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Full text of "Davis's Manual of Magnetism: Including Also Electro-magnetism, Magneto-electricity, and Thermo ..."

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DAVIS'S MANUAL 



OF 



MAGNETISM. 



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D A V I S'S 

I 



MAMJAL OF MAGNETISM. 



nrOLUSZHO ALSO 



ELECTRO-MAGNETISM, MAGNETO-ELECTRICITY, 
AND THERMO-ELECTRICITY. 



WITH A DBSORXPTION OF TBB SLBCTSOTTPB PROCBSS* 



FOR THE USE OF STUDENTS AND LITERABY INSTITUTIONS. 



WITH 100 OSIGINAL ILLUSTRATIONS. 



,^N^>^W>«^/%/NA/^«V<i^S/>^\/ 



BOSTON: 
PUBLISHED BY DANIEL DAVIS^ Ji, 

IttAGNETICAL INffTFUMteKT MAKE*, 
No. 11 CornhilU 

1842. 



'tk 



/ 




Entered according to Act of Congress in the year 1842, by 

DANIEL DAVIS, Jr. 
In the Clerk's Office of the District Court of Massachusetts. 



.» 



WILLIAM 8. DAMBfiLL, 

PRINTBB, 

90. 11 GORNHILL. 



:lnr;'. 



PREFACE. 



I 



Magnetism and Electricity have become related 
sciences within so short a period, and their growth 
has been so rapid, that many important facts which 
have been observed have not yet been collected in 
any scientific treatise, and the amount of unwritten 
knowledge has been constantly increasing. For 
this reason it has been necessary, in preparing the 
following work, which is intended as a companion 
to the apparatus manufactyred by me, to give a 
fuller view of these sciences, and more minute 
descriptions of the instruments and experiments 
designed to illustrate them, in their relation to each 
other, than would otherwise have been required. 
This Manual, therefore, will answer the purpose 
of an elementary treatise on those branches of 
science to which it relates, and may be used as a 
text-book. 



Vl PREFACE. 

The aid of several gentlemen scientifically ac- 
quainted with the subject has been obtained in 
describing the various instruments, the experiments 
which may be performed with them, and the prin- 
ciples on which they depend. The object, which 
has been kept in view, is in all cases simply to state 
the facts which have been observed, and to gener- 
alize them only so far as the progress of discovery 
has fully authorized. The theories concerning mag- 
netism and electricity in their relation to each other, 
which have been discussed in the scientific journals 
of Europe and America, must yet be regarded as 
hypothetical, and have been as far as possible 
avoided. 

It will be found that many of the observations 
recorded here, and many of the instruments de- 
scribed, are new. Wood cuts have been introduced, 
wherever, from the nature of the instrument or 
experiment under consideration, it has been deemed 
advisable in order to ensure a clear comprehension 
of the subject. 

BosToir, August, 1842. 



CONTENTS. 



INTRODUCTORY CHAPTER. 

Page 
DEnNinONS AND EXPLANATIONS, 1 

PRODUCTION OF ELECTRICITY. 

1. MECHANICAL OR FRICTIONAL ELECTRICITY, 6 

2. GALVANIC OR VOLTAIC ELECTRICITY, 7 

3. THERMO-ELECTRICITY, 21 

4. ANIMAL ELECTRICITY, 34 



MAGNETISM. 



CHAPTER I. 

DIRECTIVE TENDENCY OF THE MAGNET. 

1. IN REFERENCE TO ANOTHER MAGNET, 35 

2. IN REFERENCE TO A CURRENT OF ELECTRICITY, 43 

3. IN REFERENCE TO THE EARTH, 62 



Vm CONTENTS. 

CHAPTER II. 

INDUCTION OF MAGNETISM. 

1. BY THE INFLUENCE OF A MAGNET, 61 

2. BY THE INFLUENCE OF A CURRENT OF ELECTRICITY, 68 

3. BY THE INFLUENCE OF THE EARTH, 122 



CHAPTER III. 

INDUCTION OF ELECTRICITY. 

1. BY THE LNFLUENCE OF A CURRENT OF ELECTRICITY, 125 

2. BY THE INFLUENCE OF A MAGNET, 153 

3. BY THE INFLUENCE OF THE EARTH, 197 

I 



THE ELECTROTYPE PROCESS. 

1. ORIGIN OF THE ELECTROTYPE, 199 

2. PROCESSES FOR ELECTROTYPES IN COPPER, 200 

3. PROCESSES FOR GILDING, SILVERING, AND PLATINATING,.. 205 



INTRODUCTORY CHAPTER. 

DEFINITIONS AND EXPLANATIONS. 

1. Magnetism. The term magnetism expresses the 
peculiar properties of attraction^repulsion, &c.,possessed, 
under certain circumstances, by iron and some of its 
compounds, and in a feebler degree by the metals nickel 
and cobalt. Hammered brass is said to be sometimes 
magnetic. The science which treats of these proper- 
ties is also called magnetism. 

Electro-Magnetism. That branch of science which 
relates to the development of magnetism by means of a 
current of electricity, is called electro-magnetism. It 
will be treated of in chapter I, section 2, and in chapter 
II, section 2. 

Magneto-Electricity treats of the development of 
electricity by the influence of magnetism, and will form 
the subject of chapter III, section 2. 

2. The Magnet. Any body in which the magnetic 
phenomena manifest themselves, is called a magnet It 
may be of any form, but it must be composed in whole 
or in part of iron, nickel, or cobalt. 

Natural Magnets. Certain ores of iron are found to 
be possessed of the magnetic properties in their natural 
state. These are called natural magnets^ or loadstones. 

1 



2 DANIEL DAVIS, JR.'s MANUAL. 

Artificial Magnets. Bodies of whatever form or 
composition, in which magnetism is artificially induced, 
are called artijmal magnets. 

3. Induction of Magnetism. Whenever magnetic 
properties are developed in bodies not previously pos- 
sessed of them, the process is termed the induction of 
magnetism. When this is effected by the influence of 
a magnet, it is called magnetic induction : when by a 
current of electricity, electro-magnetic induction. 

Induction of Electricity, is whenever electricity 
is developed by the influence of other electricity in its 
neighborhood, or by the influence of magnetism. In 
order to distinguish the inductive action of an electric 
current from the static induction of electricity at rest, 
the former is called electro-dynamic induction. The 
development of electricity by the influence of a magnet 
is termed magneto-electric induction, 

4. Poles. The magnetic phenomena manifest them- 
selves principally at the two opposite extremities of the 
magnet : as may be shown with regard to the attractive 
force by the following experiment : 

Exp. 1. — ^Immerse a magnet in iron filings and then withdraw it 
A considerahle quantity of the filings will be found to adhere to 
it ; being accumulated most abundantly about its ends, while few 
or none will be attached to its middle : thus proving the attractive 
force to bo strongest at the extremities, and to diminish rapidly 
as the distance from them increases, until it becomes entirely 
insensible at the middle point These extremities are called the 
poles of the magnet 

5. The earth itself is found to possess the properties 
of a magnet, having magnetic poles corresponding nearly 
in their direction with the poles of its diurnal rotation. 



EXPLANATIONS. 



Now if a straight magnet be suspended so as to allow of 
a free horizontal motion, it will be found to place itself in 
a direction nearly north and south : as will be explamed 
hereafter. The end which turns towards the north is 
called the north pole of the magnet, the other end its 
south pole. Hence every magnet, whatever its form, is 
said to have a north and a south pole. In the figures 
to be hereafter described, the north pole is indicated by 
the point of an arrow, and the south pole by the feather. 
The poles of a galvanic battery will be described when 
treating of that instrument. 

6. Permanent Magnets. It is found that pure soft 
iron easily acquires magnetism when exposed to any 
magnetic influence, but immediately loses this magnetism 
when that influence is withdrawn. But steel, which is 
a compound of iron with a small quantity of carbon, and 
especially hardened cast-steel, though it acquirer the 
magnetic properties less readily, retains them more or less 
permanently after they are acquired. Hence a magnet 
formed of hardened steel is called a permanent magnet. 

7. Bar Magnet. An artificial permanent magnet in 
the form of a straight bar, is called a bar magnet. 

^' ^' ^ Fig.l represents a 

small case contain- 
ing two bar mag- 
nets, with two short 
pieces of soft iron 
connectmg their 
poles: these act as arm>atures (see <5> 9), and serve to 
preserve the power of the magnets. The magnets, when 




4 DAKIEL DAVIS, J B. S MANUAL. 

not in use, should be kept packed in the case, with their 
oppo^te poles connected by the armatures, in the man- 
ner shown in the cut. 

CoifFocND Bar Magnet. A magnet composed of 
several straight hars joined together, side by side, with 
their similar poles in contact, for the purpose of bcreasing 
the magnetic power, is called a compoimd bar magnet, 

^- 2. 8. HoBSE-sHOE OB U Maonet. A magnet 
which is bent into such a form as to bring 
the two opposite poles near together, so that 
they may act simultaneously upon the same 
body, is called % horseshoe or U magnet. 

Fig. 2 represents a magnet of this descrip- 
tion. The middle of the magnet is usually 
painted, as represented in the cut. 

ColfPOUND HOBSE-SUOE MaGNET. 

A magnet composed of several horse- 
shoe magnets joined together, side by 
side, as in fig. 3, for the purpose of in- 
creasing the power, is called a com- 
pound horsc'shoe magnet or magnetic 
battery. These magnets are charged 
separately, and are put together with all 
the similar poles in the same direcUon. 
9. AiutATDBf!. A piece of sofl iron, adapted to, and 
intended to connect the poles of a magnet, is called an 
armature, or keeper. Horse-shoe magnets are usoftlly 
provided with an armature, consisting of a straight bar 
of iron, for the purpose of preserving their magnetic 




EXPLANATIONS . 



power : thi^ should be kept constandy applied to the 
poles of the magnet when it is not in use ; as shown in 
fig. 3, where A is the keeper. Armatures are employed, 
in various experiments, and their forms vary with the 
purposes intended. 



jF%. 4. 




10. Magnetic Needle. A 
light and slender magnet, 
mounted upon a centre of 
motion, as in fig. 4, so as to 
allow it to traverse freely in 
certain directions, is called a 
magnetic needle. 



1 1 . The most obvious effects exhibited by magnets are 
theur power to attract iron, and their tendency, when 
fireely suspended, to assume a determinate position in 
reference to the earth. For a long time these were the 
only properties which were noticed, or at least which 
received particular attention. The attractive power of 
the loadstone over sitiall pieces of iron seems to have 
been known fix)m the remotest antiquity ; but its polarity 
with regard to the earth does not appear to have been 
observed until the eleventh or twelfth century of the 
Christian era. 



1* 



DANIEL DAVIS, J B.'s MANUAL. 



PRODUCTION OP ELECTRICrry. 

12. As a current of electricity is requisite in many of 
the experiments to be mentioned hereafter, it becomes 
necessary to describe the various means by which it 
may be produced. 

