is also a considerable mass of mathematical
memoirs which are of great importance in electrical
science, but they lie concealed in the bulky Trans
actions of learned societies ; they do not form a con
nected system ; they are of very unequal merit, and
they are for the most part beyond the comprehension
of any but professed mathematicians.
I have therefore thought that a treatise would be
useful which should have for its principal object to
take up the whole subject in a methodical manner,
and which should also indicate how each part of the
subject is brought within the reach of methods of
verification by actual measurement.
The general complexion of the treatise differs con
siderably from that of several excellent electrical
works, published, most of them, in Germany, and it
may appear that scant justice is done to the specu
lations of several eminent electricians and mathema
ticians. One reason of this is that before I began
the study of electricity I resolved to read no mathe
matics on the subject till I had first read through
Faraday s Experimental Researches on Electricity. I
was aware that there was supposed to be a difference
between Faraday s way of conceiving phenomena and
that of the mathematicians, so that neither he nor
x PREFACE.
they were satisfied with each other s language. I had
also the conviction that this discrepancy did not arise
from either party being wrong. I was first convinced
of this by Sir William Thomson *, to whose advice and
assistance, as well as to his published papers, I owe
most of what I have learned on the subject.
As I proceeded with the study of Faraday, I per
ceived that his method of conceiving the phenomena
was also a mathematical one, though not exhibited
in the conventional form of mathematical symbols. I
also found that these methods were capable of being
expressed in the ordinary mathematical forms, and
thus compared with those of the professed mathema
ticians.
For instance, Faraday, in his mind s eye, saw lines
of force traversing all space where the mathematicians
saw centres of force attracting at a distance : Faraday
saw a medium where they saw nothing but distance :
Faraday sought the seat of the phenomena in real
actions going on in the medium, they were satisfied
that they had found it in a power of action at a
distance impressed on the electric fluids.
When I had translated what I considered to be
Faraday s ideas into a mathematical form, I found
that in general the results of the two methods coin
cided, so that the same phenomena were accounted
for, and the same laws of action deduced by both
methods, but that Faraday s methods resembled those
* I take this opportunity of acknowledging my obligations to Sir
W. Thomson and to Professor Tait for many valuable suggestions made
during the printing of this work.
PREFACE. xi
in which we begin with the whole and arrive at the
parts by analysis, while the ordinary mathematical
methods were founded on the principle of beginning
with the parts and building up the whole by syn
thesis.
I also found that several of the most fertile methods
of research discovered by the mathematicians could be
expressed much better in terms of ideas derived from
Faraday than in their original form.
The whole theory, for instance, of the potential, con
sidered as a quantity which satisfies a certain partial
differential equation, belongs essentially to the method
which I have called that of Faraday. According to
the other method, the potential, if it is to be considered
at all, must be regarded as the result of a summa
tion of the electrified particles divided each by its dis
tance from a given point. Hence many of the mathe
matical discoveries of Laplace, Poisson, Green and
Gauss find their proper place in this treatise, and their
appropriate expression in terms of conceptions mainly
derived from Faraday.
Great progress has been made in electrical science,
chiefly in Germany, by cultivators of the theory of
action at a distance. The valuable electrical measure
ments of W. Weber are interpreted by him according
to this theory, and the electromagnetic speculation
which was originated by Gauss, and carried on by
Weber, Riemann, J. and C. Neumann, Lorenz, &c. is
founded on the theory of action at a distance, but
depending either directly on the relative velocity of the
particles, or on the gradual propagation of something,
xii PREFACE.
whether potential or force, from the one particle to
the other. The great success which these eminent
men have attained in the application of mathematics
to electrical phenomena gives, as is natural, addi
tional weight to their theoretical speculations, so that
those who, as students of electricity, turn to them as
the greatest authorities in mathematical electricity,
would probably imbibe, along with their mathematical
methods, their physical hypotheses.
These physical hypotheses, however, are entirely
alien from the way of looking at things which I
adopt, and one object which I have in view is that
some of those who wish to study electricity may, by
reading this treatise, come to see that there is another
way of treating the subject, which is no less fitted to
explain the phenomena, and which, though in some
parts it may appear less definite, corresponds, as I
think, more faithfully with our actual knowledge, both
in what it affirms and in what it leaves undecided.
In a philosophical point of view, moreover, it is
exceedingly important that two methods should be
compared, both of which have succeeded in explaining
the principal electromagnetic phenomena, and both of
which have attempted to explain the propagation of
light as an electromagnetic phenomenon, and have
actually calculated its velocity, while at the same time
the fundamental conceptions of what actually takes
place, as well as most of the secondary conceptions of
the quantities concerned, are radically different.
I have therefore taken the part of an advocate rather
than that of a judge, and have rather exemplified one
PREFACE. xiii
method than attempted to give an impartial description
of both. I have no doubt that the method which I
have called the German one will also find its sup
porters, and will be expounded with a skill worthy
of its ingenuity.
I have not attempted an exhaustive account of elec
trical phenomena, experiments, and apparatus. The
student who desires to read all that is known on these
subjects will find great assistance from the Traite
d Electricite of Professor A. de la Rive, and from several
German treatises, such as Wiedemann s Galvanismus,
Riess Reibiingselektricitat, Beer s Einleitung in die Elek-
trostatik, &c.
I have confined myself almost entirely to the ma
thematical treatment of the subject, but I would
recommend the student, after he has learned, experi
mentally if possible, what are the phenomena to be
observed, to read carefully Faraday s Experimental
Researches in Electricity. He will there find a strictly
contemporary historical account of some of the greatest
electrical discoveries and investigations, carried on in
an order and succession which could hardly have been
improved if the results had been known from the
first, and expressed in the language of a man who
devoted much of his attention to the methods of ac
curately describing scientific operations and their re
sults *.
It is of great advantage to the student of any
subject to read the original memoirs on that subject,
for science is always most completely assimilated when
* Life and Letters of Faraday, vol. i. p. 395.
xiv PREFACE.
it is in the nascent state, and in the case of Faraday s
Researches this is comparatively easy, as they are
published in a separate form, and may be read con
secutively. If by anything I have here written I
may assist any student in understanding Faraday s
modes of thought and expression, I shall regard it as
the accomplishment of one of my principal aims to
communicate to others the same delight which I have
found myself in reading Faraday s Researches.
The description of the phenomena, and the ele
mentary parts of the theory of each subject, will be
found in the earlier chapters of each of the four Parts
into which this treatise is divided. The student will
find in these chapters enough to give him an elementary
acquaintance with the whole science.
The remaining chapters of each Part are occupied
with the higher parts of the theory, the processes of
numerical calculation, and the instruments and methods
of experimental research.
The relations between electromagnetic phenomena
and those of radiation, the theory of molecular electric
currents, and the results of speculation on the nature
of action at a distance, are treated of in the last four
chapters of the second volume.
Feb. 1, 1873.
CONTENTS,
PRELIMINARY.
ON THE MEASUREMENT OF QUANTITIES.
Art. Page
1. The expression of a quantity consists of two factors, the nu
merical value, and the name of the concrete unit 1
2. Dimensions of derived units 1
3-5. The three fundamental units Length, Time and Mass .. 2, 3
6. Derived units 5
7. Physical continuity and discontinuity 6
8. Discontinuity of a function of more than one variable .. .. 7
9. Periodic and multiple functions 8
10. Relation of physical quantities to directions in space .. .. 8
11. Meaning of the words Scalar and Vector 9
12. Division of physical vectors into two classes, Forces and Fluxes 10
13. Relation between corresponding vectors of the two classes .. 11
14. Line-integration appropriate to forces, surface-integration to
fluxes 12
15. Longitudinal and rotational vectors 12
16. Line-integrals and potentials 13
17. Hamilton s expression for the relation between a force and its
potential 15
18. Cyclic regions and geometry of position 16
19. The potential in an acyclic region is single valued 17
20. System of values of the potential in a cyclic region 18
21. Surface-integrals 19
22. Surfaces, tubes, and lines of flow 21
23. Right-handed and left-handed relations in space 24
24. Transformation of a line-integral into a surface-integral .. .. 25
25. Effect of Hamilton s operation v on a vector function .. .. 27
26. Xature of the operation v 2 29
xvi CONTENTS.
PART I.
ELECTROSTATICS.
CHAPTER I.
DESCRIPTION OP PHENOMENA.
Art. Page
27. Electrification by friction. Electrification is of two kinds, to
which the names of Vitreous and Resinous, or Positive and
Negative, have been given .. .. 30
28. Electrification by induction 31
29. Electrification by conduction. Conductors and insulators .. 32
30. In electrification by friction the quantity of the positive elec
trification is equal to that of the negative electrification .. 33
31. To charge a vessel with a quantity of electricity equal and
opposite to that of an excited body 33
32. To discharge a conductor completely into a metallic vessel .. 34
33. Test of electrification by gold-leaf electroscope 34
34. Electrification, considered as a measurable quantity, may be
called Electricity 35
35. Electricity may be treated as a physical quantity 36
36. Theory of Two fluids .. 37
37. Theory of One fluid 39
38. Measurement of the force between electrified bodies 40
39. Relation between this force and the quantities of electricity .. 41
40. Variation of the force with the distance 42
41,42. Definition of the electrostatic unit of electricity. Its
dimensions 42
43. Proof of the law of electric force 43
44. Electric field .. .... 44
45. Electric potential 45
46. Equipotential surfaces. Example of their use in reasoning
about electricity .. ,. .. 45
47. Lines of force 47
48. Electric tension 47
49. Electromotive force 47
50. Capacity of a conductor 48
51. Properties of bodies. Resistance 48
CONTENTS. xvi i
Art. Pae*
52. Specific Inductive capacity of a dielectric 50
53. * Absorption of electricity 50
54. Impossibility of an absolute charge ..51
55. Disruptive discharge. Glow 52
56. Brush 54
57. Spark 55
58. Electrical phenomena of Tourmaline 56
59. Plan of the treatise, and sketch of its results 57
60. Electric polarization and displacement 59
61. The motion of electricity analogous to that of an incompressible
fluid 62
62. Peculiarities of the theory of this treatise 62
CHAPTER II.
ELEMENTAKY MATHEMATICAL THEORY OF ELECTRICITY.
63. Definition of electricity as a mathematical quantity .. .. .. 66
64. Volume-density, surface-density, and line-density .. .. .. 67
65. Definition of the electrostatic unit of electricity 68
66. Law of force between electrified bodies .. 69
67. Resultant force between two bodies 69
68. Resultant force at a point 69
69. Line-integral of electric force ; electromotive force 71
70. Electric potential 72
71. Resultant force in terms of the potential 72
72. The potential of all points of a conductor is the same .. .. 73
73. Potential due to an electrified system 74
74. Proof of the law of the inverse square 74
75. Surface-integral of electric induction 77
76. Introduction through a closed surface due to a single centre
of force 77
77. Poisson s extension of Laplace s equation .. ... 79
78. Conditions to be fulfilled at an electrified surface 80
79. Resultant force on an electrified surface 82
80. The electrification of a conductor is entirely on the surface .. 83
81. A distribution of electricity on lines or points is physically
impossible
82. Lines of electric induction 84
83. Specific inductive capacity 86
VOL. I. b
xviii CONTENTS.
CHAPTER III.
SYSTEMS OF CONDUCTORS.
Art.
84. On the superposition of electrified systems 88
85. Energy of an electrified system 88
86. General theory of a system of conductors. Coefficients of po
tential 89
87. Coefficients of induction. Capacity of a conductor. Dimensions
of these coefficients 90
88. Reciprocal property of the coefficients 91
89. A theorem due to Green 92
90. Relative magnitude of the coefficients of potential 92
91. And of induction 93
92. The resultant mechanical force on a conductor expressed in
terms of the charges of the different conductors of the system
and the variation of the coefficients of potential 94
93. The same in terms of the potentials, and the variation of the
coefficients of induction 94
94. Comparison of electrified systems 96
CHAPTER IV.
GENERAL THEOREMS.
95. Two opposite methods of treating electrical questions .. .. 98
96. Characteristics of the potential function 99
97. Conditions under which the volume-integral
dV dV a
vanishes 100
98. Thomson s theorem of the unique minimum of
1
^ ( 2 -f o 2 + <? 2 ) dxdydz 103
99. Application of the theorem to the determination of the dis
tribution of electricity 107
100. Green s theorem and its physical interpretation 108
101. Green s functions 113
102. Method of finding limiting values of electrical coefficients .. 115
CONTENTS. XIX
CHAPTER V.
MECHANICAL ACTION BETWEEN ELECTRIFIED BODIES.
Art. Page
103. Comparison of the force between different electrified systems .. 119
104. Mechanical action on an element of an electrified surface .. 121
105. Comparison between theories of direct action and theories of
stress 122
106. The kind of stress required to account for the phenomeuou .. 123
107. The hypothesis of stress considered as a step in electrical
science 126
108. The hypothesis of stress shewn to account for the equilibrium
of the medium and for the forces acting between electrified
bodies 128
109. Statements of Faraday relative to the longitudinal tension and
lateral pressure of the lines of force 131
110. Objections to stress in a fluid considered 131
111. Statement of the theory of electric polarization 132
CHAPTER VI.
POINTS AND LINES OF EQUILIBRIUM,
112. Conditions of a point of equilibrium 135
113. Number of points of equilibrium 136
114. At a point or line of equilibrium there is a conical point or a
line of self-intersection of the equipotential surface .. .. 137
115. Angles at which an equipotential surface intersects itself .. 138
116. The equilibrium of an electrified body cannot be stable .. .. 139
CHAPTER VII.
FORMS OF EQUIPOTENTIAL SURFACES AND LINES OF FLOW.
117. Practical importance of a knowledge of these forms in simple
cases 142
118. Two electrified points, ratio 4 : 1. (Fig. I) 143
119. Two electrified points, ratio 4 : 1. (Fig. II) 144
120. An electrified point in a uniform field offeree. (Fig. Ill) .. 145
121. Three electrified points. Two spherical equipotential sur
faces. (Fig. IV) 145
122. Faraday s use of the conception of lines of force 146
123. Method employed in drawing the diagrams 147
b 2
XX CONTENTS.
CHAPTER VIII.
SIMPLE CASES OF ELECTRIFICATION.
Art. 1 age
124. Two parallel planes 150
125. Two concentric spherical surfaces 152
126. Two coaxal cylindric surfaces 154
127. Longitudinal force on a cylinder, the ends of which are sur
rounded by cylinders at different potentials 155
CHAPTER IX.
SPHERICAL HARMONICS.
128. Singular points at which the potential becomes infinite .. .. 157
129. Singular points of different orders defined by their axes .. .. 158
130. Expression for the potential due to a singular point referred
to its axes .................... 160
131. This expression is perfectly definite and represents the most
general type of the harmonic of i degrees ........ 162
132. The zonal, tesseral, and sectorial types .......... 163
133. Solid harmonics of positive degree. Their relation to those
of negative degree .................. 165
134. Application to the theory of electrified spherical surfaces .. 166
135. The external action of an electrified spherical surface compared
with that of an imaginary singular point at its centre .. .. 167
136. Proof that if Y i and Y$ are two surface harmonics of different
degrees, the surface-integral / / Y i Yj dS = 0, the integration
being extended over the spherical surface ........ 169
137. Value of // Y i YjdS where Y L and Yj are surface harmonics
of the same degree but of different types ........ 169
138. On conjugate harmonics ................ 170
139. If Yj is the zonal harmonic and Y i any other type of the
same degree
where Y i(j) is the value of Y i at the pole of Y j ...... 171
140. Development of a function in terms of spherical surface har
monics .................... ..172
141. Surface-integral of the square of a symmetrical harmonic .. 173
CONTENTS. xxi
Art. Page
142. Different methods of treating spherical harmonics 174
143. On the diagrams of spherical harmonics. (Figs. V, Vf, VII,
VHI, IX) .. 175
144. If the potential is constant throughout any finite portion of
space it is so throughout the whole region continuous with it
within which Laplace s equation is satisfied 176
145. To analyse a spherical harmonic into a system of conjugate
harmonics by means of a finite number of measurements at
selected points of the sphere 177
146. Application to spherical and nearly spherical conductors .. 178
CHAPTER X.
COXFOCAJL SURFACES OF THE SECOND DEGREE.
147. The lines of intersection of two systems and their intercepts
by the third system 181
148. The characteristic equation of V in terms of ellipsoidal co
ordinates 182
149. Expression of a, 0, y in terms of elliptic functions 183
150. Particular solutions of electrical distribution on the confocal
surfaces and their limiting forms 184
151. Continuous transformation into a figure of revolution about
the axis of 187
152. Transformation into a figure of revolution about the axis of x 188
153. Transformation into a system of cones and spheres 189
154. Confocal paraboloids 189
CHAPTER XI.
THEORY OF ELECTRIC IMAGES.
155. Thomson s method of electric images 191
156. When two points are oppositely and unequally electrified, the
surface for which the potential is zero is a sphere .. .. 192
157. Electric images 193
158. Distribution of electricity on the surface of the sphere .. .. 195
1 59. Image of any given distribution of electricity 196
160. Resultant force between an electrified point and sphere .. .. 197
161. Images in an infinite plane conducting surface 198
162. Electric inversion 199
163. Geometrical theorems about inversion 201
164. Application of the method to the problem of Art. 158 .. .. 202
xxii CONTENTS.
Art. Page
165. Finite systems of successive images 203
166. Case of two spherical surfaces intersecting at an angle ^ ..204
167. Enumeration of the cases in which the number of images is
finite 206
168. Case of two spheres intersecting orthogonally 207
169. Case of three spheres intersecting orthogonally 210
170. Case of four spheres intersecting orthogonally 211
171. Infinite series of images. Case of two concentric spheres . . 212
172. Any two spheres not intersecting each other 213
173. Calculation of the coefficients of capacity and induction .. .. 216
174. Calculation of the charges of the spheres, and of the force
between them 217
175. Distribution of electricity on two spheres in contact. Proof
sphere 219
176. Thomson s investigation of an electrified spherical bowl .. .. 221
177. Distribution on an ellipsoid, and on a circular disk at po
tential V 221
178. Induction on an uninsulated disk or bowl by an electrified
point in the continuation of the plane or spherical surface .. 222
179. The rest of the sphere supposed uniformly electrified .. .. 223
180. The bowl maintained at potential V and uninfluenced .. .. 223
181. Induction on the bowl due to a point placed anywhere .. .. 224
CHAPTER XII.
CONJUGATE FUNCTIONS IN TWO DIMENSIONS.
182. Cases in which the quantities are functions of x and y only .. 226
183. Conjugate functions 227
184. Conjugate functions may be added or subtracted 228
185. Conjugate functions of conjugate functions are themselves
conjugate 229
186. Transformation of Poisson s equation 231
187. Additional theorems on conjugate functions 232
188. Inversion in two dimensions .. 232
189. Electric images in two dimensions 233
190. Neumann s transformation of this case 234
191. Distribution of electricity near the edge of a conductor formed
by two plane surfaces 236
192. Ellipses and hyperbolas. (Fig. X) 237
193. Transformation of this case. (Fig. XI) 238
CONTENTS. xxiu
Art.
194. Application to two cases of the flow of electricity in a con
ducting sheet .................... 239
195. Application to two cases of electrical induction ...... 239
196. Capacity of a condenser consisting of a circular disk between
two infinite planes .............. -*- * - 240
197. Case of a series of equidistant planes cut off by a plane at right
angles to them .............. ,. 242
198. Case of a furrowed surface .............. 243
199. Case of a single straight groove ............ 243
200. Modification of the results when the groove is circular .. .. 244
201. Application to Sir W. Thomson s guard-ring ........ 245
202. Case of two parallel plates cut off by a perpendicular plane.
(Fig. XII) .................... 246
203. Case of a grating of parallel wires. (Fig. XIII) ...... 248
204. Case of a single electrified wire transformed into that of the
grating ...................... 248
205. The grating used as a shield to protect a body from electrical
influence .................... 249
206. Method of approximation applied to the case of the grating .. 251
CHAPTER XIII.
ELECTROSTATIC INSTRUMENTS.
207. The frictional electrical machine 254
208. The electrophorus of Volta 255
209. Production of electrification by mechanical work. Nicholson s
Revolving Doubler 256
210. Principle of Varley s and Thomson s electrical machines .. .. 256
211. Thomson s water-dropping machine ..259
212. Holtz s electrical machine 260
213. Theory of regenerators applied to electrical machines .. .. 260
214. On electrometers and electroscopes. Indicating instruments
and null methods. Difference between registration and mea
surement 262
215. Coulomb s Torsion Balance for measuring charges 263
216. Electrometers for measuring potentials. Snow Harris s and
Thomson s 266
217. Principle of the guard-ring. Thomson s Absolute Electrometer 267
218. Heterostatic method 269
219. Self-acting electrometers. Thomson s Quadrant Electrometer 271
220. Measurement of the electric potential of a small body .. .. 274
221. Measurement of the potential at a point in the air 275
xxvi CONTENTS.
Art. Page
269. Dissipation of the ions and loss of polarization 321
270. Limit of polarization 321
271. Bitter s secondary pile compared with the Leyden jar .. .. 322
272. Constant voltaic elements. Daniell s cell 325
CHAPTER VI.
MATHEMATICAL THEORY OF THE DISTRIBUTION OF ELECTRIC CURRENTS.
273. Linear conductors 329
274. Ohm s Law 329
275. Linear conductors in series 329
276. Linear conductors in multiple arc 330
277. Resistance of conductors of uniform section 331
278. Dimensions of the quantities involved in Ohm s law .. .. 332
279. Specific resistance and conductivity in electromagnetic measure 333
280. Linear systems of conductors in general 333
281. Reciprocal property of any two conductors of the system .. 335
282. Conjugate conductors .. .. 336
283. Heat generated in the system 336
284. The heat is a minimum when the current is distributed ac
cording to Ohm s law 337
CHAPTER VII.
CONDUCTION IN THREE DIMENSIONS.
285. Notation 338
286. Composition and resolution of electric currents 338
287. Determination of the quantity which flows through any surface 339
288. Equation of a surface of flow 340
289. Relation between any three systems of surfaces of flow .. .. 340
290. Tubes of flow 340
291. Expression for the components of the flow in terms of surfaces
offlow 341
292. Simplification of this expression by a proper choice of para
meters .. 341
293. Unit tubes of flow used as a complete method of determining
the current .. 341
294. Current-sheets and current-functions 342
295. Equation of continuity 342
296. Quantity of electricity which flows through a given surface .. 344
CONTENTS. xxvii
CHAPTER VIII.
RESISTANCE AND CONDUCTIVITY IX THEEE DIMENSIONS.
Art. Page
297. Equations of resistance 345
298. Equations of conduction 346
299. Kate of generation of heat .. .. -.. ... *.. ".. .. 346
300. Conditions of stability - ... .. 347
301. Equation of continuity in a homogeneous medium 348
302. Solution of the equation 348
303. Theory of the coefficient T. It probably does not exist .. 349
304. Generalized form of Thomson s theorem .. .. 350
305. Proof without symbols 351
306. Strutt s method applied to a wire of variable section. -Lower
limit of the value of the resistance 353
307. Higher limit 356
308. Lower limit for the correction for the ends of the wire .. .. 358
309. Higher limit 358
CHAPTER IX.
CONDUCTION THROUGH HETEROGENEOUS MEDIA.
310. Surface-conditions 360
311. Spherical surface 362
312. Spherical shell 363
313. Spherical shell placed in a field of uniform flow 364
314. Medium in which small spheres are uniformly disseminated .. 365
315. Images in a plane surface 366
316. Method of inversion not applicable in three dimensions .. .. 367
317. Case of conduction through a stratum bounded by parallel
planes 367
318. Infinite series of images. Application to magnetic induction .. 368
319. On stratified conductors. Coefficients of conductivity of a
conductor consisting of alternate strata of two different sub
stances 369
320. If neither of the substances has the rotatory property denoted
by I 7 the compound conductor is free from it 370
321. If the substances are isotropic the direction of greatest resist
ance is normal to the strata 371
322. Medium containing parallelepipeds of another medium .. .. 371
323. The rotatory property cannot be introduced by means of con
ducting channels 372
324. Construction of an artificial solid having given coefficients of
longitudinal and transverse conductivity 373
xxviii CONTENTS.