I. MECHANICAL OR FRICTIONAL ELECTRICITY. 

The electricity developed by the electrical machine 
is called mechanical or fnctional, from the mechanical 
force or friction by which it is obtained. It possesses 
properties differing much in degree from those exhibited 
by the galvanic arrangements described below, and is 
altogether less capable of producing magaetieal effects. 
Mechanical electricity is also developed, though not in 
so striking a mann^, by the pressure of some minerals^ 
and of certain elastic substances, such as India rubber. 

13. The great development of electricity recently 
observed during the escape of steam f]x>m high pressure 
boilers, may also be mentioned here. This is collected 
for purposes of experiment, by plunging into the steam, 
escaping from a safety valve, a brass rod (fig. 5) fur- 
nished with a brush of points P, at one end, to collect 

IHg.5. 





the electricity, and held by means of a glass insulating 
handle attached to the other end. A length of six or 
eight feet is found advantageous in this instrument, to 



PRODUCTION OP ELECTBICITY. 7 

convey and insulate the electricity, which may be con- 
veniently drawn from the lower part of the rod. In the 
cut, the brass rod is represented as terminating in a brass 
ball B, and insulated from the wooden handle H by a 
stout glass rod 6. 

The electricity obtained in this way from steam is of 
high intensity, affording sparks of an inch or more in 
length, and charging the Leyden jar so as to give strong 
shocks. It is almost always positive, and is not obtained 
unless the steam is of high pressure so Bs to issue from the 
valve as a transparent vapor. 

n. GALVANIC OR VOLTAIC ELECTRICITY. 

14. These names are given to that form of electricity 
which is produced by chemical action. It is found, that 
when two metals are placed in connection with some 
liquid capable of acting more poweriiilly upon one than 
upon the other, electricity of a peculiar character is 
developed. The metals usually employed are zinc and 
copper, and the chemical agent some liquid containing 
an acid having a powerfiil affinity for the zinc. The 
phraseology used in describing the effect is founded upon 
the idea, that electricity is given out to the copper from 

■f%r« ^ the zmc, through the liquid between 

them ; as is shown in the ac^ining 
cut, fig. 6, which represents a vessel 
of some non-conducting substance, 
as glass, partly filled with the fluid,, 
and containing a zinc plate marked 
Z, and one of copper, C. Now the supposed motion of 
the electric current within the vessel is from^ Z to C ; 




8 DANIEL DAVIS, J R.'s MANUAL. 

then, if a wire passing from C is brought in contact with 
another from Z, as represented in the figure, the elec- 
tricity will pass around through the wires from the copper 
to the zinc again. Thus the current is considered as 
passing from zinc to copper vnthin the series, and firom 
copper to zinc without it. C is therefore called the 
positive or delivering pole of the arrangement, and Z 
the n^ative or receiving pole. This, however, must 
not be considered as an established theory, but only 
as the idea on which the phraseology is founded. For 
whether there is one fluid flowing in the direction above 
described, or two flowing in opposite directions, or no 
motion of a fluid at all, is still a matter of discussion 
among philosophers. 

15. In order to avoid the inconvenience of having 
phraseology in use which is based upon a doubtful 
theory, some philosophers call the two opposite extrem- 
ities of the galvanic arrangement electrodes^ that is, ways 
or paths of electricity. To distinguish the two, they 
call the copper end the anode^ and the zinc end the 
cathode. The terms positive pole and negative pole 
are, however, still most frequently employed to designate 
these extremities ; and the wire without, when in con- 
nection with these poles, is spoken of as the channel of 
a positive current passing fix)m the former to the latter. 
This language, however, as has been already remarked, 
must be considered as conventional, and not as an ex- 
pression of actual facts. 

16. Instead of using two metals to form the galvanic 
circuit, one metal in different conditions may be used 
on the same principle ; the necessary condition of this 



PBODUCTION OF £ L £ CT RI C IT T. 9 

current being only that one part of a conductor of elec- 
tricity shall be more corroded by some chemical agent 
than another part. Thus, if a galvanic pair be made of 
the same metal, one part of which shall be softer than 
another, as of cast and rolled zinc, so as to be differently 
corroded, or if a greater amount of surface be exposed 
to corrosion on one side than on the other, or a more 
corrosive chemical agent be used on one side, a current 
will be determined from the part most corroded through 
the liquid to the part least corroded, whenever the circuit 
of the poles is completed. 

17. There are two modes by which the peculiar 
powers of a galvanic arrangement, like the one previous- 
ly described, may be increased. First, by increasing 
the size of the plates used, and secondly, by increasing 
their number. 1 . The extension of the size of the plates. 
If the size of the plates, that is the extent of the surfaces 
acted upon by the chemical agent, is increased, some of 
the resulting effects become more powerful in the same 
ratio, while others do not. The power to develop heat 
and magnetism is increased, while the power to de- 
compose chemical compounds and to affect the animal 
system is very slightly or not at all augmented. Bat- 
teries constructed in this way, of large plates, are some- 
times called calorimotors, fh)m their great power of 
producing heat ; and they usually consist of from one to 
eight pairs of plates. They are made of various forms. 
Sometimes the sheets of copper and zinc are coiled in 
concentric spirals, sometimes placed side by side ; and 
they may be divided into a great number of small plates, 
provided that all the zinc plates are connected together, 



10 DANIEL DAVIS, JR«'s MANUAL. 

and all the copper plates together, and then, finally, that 
the experiments are performed in a channel of electrical 
communication opened between the one congeries and 
the other ; for it is immaterial whether one large surface 
be used, or many small surfaces, electrically connected 
together. The modification of electricity which such 
arrangements develop, is said to be great in quantity. 
2. The extension of the number of the plates consecutive- 
ly. That is, by connecting the copper plate of each pair 
with the zinc plate of the next pair. By this arrange- 
ment, the electricity is obliged to traverse a longer or 
shorter series of pairs ; each pair being separated from 
the adjoining ones by a stratum of imperfectly conduct- 
ing liquid. The result is, that the electricity acquires 
what is called intensity. It has greater power to pass 
through imperfect conductors, or through intervals in the 
circuit, to give shocks to the animal system and to de- 
compose chemical compounds ; and when the number 
of consecutive pairs of plates is mcreased to some thou- 
sands or even hundreds, the electricity developed ap- 
proaches very near in its character to that produced by 
the electrical machine ; it manifests similar attractions 
and repulsions, and in fact the Leyden jar may be 
charged with it. These different modifications of elec- 
tricity are therefore spoken of as characterized by differ- 
ent degrees of intensity. That which is obtained from 
one pair of plates has a very low intensity. As the 
number of consecutive pairs is multiplied, the intensity 
increases, until at length it approximates to that of 
frictional electricity, which is able to strike across a 
considerable interval of air, and to fracture solid non- 
conductors interposed in its circuit. 

A 

4 



PRODUCTION OF ELECTRICITY. 11 

18. In consequence of the low intensity of the elec- 
tricity required for electro-magnetic experiments^ it is 
very easy of insulation. This is a great advantage in 
regard to the practical construction of magnetic appara- 
tus. Where electricity exists in a state of high intensity^ 
it has a strong tendency to pass off and dissipate itself 
through imperfect conductors ; but where it exists only 
in great quantity, h requires nearly perfect conductors 
to allow it a passage. The electricity developed by a 
single pair of plates, however much its power may 
be increased by increasing the size of the plates, will 
scarcely pass across the smallest interval of air, and a 
wire conveying the current may be perfectly insulated 
by a covering of varnish. In working the electrical 
machine, on the other hand, the electrified parts of the 
apparatus must be kept at a distance from each other, 
raised on tall glass supports, or suspended by long silken 
lines ; and then, unless the atmosphere is very dry, the 
electricity will be very rapidly dissipated. But in the 
case of currents of low mtensity, however great what is 
called the quantity may be, two wires may lie side by 
side, with a coating of varnish or wax between them, 
and convey different and opposite currents, without any 
perceptible electrical intercommunication. 

19. Now for the purposes of magnetic experiments, 
electricity of a low intensity is required ; for the power 
of the magnetical effects of a currentfof electricity de- 
pends upon an increase of its quantity, mainly. Increas- 
mg the number of consecutive pairs, would only add to 
the intensity of the current, making it more unmanage- 
able in respect to insulation, without adding much to 



12 



DANIEL DAVIS^ J R.*S MANUAL. 



its magnetic effects. Galvanic batteries having many 
pairs of plates, are therefore unsuitable for these experi- 
ments. The maximum magnetic effect is produced by 
a smgle galvanic combination, or at most by three or 
four; the condition for the production of the effect 
being the extent of the surface acted upon. The form 
found most convenient is the following. 

20. Cylindrical Battery. This battery, a vertical 
-^' '^' section of which is repre- 

sented in fig. 7, consists 
of a double cylinder of 
copper, C C, with a bot- 
tom of the same metal; 
which answers the pur^ 
pose both of a galvanic 
plate and of a vessel to 
contain the chemical solution. The space between the 
two copper cylinders is the receptacle for the solution. 
There is a movable cylinder of zinc, marked Z in the 
section, which is to be let down into this solution when- 
ever the battery is to be put in action. It is, of course, 
intermediate in size, as well as in position, between the 
two copper cylinders ; and is made to rest upon the 
exterior one by means of three insulating branches of 
wood or ivory, projecting from it outwardly. Thus it 
hangs suspended in the solution, and presents its two 
opposite surfaces to the action of the liquid, and to the 
inner and outer cylinders of copper respectively. There 
is a binding screw cup N connected with the zinc cylin- • * 
der, and also one marked P, with the copper cylinder ; 
and, accordmg to the principles heretofore explained, 




PRODUCTION OF ELECTRICITY. 13 

when a communication is made between these cups, the 
electricity developed by the action within the battery 
will pass from one to the other. 

21, Chemical agent. The liquid employed for put- 
ting this battery in action is a solution of the sulphate of 
copper (the common blue vitriol) in water. To prepare 
it, a saturated solution of the salt is first made, and to this 
solution is then added as much more water. It may be 
convenient to know, that a pint of water, at the ordinary 
temperatures of the atmosphere, is capable of dissolvmg 
one fourth of a pound of blue vitriol ; so that the half 
saturated solution employed will contain about two ounces 
of the salt to the pint. The zinc is oxydized by the 
oxygen of the water ; the oxide combines with the acid 
of the salt, forming sulphate of zinc, which remains in so- 
lution ; while the oxide of copper, which was previously 
combined with the acid, being set free, partly adheres 
to the surface of the zinc cylinder, or falls to the bottom 
of the solution as a black powder, and partly is reduced 
to metallic copper, which is precipitated on the surface 
of the copper cylinder, or falls to the bottom in fine 
grains. This reduction of the oxide to the metallic 
state takes place in the following manner. The water 
of the solution furnishes oxygen to the zinc, and thus 
enables it to combine with the acid, while the hydrogen 
which is liberated, again forms water with the oxygen 
of any oxide of copper with which it may come in con- 
tact, leaving the metal free. Hence but littie gas is 
given off during the action of a battery charged by sul- 
phate of copper, as the hydrogen, which usually escapes, 
b in this case mostly absorbed. The coating of oxide 

2 



14 DANIEL DAVIS, J R.'s MANUAL. 

of copper should always be removed from the zinc aft^ 
using the battery. For this purpose a card brush is 
provided. With this the surface of the zinc should be 
thoroughly cleansed, with the aid of plenty of water, 
whenever it has been in use. If this has been neg- 
lected, so that the zinc has become covered, in whole 
or in part, with a hard coating, it will be necessary to 
scrape or file it to obtain a clean metallic surface. The 
deposit of copper, also, which will gradually accumulate 
below, must be removed from time to time. 

22. The zinc cylinder should of course be always 
taken out of the solution when the battery is not in use, 
but the solution itself may remain in the battery, as it 
has no chemical action upon the copper, but tends to 
keep its surface in good condition. When the solution 
has lost its power, as it will do, of course, after a time, 
it is not best to attempt to renew its efficiency by adding 
a fresh quantity of the salt. It should be thrown away, 
and a new solution be prepared, according to the fore- 
going directions. 

23. These cylindrical batteries are made, for the pur- 
poses of magnetical experiments, of three sizes, called 
the large, small, and medium sizes. 

24. When a current of electricity is passed through 
a metallic wire in greater quantity than it can readily 
transmit, the wire becomes more or less heated ; if its 
length and thickness be proportioned to the power of 
the battery, it may readily be melted. A single pair 
of plates would be the most efficient arrangement for 
producing this effect, were it not that an increase of 
intensity enables a greater quantity of electricity to 
traverse the wire. Hence, for igniting a great length 



PBODUCTION OF ELECTBICITY. 15 

of wire, a battery of a coDsiderable number of pairs is 
necessary ; but a much thicker wire may be ignited by 
a few pairs of large size. When a very extensive series 
of small plates is used, the current acquires so high an 
int^isity that its power of producing ignition is dimin- 
ished, as it becomes capable of traversing a pretty fine 
wire without obstruction. 

25. Metals differ very much in their power of con- 
ducting galvanic electricity. The following are several 
of the most useful metals, in the order of their conduct- 
ing power; viz. silver, copper, brass, iron, platinum. 
For conducting wires, (^ifpper is generally used; for 
delicate connections, silver. Iron and platinum are 
used where it is an object to employ the poorest con- 
ductors, as in the following experiment. 

Exp. 2. — ^Either of the batteries mentioned in § 23 has sufficient 
power to ignite a fine wire of iron or other metal, through which 
the current is made to pass. This effect is most easily produced 
in those metals which offer the greatest resistance not only to the 
passage of electricity, but also to that of heat ; hence a larger 
wire of platinum may be ignited than of perhaps any, other metal, 
as that is a poor conductor both of electricity and of heat A steel 
wire, when intensely heated in this way, bums with beautiful scin- 
tillations. The shorter and finer the wire, within certain limits, 
the greater is the effect produced. 

^.8. 




26. The Powder Cup. Fig. 8, No. 1, represents a 
little instrument designed to show the heating power of the 



16 DANIEL DAVIS, J R.'s MANUAL. 

battery current. Two copper wires, W and W', wound 
with cotton thread, except at their ends, are joined by 
a short piece of fine platinum wire P, No. 2. These wires 
pass through the bottom of a small glass cup, C, so that 
the platinum wire lies free in its cavity. On putting a 
little gunpowder mto the cup C, and then connecting W 
and W^ with the poles of the battery, the platinum will 
become Ideated, in consequence of the flow of the current 
through it, so as to inflame the powder. 

27. The Voltaic Gas Pistol, represented in fig. 9, is 

constructed on the same principle as the last described in- 

Fig.9. . ^ strument. The wire W 

^Mk A passes up through a brass 

^^^^^^^^^^ ^^^^ piece which screws into 

"W I ' the barrel;, the wire being 

completely insulated from the brass. A sectional view 
of this part is annexed. One end of the fine platinum 
wire P is connected with W, the* other with the brass 
piece. A stop-cock C is added, to insure the introduc- 
tion of a proper quantity of hydrogen. T^ object is 
effected in the following manner : Conn^ with a self- 
regulating reservoir of hydrogen, a leaden or other tube, 
so bent as to deliver the gas under the surface of water 
in a jar. The pistol being uncorked and the stop-cock 
open, immerse the niuzzle in the jar to such a depth 
that the water may fill one quarter of the barrel. Then 
close C, and bringing the muzzle over the end of the 
tube, open the stop-cock of the reservoir. When the 
escape of bubbles shows the pistol to be full of gas, 
withdraw it, and insert the cork. In this way it will 
contain one volume of hydrogen to three of air, which 



PBODUCTION OP SLECTBICITY. 17 

is the best proportion. If too much hydrogen is in* 
tioduced^ no explosion will occur ; it b not, however, 
necessary to be very particular; and it will answer the 
purpose, if the pistol is held for a few moments over a 
jet of the gas. The explosion is louder and more cer- 
tain to occur, if it is filled with a mixture of oxygen and 
hydrogen, in the proportion of one volume of the former 
to two of the latter. 

28. The pistol being corked and the stop-cock closed, 
connect W with one pole of the battery and bring the 
wire fix)m the other pole in contact with the stop-cock, 
or any part of the barrel. The circuit will now be 
completed through the platinum wire ; this will instantly 
be ignited, setting fire to the gas, which will expel the 
cork with a loud report. The stop-cock C allows the 
mixed gases to be fired by the application of flame when 
desired. 

29. By connecting two or three batteries (<5> 20) of the 
same size together consecutively, that is to say, the zinc of 
one with the copper of the other, the power of the current 
will be greatly increased. For most experiments relat- 
ing to magnetism there is no advantage in extending the 
series beyond this. Any number, however, of single 
batteries may be usefully combined, where great power 
is desired, by dividing them into two or three sets, and 
uniting the plates of each set among themselves, copper 
with copper, and zinc with zinc ; the sets may then be 
connected consecutively. 

30. Where a battery of a number of pairs is wanted, 
the arrangement represented in fig. 10 is very convenient. 
The zinc plates are flat, and are enclosed in copper 




18 DANICL DATIS, JB.S MAKDAL. 

cases, open only at top and bottom ; each ziac plate 
■^> 10. being iosulated from 

the suiiounduig copper 
by slips of wood at the 
edges, and cotmected 
by a strip of copper 
soldered to it, with the 
case belonging to the 
next pair. The whole series is fiimly fixed in a woodra 
frame B ; pieces of pasteboard soaked in melted wax 
being interposed between the adjacent copper cases. By 
means of the windlass C, the frame, with the plates, may 
be raised out of the trough A, containmg the excitjng 
liquid, or allowed to descend into it at pleasure. Diluted 
acid is employed for the charge, in preference to a solu- 
tion of sulphate of coppa' : sulphuric acid, one part, with 
forty or fifty parts of water, is very good ; if greater power 
is desired, a little nitric acid may be added. E E aie 
small hand-vices, connected with the poles, for the pui> 
pose of holding wires, &c. The battery represented in 
the cut, consisting of twenty-five pairs of plates, is able 
to ignite a considerable length of wire, to decompose 
acidulated water with rapidity, and to give a brilliant 
light with charcoal points. 

31. Fig. 11 represents a still more powerful battery- 
There are two distinct series of fifty pairs, each connect- 
ed with two of the cups on the table above the battery. 
In thb way the whole may be used as a single series of 
one hundred pairs, or as a battery of fifty pairs of double 
size, by establishing proper connections between these 
cups. Or only half the battery may be put in action ; 




tJ cr 

each haviDg a separate trough to contain the acid. The 
plates are staUonary, and the troughs are raised up to 
them by means of two racks moved by the crank and 
handle H, which lift the platform on which the troughs 
stand : either trough may be removed from the platfotm 
at pleasure, when it is wished to use only half of the 
battery. 

32. In the cut, the arrangement for producing the 
arch of flame between charcoal points is shown. Two ' 
pointed pieces of prepared boxwood charcoal are fixed 
in the pincers at A, and the battery being put in action, 
are brought in contact. The spark passes and the 
points become ignited; they may then be separated to 
a greater or less distance, in proportion to the power 
of the battery, and the current will continue to flow 
through the interval with the production of intense light 
and heat. 



so DA9ISL DATIS^ J B.'s MANUAL. 

33. In the batteries described in <^ 30 and 31 in which 
the plates are fixed pennanently in a frame, the solution of 
sulphate of copper cannot be employed, on account of the 
deposit which it forms. Hence diluted acid is used ; and 
the batteries will not maintain a good action for more 
than a few minutes at a time ; in fact their highest rate of 
action only continues for a few seconds after immersion. 
The plates require to be taken out of the acid occasion- 
ally during the experiments, and exposed to the air a 
minute or two. The batteries worked by sulphate of 
copper will keep in good action for fifteen or thirty 
minutes at a time. 

34. When the zinc and copper plates are separated 
by a porous partition or membrane, on each side of 
which a different solution b put, so that one solution 
comes in contact with the copper, and the other with 
the zinc plate, the battery is called a mstaining or con- 
stant battery, because it maintains a nearly uniform 
power for hours and days in succession. This arrange- 
ment is very useful for many purposes, and will be more 
particularly described hereafter when we come to speak 
of experiments which require a steady and constant 
current. 

35. The wires used for conveying the electrical cur- 
rent in electro-magnetic and magneto-electric experi- 
ments are wound with cotton thread, and sometimes, m 
addition, covered with varnish. This is sufiicient for 
their perfect insulation, as the electrical current employ- 
ed is one of very low intensity. The extremities of the 
communicating wires should be kept clean and bright ; 
it is often advantageous to tin them, or cover them with 



PRODUCTION OF ELECTRICITY. 21 

soft solder, when the connectioDS are made by means of 
mercury cups, as they then become amalgamated when 
dipped into the mercury, and thus form a perfect metallic 
contact. 



III. THERMO-ELECTRICITY. 

36. The term Thermo-Electricity expresses the de- 
velopment of electricity by the agency of heat. It was 
discovered by Prof. Seebeck, of Berlin, in 1822, that 
if the junction of two dissimilar metals was heated, an 
electrical current would flow from one to the other. 
Thus, if the ends of two wires, or strips of German silver 
and brass are made to touch each other, or are brazed 
together, and the junction heated, a current will flow 
from the German silver to the brass, if the free extremi- 
ties of the wires are connected by any conductor of 
electricity, and an electrical circuit will be established, 
as the galvanic circuit is established by connecting the 

Fig. 12. 
G 




B 

poles of the battery. In the cut, fig. 12, G represents 
the German silver, and B the brass ; the direction of the 
current being indicated by the arrows. 

37. In thermo-electricity, as in galvanism, instead of 
two metals, one metal, in different conditions, can be 
used to excite a current. Thus, merely twisting the 
middle of -an iron or platinum wire, and heating it on 



22 DANISL DAVIS, J R«'s MANUAL. 

one side of the twisted portion, will produce a current 
flowing, at the heated part, from the untwisted to the 
twisted portion, whenever the extremes are connected. 