CHAPTER X.
CONDUCTION IN DIELECTRICS.
Art. Page
325. In a strictly homogeneous medium there can be no internal
charge 374
326. Theory of a condenser in which the dielectric is not a perfect
insulator 375
327. No residual charge due to simple conduction 376
328. Theory of a composite accumulator 376
329. Residual charge and electrical absorption 378
330. Total discharge 380
331. Comparison with the conduction of heat 381
332. Theory of telegraph cables and comparison of the equations
with those of the conduction of heat 381
333. Opinion of Ohm on this subject 384
334. Mechanical illustration of the properties of a dielectric .. .. 385
CHAPTER XI.
MEASUREMENT OF THE ELECTRIC RESISTANCE OF CONDUCTORS.
335. Advantage of using material standards of resistance in electrical
measurements 388
336. Different standards which have been used and different systems
which have been proposed 388
337. The electromagnetic system of units .. 389
338. Weber s unit, and the British Association unit or Ohm .. ,. 389
339. Professed value of the Ohm 10,000,000 metres per second .. 389
340. Reproduction of standards 390
341. Forms of resistance coils 391
342. Coils of great resistance 392
343. Arrangement of coils in series 392
344. Arrangement in multiple arc 393
345. On the comparison of resistances. (1) Ohm s method .. .. 394
346. (2) By the differential galvanometer 394
347. (3) By Wheatstone s Bridge 398
348. Estimation of limits of error in the determination 399
349. Best arrangement of the conductors to be compared .. .. 400
350. On the use of Wheatstone s Bridge 402
351. Thomson s method for small resistances 404
352. Matthiessen and Hockin s method for small resistances ., .. 406
CONTENTS. xxix
Art. Page
353. Comparison of great resistances by the electrometer .. .. 408
354. By accumulation in a condenser 409
355. Direct electrostatic method 409
356. Thomson s method for the resistance of a galvanometer .. .. 410
357. Mance s method of determining the resistance of a battery .. 411
358. Comparison of electromotive forces 413
CHAPTER XII.
ELECTRIC RESISTANCE OF SUBSTANCES.
359. Metals, electrolytes, and dielectrics 415
360. Resistance of metals 416
361. Resistance of mercury 417
362. Table of resistance of metals .. 418
363. Resistance of electrolytes 419
364. Experiments of Paalzow - 419
365. Experiments of Kohlrausch and Nippoldt ... 420
366. Resistance of dielectrics 421
367. Gutta-percha .. 423
368. Glass .. .. " .. ., 423
369. Gases .* 424
370. Experiments of Wiedemann and Riihlinann 425
ERRATA. VOL. I.
Page 26, 1. 3 from bottom, dele As we have made no assumption , &c.
down to 1. 7 of p. 27, the expression may then be written , and
substitute as follows :
Let us now suppose that the curves for which a is constant
form a series of closed curves, surrounding the point on the surface
for which a has its minimum value, a , the last curve of the series,
for which a = a lt coinciding with the original closed curve s.
Let us also suppose that the curves for which /3 is constant form
a series of lines drawn from the point at which a = a to the
closed curve s, the first, /3 , and the last, fa, being identical.
Integrating (8) by parts, the first term with respect to a and
the second with respect to /3, the double integrals destroy each
other. The line integral,
f^ 1 (X
/h d fi
.
is zero, because the curve a = a is reduced to a point at which
there is but one value of X and of x.
The two line integrals,
*
destroy each other, because the point (a, /3J is identical with the
point (a, /3 ).
The expression (8) is therefore reduced to
Since the curve a = a l is identical with the closed curve s, we
may write this expression
p. 80, in equations (3), (4), (6), (8), (17), (18), (19), (20), (21), (22), for
R read N.
p. 82, 1. 3, for Rl read Nl.
d-V d*V
p. 83, in equations (28), (29), (30), (31), for -^ read j^*
in equation (29), insert before the second member.
p. 105, 1. 2, for Q read 8irQ.
p. 108, equation (1), for p read //.
(2), for p read p.
(3), for a- read (/.
(4), for a- read <r.
p. 113, 1. 4, for KR read ^- KR.
1. 5, for KRRfcosc read --KRRfwse.
T: 7T
p. 114, 1. 5, for S l read S.
p. 124, last line, for e l -\-e l read e l + e 2 .
p. 125, lines 3 and 4, transpose within and without; 1. 16, for v
read V ; and 1. 18, for V read v.
p. 128, lines 11, 10, 8 from bottom, for dx read dz.
p. 149, 1. 24, for equpotential read equipotential.
2 ERRATA. VOL. I.
p. 159, 1. 3, for F read f.
,, 1. 2 from bottom, for M read M 2 .
p. 163, 1. 20, for \i- s +i read AJ-^+I.
p. 164, equation (34), Jor (_iy- J= read (_!)*-*_
p. 179, equation (76), for i+l read 27+1.
X 2 Z 2 X 2 Z 2
p. 185, equation (24), for ~ ~=l read -^ T^r 2 = 1 -
p. 186, 1. 5 from bottom, for The surface-density on the elliptic plate
read The surface-density on either side of the elliptic plate,
p. 186, equation (30), for 2n read 4ir.
p. 188, equation (38), for v 2 read 2n 2 .
p. 196, 1. 27, for e..e read e 1 ..e 2 .
Ee e 2 a 3
p. 197, equation (10) should be M = 1
p. 204, 1. 15 from bottom, dele either,
p. 215, 1. 4, for \/2k read */2k.
E
p. 234, equation (13), for 2JZ read
p. 335, dele last 14 lines,
p. 336, 1. 1, dele therefore.
1. 2, for l the potential at C to exceed that at D by P, read a
current, (7, from X to Y.
1. 4, for C to D will cause the potential at A to exceed that at
B by the same quantity P, read X to Y will cause an equal
current G from A to B.
p. 351, 1. 3, for R 2 y? + R 2 v 2 + R 2 w 2 read R^
dt>
1. 5, read +
p. 355, last line, for S read S.
~db*
p. 356, equation (12), for read ~
d
p. 365, in equations (12), (15), (16), for A read Ar.
E E
p. 366, equation (3), for ~ read
r i r z
p. 367, 1. 5, for 2^8 read 2k 2 S.
p. 368, equation (14), for <// read //.
p. 397, 1. 1, for ~8 read ~S .
FJ rj
p. 404, at the end of Art. 350 insert as follows :
When y, the resistance to be measured, a, the resistance of the
battery, and a, the resistance of the galvanometer, are given, the
best values of the other resistances have been shewn by Mr. Oliver
Heaviside (Phil. Mag., Feb. 1873) to be
a+y a+ y
ELECTRICITY AND MAGNETISM.
ELECTEICITY AND MAGNETISM.
PRELIMINARY.
ON THE MEASUREMENT OF QUANTITIES.
1.] EVERY expression of a Quantity consists of two factors or
components. One of these is the name of a certain known quan
tity of the same kind as the quantity to be expressed, which is
taken as a standard of reference. The other component is the
number of times the standard is to be taken in order to make up
the required quantity. The standard quantity is technically called
the Unit, and the number is called the Numerical Value of the
quantity.
There must be as many different units as there are different
kinds of quantities to be measured, but in all dynamical sciences
it is possible to define these units in terms of the three funda
mental units of Length, Time, and Mass. Thus the units of area
and of volume are defined respectively as the square and the cube
whose sides are the unit of length.
Sometimes, however, we find several units of the same kind
founded on independent considerations. Thus the gallon, or the
volume of ten pounds of water, is used as a unit of capacity as well
as the cubic foot. The gallon may be a convenient measure in
some cases, but it is not a systematic one, since its numerical re
lation to the cubic foot is not a round integral number.
2.] In framing a mathematical system we suppose the funda
mental units of length, time, and mass to be given, and deduce
all the derivative units from these by the simplest attainable de
finitions.
The formulae at which we arrive must be such that a person
VOL. i. B
2 PRELIMINARY. [3.
of any nation, by substituting for the different symbols the nu
merical value of the quantities as measured by his own national
units, would arrive at a true result.
Hence, in all scientific studies it is of the greatest importance
to employ units belonging to a properly defined system, and to
know the relations of these units to the fundamental units, so that
we may be able at once to transform our results from one system to
another.
This is most conveniently done by ascertaining the dimensions
of every unit in terms of the three fundamental units. When a
given unit varies as the ^th power of one of these units, it is said
to be of n dimensions as regards that unit.
For instance, the scientific unit of volume is always the cube
whose side is the unit of length. If the unit of length varies,
the unit of volume will vary as its third power, and the unit of
volume is said to be of three dimensions with respect to the unit of
length.
A knowledge of the dimensions of units furnishes a test which
ought to be applied to the equations resulting from any lengthened
investigation. The dimensions of every term of such an equa
tion, with respect to each of the three fundamental units, must
be the same. If not, the equation is absurd, and contains some
error, as its interpretation would be different according to the arbi
trary system of units which we adopt *.
The Three Fundamental Units.
3.] (1) Length. The standard of length for scientific purposes
in this country is one foot, which is the third part of the standard
yard preserved in the Exchequer Chambers.
In France, and other countries which have adopted the metric
system, it is the metre. The metre is theoretically the ten mil
lionth part of the length of a meridian of the earth measured
from the pole to the equator ; but practically it is the length of
a standard preserved in Paris, which was constructed by Borda
to correspond, when at the temperature of melting ice, with the
value of the preceding length as measured by Delambre. The metre
has not been altered to correspond with new and more accurate
measurements of the earth, but the arc of the meridian is estimated
in terms of the original metre.
* The theory of dimensions was first stated by Fourier, Tkeorie de Chaleur, 160.
5-] THE THREE FUNDAMENTAL UNITS. 3
In astronomy the mean distance of the earth from the sun is
sometimes taken as a unit of length.
In the present state of science the most universal standard of
length which we could assume would be the wave length in vacuum
of a particular kind of light, emitted by some widely diffused sub
stance such as sodium, which has well-defined lines in its spectrum.
Such a standard would be independent of any changes in the di
mensions of the earth, and should be adopted by those who expect
their writings to be more permanent than that body.
In treating of the dimensions of units we shall call the unit of
length []. If I is the numerical value of a length, it is under
stood to be expressed in terms of the concrete unit [_Z/], so that
the actual length would be fully expressed by I \L~\.
4.] (2) Time. The standard unit of time in all civilized coun
tries is deduced from the time of rotation of the earth about its
axis. The sidereal day, or the true period of rotation of the earth,
can be ascertained with great exactness by the ordinary observa
tions of astronomers ; and the mean solar day can be deduced
from this by our knowledge of the length of the year.
The unit of time adopted in all physical researches is one second
of mean solar time.
In astronomy a year is sometimes used as a unit of time. A
more universal unit of time might be found by taking the periodic
time of vibration of the particular kind of light whose wave length
is the unit of length.
We shall call the concrete unit of time \_T~\, and the numerical
measure of time t.
5.] (3) Mass. The standard unit of mass is in this country the
avoirdupois pound preserved in the Exchequer Chambers. The
grain, which is often used as a unit, is defined to be the 7000th
part of this pound.
In the metrical system it is the gramme, which is theoretically
the mass of a cubic centimetre of distilled water at standard tem
perature and pressure, but practically it is the thousandth part
of a standard kilogramme preserved in Paris.
The accuracy with which the masses of bodies can be com
pared by weighing is far greater than that hitherto attained in
the measurement of lengths, so that all masses ought, if possible,
to be compared directly with the standard, and not deduced from
experiments on water.