38. A current may also be excited with two wires of 
the same metal, by heating the end of one and bringing 
it in contact with the other. It is difficult to succeed 
in this experiment when metals are used whose con- 
ducting power for heat is great. Thus copper or silver 
wires produce a very feeble current, but iron or platinum 
an energetic one, especially when the ends, which are 
brought in contact, are twisted into a spiral. The di- 
rection of the current at the junction is from the cold to 
the hot wire ; and it ceases as soon as an equilibrium of 
temperature is established between the two. A consid- 
erable current is also produced by heating the junction 
of two platinum wires of different diameters. The cur- 
rent flows from the fine to the coarse wire, whether the 
heat is applied at the point of junction or to either wire 
at a little distance from it. In large arrangements, plates 
or strips of dissimilar metals are generally used. 

39. The cause of the thermo-electric current, thus 
excited between two metals, is generally referred to the 
difference in their conducting power for heat, and to the 
different orders of crystallization to which their particles 
belong, the laws of crystallization being supposed to 
result from the electrical character of the particles. 
Where the same metal in different conditions is used, 
the production of electricity is referred to the unequal 
propagation of heat on each side of the heated point, 
caused in the single wire by the obstruction occasioned 
by the twist, and in the case of two wires, by the contact 



PRODUCTION OF ELECTRICITY. 23 

of the cold wire, or where they are connected together, 
by the difference in their diameters. The causes, how- 
evOT, have not yet heen fidly investigated, and many 
points are involved in great obscurity. 

40. Metals differ greatly in their power to excite a 
current, when associated together in thermo-electric pairs. 
Sonie of the peculiarities in the combinations of the more 
useful metals are given in <5> 43. It is necessary, however, 
to say a few words with regard to the galvanormiery an 
instrument to indicate or measure electrical currents, 
and which is more folly described in chapter I, section 2. 
A current of electricity passing through a wire or coil of 
wire, is found to deflect a magnetic needle in its neigh- 
borhood. By an arrangement, such as fig. 13, where G 
is the galvanometer, consisting of a magnetic needle in 

Fig. 13. 




close proximity to a coil of wire, above which is 
fixed a graduated circle, the direction of an electrical 
current made to pass through the wire is indicated by 
the deflection of the needle from the north and south 
line, in one direction or the other, and its strength is 
measured by the number of degrees to which it is de- 
flected. The deflection of the needle wiirbje frequently 



.4- 



24 DANIEL DAT is, JR.'s MANUAL. 

alluded to hereafter. In the figure, a thermo-electric 
pair, of bismuth and antimony, heated by a spirit lamp, 
b shown in connection with the galvanometer. The 
arrows indicate the course of tb^ current from the anti- 
mony A to the bismuth B, in the exterior circuit ; its 
direction being of course the reverse of that at the junc- 
tion, where it flows from B to A. 

41. The character of the juncture between the plates 
or wires has an important influence on the amount of 
the ciirrent with the same metals. Frequently, when 
the elements of the pair are merely made to touch each 
other, the current is greater than when they are brazed 
or soldered together. Generally, the slighter the con- 
nections are, the better. They must be sufficient to 
conduct all the electricity generated, but no more, for if 
they are unnecessarily large, they allow the electricity 
to return to the metal whence it proceeded, without ac- 
complishing the circuit. 

42. The metal from which the current proceeds 
through the heated junction is exactly analogous in situ- 
ation to the zinc or positive plate in the galvanic pair, 
from which the current proceeds through the liquid of the 
battery, ^ 14. The metal to which the current proceeds 
through the junction is analogous to the copper or nega- 
tive plate. The positive or delivering pole of the thermo- 
electric pair is the extremity of the negative or receiving 
metal,as the copper pole is the positive pole of the battery. 
The negative thermo-electric pole is the extremity of 
the positive metal. In the observations and table which 
follow, the positive element of the pair, answering to the 
zinc in a galvanic pair, will always be placed first. 



P&ODUClriON or ELECTRICITT. 25 

43. Oerman Silver and Antinumy. The current ex- 
cited by these is greater than that from bismuth and 
antimony at the same temperature. Their junctions 
being put into hot oil, of a fixed temperature, and the 
free ends of the plates connected with the galvanometer 
used in these experiments, the bismuth and antimony 
occasioned a constant deflection of the needle of 75^ ; 
the German silver and antimony, a deflection of 85^'; 
the heat being increased with the bismuth and antimony 
to the melting point of bismuth, the deflection was 82^, 
while the German silver and antimony, heated in a spirit 
lamp, gave a deflection of 88^. 

Bismuth and Antimony. Plates of these metalb 
have been heretofore generally used in large thermo-- 
electric arrangements. The current excited by heating 
their junctions is greater than from many other metab, 
when a feeble heat is used ; but from the fusibility of 
bismuth, the heat can never be raised very high. The 
current flows through the junction' from the bismuth te 
the antimony. 

44. Oerman Silver and Carbon. A current of con** 
siderable energy was produced by this combination. In 
this and in the succeeding experiments, where the use of 
carbon is mentioned, the kind employed was the com- 
pact carbon deposited from the gas in the retorts of the 
gas works. It is nearly or quite pure, and is a better 
conductor, both of heat and electricity, than ordinary 
charcoal. 

45. German silver is an alloy of nickel with copper 
and zinc, the proportion of nickel being about twenty or 
twenty-five per cent. This alloy is not magnetic. Its 

3 



26 DANIEL DAVISy JR.'s MANUAL. 

value in thermo-electric combinations has cmly recentlj 
been observed. It will be used in many of the thermo- 
electric instruments, to be hereafter described. Germin 
silver is positive to all the metals that have been tried, 
even to nickel itself; with the exception of bismuth, to 
which it is negative. 

Carbon and Silver y or iron. In these combinations, 
and also with antimony, the carbon is positive, the cut- 
rent being rather feeble. 

46. The deflections given in the following table admh 
of comparison with each other to a considerable extent, 
though not so strictly as if wires of the same sdze had 
been employed in all the experiments. It must be 
remembered, too, that as the needle approaches the ex- 
treme angle of deflection 90^, a much greater increase 
of the current is required to carry it a few degrees fiu> 
ther towards 90^ than when it is near the zero. Hence, 
a deflection of 40^ does not indicate a current of half 
the power of one of 8(P, but considerably less. Nor 
can momentary deflections be compared with permaneDt 
ones, in estimating the power of the current ; as a current 
which by its first impulse causes the needle to traverse 
a large arc, may not be able to maintain more than a 
few degrees of steady deflecticMi. 

47. The wires were not soldered together, but thdr 
ends were brought in contact before the application of 
the heat, and kept so to the end of the experiment 
With the more fusible metals, the greatest heat was 
employed which was consistent with their fiisibility. 
The object was to produce the greatest current that 
could easily be obtained from each combination. It 



PRODUCTION OF ELECTBICITT. 



27 



will be found that there b an entire difTerence between 
the series of positive and negative metals for thermo- 
electricity and for galvanism. 



ObRBBMT FLOWS THROUGH HBATBD JUNCTION. 



From potitire. 



Gennan Silver, 

German Silver, 

Gprman Silver, 

German Silver, •••>.. 

Grerman Silver, 

German Silver, 

Grerman Silver, 

German Silver, 

German Silver, 

German Silver, 

Silver,. 

Bismnth,. 

Bismntii,. 

ffismnth^ 

Bismuth^ 

Bismnthj. 

Platinum^ 

Carbon, 



To negative. 



Antimony, . • . . . 

Silver, 

^rass,. 

Iron,. 

Palladium, . . . • < 

Copper,. 

Cadmiam,. 

Zinc, 

Platinum, ..... 

Carbon, 

Antimony, 

Antimony, . . . . , 

Silver,. 

Palladium,.. ... 

Carbon, 

German SUver,. 

Carbon, 

Antimony...... 



Dbflbction of 

THE NbBDLB. 



88* 
85* 
85* 
85* 
85*» 
85** 
85* 
84* 
81* 
82* 
88* 
82* 
78* 
85* 
85* 
83* 
78* 
75* 



48. In some cases, the direction of the current is re- 
versed, either by raising the heat at the junction to a 
high degree, or by heatbg one metal more than the 
other* The following are instances of this kind. The 
metal of each combination, which is positive at low 
temperatures, is named first. Increasing the temperature 
of the negative metal generally increases the amount of 
deflection, produced by heating the junction ; while, if 
the higher heat is applied to the metal which is positive 
at moderate temperatures, a current in the opposite 
direction is established. The direction of the current 
in these combinations b, however, often uncertain, and 
the few experiments which have been made, afibrd no 
explanation of the cause of the changes. 



38 DANIEL DAVIS, JR^'s MANUAL. 

49. iron and PlaHtmau When heat b applied to 
the junction, or to the platinum a little one side of it, a 
deflection of about 50^ is obtained ; when to the iron 
near the junction, or when the junction itself is raised 
to a red heat, the direction of the current is immediately 
reversed, it now flowing from the platinum to the iroD, 
and the needle is deflected 60^ or 70^ in the opposite 
direction. 

50. Copper and iron. With fine wires the current 
is feeble, with large ones tolerably powerful. The de- 
flection is increased by heating the iron near the junction. 
When the junction is raised to a red heat, the current is 
reversed, and still more readily when the heat b applied 
to the copper near it. 

Siher and iron. Deflection considerable. On heat- 
ing the silver, an energetic current ensues in the opposite 
direction ; also, in a less degree, by rabing the junction 
to a red heat. 

Brass and iron. Current moderate ; reversed at a 
red heat, and still more efiectually by heating the brass. 

2!inc and iron. Current moderate, and on heating 
the zinc near the junction to its melting point, changes 
its direction. 

51. Platinum and Sther, Deflection 70^. Onheat« 
ing the platinum a strong current flows in the oppoate 
direction. 

Brass and Silver. The current is reversed at a red 
heat, or by applying the heat to the brass, near the 
junction. 

53.. In qiMntitj/, the thermo-electric current much 
resembles a feeble galvanic current. In intensity^ it b 



FBODUCTION OF ELECTRICITY. 29 

somewliiat less. In a single galvanic pair, electricity is 
set in motion in a certain direction, and cannot return 
in the same path to the zinc, fiom which it proceeded, 
without passing through the fluid between the plates, 
which b a poor conductor. It is, therefore, partially, 
though very imperfectly, insulated. In a thermo-electric 
pair, the electricity is set in motion fjx)m one of the metals 
to the other, through the metallic junction. Here there 
is no insulation. The current flows through a perfect 
conductor, and can only be the excess of the force which 
sets the electricity in motion over its constant eflbrt to 
return to equilibrium. It is probably for this reason 
that the intensity of thermo-electricity is less than that 
of galvanism. 

ExF. 3. — A single galvanic and thermo-electric pair were (taken, 
each of which deflected the needle 75", permanently. The gal- 
vanic current was then made to flow through a hundred feet of 
fine steel wire 1-150 of an inch in diameter. From the poor 
conduction of the wire, the needle was only deflected 60*. By 
experiment it was found that the thermo-electric current deflect- 
ed the needle 60**, when it was passed through only fourteen feet 
of the wire. As the conducting power of a wire is in proportion 
to the intensity of the current, some estimate may therefore be 
made of the relative intensity of the two currents by the respective 
nambers 100 and 14. 

53. In soldering the wires or plates together, they 
are not usually connected in a straight line, but at an 
acute angle with each other. If several of these single 
pairs be associated together consecutively, that is, by 
connecting the German silver of the one to the brass of 
the next, or the bismuth of one to the antimony of the 
next, and so on, we have a thermo-electric battery, in 

3* 



DAKIKL OkriB 



JB.S MA HO 



which the powers of thermo-electricity are much exalted. 
It will be undeiYtood that in these cases there is Gennan 
silver and brass alternately, or bismuth and antimony 
alternately, be, throughout the whole series. For the 
sake of compactness, the wires or plates are laid side by 
side, and soldered by their alternate ends, while they 
are insulated or separated Trom each other by paper tx 
pasteboard, which prevents all passage of electricity 
from one to the other. 

Fig. i4. 




54. Fig. 14 represents a series, con»sting of eleven 
pairs of Gennan silver and brass wire, arranged in 
two rows, one behind the other. When seveml pairs 
are connected in ihis manner, it is necessary that the 
junctions should be somewhat larger than in the case 
of a single pair. Then, the slighter the junction the 
better ; but as the current has to flow through all the 
junctions in a series of pairs, the electricity generated 
would scarcely be conducted through them at all, were 
they all imperfect. By beating the junctions of the 
wires on one side of the series with a spirit lamp, a cur- 
rent is produced which mcreases or diminishes as the heat 
13 applied, depending altogether for its existence on the 
difference of temperature in the opposite juncuons of 
the wes. By grasping the junctions on one side in the 



- PBODUCTION OT ELECT BICITT. 31 

fingen, even the wanntli of the hand produces a senaible 
effect. It is evident that, if the junctions on both sides 
of the series were heated, currents would be produced 
in opposite directions, which would neutralize each 
other. , 

55. Fig. 15 represents a battery, consisting of sixty 

pairs of bismuth and antimony plates, each three inches 

Fig. 15. 




long, three-fourths of an inch broad, and one-fourth of an 
mcb thick. They are arranged side by side, in an ex- 
terior case, so that one series of junctions underneath 
the battery may be heated by the radiation of a hot iron 
plate, I, shown separately in the cut, while the opposite 
junctioDS seen at A are kept cool by water or ice placed 
in the receiver, which forms the upper part of the battery. 
A still greater depression of temperature is produced by 
a nuxtnre of snow or pounded ice with half its weight of 
coaunon salt. In order to make a water-tight receiver, 
the plates are cemented into the case with plaster. 
Refrigeration at one end of the pairs, as would be 
anticipated, is found to produce a current in the same 
direction, and equal to that which would be produced 
by a similar excess of heat at the other end ; diSerence 
of heat at the (Uflerent ends, however produced, being 



32 DANIEL DAVIS, J R*'s MANUAL. 

V 

the occasion of the current. By associating both of 
these causes in this battery, there is a corresponding 
increase of power. As the metals employed in the 
battery are fusible, the radiant heat of the iron ought 
never to exceed 300^ Fahrenheit. The iron plate 
being laid upon a large tile, the battery is placed over 
it, the iron being pretty near the ends of the bars, but 
not in contact with them. 

56. The terminal plates of the battery are connected 
with two binding screw cups, passing through the exte- 
rior case. In the cut, the battery is seen in connection 
with an apparatus to be described in chapter U, sect. 2, 
by which die magnetizing power of the current is shown. 
The ends of die coil of insulated wire C being fixed in 
the cups, the current is obliged to traverse the coU, and 
the two semicircular armatures of iron seen at D, are 
held together by the magnetism thus induced, with so 
much force as to require a weight of forty or fifty pounds 
to separate them. This battery has sufficient power to 
give shocks and sparks, and produce various magnetic 
phenomena, with the appropriate apparatus, which will 
be described hereafter, when the principles on which 
those effects depend have been explained. 

57. A thermo-electric battery of considerable energy 
can also be constructed of strips of German silver and 
brass. It will bear contact with red hot iron, and is 
very compact. This has not yet been fully brought to 
perfection ; so that a comparison cannot be instituted 
here between its powers and those of the bismuth and 
antimony battery described m sect. 55. 

58. By forming a bundle or small battery, consisting 



PRODUCTION or ELECTBICITT. 33 

of many pairs of wires, the slightest increase of heat at 
one end produces a sensible current of electricity. This 
forms an instrum^it for measuring heat far more delicate 
than any other which has been contrived. It has been 
used m ascertwning the temperature of insects, and of 
various parts of the animal system. 

59. In thermo-electricity, an electrical current is pro- 
duced by heating unequally the opposite ends of metallic 
plates, associated in a thermo-electric series. The con- 
verse of this is found true. If a galvanic current is 
made to pass through the same series, the opposite 
junctions will acquire heat on the one side and lose it 
on the other. 

60. Fig. 16 represents an instrument for showing 
the simultaneous production of heat and cold by the 

Fig' 16- galvanic current. It 

consists of three bars, 
two of bismuth and one 
of antimony, arranged 
as seen in the figure? 
where the antimony is 
shown at A, and the 
two bars of bismuth at 
B B', the bars being soldered together under the bulbs 
of two air thermometers, T and T'; a little cavity being 
made to receive the bulb of each thermometer ; a drop 
of water is put in each cavity, in order to facilitate the 
conduction of heat from the metals to the thermometers. 
The galvanic current being sent through the metals, in 
the direction indicated by the airows, from the bismuth 
B', through the antimony, to the oth^ bar of bismuth, 




34 DANISL DAYIS, J R*'s MANUAI^. 

and thence back to the battery, at the junction of A 
with B', cold is produced, as will be indicated by the 
thermometer T^, and heat at the junction between A 
and B, as the thermometer T will show ; by reversbg 
the direction of the battery current, the effect on the two 
thermometers will be reversed. The elevation of tem- 
perature produced is always greater than the depression ; 
this difference is probably due to the low conducting 
power of the metals for electricity, which causes them 
to become slightly heated by the current, a phenome- 
non altogether distmct from the heatmg of the junction 
by it. It will be observed in the figure that the current 
has the same direction as that which would be produced, 
were the battery removed, by the applicatbn of beat at 
the junction of A with B^, or of cold to that between B 
and A ; the current which produces heat flowing in the 
opposite direction to the current which would be pro- 
duced by it. 

IV. ANIMAL ELECTRICITY. 

61. The torpedo, on the shores of Europe, the gym- 
notus, or electrical eel, inhabiting the fresh waters of 
South America, and the silurus electricus, living in the 
rivers of AfHca, have been celebrated for their powers 
of producmg electricity. As it appears to be dependent 
on will, although associated with certain organs, it has 
received the name of animal electricity. It possesses 
considerable intensity, and is capable, to a certain ex- 
tent, of producing all the magnetic phenomena. The 
production of electricity by animal life, has been occa- 
sionally noticed under other circumstances. 



MAGNETISM. 



L 



DIRECTIVE TENDENCY OF THE MAGNET. 



I. IN REFERENCE TO ANOTHER MAGNET. 



62. Attractions and Repulsions. — ^The effects 
produced by the opposite poles of a magnet, though in 
some respects similar, are in others contrary to each 
other ; the one attractmg what the other repels. Poles 
of different magnets, of the same name, that is, both 
north or both south, are found to repel, while those of 
an opposite name attract each other. 

Exp. 4 — ^Let N. S. (fig. 17,) be a magnetic needle poised npon 
Fig. 17. a pivot Let N be the north and 

^fli !M1 S the south pole. Then bring near 
* "p to its north pole the north pole of 

the bar magnet M. The north 
pole of the needle will be repelled, 
causing the needle to assume the 
position r r. If now the magnet M 
is reversed, so that its south pole 
is made to approach the north pole 
of the needle, the latter will be 
attracted, and the needle will be 
drawn to the position a a. The 
south pole of the needle, on the contrary, would be attracted by 
the north pole of M, and repeUed by its south pole. 




36 DANIEL DAYIS, JR*'s MANUAI.. 

63. The intensity of the attraction or repulsion exerted 
between two magnetic poles, yaries in the inverse ratio 
of the square of their distance ; that b, if the distance of 
the poles is doubled, the force with which they attract or 
repel each other b reduced to one quarter of its previoas 
amount ; if their dbtance b trebled, to one ninth ; and 
so on. 

64. These attractions and repulsions are not af- 
fected by the interposition of glass or metal, or any 
substance whatever between the two magnets ; unless 
the interposed body is itself susceptible of magnetbm. 

65. Whenever a piece of iron, as B (fig. 18) is 
brought near to one of the poles of a magnet, M, the iron 

Fig. 18. becomes mag- 

^ ^ ^ netized by in- 

duction,aswill 



\: mm > |j is n 



be explained hereafter, chapter II, sect. 1 ; and the ex- 
tremity nearest to the pole acquires an opposite polarity 
to that of the pole, while the end farthest oS acquires the 
same polarity. Thus, in the figure, the point of the 
arrow indicates the north pole of the magnet, and the 
extremity S of the iron bar will acquire a south polarity. 
It follows from thb, that it is only that part of a firag- 
ment of iron nearest to the pole of a magnet, which can 
be attracted by that pole, while the part, most distant 
must be repelled. If the fragment of iron has any con- 
siderable lengdi in proportion to its breaddi, the end 
which is repelled will be at such a dbtance firom the 
influence of the magnet that its repulsion will be over- 
powered by the attraction of the extremity which b near 
it. If, however, the fragment b y&j short, so that the 



DIBECTIVB TENDENCY OF HAONET. 37 



repelled pole is brought very near to the magnet, the 
repulsion will be proportionally stronger, and the attrac- 
tion will be neutralized to a considerable extent ; and, 
finally, if the fragment of iron is made of such a form as 
to bring the two opposite poles as near together as pos-. 
sible, so as to expose them both nearly equally to the 
influence of the pole of the magnet, the attraction will 
become scarcely perceptible. This may be shown very 
satisfactorily in the following manner. 

£xF. 5. — ^TiOt M (%. 19) be the south pole of a bar or horse- 
ghoe magnet, and A a piece of sheet iron, somewhat smaller 
Fig, 19. than the end of the magnet When thia 

iron plate is placed in the position repre- 
sented in the upper figure, the surface next 
the pole of the magnet will acquire north 
polarity, while the opposite surface will 
become south ; and the iron being thin, the 
two surfaces are both so near to the pole of 
the magnet that one is repelled nearly as 
much as the other is attracted. The thin 
plate will be found to adhere to the pole 
with a very slight force, and will tend to 
slip down into the position represented in 
the lower figure. In this position it will be much more strongly 
attracted ; for the two opposite ends, instead of the two opposite 
gwrfaots, will become the poles, and the end in contact will be at- 
tracted, and the remote end will be repelled. The same effect 
will be produced if the plate is applied to the pole of the magnet 
by its edge, instead of by one of its surfaces ; by this means the 
repelled pole of the plate is removed to a distance from the mag- 
net, leaving the latter to attract the other pole, with a less inter- 
ference from the counteraction which operated in the former case. 