In descriptive astronomy the mass of the sun or that of the
B 2
4 PRELIMINARY. [5.
earth is sometimes taken as a unit, but in the dynamical theory
of astronomy the unit of mass is deduced from the units of time
and length, combined with the fact of universal gravitation. The
astronomical unit of mass is that mass which attracts another
body placed at the unit of distance so as to produce in that body
the unit of acceleration.
In framing a universal system of units we may either deduce
the unit of mass in this way from those of length and time
already defined, and this we can do to a rough approximation in
the present state of science ; or, if we expect * soon to be able to
determine the mass of a single molecule of a standard substance,
we may wait for this determination before fixing a universal
standard of mass.
We shall denote the concrete unit of mass by the symbol \M ]
in treating of the dimensions of other units. The unit of mass
will be taken as one of the three fundamental units. When, as
in the French system, a particular substance,, water, is taken as
a standard of density, then the unit of mass is no longer inde
pendent, but varies as the unit of volume, or as [I/ 3 ].
If, as in the astronomical system, the unit of mass is defined
with respect to its attractive power, the dimensions of [If] are
[Z 3 ? 7 - 2 ].
For the acceleration due to the attraction of a mass m at a
fflL
distance r is by the Newtonian Law -j . Suppose this attraction
to act for a very small time t on a body originally at rest, and to
cause it to describe a space s, then by the formula of Galileo,
whence m = 2 -^ . Since r and s are both lengths, and t is a
t
time, this equation cannot be true unless the dimensions of m are
[i/ 3 ^? 7 " 2 ]. The same can be shewn from any astronomical equa
tion in which the mass of a body appears in some but not in all
of the terms f.
* See Prof. J. Loschmidt, Zur Grosse der Luftmolecule, Academy of Vienna,)
Oct. 12, 1865; G. J. Stoney on The Internal Motions of Gases, Phil. Mag., Aug.
1868 ; and Sir W. Thomson on The Size of Atoms, Nature, March 31, 1870.
f If a foot and a second are taken as units, the astronomical unit of mass would
be about 932,000,000 pounds.
6.] DERIVED UNITS. 5
Derived Units.
6.] The unit of Velocity is that velocity in which unit of length
is described in unit of time. Its dimensions are [j&T 7 " 1 ].
If we adopt the units of length and time derived from the
vibrations of light, then the unit of velocity is the velocity of
light.
The unit of Acceleration is that acceleration in which the velo
city increases by unity in unit of time. Its dimensions are [I/T~ 2 ].
The unit of Density is the density of a substance which contains
unit of mass in unit of volume. Its dimensions are [J/.Z/~ 3 ].
The unit of Momentum is the momentum of unit of mass moving
with unit of velocity. Its dimensions are [MLT~ l ~].
The unit of Force is the force which produces unit of momentum
in unit of time. Its dimensions are [MLT~ 2 ].
This is the absolute unit of force, and this definition of it is
implied in every equation in Dynamics. Nevertheless, in many
text books in which these equations are given, a different unit of
force is adopted, namely, the weight of the national unit of mass ;
and then, in order to satisfy the equations, the national unit of mass
is itself abandoned, and an artificial unit is adopted as the dynamical
unit, equal to the national unit divided by the numerical value of
the force of gravity at the place. In this way both the unit of force
and the unit of mass are made to depend on the value of the
force of gravity, which varies from place to place, so that state
ments involving these quantities are not complete without a know
ledge of the force of gravity in the places where these statements
were found to be true.
The abolition, for all scientific purposes, of this method of mea
suring forces is mainly due to the introduction of a general system
of making observations of magnetic force in countries in which
the force of gravity is different. All such forces are now measured
according to the strictly dynamical method deduced from our
definitions, and the numerical results are the same in whatever
country the experiments are made.
The unit of Work is the work done by the unit of force acting
through the unit of length measured in its own direction. Its
dimensions are [J/Z 2 T~ 2 ].
The Energy of a system, being its capacity of performing work,
is measured by the work which the system is capable of performing
by the expenditure of its whole energy.
6 PRELIMINARY. [7.
The definitions of other quantities, and of the units to which
they are referred, will be given when we require them.
In transforming the values of physical quantities determined in
terms of one unit, so as to express them in terms of any other unit
of the same kind, we have only to remember that every expres
sion for the quantity consists of two factors, the unit and the nu
merical part which expresses how often the unit is to be taken.
Hence the numerical part of the expression varies inversely as the
magnitude of the unit, that is, inversely as the various powers of
the fundamental units which are indicated by the dimensions of the
derived unit.
On Physical Continuity and Discontinuity.
7.] A quantity is said to vary continuously when, if it passes
from one value to another, it assumes all the intermediate values.
We may obtain the conception of continuity from a consideration
of the continuous existence of a particle of matter in time and space.
Such a particle cannot pass from one position to another without
describing a continuous line in space, and the coordinates of its
position must be continuous functions of the time.
In the so-called equation of continuity, as given in treatises
on Hydrodynamics, the fact expressed is that matter cannot appear
in or disappear from an element of volume without passing in or out
through the sides of that element.
A quantity is said to be a continuous function of its variables
when, if the variables alter continuously, the quantity itself alters
continuously.
Thus, if u is a function of x, and if, while x passes continuously
from # to fl? 1} u passes continuously from n to u lt but when x
passes from x l to # 2 , u passes from u-f to u 2 , % being different from
%, then u is said to have a discontinuity in its variation with
respect to x for the value x = x l} because it passes abruptly from u^
to u{ while x passes continuously through # r
If we consider the differential coefficient of u with respect to x for
the value x = x^ as the limit of the fraction
when # 2 and # are both made to approach ^ without limit, then,
if X Q and x 2 are always on opposite sides of as ly the ultimate value of
the numerator will be u^u ly and that of the denominator will
be zero. If u is a quantity physically continuous, the discontinuity
8.] CONTINUITY AND DISCONTINUITY. 7
can exist only with respect to the particular variable x. We must
in this case admit that it has an infinite differential coefficient
when x X L . If u is not physically continuous, it cannot be dif
ferentiated at all.
It is possible in physical questions to get rid of the idea of
discontinuity without sensibly altering the conditions of the case.
If # is a very little less than x l} and x. 2 a very little greater than
.r 15 then U Q will be very nearly equal to u^ and u 2 to u{. We
may now suppose u to vary in any arbitrary but continuous manner
from ?/ to u 2 between the limits X Q and x 2 . In many physical
questions we may begin with a hypothesis of this kind, and then
investigate the result when the values of # and x 2 are made to
approach that of ^ and ultimately to reach it. The result will
in most cases be independent of the arbitrary manner in which we
have supposed u to vary between the limits.
Discontinuity of a Fimction of more than One Variable.
8.] If we suppose the values of all the variables except x to be
constant, the discontinuity of the function will occur for particular
values of #, and these will be connected with the values of the
other variables by an equation which we may write
$ = < (x, y, z 3 &c.) = 0.
The discontinuity will occur when <f> = 0. When $ is positive the
function will have the form F 2 (x } y, z, &c.). When < is negative
it will have the form F 1 (x, y, z, &c.). There need be no necessary
relation between the forms F and F 2 .
To express this discontinuity in a mathematical form, let one of
the variables, say .r, be expressed as a function of </> and the other
variables, and let F 1 and F 2 be expressed as functions of <, y, z, &c.
We may now express the general form of the function by any
formula which is sensibly equal to F 2 when <p is positive, and to
F when c/> is negative. Such a formula is the following
F n< ^ 2
As long as n is a finite quantity, however great, F will be a
continuous function, but if we make n infinite F will be equal to
F 2 when < is positive, and equal to F^ when </> is negative.
Discontinuity of the Derivatives of a Continuous Function.
The first derivatives of a continuous function may be discon-
8 PRELIMINARY. [9.
tinuous. Let the values of the variables for which the discon
tinuity of the derivatives occurs be connected by the equation
< = <(#,y, 2...) = 0,
and let F L and F 2 be expressed in terms of $ and nl other
variables, say (y> z . . .).
Then, when $ is negative, F l is to be taken, and when $ is
positive F 2 is to be taken, and, since F is itself continuous, when
</> is zero, F^ = F 2 .
Hence, when d> is zero, the derivatives - and -~ may be
d(p dfy
different, but the derivatives with respect to any of the other
variables, such as 7^ and =- must be the same. The discon-
du dy
t/ */
tinuity is therefore confined to the derivative with respect to 0, all
the other derivatives being- continuous.
Periodic and Multiple Functions.
9.] If u is a function of x such that its value is the same for
a?, x + a t x + na, and all values of x differing by a, u is called a
periodic function of x, and a is called its period.
If x is considered as a function of u, then, for a given value of
U, there must be an infinite series of values of x differing by
multiples of a. In this case x is called a multiple function of u,
and a is called its cyclic constant.
dx
The differential coefficient - has only a finite number of values
du
corresponding to a given value of u.
On the Relation of Physical Quantities to Directions in Space.
10.] t ln distinguishing the kinds of physical quantities, it is of
great importance to know how they are related to the directions
of those coordinate axes which we usually employ in defining the
positions of things. The introduction of coordinate axes into geo
metry by Des Cartes was one of the greatest steps in mathematical
progress, for it reduced the methods of geometry to calculations
performed on numerical quantities. The position of a point is made
to depend on the length of three lines which are always drawn in
determinate directions, and the line joining two points is in like
manner considered as the resultant of three lines.
But for many purposes in physical reasoning, as distinguished
II.] VECTORS, OR DIRECTED QUANTITIES. 9
from calculation, it is desirable to avoid explicitly introducing the
Cartesian coordinates, and to fix the mind at once on a point of
space instead of its three coordinates, and on the magnitude and
direction of a force instead of its three components. This mode
of contemplating geometrical and physical quantities is more prim
itive and more natural than the other, although the ideas connected
with it did not receive their full development till Hamilton made
the next great step in dealing with space, by the invention of his
Calculus of Quaternions.
As the methods of Des Cartes are still the most familiar to
students of science, and as they are really the most useful for
purposes of calculation, we shall express all our results in the
Cartesian form. I am convinced, however, that the introduction
of the ideas, as distinguished from the operations and methods of
Quaternions, will be of great use to us in the study of all parts
of our subject, and especially in electrodynamics, where we have to
deal with a number of physical quantities, the relations of which
to each other can be expressed far more simply by a few words of
Hamilton s, than by the ordinary equations.
11.] One of the most important features of Hamilton s method is
the division of quantities into Scalars and Vectors.
A Scalar quantity is capable of being completely defined by a
single numerical specification. Its numerical value does not in
any way depend on the directions we assume for the coordinate
axes.
A Vector, or Directed quantity, requires for its definition three
numerical specifications, and these may most simply be understood
as having reference to the directions of the coordinate axes.
Scalar quantities do not involve direction. The volume of a
geometrical figure, the mass and the energy of a material body,
the hydrostatical pressure at a point in a fluid, and the potential
at a point in space, are examples of scalar quantities.
A vector quantity has direction as well as magnitude, and is
such that a reversal of its direction reverses its sign. The dis
placement of a point, represented by a straight line drawn from
its original to its final position, may be taken as the typical
vector quantity, from which indeed the name of Vector is derived.
The velocity of a body, its momentum, the force acting on it,
an electric current, the magnetization of a particle of iron, are
instances of vector quantities.
There are physical quantities of another kind which are related
1 PRELIMINARY. [ I 2.
to directions in space, but which are not vectors. Stresses and
strains in solid bodies are examples of these, and the properties
of bodies considered in the theory of elasticity and in the theory
of double refraction. Quantities of this class require for their
definition nine numerical specifications. They are expressed in the
language of Quaternions by linear and vector functions of a vector.
The addition of one vector quantity to another of the same kind
is performed according to the rule given in Statics for the com
position of forces. In fact, the proof which Poisson gives of the
parallelogram of forces is applicable to the composition of any
quantities such that a reversal of their sign is equivalent to turning
them end for end.
When we wish to denote a vector quantity by a single symbol,
and to call attention to the fact that it is a vector, so that we must
consider its direction as well as its magnitude, we shall denote
it by a German capital letter, as fy, 33, &c.