66. Magnetic Toys. Various magnetical toys are 
constructed to exhibit the effects of the attractions and 
repulsioQS, described in ^ 6^ such as swans, ships, fishes, 

4 






8 






I 

1^ 



38 DANIEL DATIS, JR.9 HANDAL. 

and Other Bgures, with magnets concealed within them, 
and intended to float upon the water. When thua 
floating, they may be attracted or repelled over the 
surface of the water at pleasure by means of another 
magnet held in the hand. 

67. Floating Needle. A rery fine and perfectly 
dry sewing-needle, being previously magnetized and 
then laid carefully upon the surface of water, will float, 
and being thus at liberty to move freely in any direclioa, 
may be conveniently used to show the above-described 
attractions and repulsions. A larger needle will answer 
equally well, if passed through a small piece of cork, 
that it may float. 

63. RoLLiNQ Aruatdre. This apparatus consists 
of a compound horse-shoe magnet and an armature con- 
sisting of an iron wire whose length is a litde greater than 



the breadth of the magnet, so that when applied to it 
the extremities may project a little beyond its sides. To 
each of these extremities a small fly-wheel is attached. 



DIRECTIYE TENDENCY OF MAGNET. 39 



This armature is then placed across the magnet, at 
some distance from the poles, as seen at A, and the 
magnet is held in such a position, with the poles down- 
ward, that the armature may roll towards them. When 
it reaches the poles, the magnetic attraction for the iron 
axis will prevent its falling off, while the momentum 
acquired by the fly-wheels will carry it forward and roll 
it some distance up the under side of the magnet to B in 
the figure ; and by varying the inclination of the magnet 
N S, the armature may be made to roll from A to B, 
and fix)m B to A, at pleasure. 

^ 69. It results fix)m what was said in <^ 65, that the 
action of a magnet upon a mass of iron is not simply 
an attraction or a repulsion of it as a mass, causing 
it merely to approach or to recede ; but that there is a 
complicated reciprocal action between the poles of the 
magnet and those which the mass of iron has assumed. ^ 

Exp. 6. — ^Let M (fig. 21) be a magnet, the position of the north 
pole being indicated by the arrow. Now if the small bar of iron 
S N, suspended by a thread, is placed in the position marked 1, 
it becomes magnetized by induction from the fixed magnet, so 
IHg. 21. _ ^ that the extremity S will be 

attracted by the north pole 
of the magnet, and the ex- 
tremity N will be repelled 
by it,a8 has already been ex- 
plained. Both these forces 
will conspire to retain the 
^ body in the direction rep- 
resented in the drawing; 



M 




fTBF 



g 



while the influence of the remote extremity of the magnet M, 
will be insensible. Now if the bar S N is removed to the position 
marked 2, the north pole of the magnet will attract the south pole 
of the bar, and will repel the north pole, as before ; but then, on 



40 



DANIEL DAYIS, J R* S MANUAL. 



account of the inclined position of the bar, the attractive force 
between the south extremity of ^ the magnet and the north ex- 
tremity of the bar will come into action ; so that the north pole of 
the bar will be drawn towards the south pole of the magnet, and 
the bar will be deflected somewhat from the position which it 
would otherwise have assumed. This tendency of the bar to 
place itself in a certain determinate direction, in reference to the 
other magnet to whose influence it is exposed, is called its diree- 
iive tendency. 

70. This efiect of the remote pole of the magnet in 
giving direction to the bar, will be quite decided when 
the suspended bar is carried still farther from the north 

2^.22. pole: for example ; near- 

ly opposite the centre of 
the magnet, as in fig. 22, 
where M represents the 
magnet as before. Now 
hi this case, if the sus- 
pended bar were acted 
upon solely by the north pole of the magnet, it would 
assume the position A B ; for the pole S being attracted, 
and the pole N repelled, the bar would place itself in a 
line directed towards the north pole of the magnet. But 
instead of this, the bar is in such a position that the 
south pole of the magnet acts powerfully upon it also ; and 
if the magnetic forces of the two poles of the magnet are 
of equal intensity, the south pole will act upon the end 
marked N, as strongly as the north pole acts upon S ; 
and the suspended bar will assume the position marked 
N S, that is, parallel to the magnet. 

71. The directions thus assumed by an iron rod 
brought near a magnet depend upon the much greater 
facility with which the bar receives polarity in the direc- 




DIRECTIVE TENDENCY OF MAGNET. 41 

tion of its length than transversely. Thus if the bar is 
placed on one side of the magnet, at right angles to it, 
and opposite its middle, it would remain in this position 
instead of turning itself parallel to the magnet, were it 
not for the difficulty of developing the two polarities on 
its opposite sides. 

72. A steel magnet does not experience that change 
in the distribution of its polarity, by altering its position 
with regard to the fixed magnet, which the iron bar does. 
Hence the experiments above described are better per- 
formed "mth B. magnetic needle, which may be suspended 
by a thread, or, which is better, supported by a pivot, 
and thus held in various positions near to a bar magnet. 
The needle being a permanent magnet, and having been 
powerfully magnetized by the process to which it has 
been subjected in the manufacture, the action of its 
poles will be more decided than that of the poles of a 
bar of iron magnetized only by temporary induction. 




By passing such a needle carefully around a bar magnet 
it will be found that it will assume positions in relation 
to it, as represented in the above cut, fig. 23. 

73. These effects, produced by the combined attrac- 

4* 



/ 






43 DANIEL DATI8, JB.'b MAHtTAI. 

tions and repulsions of the magneUcal poles, may be 
also reodered sensible in a very satis&ctorr manDer by 
the following experiment. 
Exr. 7. — Spread ft thin covering of iron filing! en femiginoai 
I^. 24. sand over a iheet of paper, ud 

■■.S1.''V.'.'.'.' .■'.--:-:• .-.--.v^^, „,- P'oce a powerfnl hone-^hoa 
s^;^''',''^!;,'' magnet vertically beneath i^ 
Ai','.y:|;''."''/ with the polea very near to the 
i paper. The dotted linea in-tha 

.'^. J cut {&g. 24) ahow the amnge- 

'/'y'/,','i'-X\'}. '■ ■/■"-■ .;' ' ' . ■ ment wliich the particlea of 
////,' ;',\>;i>;(^^^~ -'J-'y/-'fi\\',y\\\ iron will asaame. Each ons 
'' ' ''W^^ '-'/,'•",'.'.','.- becomea a magnet with its two 
polea, and connecta itaelf with thoae adjoining it so aa to form 
carved linea of a peculiar cfaaiacter. Thia experiment may be 
perfwrned in a atill more aUiafactory manner, bj aepportiug the 
paper, with the magnet in contact with ita under auftce, and 
then showering down iron aand or iron filinga from n aand-box 
held some inches above. The particlea of iron, as they etrfte 
the paper can thns more readilj assume the poaitiona to which 
they tend under the magneto inSnence. 

74. The lines formed by the Slings aSbrd a good ex- 
perimental illustration of what are called magneftc Oilfvtt, 
that is, the curves into which an infinite number of very 
minute magnetic needles suspended freely would arrange 
themselves, if placed in all possible positions about a 
magnet. When the particles are very small, the attfex- 
tive force exerted upon them by the magnet, being the 
difierence of its action upon the two poles of each 
particle, is exceedingly slight ; while the directive force 
is vety condderable. The direction assumed by each 
particle, and consequently the form of the magnetic 
curve, connecting any point on one half of the magnet, 
with the corresponding point of the other half, is de- 



DIRECTIVE TEKDENCT OF MAGNET. 43 

ducible on strict mathematical principles from the laws 
of magnetic attraction and repulsion. The curvature of 
the lines is due to the combined action of the two poles 
of the magnet. If only one pole acted on the minute 
particles, they would arrange themselves in straight lines^ 
diverging in all directions from the pole, like radii from 
the centre of a sphere. This may be partially shown 
by placing a bar magnet perpendicularly under the paper 
which is strewed with filings, with its upper pole close 
to the sheet. 

II. IN REFERENCE TO A CURRENT OF ELECTRICITY. 

75. It was discovered by Prof. CErsted, of Copenha- 
gen, in the year 1819, that a magnet, freely suspended, 
tends to assume a position at right angles to the direction 
of a current of electricity passing near it. This may be 
made manifest as follows. 

Exp. S. — ^Let N S, fig. 35, be a magnetic needle poised upon 

a pivot so as to allow of a free horizontal motion, and W R a 

Fig, 25. wire passing directly over and 

W— ** "*" Tt parallel to it Of course, the 

direction of the wire must be 
north and south, as the needle 
will necessarily assume that 
direction, on account of the 
influence of the earth. If now 
the extremities of the wire are 
put in connection with the 
poles of a galvanic battery, in 
such a manner as to cause a 
current of electricity to pass 
through it, tiie needle N S will 
be deflected and will turn towards the position ab or cd, according 
to the direction of the current of positive electricity, whether from 




44 DANIEL DAVIS, Jb/sKANUAL. 

W to R, or from R to W. If the wire be placed in the same direc- 
tion below the needle, the deflections will be the reverse of those 
produced by the same current when flowing above. If the positive 
current is passing from south to north in the wire, as shown by 
the arrow in the cut, the north pole of the needle will turn to the 
west, if it be below the wire ; and to the east if above it. 

76. In these cases the needle will not be deflected so 
far as to assume a position really at right angles with 
the wire, on account of the influence of the earth, which 
still acts upon the magnet, and tends to draw it back to 
its original position. It will accordingly come to rest 
in a state of equilibrium between the forces, in a direc- 
tion intermediate between a line at right angles to the 
wire and that of the needle when controlled by the 
magnetism of the earth alone. 

77. The same experiment may be performed with 
the dipping needle, the wire being placed parallel with 
the needle. By thus varying the positions of the wire 
and the needle, it will be found that in all cases the 
needle tends to place itself at right angles with the wire, 
and to turn its north pole in a determinate direction 
with regard to the wire. 

78. Tne action of a conducting wire upon a magnet 
exhibits in one respect a remarkable peculiarity. All 
other known forces exerted between two points, act in 
the direction of a line joining these points ; such is the 
case with the electric and magnetic actions separately 
considered. But the electric current exerts its magnetic 
influence laterally, at right angles to its own course. 
Nor does the magnetic pole move either directly towards 
or directly from the conducting wire, but tends to revolve 
around it without changing its distance. Hence the force 



I 



DIBECTIYE TENDENCY OF MAGNET. 45 



must be considered as acting in the direction of a tangent 
to the circle in which the magnetic pole would move. 
It is true, that in many positions of the magnet with 
regard to the wire, apparent attractions and repulsions 
occur ; but they are all referable to a force acting tan- 
gentially upon the magnetic poles, and in a plane per- 
pendicular to the direction of the current. This peculiar 
action may be better understood by means of a figure. 

79. Thus, let p n (fig. 26) be a wire, placed in a 
vertical position, and conveying a current downwards 
(p being connected with the positive pole of the battery). 