In the calculus of Quaternions, the position of a point in space
is defined by the vector drawn from a fixed point, called the origin,
to that point. If at that point of space we have to consider any
physical quantity whose value depends on the position of the point,
that quantity is treated as a function of the vector drawn from
the origin. The function may be itself either scalar or vector.
The density of a body, its temperature, its hydrostatic pressure,
the potential at a point, are examples of scalar functions. The
resultant force at the point, the velocity of a fluid at that point,
the velocity of rotation of an element of the fluid, and the couple
producing rotation, are examples of vector functions.
12.] Physical vector quantities may be divided into two classes,
in one of which the quantity is defined with reference to a line,
while in the other the quantity is defined with reference to an
area.
For instance, the resultant of an attractive force in any direction
may be measured by finding the work which it would do on a
body if the body were moved a short distance in that direction
and dividing it by that short distance. Here the attractive force
is defined with reference to a line.
On the other hand, the flux of heat in any direction at any
point of a solid body may be defined as the quantity of heat which
crosses a small area drawn perpendicular to that direction divided
by that area and by the time. Here the flux is defined with
reference to an area.
13.] FORCES AND FLUXES. 11
There are certain cases in which a quantity may be measured
with reference to a line as well as with reference to an area.
Thus, in treating- of the displacements of elastic solids, we may
direct our attention either to the original and the actual position
of a particle, in which case the displacement of the particle is
measured by the line drawn from the first position to the second,
or we may consider a small area fixed in space, and determine
what quantity of the solid passes across that area during the dis
placement.
In the same way the velocity of a fluid may be investigated
either with respect to the actual velocity of the individual parti
cles, or with respect to the quantity of the fluid which flows through
any fixed area.
But in these cases we require to know separately the density of
the body as well as the displacement or velocity, in order to apply
the first method, and whenever we attempt to form a molecular
theory we have to use the second method.
In the case of the flow of electricity we do not know anything
of its density or its velocity in the conductor, we only know the
value of what, on the fluid theory, would correspond to the product
of the density and the velocity. Hence in all such cases we must
apply the more general method of measurement of the flux across
an area.
In electrical science, electromotive force and magnetic force
belong to the first class, being defined with reference to lines.
When we wish to indicate this fact, we may refer to them as
Forces.
On the other hand, electric and magnetic induction, and electric
currents, belong to the second class, being defined with reference
to areas. When we wish to indicate this fact, we shall refer to them
as Fluxes.
Each of these forces may be considered as producing, or tending
to produce, its corresponding flux. Thus, electromotive force pro
duces electric currents in conductors, and tends to produce them
in dielectrics. It produces electric induction in dielectrics, and pro
bably in conductors also. In the same sense, magnetic force pro
duces magnetic induction.
13.] In some cases the flux is simply proportional to the force
and in the same direction, but in other cases we can only affirm
that the direction and magnitude of the flux are functions of the
direction and magnitude of the force.
12 PRELIM1NAKY. [14.
The case in which the components of the flux are linear functions
of those of the force is discussed in the chapter on the Equations
of Conduction, Art. 296. There are in general nine coefficients
which determine the relation between the force and the flux. In
certain cases we have reason to believe that six of these coefficients
form three pairs of equal quantities. In such cases the relation be
tween the line of direction of the force and the normal plane of the
flux is of the same kind as that between a diameter of an ellipsoid
and its conjugate diametral plane. In Quaternion language, the
one vector is said to be a linear and vector function of the other, and
when there are three pairs of equal coefficients the function is said
to be self-conjugate.
In the case of magnetic induction in iron, the flux, (the mag
netization of the iron,) is not a linear function of the magnetizing
force. In all cases, however, the product of the force and the
flux resolved in its direction, gives a result of scientific import
ance, and this is always a scalar quantity.
14.] There are two mathematical operations of frequent occur
rence which are appropriate to these two classes of vectors, or
directed quantities.
In the case of forces, we have to take the integral along a line
of the product of an element of the line, and the resolved part of
the force along that element. The result of this operation is
called the Line-integral of the force. It represents the work
done on a body carried along the line. In certain cases in which
the line-integral does not depend on the form of the line, but
only on the position of its extremities, the line-integral is called
the Potential.
In the case of fluxes, we have to take the integral, over a surface,
of the flux through every element of the surface. The result of
this operation is called the Surface-integral of the flux. It repre
sents the quantity which passes through the surface.
There are certain surfaces across which there is no flux. If
two of these surfaces intersect, their line of intersection is a line
of flux. In those cases in which the flux is in the same direction
as the force, lines of this kind are often called Lines of Force. It
would be more correct, however, to speak of them in electrostatics
and magnetics as Lines of Induction, and in electrokinematics as
Lines of Flow.
15.] There is another distinction between different kinds of
directed quantities, which, though very important in a physical
1 6.] LINE-INTEGRALS. 1 3
point of view, is not so necessary to be observed for the sake of
the mathematical methods. This is the distinction between longi
tudinal and rotational properties.
The direction and magnitude of a quantity may depend upon
some action or effect which takes place entirely along a certain
line, or it may depend upon something of the nature of rota
tion about that line as an axis. The laws of combination of
directed quantities are the same whether they are longitudinal or
rotational, so that there is no difference in the mathematical treat
ment of the two classes, but there may be physical circumstances
which indicate to which class we must refer a particular pheno
menon. Thus, electrolysis consists of the transfer of certain sub
stances along a line in one direction, and of certain other sub
stances in the opposite direction, which is evidently a longitudinal
phenomenon, and there is no evidence of any rotational effect
about the direction of the force. Hence we infer that the electric
current which causes or accompanies electrolysis is a longitudinal,
and not a rotational phenomenon.
On the other hand, the north and south poles of a magnet do
not differ as oxygen and hydrogen do, which appear at opposite
places during electrolysis, so that we have no evidence that mag
netism is a longitudinal phenomenon, while the effect of magnetism
in rotating the plane of polarized light distinctly shews that mag
netism is a rotational phenomenon.
On Line-integrals.
16.] The operation of integration of the resolved part of a vector
quantity along a line is important in physical science generally,
and should be clearly understood.
Let X) y, z be the coordinates of a point P on a line whose
length, measured from a certain point A, is s. These coordinates
will be functions of a single variable s.
Let R be the value of the vector quantity at P, and let the
tangent to the curve at P make with the direction of R the angle e,
then .72 cose is the resolved part of R along the line, and the
integral
C*
= /
^o
R cos e
is called the line-integral of R along the line s.
We may write this expression
14 PRELIMINARY. [l6.
where X, T, Z are the components of R parallel to #, y, z respect
ively.
This quantity is, in general, different for different lines drawn
between A and P. When, however, within a certain region, the
quantity
Xdx+ Ydy + Zdz = -DV,
that is, is an exact differential within that region, the value of L
becomes Jj = \^ A \^ P)
and is the same for any two forms of the path between A and P,
provided the one form can be changed into the other by con
tinuous motion without passing out of this region.
On Potentials.
The quantity ^ is a scalar function of the position of the point,
and is therefore independent of the directions of reference. It is
called the Potential Function, and the vector quantity whose com
ponents are X, Y, Z is said to have a potential ^, if
*-- --($ --<>
When a potential function exists, surfaces for which the po
tential is constant are called Equipotential surfaces. The direction
of R at any point of such a surface coincides with the normal to
dty
the surface, and if n be a normal at the point P. then R = =-
dn
The method of considering the components of a vector as the
first derivatives of a certain function of the coordinates with re
spect to these coordinates was invented by Laplace * in his treat
ment of the theory of attractions. The name of Potential was first
given to this function by Green f, who made it the basis of his
treatment of electricity. Green s essay was neglected by mathe
maticians till 1846, and before that time most of its important
theorems had been rediscovered by Gauss, Chasles, Sturm, and
Thomson J.
In the theory of gravitation the potential is taken with the
opposite sign to that which is here used, and the resultant force
in any direction is then measured by the rate of increase of the
* Mec. Celeste, liv. iii.
t Essay on the Application of Mathematical Analysis to the Theories of Electricity
and Magnetism, Nottingham, 1828. Eeprinted in Crelle s Journal, and in Mr. Ferrer s
edition of Green s Works.
J Thomson and Tait, Natural Philosophy, 483.
17.] RELATION BETWEEN FORCE AND POTENTIAL. 15
potential function in that direction. In electrical and magnetic
investigations the potential is defined so that the resultant force
in any direction is measured by the decrease of the potential in
that direction. This method of using the expression makes it
correspond in sign with potential energy, which always decreases
when the bodies are moved in the direction of the forces acting
on them.
17.] The geometrical nature of the relation between the poten
tial and the vector thus derived from it receives great light from
Hamilton s discovery of the form of the operator by which the vector
is derived from the potential.
The resolved part of the vector in any direction is, as we have
seen, the first derivative of the potential with respect to a co
ordinate drawn in that direction, the sign being reversed.
Now if i t /, k are three unit vectors at right angles to each
other, and if X, Y, Z are the components of the vector 5 resolved
parallel to these vectors, then
9 = IX+jY+kZ; (1)
and by what we have said above, if ^ is the potential,
If we now write V for the operator,
(4)
The symbol of operation V may be interpreted as directing us
to measure, in each of three rectangular directions, the rate of
increase of &, and then, considering the quantities thus found as
vectors, to compound them into one. This is what we are directed
to do by the expression (3). But we may also consider it as directing
us first to find out in what direction ^ increases fastest, and then
to lay off in that direction a vector representing this rate of
increase.
M. Lame, in his Traite des Fonctions Inverses, uses the term
Differential Parameter to express the magnitude of this greatest
rate of increase, but neither the term itself, nor the mode in which
Lame uses it, indicates that the quantity referred to has direction
as well as magnitude. On those rare occasions in which I shall have
to refer to this relation as a purely geometrical one, I shall call the
vector g the Slope of the scalar function ^, using the word Slope
16 PRELIMINARY. [l8.
to indicate the direction, as well as the magnitude, of the most
rapid decrease of #.
18.] There are cases, however, in which the conditions
dZ dY dX dZ dY dX
;- = 0, -j = 0, and =- = 0,
dy dz dz ax dx dy
which are those of Xdx + Ydy -f Zdz being a complete differential,
are fulfilled throughout a certain region of space, and yet the line-
integral from A to P may be different for two lines, each of
which lies wholly within that region. This may be the case if
the region is in the form of a ring, and if the two lines from A
to P pass through opposite segments of the ring. In this case,
the one path cannot be transformed into the other by continuous
motion without passing out of the region.
We are here led to considerations belonging to the Geometry
of Position, a subject which, though its importance was pointed
out by Leibnitz and illustrated by Gauss, has been little studied.
The most complete treatment of this subject has been given by
J. B. Listing *.
Let there be p points in space, and let I lines of any form be
drawn joining these points so that no two lines intersect each
other, and no point is left isolated. We shall call a figure com
posed of lines in this way a Diagram. Of these lines, p 1 are
sufficient to join the p points so as to form a connected system.
Every new line completes a loop or closed path, or, as we shall
call it, a Cycle. The number of independent cycles in the diagram
is therefore K = I jo+1.
Any closed path drawn along the lines of the diagram is com
posed of these independent cycles, each being taken any number of
times and in either direction.
The existence of cycles is called Cyclosis, and the number of
cycles in a diagram is called its Cyclomatic number.
Cyclosis in Surfaces and Regions.
Surfaces are either complete or bounded. Complete surfaces are
either infinite or closed. Bounded surfaces are limited by one or
more closed lines, which may in the limiting cases become finite
lines or points.
A finite region of space is bounded by one or more closed
surfaces. Of these one is the external surface, the others are
* Der Census RaumlicTier Complexe, Gott. Abh., Bd. x. S. 97 (1861).
19.] CYCLIC REGIONS. 17
included in it and exclude each other, and are called internal
surfaces.
If the region has one bounding- surface, we may suppose that
surface to contract inwards without breaking its continuity or
cutting itself. If the region is one of simple continuity, such as
a sphere, this process may be continued till it is reduced to a
point; but if the region is like a ring, the result will be a closed
curve; and if the region has multiple connexions, the result will
be a diagram of lines, and the cyclomatic number of the diagram
will be that of the region. The space outside the region has the
same cyclomatic number as the region itself. Hence, if the region
is bounded by internal as well as external surfaces, its cyclomatic
number is the sum of those due to all the surfaces.
When a region encloses within itself other regions, it is called a
Periphractic region.
The number of internal bounding surfaces of a region is called
its periphractic number. A closed surface is also periphractic, its
number being unity.
The cyclomatic number of a closed surface is twice that of the
region which it bounds. To find the cyclomatic number of a
bounded surface, suppose all the boundaries to contract inwards,
without breaking continuity, till they meet. The surface will then
be reduced to a point in the case of an acyclic surface, or to a linear
diagram in the case of cyclic surfaces. The cyclomatic number of
the diagram is that of the surface.
19.] THEOREM I. If throughout any acyclic region
Xdx+Ydy + Zch =-*,
the value of the line-integral from a point A to a point P taken
along any path within the region will le the same.
We shall first shew that the line-integral taken round any closed
path within the region is zero.
Suppose the equipotential surfaces drawn. They are all either
closed surfaces or are bounded entirely by the surface of the region,
so that a closed line within the region, if it cuts any of the sur
faces at one part of its path, must cut the same surface in the
opposite direction at some other part of its path, and the corre
sponding portions of the line-integral being equal and opposite,
the total value is zero.
Hence if AQP and AQ P are two paths from A to P, the line-
integral for AQ P is the sum of that for AQP and the closed path
VOL. i. c
18 PRELIMINARY. [20.
AQ PQA. But the line-integral of the closed path is zero, there
fore those of the two paths are equal.
Hence if the potential is given at any one point of such a
region, that at any other point is determinate.
20.] THEOREM II. In a cyclic region in which the equation
Xdx+Ydy + Zdz = -D*
is everywhere fulfilled, the line-integral from, A to P, along a
line drawn within the region, will not in general be determinate
unless the channel of communication between A and P be specified,
Let K be the cyclomatic number of the region, then K sections
of the region may be made by surfaces which we may call Dia
phragms, so as to close up K of the channels of communication,
and reduce the region to an acyclic condition without destroying
its continuity.
The line-integral from A to any point P taken along a line
which does not cut any of these diaphragms will be, by the last
theorem, determinate in value.
Now let A and P be taken indefinitely near to each other, but
on opposite sides of a diaphragm, and let K be the line-integral
from A to P.
Let A and P be two other points on opposite sides of the same
diaphragm and indefinitely near to each other, and let K be the
line-integral from A to F. Then K = K.
For if we draw AA and PP 7 , nearly coincident, but on opposite
sides of the diaphragm, the line-integrals aloug these lines will be
equal. Suppose each equal to Z, then the line-integral of A P* is
equal to that of A A + AP + PP= -L+K+L = K = that of AP.
Hence the line-integral round a closed curve which passes through
one diaphragm of the system in a given direction is a constant
quantity K. This quantity is called the Cyclic constant corre
sponding to the given cycle.
Let any closed curve be drawn within the region, and let it cut
the diaphragm of the first cycle p times in the positive direction
and p times in the negative direction, and let p p = % . Then
the line-integral of the closed curve will be % K r
Similarly the line-integral of any closed curve will be
where n K represents the excess of the number of positive passages
of the curve through the diaphragm of the cycle K over the
number of negative passages.
21.] SURFACE-INTEGRALS. 19
If two curves are such that one of them may be transformed
into the other by continuous motion without at any time passing*
through any part of space for which the condition of having a
potential is not fulfilled,, these two curves are called Reconcileabje
curves. Curves for which this transformation cannot be effected
are called Irreconcileable curves *.
The condition that Xdx+Ydy -\- Zdz is a complete differential
of some function ^ for all points within a certain region, occurs in
several physical investigations in which the directed quantity and
the potential have different physical interpretations.
In pure kinematics we may suppose X, Y, Z to be the com
ponents of the displacement of a point of a continuous body whose
original coordinates are x, y, z, then the condition expresses that
these displacements constitute a non-rotational strain f.
If X, Y, Z represent the components of the velocity of a fluid at
the point x, y, z, then the condition expresses that the motion of the
fluid is irrotational.
If X, Y, Z represent the components of the force at the point
#, i/j z, then the condition expresses that the work done on a
particle passing from one point to another is the difference of the
potentials at these points, and the value of this difference is the
same for all reconcileable paths between the two points.
On Surface-Integrals.
21.] Let dS be the element of a surface, and c the angle which
a normal to the surface drawn towards the positive side of the
surface makes with the direction of the vector quantity R, then
R cos tdS is called the surface-integral of It over the surface S.
ff
THEOREM III. The surface-integral of the flux through a closed
surface may be expressed as the volume-integral of its convergence
taken within the surface. (See Art. 25.)
Let X, Y, Z be the components of R, and let I, m, n be the
direction-cosines of the normal to S measured outwards. Then the
surface-integral of R over S is
{(R cos e dS = ffxidS + JJYmdS + j j ZndS
= IJXdydz+jJYdzdx -f- ft Zdxdy, (1)
* See Sir W. Thomson C 0n Vortex Motion, Trans. R. S. Edin., 1869.
t See Thomson and Tait s Natural Philosophy, 190 ().
C 2
20 PRELIMINARY. [21.
the values of X, Y, Z being those at a point in the surface, and
the integrations being extended over the whole surface.
If the surface is a closed one, then, when y and z are given,
the coordinate sc must have an even number of values, since a line
parallel to x must enter and leave the enclosed space an equal
number of times provided it meets the surface at all.
Let a point travelling from # = oo to # = +oo first enter
the space when os = as l9 then leave it when x = a? 2 , and so on;
and let the values of X at these points be X 1 , X 2 , &c., then
,- X 3 ) + &c. 4 (li.-.Xi.-,)} <fyfe. (2)
If Jf is a quantity which is continuous, and has no infinite values
between and # 2 , then
dX
where the integration is extended from the first to the second
intersection, that is, along the first segment of x which is within
the closed surface. Taking into account all the segments which lie
within the closed surface, we find
the double integration being confined to the closed surface, but
the triple integration being extended to the whole enclosed space.
Hence, if X, J, Z are continuous and finite within a closed surface
$, the total surface-integral of R over that surface will be
IT*, ffr/dX dY dz \ -
JJ****-JJJfc + ^ + jg)*** (5)
the triple integration being extended over the whole space within &
Let us next suppose that X, Y, Z are not continuous within the
closed surface, but that at a certain surface F (x, y> z] = the
values of X } Y y Z alter abruptly from X, Y, Z on the negative side
of the surface to X , Y , Z on the positive side.
If this discontinuity occurs, say, between a? t and # 2 , the value
J x . dx
where in the expression under the integral sign only the finite
values of the derivative of X are to be considered.
In this case therefore the total surface-integral of R over the
closed surface will be expressed by
22.] SOLENOIDAL DISTRIBUTION. 21
+
jj(Y -Y)dzdx + ff(Z -Z)dxdy; (7)
or, if / , m , ft are the direction-cosines of the normal to the surface
of discontinuity, and dS an element of that surface,
, (8)
where the integration of the last term is to be extended over the
surface of discontinuity.
If at every point where X, Y, Z are continuous
dX dY dZ
^ + ^ + ^ = > (9)
and at every surface where they are discontinuous
I X + m T + n Z = I X+ m Y+ n Z, (10)
then the surface-integral over every closed surface is zero, and the
distribution of the vector quantity is said to be Solenoidal.
We shall refer to equation (9) as the General solenoidal con
dition, and to equation (10) as the Superficial solenoidal condition.
22.] Let us now consider the case in which at every point
within the surface 8 the equation
+ + *?.0 (11)
dx dy dz
is fulfilled. We have as a consequence of this the surface-integral
over the closed surface equal to zero.
Now let the closed surface S consist of three parts S lt S , and
S 2 . Let S 1 be a surface of any form bounded by a closed line L r
Let S Q be formed by drawing lines from every point of L always
coinciding with the direction of E. If I, m, n are the direction-
cosines of the normal at any point of the surface $ , we have
RcoB* = Xl+Ym + Zn = 0. (12)
Hence this part of the surface contributes nothing towards the
value of the surface-integral.
Let # 2 be another surface of any form bounded by the closed
curve L. 2 in which it meets the surface S .
Let Q 1} Q , Q 2 be the surface-integrals of the surfaces S IS S ,S 2 ,
and let Q be the surface-integral of the closed surface S. Then
22 PRELIMINARY. [2,2.
and we know that Q Q = ; (14)
therefore Q 2 = - Q l ; (15)
or, in other words, the surface-integral over the surface $ 2 is equal
and opposite to that over S L whatever be the form and position
of 2 , provided that the intermediate surface S is one for which R
is always tangential.
If we suppose Z^ a closed curve of small area,, $ will be a
tubular surface having the property that the surface-integral over
every complete section of the tube is the same.
Since the whole space can be divided into tubes of this kind
provided dX dY dZ
-T- + -T- + -j- = 0, (16)
dx du dz
J
a distribution of a vector quantity consistent with this equation is
called a Solenoidal Distribution.
On Tubes and Lines of Flow.
If the space is so divided into tubes that the surface-integral
for every tube is unity, the tubes are called Unit tubes, and the
surface-integral over any finite surface S bounded by a closed
curve L is equal to the number of such tubes which pass through
S in the positive direction, or, what is the same thing, the number
which pass through the closed curve L.
Hence the surface-integral of S depends only on the form of
its boundary L, and not on the form of the surface within its
boundary.
On Peripkractic Regions.
If, throughout the whole region bounded externally by the single
closed surface S lt the solenoidal condition
dX dY dZ^_
dx dv dz
t/
is fulfilled, then the surface-integral taken over any closed surface
drawn within this region will be zero, and the surface-integral
taken over a bounded surface within the region will depend only
on the form of the closed curve which forms its boundary.
It is not, however, generally true that the same results follow
if the region within which the solenoidal condition is fulfilled is
bounded otherwise than by a single surface.
For if it is bounded by more than one continuous surface, one of
these is the external surface and the others are internal surfaces,
22.] PERIPHRACTIC REGIONS. 23
and the region S is a periphractic region, having within it other
regions which it completely encloses.
If within any of these enclosed regions, S^ the solenoidal con
dition is not fulfilled, let
*-//
R cos e dS l
be the surface-integral for the surface enclosing this region, and
let Q 2 , Q 3 , &c. be the corresponding quantities for the other en
closed regions.
Then, if a closed surface S is drawn within the region S t the
value of its surface-integral will be zero only when this surface
/S" does not include any of the enclosed regions S 19 S 2 , &c. If it
includes any of these, the surface-integral is the sum of the surface-
integrals of the different enclosed regions which lie within it.
For the same reason, the surface-integral taken over a surface
bounded by a closed curve is the same for such surfaces only bounded
by the closed curve as are reconcileable with the given surface by
continuous motion of the surface within the region S.
When we have to deal with a periphractic region, the first thing
to be done is to reduce it to an aperiphractic region by drawing
lines joining the different bounding surfaces. Each of these lines,
provided it joins surfaces which were not already in continuous
connexion, reduces the periphractic number by unity, so that the
whole number of lines to be drawn to remove the periphraxy is
equal to the periphractic number, or the number of internal sur
faces. When these lines have been drawn we may assert that if
the solenoidal condition is fulfilled in the region S, any closed surface
drawn entirely within S, and not cutting any of the lines, has its
surface-integral zero.
In drawing these lines we must remember that any line joining
surfaces which are already connected does not diminish the peri
phraxy, but introduces cyclosis.
The most familiar example of a periphractic region within which
the solenoidal condition is fulfilled is the region surrounding a mass
attracting or repelling inversely as the square of the distance.
In this case we have
/>* y* g
X = m > Y= m > Z = m ;
r 3 r 3 r 3
where m is the mass supposed to be at the origin of coordinates.
At any point where r is finite
dX dY dZ
T + -j- + -r =
ax ay dz
24 PRELIMINARY. [23.
but at the origin these quantities become infinite. For any closed
surface not including the origin, the surface-integral is zero. If
a closed surface includes the origin, its surface-integral is 4?m.