Now suppose the north pole of 
a magnet N to be brought near 
the wire, and to be perpen- 
dicular to any pomt C. If free 
to move, the pole will revolve 
around C as a centre in the di- 
rection indicated by the arrows 
in the cut ; that is, in the same 
direction as the hands of a 
watch, when its face i^upwards. 
The plane of the circle which 
the pole describes is horizontal. On causing the current 
to ascend in the wire, the pole will rotate in the opposite 
direction. If the wire be placed in a horizontal position, 
the plane in which the pole revolves will, of course, be 
vertical. The actions of either a descending or an as- 
cending current upon the south pole are exactly the 
reverse of those exerted on the north pole. If the wire 
is movable and the magnet fixed, the former will revolve 
around the latter m a similar manner, and in the same 




46 DANIEL DAVIS, J R.'s MANUAL. 

directions. Thus, a wire conveying a descentling current 
tends to rotate round the north pole of a magnet, in the 
direction of the hands of a watch. In the experiment 
given in <^ 75, no revolution occurs, because the current, 
acting at once on both poles, tends to give them modon 
in opposite directions ; so that the magnet comes to rest 
in a position of equilibrium between these two forces, 
across the wire. It will be shown hereafter (chap. II, 
sect. 2) that, by confining the action to one pole, a con- 
tinued rotation is produced. 

80. The following apparatus illustrates the directive 
tendency of the magnet in respect to a current of 
electricity. 

Magnetic Needle, half brass. In thb instrument 
the steel needle is wholly upon one side of the point of 
support, and is counterpoised by a brass weight on the 
other side. By this arrangement the action of a current 
u])on the pole which is situated at the centre of motion 
can have no influence in turning the magnet in any 
particular direction ; and its motion will be determined 
sololy by the action upon the other pole ; no rotation, 
however, can be obtained. The object of the instru- 
niont is to sliow tlie directive tendericy of a single pok 
NN iih reforiMico to the electrical current. 

v^L Astatic Needle. A needle so contrived that 
its dinvtivo tendency in respect to the earth is neutral- 
i.-.t'd, so that it shall remain at rest in any position, is 
{{\\Uh\ an astatic needle. It is constructed as represented 
in flio following cut, fig. J27, consisting essentially of two 
notMllt\^. ont^ above the other, placed in positions the 
ix^MM'M^ o( oach otlior in respect to their poles. Such a 



DIBECTIVE TENDENCY OP MAGNET. 47 





system will of course not be affected by the magnetic 
influence of the earth, as whatever forces may be 
Fig, 27. exerted upon the upper needle, 

^ will be counteracted by equal 
^'-' ■■ — ^N forces exerted in reverse direc- 

tions upon the lower. It would 
be the same, indeed, with the 
influence exerted by the current 
of electricity, if the wire were 
to be placed in such a position 
as to act equally on both needles. 
But by placing the wire parallel 
to and above the upper needle, the influence of the wire 
will be, of course, far more powerful upon the upper 
than upon the lower one, and the action of terrestrial 
magnetism being neutralized, the needle will assume a 
position at right angles with the conducting wire. If the 
wire be placed as nearly as possible between the needles 
and parallel to them, the influence of the upper side of 
the wire will deflect the upper needle in the same direc- 
tion as the lower needle will be deflected by the action of 
the lower side of the wire, causing a more powerful effect. 
J%. 28. 82. Fig. 28 represents another 

■^ astatic needle, similar to the above, 
but consisting of two horse-shoe or 
U magnets united at the bend, so 
as to have their opposite poles in 
the same line, and delicately ^p- 
ported upon an agate cup. These 
needles need not be perfectly astatic, 
nor is it easy to make them so. 



^ 




;^ 




48 



DANIEL DAVISy J R* S KANUAL 




83. If the wire transmitting the electrical current, after 
passing over the needle, is bent and returned under it, as 

in fig. 29, it might 
be supposed that 
as the electricity 
which flows firoffl 
C to A in the 
upper part of the 
wire, must pass in 
a contrary direc- 
tion, in returning 
from A to B, be- 
low (the cup C 
being connected 
with the positive 
pole of the battery, and B with the negative), the in*- 
fluence of the one part of the wire would neutralize that 
of the other, for it has already been stated that the 
needle is deflected to one side or the other according to 
the direction of the electrical current. And this would 
in fact be the case, if the returning part of the wire were 
upon the same side of the needle with the other part, 
and at an equal distance from it. But a wire transmit- 
ting an electrical current, when passing below the needle, 
will produce an eflfect the reverse of that produced by 
one passing above, if the current in both cases flows in 
the same direction. And of course it follows, that if the 
direction of the electric current is reversed in the wire 
which passes below, it will exert a force auxiliary, and 
not antagonist, to that of the wire passing above. This 
is the case with the arrangement here represented. 



DIBECTIVE TENDENCY OF MAGNET. 49 

The electric current flows, it is true, in a contrary dii:ec- 
tion, below the needle, but then it is on the opposite side 
of it, and therefore the effect produced by the lower 
porUon of the wire will conspire with that of the upper 
part* It should be stated, that the two portions of the 
wire are not allowed to touch each other where they 
cross, but are insulated at that point by some non- 
conductor of electricity, as by being wound with thread. 

84. The vertical portions of the wire also aid in 
deflecting the needle ; as may be shown by connecting 
both the cups B and C with one pole of the battery by 
two wires of equal length and thickness, and the cup A 
with the other pole (say the positive). The current 
will then be divided into two portions very nearly equal, 
both flowing in the same direction and at the same dis- 
tance from the> magnet M, but one below and the other 
above it. Now if the horizontal portions of the wire 
alone acted on the needle,, it would remain unaflfected; 
but it will be found to be deflected to a considerable 
extent by the current which is descending in the vertical 
portion of the wire near A, and ascending in that below 
B, as these conspire in their influence. 

85. The Galvanoscope or Galvanometer. In- 
struments of a variety of forms are constructed on the 
above principles, and are called galvanoscopes or gal* 
variometers^ as they serve to indicate the presence of a 
current of electricity and in some degree to measure its 
quantity. If the wire is carried many times around the 
needle, as in fig. 30, the power of the instrument is much 
increased, as each turn of the wire adds its influence ; 
provided the wire is not so long or of so small a size as 

5 



50 DAHIEL DATIS, J B.'S KAITVAL. 

to be unable to coavey the whole of the current. The I 
instninient thus becomes a delicate test of the preHntce ■ 
of a current of electricity. The coil of wire is supported I 
Jfe-aft 




Fig. SI. 



on a tripod stand, with levelmg screws ; the ends C and 
D of the wires being connected with the binding screw 
cups A and B. 

86. Uphight Galtanohetei. 
In this instrument, represented is 
fig. 3 1 , both the coil of wire anc^the 
needle are placed in a vertical po- 
sition, the north pole being niade i 
little heavier, in order to keep die 
magnet perpendicular. When a cur- 
rent is passed through the coil, die 
deflection is towards a horizontal po- 
sition. The needle is made of large 
^ze, for the purpose of exlubiting 
the deflections befiae an audience. 




DIRECTIVE TENDENCY OF KAGNET. 51 

87. Galvanometer with Astatic Needle. This 
instrument is similar in construction to the precedingi 
except that the needle is nearly astatic. The slight 
degree of directive tendency which is allowed to remain 
becomes the measure of the force of the electric current, 
as the angle of deflection from the north and south line 
shows how far this resistance is overcome. This instru- 
ment may be made so extremely delicate in its indica- 
tions, that if two fine wires, one of copper and one of 
zinc, are connected with it, and their ends immersed in 
diluted acid, or even placed in the mouth, it will be very 
perceptibly aflfected. Before proceeding to experiment 
with any galvanometer, it should be so placed that the 
direction of the coil may coincide with that of the needle, 
as this is the position of greatest sensibility. 

88. The galvanometer is a measurer of what is called 
the quantity of electricity, but takes no cognizance of 
intensity. Mechanical electricity which possesses great 
intensity and but little quantity, very slightly deflects the 
needle of the galvanometer. The current fix)m one gal- 
vanic pair influences the needle powerfully, the quantity 
being very great, and the intensity small. If a hundred 
pairs be connected together in a single series, the inten- 
sity is increased a hundred fold, but the quantity remains 
the same, and the needle is but little more deflected than 
by one pair. The reason that there is any diflference in 
this respect is, that when the electricity is of high ten- 
sion, the wire of the galvanometer obstructs the current 
less, and more actually passes through it. In thermo- 
electricity, with a single pair, the intensity is less in 
proportion to the quantity than with a single galvanic 
panr, and the current is strongly indicated by the galva- 



52 



DANIEL DAVIS, J R.'S MANUAL. 



nometer. The amoont of decomposing power in a 
current of electricity is always exactly as its quantity. 
The galvanometer indicates therefore the electro- 
magnetic and the decomposing capacity of a current of 
electricity. An intense electrical current decomposes 
more easily than one of little intensity, but the amount 
of matter decomposed is proportional merely to the 
quantity of the current. Besides the galvanometers in 
which a magnetic needle is used, the gold-leaf galvano- 
scope, an instrument possessing great delicacy in its 
indications, will be described hereafter. 



in. IN REFERENCE TO THE EARTH. 

89. The exact period of the discovery of the directive 
tendency of the magnet with respect to the earth, and of 
its employment as a guide to the mariner, cannot be as- 
certained witli certainty ; but it was used for this purpose 
by the nations in the north of Europe, at least as early 
as the twelfth or latter part of the eleventh century.* 

Fig. 32. 90. Fig. 32 represents 

a magnet poised upon a 
pivot so as to turn hori- 
zontally. This arrange- 
ment is essentially on 
the same principle as 
the compass-needle ; the 
latter, however, being 
fixed to a circular card 
on which the cardinal 
points are marked. 

* The Chinese claim to have known the polarity and use of the magnet ip the 
second century or earlier. 





DIBSCTIVE TENDENCY OF MAGNET. 53 



91. It is found that a magnetic needle, so suspended 
as to allow of a free horizontal motion, spontaneously 
assumes a durection neariy north and south ; and if dis* 
placed from this position returns to it after a number of 
oscillations. 



S JF%.33. 




92. If the needle be suspended so 
as to have freedom of motion in a ver- 
tical direction, it is found not to main- 
tain a horizontal position, but one of 
its poles (in this hemisphere the north) 
inclines downwards towards the earth. 
At the magnetic poles of the earth the 
direction of the needle would be verti- 
cal ; but the inclination diminishes as 
we recede fix)m the poles towards the 
equator, and at the magnetic equator, 
which is near the geographical one, the 
needle becomes horizontal. A needle 
properly prepared for exhibiting this 
inclination, is called a dippirig needle. 



93. Fig. 33 represents a dipping needle whose mode 
of suspension allows of its turning freely in any direction. 
It is fixed by means of a universal joint to a brass cap 
containing an agate, which rests upon the pivot. The 
usual arrangement allows only of motion in a vertical 
plane, the needle having an axis passing through its 
middle at right angles to its length, which axis is sup- 
ported horizontally. The small needles shown in fig. 34 
are suspended in this manner. Sometimes a vertical 
graduated circle is added, to measure the angle which 
the needle makes with the horizon. In using a needle 

5* 



54 DANIEL DATIS, JB.S VARCAL. 

whose motion is cODfined to a single plane, il must be so 
placed that tliis plane may be directed north and south, 
coinciding with the plane of the magnetic meridian. A 
dipping needle, before being magnetized, should be at 
equally balanced as possible, so as to remain at rest ia 
any direc^on in which it may be placed ; a high degree 
of accuracy is, however, difficult of attainment. 