If, for any reason, we wish to treat the region round m as if it
were not periphractic, we must draw a line from m to an infinite
distance, and in taking surface-integrals we must remember to add
4Trm whenever this line crosses from the negative to the positive
side of the surface.
On Right-handed and Left-handed Relations in Space.
23.] In this treatise the motions of translation along any axis
and of rotation about that axis, will be assumed to be of the same
sign when their directions correspond to those of the translation
and rotation of an ordinary or right-handed screw *.
For instance, if the actual rotation of the earth from west to east
is taken positive, the direction of the earth s axis from south to
north will be taken positive, and if a man walks forward in the
positive direction, the positive rotation is in the order, head, right-
hand, feet, left-hand.
If we place ourselves on the positive side of a surface, the positive
direction along its bounding curve will be opposite to the motion
of the hands of a watch with its face towards us.
This is the right-handed system which is adopted in Thomson
and Tait s Natural Philosophy, 243. The opposite, or left-handed
system, is adopted in Hamilton s and Tait s Quaternions. The
operation of passing from the one system to the other is called, by
Listing, Perversion.
The reflexion of an object in a mirror is a perverted image of the
object.
"When we use the Cartesian axes of %, y, z, we shall draw them
* The combined action of the muscles of the arm when we turn the upper side of
the right-hand outwards, and at the same time thrust the hand forwards, will
impress the right-handed screw motion on the memory more firmly than any verbal
definition. A common corkscrew may be used as a material symbol of the same
relation.
Professor W. H. Miller has suggested to me that as the tendrils of the vine are
right-handed screws and those of the hop left-handed, the two systems of relations in
space might be called those of the vine and the hop respectively.
The system of the vine, which we adopt, is that of Linnseus, and of screw-makers
in all civilized countries except Japan. De Candolle was the first who called the
hop-tendril right-handed, and in this he is followed by Listing, and by most writers
on the rotatory polarization of light. Screws like the hop-tendril are made for the
couplings of railway-carriages, and for the fittings of wheels on the left side of ordinary
carriages, but they are always called left-handed screws by those who use them.
24.] LINE-INTEGRAL AND SURFACE-INTEGRAL. 25
so that the ordinary conventions about the cyclic order of the
symbols lead to a right-handed system of directions in space. Thus,
if x is drawn eastward and y northward, z must be drawn upward.
The areas of surfaces will be taken positive when the order of
integration coincides with the cyclic order of the symbols. Thus,
the area of a closed curve in the plane of xy may be written either
\xdy
or
the order of integration being x, y in the first expression, and y^ x
in the second.
This relation between the two products dx dy and dy d-x may
be compared with that between the products of two perpendicular
vectors in the doctrine of Quaternions, the sign of which depends
on the order of multiplication, and with the reversal of the sign
of a determinant when the adjoining rows or columns are ex
changed.
For similar reasons a volume-integral is to be taken positive when
the order of integration is in the cyclic order of the variables x t y, z,
and negative when the cyclic order is reversed.
We now proceed to prove a theorem which is useful as esta
blishing a connexion between the surface-integral taken over a
finite surface and a line-integral taken round its boundary.
24.] THEOREM IV. A line-integral taken round a closed curve
may be expressed in terms of a surface-integral taken over a
surface bounded by the curve.
Let X, Y } Z be the components of a vector quantity 51 whose line-
integral is to be taken round a closed curve s.
Let S be any continuous finite surface bounded entirely by the
closed curve s, and let f, 77, f be the components of another vector
quantity 33, related to X, Y, Z by the equations
_
dy dz ~ dz dx ~ dx dy
Then the surface-integral of 3 taken over the surface S is equal to
the line-integral of 51 taken round the curve s. It is manifest that
, YJ, f fulfil of themselves the so lenoidal condition
d( drj dC
_z i __ I _j __ __ o.
dx dy dz
Let /, m, n be the direction-cosines of the normal to an element
26 PEELIMINARY. [24.
of the surface dS t reckoned in the positive direction. Then the
value of the surface-integral of 33 may be written
(2)
In order to form a definite idea of the meaning of the element
dS, we shall suppose that the values of the coordinates x, y, z for
every point of the surface are given as functions of two inde
pendent variables a and p. If ft is constant and a varies, the point
(>, y, z) will describe a curve on the surface, and if a series of values
is given to /3, a series of such curves will be traced, all lying on
the surface S. In the same way, by giving a series of constant
values to a, a second series of curves may be traced, cutting the
first series, and dividing the whole surface into elementary portions,
any one of which may be taken as the element dS.
The projection of this element on the plane of y, z is, by the
ordinary formula,
, 70 ,dy dz dy dz^ _ . *
IdS = (-T- TS - -A -T-) dP da - (3)
\a dp dp da
The expressions for mdS and ndS are obtained from this by sub
stituting x, y> z in cyclic order.
The surface-integral which we have to find is
(4)
or, substituting the values of , 77, f in terms of X, Y, Z,
dX dX dY dY dZ dZ
The part of this which depends on X may be written
dXsdzdx dz dx dX ,dx dy dx dy
~
, ,. , , . dXdx dx . . .
adding and subtracting -= 7 - , this becomes
3 dx da dp
f C ( dx ,dX dx dX dy dX dz^
JJ (dp \da da dy da dz do)
dx ,dX dx dX dy dX ,
~^" + ~ + ~ P a
As we have made no assumption as to the form of the functions
a and /3, we may assume that a is a function of X, or, in other
words, that the curves for which a is constant are those for which
25.] HAMILTON S OPERATOR v. 27
7 1?"
X is constant. In this case -^- = 0, and the expression becomes
dp
by integration with respect to a,
C CdX dx -, f ^r dx ,
/ / dQda. = X-T-d8; (9)
JJ da dp J dp
where the integration is now to be performed round the closed
curve. Since all the quantities are now expressed in terms of one
variable & we may make s, the length of the bounding curve, the
independent variable, and the expression may then be written
AS* < io >
where the integration is to be performed round the curve s. We
may treat in the same way the parts of the surface -integral which
depend upon T and Z, so that we get finally,
where the first integral is extended over the surface , and the
second round the bounding curve s *.
On the effect of the operator V on a vector function.
25.] We have seen that the operation denoted by V is that by
which a vector quantity is deduced from its potential. The same
operation, however, when applied to a vector function, produces
results which enter into the two theorems we have just proved
(III and IV). The extension of this operator to vector displacements,
and most of its further development, is due to Professor Tait f.
Let o- be a vector function of p, the vector of a variable point.
Let us suppose, as usual, that
p = ix+jy + kz,
and o- = iX+jY+kZ;
where X y Y, Z are the components of o- in the directions of the
axes.
We have to perform on cr the operation
. d . d 7 d
V = i-j- +J-T +b-r
dx ay dz
Performing this operation, and remembering the rules for the
* This theorem was given by Professor Stokes. Smith s Prize Examination, 1854,
question 8. It is proved in Thomson and Tait s Natural Philosophy, 190 (f).
t See Proc. R. S. Edin., April 28, 1862. On Green s and other allied Theorems,
Trans. R. S. Edin., 1869-70, a very valuable paper ; and On some Quaternion
Integrals, Proc. R. S. Edin., 1870-71.
28 PRELIMINARY. [25.
multiplication of i, /, /, we find that V a- consists of two parts,
one scalar and the other vector.
The scalar part is
,dX dY dZ. TTT
#V <T = (-7- + -j- + -j-}t see Theorem III,
\dx dy dz
and the vector part is
. ( dZ dY. ,,dX d2\ /./^_^\
* VJJT ~ ~3i) + <? \dz~~fa> + ^ ~dgJ
If the relation between X, Y } Z and , r/, f is that given by
equation (1) of the last theorem, we may write
F V o- = & + y rj + Jc C See Theorem IV.
It appears therefore that the functions of X, Y, Z which occur
in the two theorems are both obtained by the operation V on the
vector whose components are X, Y, Z. The theorems themselves
may be written
jjjSVvds =jfs.vUvds, (III)
and fsvdp =JJ8.V<FUvd9i (IV)
where d 9 is an element of a volume, ds of a surface, dp of a curve,
and Uv a unit- vector in the direction of the normal.
To understand the meaning 1 of these functions of a vector, let us
suppose that o- is the value of o- at a point P, and let us examine
the value of o- o- in the neighbourhood of P.
If we draw a closed surface round P } then, if the
I/ surface-integral of o- over this surface is directed
inwards, S V o- will be positive, and the vector
p O-O-Q near the point P will be on the whole
/ X^ directed towards P, as in the figure (1).
I propose therefore to call the scalar part of
jv i V (T the convergence of o- at the point P.
To interpret the vector part of Vo-, let us
suppose ourselves to be looking in the direction of the vector
whose components are f, 77, and let us examine
-* the vector o- o- near the point P. It will appear
I p . as in the figure (2), this vector being arranged on
the whole tangentially in the direction opposite to
the hands of a watch.
I propose (with great diffidence) to call the vector
part of V o- the curl, or the version of o- at the point P.
26.] CONCENTRATION. 29
At Fig. 3 we have an illustration of curl combined with con
vergence.
Let us now consider the meaning of the equation ,
VV a = 0. \
This implies that V <r is a scalar, or that the vector X
o- is the slope of some scalar function $?. These f
applications of the operator V are due to Professor ^
Tait *. A more complete development of the theory
is given in his paper On Green s and other allied Theorems f/
to which I refer the reader for the purely Quaternion investigation
of the properties of the operator V.
26.] One of the most remarkable properties of the operator V is
that when repeated it becomes
an operator occurring in all parts of Physics, which we may refer to
as Laplace s Operator.
This operator is itself essentially scalar. When it acts on a
scalar function the result is scalar, when it acts on a vector function
the result is a vector.
If, with any point P as centre, we draw a small sphere whose
radius is r, then if q is the value of q at the centre, and q the
mean value of q for all points within the sphere,
2o-2 = iV>- 2 v 2 2 ;
so that the value at the centre exceeds or falls short of the mean
value according as V 2 q is positive or negative.
I propose therefore to call V 2 <? the concentration of q at the
point P, because it indicates the excess of the value of q at that
point over its mean value in the neighbourhood of the point.
If q is a scalar function, the method of finding its mean value is
well known. If it is a vector function, we must find its mean
value by the rules for integrating vector functions. The result
of course is a vector.
* Proceedings R. S. Edin., 1862. t Trans. R. 8. Edin., 1869-70.
PART I.
ELECTROSTATICS.
CHAPTEE I.
DESCRIPTION OF PHENOMENA.
Electrification by Friction.
27.] EXPERIMENT I *". Let a piece of glass and a piece of resin,
neither of which exhibits any electrical properties, be rubbed to
gether and left with the rubbed surfaces in contact. They will
still exhibit no electrical properties. Let them be separated. They
will now attract each other.
If a second piece of glass be rubbed with a second piece of
resin, and if the pieces be then separated and suspended in the
neighbourhood of the former pieces of glass and resin,, it may be
observed
(1) That the two pieces of glass repel each other.
(2) That each piece of glass attracts each piece of resin.
(3) That the two pieces of resin repel each other.
These phenomena of attraction and repulsion are called Elec
trical phenomena, and the bodies which exhibit them are said to
be electrified, or to be charged with electricity.
Bodies may be electrified in many other ways, as well as by
friction.
The electrical properties of the two pieces of glass are similar
to each other but opposite to those of the two pieces of resin,
the glass attracts what the resin repels and repels what the resin
attracts.
* See Sir W. Thomson On the Mathematical Theory of Electricity, Cambridge
and Dublin Mathematical Journal, March, 1848.
28.] ELECTRIFICATION. 31
If a body electrified in any manner whatever behaves as the
glass does, that is, if it repels the glass and attracts the resin, the
body is said to be vitreously electrified, and if it attracts the glass
and repels the resin it is said to be resinously electrified. All
electrified bodies are found to be either vitreously or resinously
electrified.
It is the established practice of men of science to call the vitreous
electrification positive, and the resinous electrification negative.
The exactly opposite properties of the two kinds of electrification
justify us in indicating them by opposite signs, but the applica
tion of the positive sign to one rather than to the other kind must
be considered as a matter of arbitrary convention, just as it is a
matter of convention in mathematical diagrams to reckon positive
distances towards the right hand.
No force, either of attraction or of repulsion, can be observed
between an electrified body and a body not electrified. When, in
any case, bodies not previously electrified are observed to be acted
on by an electrified body, it is because they have become electrified
by induction.
Electrification by Induction.
28.] EXPERIMENT II *. Let a hollow vessel of metal be hung
up by white silk threads, and let a similar thread
be attached to the lid of the vessel so that the vessel
may be opened or closed without touching it.
Let the pieces of glass and resin be similarly sus
pended and electrified as before.
Let the vessel be originally unelectrified, then if
an electrified piece of glass is hung up within it by
its thread without touching the vessel, and the lid
closed, the outside of the vessel will be found to
be vitreously electrified, and it may be shewn that
the electrification outside of the vessel is exactly the
same in whatever part of the interior space the glass
is suspended.
If the glass is now taken out of the vessel without touching it,
the electrification of the glass will be the same as before it was
put in, and that of the vessel will have disappeared.
This electrification of the vessel, which depends on the glass
* This, and several experiments which follow, are due to Faraday, On Static
Electrical Inductive Action, 1 Phil. Mag., 1843, or Exp. Res., vol. ii. p. 279.
32 ELECTROSTATIC PHENOMENA. [29.
being within it, and which vanishes when the glass is removed, is
called Electrification by induction.
Similar effects would be produced if the glass were suspended
near the vessel on the outside, but in that case we should find
an electrification vitreous in one part of the outside of the vessel
and resinous in another. When the glass is inside the vessel
the whole of the outside is vitreously and the whole of the inside
resinously electrified.
Electrification ly Conduction.
29.] EXPERIMENT III. Let the metal vessel be electrified by
induction, as in the last experiment, let a second metallic body
be suspended by white silk threads near it, and let a metal wire,
similarly suspended, be brought so as to touch simultaneously the
electrified vessel and the second body.
The second body will now be found to be vitreously electrified,
and the vitreous electrification of the vessel will have diminished.
The electrical condition has been transferred from the vessel to
the second body by means of the wire. The wire is called a con
ductor of electricity, and the second body is said to be electrified
by conduction.
Conductors and Insulators.
EXPERIMENT IV. If a glass rod, a stick of resin or gutta-percha,
or a white silk thread, had been used instead of the metal wire, no
transfer of electricity would have taken place. Hence these latter
substances are called Non-conductors of electricity. Non-conduc
tors are used in electrical experiments to support electrified bodies
without carrying off their electricity. They are then called In
sulators.
The metals are good conductors ; air, glass, resins, gutta-percha,
vulcanite, paraffin, &c. are good insulators; but, as we shall see
afterwards, all substances resist the passage of electricity, and all
substances allow it to pass, though in exceedingly different degrees.
This subject will be considered when we come to treat of the
Motion of electricity. For the present we shall consider only two
classes of bodies, good conductors, and good insulators.
In Experiment II an electrified body produced electrification in
the metal vessel while separated from it by air, a non-conducting
medium. Such a medium, considered as transmitting these electrical
effects without conduction, has been called by Faraday a Dielectric
31.] SUMMATION OF ELECTRIC EFFECTS. 33
medium, and the action which takes place through it is called
Induction.
In Experiment III the electrified vessel produced electrification
in the second metallic body through the medium of the wire. Let
us suppose the wire removed, and the electrified piece of glass taken
out of the vessel without touching it, and removed to a sufficient
distance. The second body will still exhibit vitreous electrifica
tion, but the vessel, when the glass is removed, will have resinous
electrification. If we now bring the wire into contact with both
bodies, conduction will take place along the wire, and all electri
fication will disappear from both bodies, shewing that the elec
trification of the two bodies was equal and opposite.
30.] EXPERIMENT V. In Experiment II it was shewn that if
a piece of glass, electrified by rubbing it with resin, is hung up in
an insulated metal vessel, the electrification observed outside does
not depend on the position of the glass. If we now introduce the
piece of resin with which the glass was rubbed into the same vessel,
without touching it or the vessel, it will be found that there is
no electrification outside the vessel. From this we conclude that
the electrification of the resin is exactly equal and opposite to that
of the glass. By putting in any number of bodies, electrified in
any way, it may be shewn that the electrification of the outside of
the vessel is that due to the algebraic sum of all the electrifica
tions, those being reckoned negative which are resinous. We have
thus a practical method of adding the electrical effects of several
bodies without altering the electrification of each.
31.] EXPERIMENT VI. Let a second insulated metallic vessel, J5,
be provided, and let the electrified piece of glass be put into the
first vessel A, and the electrified piece of resin into the second vessel
B. Let the two vessels be then put in communication by the metal
wire, as in Experiment III. All signs of electrification will dis
appear.
Next, let the wire be removed, and let the pieces of glass and of
resin be taken out of the vessels without touching them. It will
be found that A is electrified resinously and B vitreously.
If now the glass and the vessel A be introduced together into a
larger insulated vessel C, it will be found that there is no elec
trification outside C. This shews that the electrification of A is
exactly equal and opposite to that of the piece of glass, and that
of B may be shewn in the same way to be equal and opposite to that
of the piece of resin.
VOL. I. D
34 ELECTROSTATIC PHENOMENA. [32.
We have thus obtained a method of charging a vessel with a
quantity of electricity exactly equal and opposite to that of an
electrified body without altering the electrification of the latter,
and we may in this way charge any number of vessels with exactly
equal quantities of electricity of either kind, which we may take
for provisional units.
32.] EXPERIMENT VII. Let the vessel B, charged with a quan
tity of positive electricity, which we shall call, for the present,
unity, be introduced into the larger insulated vessel C without
touching it. It will produce a positive electrification on the out
side of C. Now let B be made to touch the inside of C. No change
of the external electrification will be observed. If B is now taken
out of C without touching it, and removed to a sufficient distance,
it will be found that B is completely discharged, and that C has
become charged with a unit of positive electricity.
We have thus a method of transferring the charge of B to C.
Let B be now recharged with a unit of electricity, introduced
into C already charged, made to touch the inside of C, and re
moved. It will be found that B is again completely discharged,
so that the charge of C is doubled.
If this process is repeated, it will be found that however highly
C is previously charged, and in whatever way B is charged, when
B is first entirely enclosed in C, then made to touch C, and finally
removed without touching C, the charge of B is completely trans
ferred to C, and B is entirely free from electrification.
This experiment indicates a method of charging a body with
any number of units of electricity. We shall find, when we come
to the mathematical theory of electricity, that the result of this
experiment affords an accurate test of the truth of the theory.
33.] Before we proceed to the investigation of the law of
electrical force, let us enumerate the facts we have already esta
blished.
By placing any electrified system inside an insulated hollow con
ducting vessel, and examining the resultant effect on the outside
of the vessel, we ascertain the character of the total electrification
of the system placed inside, without any communication of elec
tricity between the different bodies of the system.
The electrification of the outside of the vessel may be tested
with great delicacy by putting it in communication with an elec
troscope.
We may suppose the electroscope to consist of a strip of gold
34-] ELECTRICITY AS A QUANTITY. 35
leaf hanging between two bodies charged, one positively, and the
other negatively. If the gold leaf becomes electrified it will incline
towards the body whose electrification is opposite to its own. By
increasing the electrification of the two bodies and the delicacy of
the suspension, an exceedingly small electrification of the gold leaf
may be detected.
When we come to describe electrometers and multipliers we
shall find that there are still more delicate methods of detecting
electrification and of testing the accuracy of our theorems, but at
present we shall suppose the testing to be made by connecting the
hollow vessel with a gold leaf electroscope.
This method was used by Faraday in his very admirable de
monstration of the laws of electrical phenomena *.
34.] I. The total electrification of a body, or system of bodies,
remains always the same, except in so far as it receives electrifi
cation from or gives electrification to other bodies.
In all electrical experiments the electrification of bodies is found
to change, but it is always found that this change is due to want
of perfect insulation, and that as the means of insulation are im
proved, the loss of electrification becomes less. We may therefore
assert that the electrification of a body placed in a perfectly in
sulating medium would remain perfectly constant.
II. When one body electrifies another by conduction, the total
electrification of the two bodies remains the same, that is, the one
loses as much positive or gains as much negative electrification as
the other gains of positive or loses of negative electrification.
For if the two bodies are enclosed in the hollow vessel, no change
of the total electrification is observed.
III. When electrification is produced by friction, or by any
other known method, equal quantities of positive and negative elec
trification are produced.
For the electrification of the whole system may be tested in
the hollow vessel, or the process of electrification may be carried
on within the vessel itself, and however intense the electrification of
the parts of the system may be, the electrification of the whole,
as indicated by the gold leaf electroscope, is invariably zero.
The electrification of a body is therefore a physical quantity
capable of measurement, and two or more electrifications can be
combined experimentally with a result of the same kind as when
* On Static Electrical Inductive Action, Phil. Mag., 1843, or Exp. Res., vol. ii.
p. 249.
D 2
36 ELECTROSTATIC PHENOMENA. [35.
two quantities are added algebraically. We therefore are entitled
to use language fitted to deal with electrification as a quantity as
well as a quality, and to speak of any electrified body as ( charged
with a certain quantity of positive or negative electricity/
35.] While admitting electricity, as we have now done, to the
rank of a physical quantity, we must not too hastily assume that
it is, or is not, a substance, or that it is, or is not, a form of
energy, or that it belongs to any known category of physical
quantities. All that we have hitherto proved is that it cannot
be created or annihilated, so that if the total quantity of elec
tricity within a closed surface is increased or diminished, the in
crease or diminution must have passed in or out through the closed
surface.
This is true of matter, and is expressed by the equation known as
the Equation of Continuity in Hydrodynamics.
It is not true of heat, for heat may be increased or diminished
within a closed surface, without passing in or out through the
surface, by the transformation of some other form of energy into
heat, or of heat into some other form of energy.
It is not true even of energy in general if we admit the imme
diate action of bodies at a distance. For a body outside the closed
surface may make an exchange of energy with a body within
the surface. But if all apparent action at a distance is the
result of the action between the parts of an intervening medium,
and if the nature of this action of the parts of the medium is
clearly understood, then it is conceivable that in all cases of the
increase or diminution of the energy within a closed surface we
may be able to trace the passage of the energy in or out through
that surface.
There is, however, another reason which warrants us in asserting
that electricity, as a physical quantity, synonymous with the total
electrification of a body, is not, like heat, a form of energy. An
electrified system has a certain amount of energy, and this energy
can be calculated by multiplying the quantity of electricity in
each of its parts by another physical quantity, called the Potential
of that part, and taking half the sum of the products. The quan
tities Electricity and Potential, when multiplied together,
produce the quantity Energy. It is impossible, therefore, that
electricity and energy should be quantities of the same category, for
electricity is only one of the factors of energy, the other factor
being Potential.
36.] THEORIES OF ELECTRICITY. 37
Energy, which is the product of these factors, may also be con
sidered as the product of several other pairs of factors, such as
A Force x A distance through which the force is to act.
A Mass X Gravitation acting through a certain height.
A Mass x Half the square of its velocity.
A Pressure X A volume of fluid introduced into a vessel at
that pressure.
A Chemical Affinity x A chemical change, measured by the number
of electro-chemical equivalents which enter
into combination.
If we obtain distinct mechanical ideas of the nature of electric
potential, we may combine these with the idea of energy to
determine the physical category in which Electricity is to be
placed.
36.] In most theories on the subject, Electricity is treated as
a substance, but inasmuch as there are two kinds of electrification
which, being combined, annul each other, and since we cannot
conceive of two substances annulling each other, a distinction has
been drawn between Free Electricity and Combined Electricity.
Theory of Two Fluids.
In the theory called that of Two Fluids, all bodies, in their
unelectrified state, are supposed 