94. The dipping needle will assume, also, in various 

Fig. a. latitudes the directions 

exhibited in the annexed 
diagram, fig. 34, where 
the point of the arrow 
indicates the north pole 
■f .^f I ***' I ^^^ 1 ^— L-Jn t ^'"' '^® feather the 
"q *^\ ifife''W^ W^ ^"^'^ pole of the needles 
placed around the globe. 
^^V^ \ f / T^y^ '^^^ angle which the 
* ^"^SgjE^S?^ needle makes with the 

f^ K horizon at any place is 

called the dip, at that place. The tendency of the 
needle to dip is counteracted in the mariner's and sur- 
veyor's compasses, by making the south ends of needles 
intended to be used in northern latitudes, somewhat 
heavier than the north euds. 

95. In 6g. 34, M represents the North American 
magnetic pole near S the north pole of the earth. The 
line L V b nearly the present line of no variation, (see 
^ 98) and the curved line at the centre is the magnetic 
equator, or where the dip is at zero, and the direction of 
the dipping needle is the same as that of the horizontal 
needle. 




DIBECTITE TEND.ENCr OF KAONET. 55 

96. By companDg the directions assumed by the 
needle in its various positions in respect to the earth, as 
represented in fig. 34, with those assumed by a magnet 
in reference to another magnet, as illustrated m sect. 72, 
it will be found that there is a great analogy between 
them. This analogy led to the opinion, w^ich was for 
a long time entertained, that the earth was itself a mag- 
net, or that it contained within it large magnetic bodies, 
under the influence of which the magnetic needle as- 
sumed these various directions ; just as a small needle 
assumes such directions when brought in various posi- 
tions near to a bar magnet. 

97. But there is another mode of accounting for the 
directive tendency of the magnet in respect to the earth ; 
and that is by supposing, instead of magnetized bodies 
within the earth, lying parallel to the direction of the 
needle, currents of electricity passing around the earth, 
within it, but near the surface, at right angles with that 
direction. This would identify the directive power of 
the needle in respect to the earth, with its directive ten- 
dency in regard to a current of electricity, as described 
under the last head, instead of with respect to another 
magnet. And this is, in fact, the view which philoso- 
phers are now inclined to take of the subject. The 
theory, however, is yet unsettled ; and in fact all these 
three fonns of directive tendency may hereafter be 
shown to be identical. In the mean time the phe- 
nomena being distinct, they may properly be arranged 
in difierent classes. 

• 

Exp. 9. — ^Lay a fine sewing-needle, unmagnetized, upon the 
surface of water, where, if it is perfectly dry, it will float, and it 
will be found that it will lie nearly indifferently, in any positioD. 



56 DANIEL DATIS, Jiu's MANUAL. 

Then magrnetize it, by toaching it with anj magnet, and replace 
it upon the water, in a direction east and west It will imme- 
diately turn and assume a position in the magnetic meridian, 
that is, nearly north and south. 

Exp. 10. — ^Place a magnetic needle upon its pivot so tfaatiti 
north pole turns towards the north. Then take it off its pivot 
and draw the north pole across the north pole of a strong magnet, 
and the south pole of the needle across the south pole of the 
magnet On replacing it upon its pivot, it will be found that the 
polo which was previously north will now turn towards the south, 
and the south pole towards the north. In this way the poles of 
the needle may be reversed at pleasure. 

Exp. 11. — ^To prove that the inclination of the dipping needle 
is not occasioned by the greater weight of the north eztremi^of 
the needle used, reverse its poles, as described under the last ez- 
perimemt, and then what was before the south pole will be de- 
pressed, the pole which was previously north being elevated. 

98. The direction of the needle^ in respect to the 
earth is not fixed. Its variation^ that is, its deviation 
from the true geographical meridian, is subject to several 
changes, more or less regular. So also is the intensity 
of the action exerted on it by the earth, as shown bj 
the number of oscillations made by it in a given time. 
When examined also by means of apparatus constructed 
with great delicacy, the needle is found to be seldom at 
rest, but to be actuated with incessant fluctuations and 
tremulous motions, a phenomena supposed to comport 
more easily with the idea that electric currents consti- 
tute the influence by which it is controlled, than that its 
position is governed by the power of fixed permanent 
magnets in the earth. 

99. The instrument represented in fig. 35 is intended 
to illustrate the magnetism of the earth on the latter 
supposition. (See section 96.) The compound bar 



I 







i>iBBCi'it& TSNDsiret OF afAQNeT. 57 

magnet, n s, is placed 
in the magnetic aTLis of 
the earth, not coinciding 
exactly with the axis of 
rotation, N S. A small 
L t magnetic needle placed at 
B*B on the magnetic meri- 
dian, will point both to 
the magnetic pole s, and 
to the north pole N, both 
being in the same line. 
But if the needle be placed at A, or any where except on 
the magnetic meridian, it will point to the magnetic pole 
alone, the two poles not being in the same direction. 
The several magnets represented at n » are not fastened 
together, but only fixed on one axis. This allows their 
poles to be separated a little, to imitate more closely the 
dbtribution of terrestrial magnetbm : the earth really 
having four magnetic poles, two strong and two weak ; 
the strongest north pole is in America, the weakest in 
Asia. The line of no variation on the earth differs, 
however, considerably fnnn the magnetic meridian, and 
the lines of equal variation and equal dip are not exactly 
meridians and parallels of latitude to the magnetic pole. 
The action of the magnetism of the earth at its surface 
is therefore irregular. The temporary fluctuations, how- 
ever, are so slight as not to interfere with the use of the 
compass, and the variation of the needle is observed and 
noted on charts for different parts of the earth. 

100. The variation of the needle at any place is found 
by observing the magnetic bearing of any heavenly body 




58 DAlflKL DATIB, Ja.S HAIfUAL. 

whose true position at the time is Icdowd. It is imme- 
diately obtained by comparing the direction of the needle 
Fig;^ with the north sUr 

when it crosses the 
meridian or by cal- 
culation when the 
north star is at its 
greatest elongatioo. 
An observation of 
the sun, however, 
is usually preferred. 
The latitude of a 
place A (Gg. 36)heing known, the exact bearing of the 
, sun S, east or west, can be obtained by calculation,* fiw 
any given moment of lime at that place. If the needle 
at A points to M, instead of N, the true north, the angle 
MAS will be the magnetic bearing of the sun west. 
Suppose this angle to be observed by the surveyor's 
compass, and found equal to 76°, the rime being exactly 
noted. The angle N A S, the true bearing of the sun 
at the time, is then calculated. Suppose it equal to 
85^^ 30'. The difference between the magnetic hearing 
and the true bearing, represented by the angle MAN, 
is the variation of the needle, and equals 9° 30'.'t' 

101. Fig. 37 represents an instrument contrived to 
illustrate the theory which ascribes the magnetism of 
the earth to electrical currents circulating around it at 
right angles to its axb. N S is merely a wooden axis to 
the globe. When a galvanic current is sent through the 



~ Jm Bowditch'a Nirlgmltn. 

t Ths pruant Tuluion »i BoiLon la 9 deg, 30 mln. wan. Tba waa 
ippnn to Iw locrauhif. Tba pmam dip la 71 dag. SO mla, north. 



raatarl J nrikUoa 



DIBECTITE TKNDKHCY Or HAGNST. 59 

coil of wire about the equatorial regions, small needles 
placed in different situations will arrange themselves as 




they would in similar terrestrial latitudes. By compar- . 
iug this figure with fig. 35, representing the globe ^h 
the included magnet, a compaiison may be made- be- 
tween the two theories of magnetism. The small needle 
arranges itself similarly on both globes. With a small 
dipping needle the resemblance between its positions on 
hoth, and those assumed by it on the earth's surface are 
verj striking. 

lOS. It will be observed that, in Sg. 35, the touih 
pole of the included magnet is represented at the nmrtk 
geographical pole of the earth. So also, in fig. 37, the 
wooden rod N S, passed through the am of the globe, 
shows the direction of the polarity induced by the cur- 
rent to he contrary to that of the geographical poles. 
The reason of this may be easily understood. The 
northern magnetic pole is the one which attracts the 
north pole of a magnet, and therefore must itself possess 
south polarity and not north, as its name might seem to 
indicate. In the figure the battery current is of course 



60 DANIEL DATIS9 JB.'s MANUAL* 

considered as Bowing round the globe in the same direc- 
tion as the supposed currents in the earth ; that is to say, 
from east to west, in the opposite direction to that of the 
earth's rotation. The principle on which the coil acts in 
inducing polarity will be explained in chap. II, sect. 2. 

103. The aurora borealis is found to affect a deli- 
cately suspended magnetic needle, causing it to vibrate 
constantly but irregularly during its continuance, and 
especially when the auroral beams rise to the zenith ; if 
the aurora is near the horizon the disturbance of the 
needle is very slight. When the beams unite to form 
a corona, its centre is often in or near the magnetic 
meridian. 

104. Within a few years a considerable number of 
magnetic observatories have been established in various 
parts of the world, for the purpose of making systematic 
and corresponding observations in relation to terrestrial 
magnetism. At these stations the variation of the 
needle and the intensity of the earth's action qpon 
it are observed and recorded almost hourly, and on 
stated days at intervals of a few minutes only. These 
observations made by means of excellent instruments, 
and at the same time in widely remote regions, admit ef 
comparison with each other, and can hardly fail to 
throw light on many parts of this important and intricate 
subject. 



MAGNETISM. 

n. 

INDUCTION OF MAGNETISM. 

I. BY THE INFLUENCE OF A MAGNET. 

105. If a magnet is brought near to a piece of iron 
of any form, the latter becomes itself magnetic by the 
influence of the former. 

Exp. 12. — ^Let M, fig. 39, be a bar magnet, the point of the arrow 
indicating the north pole and the feather the south pole ; and B a bar 

Fig. 39. of iron brought 

J^ g near to it Now 

Izmiii — Sm I o -Kjll ^y the influence 

I U I a}) of the magnet 



the bar will become magnetized; the end towards the north pole 
will become south, and the end remote from it, north. The mag- 
netical induction is stronger when the bar is brought in contact 
with the pole of the magnet ; a decided eflrect,however,i8 produced 
by the mere proximi^ of the magnet to the iron. That the iron bar 
while under the influence of the magnet actually possesses mag- 
netic properties, may be shown by presenting to it some iron 
filings or small nails, which will adhere to each extremity ; and 
also by bringing near to it a small magnetic needle balanced on 
a pivot, the north pole of which will be repelled by the end of the 
bar farthest from the magnet M, and attracted by the end nearest 
to M. This induced magnetism will immediately disappear when 
the iron is removed from the vicinity of the magnet If a small 
bar of steel, a large sewing-needle for instance, be substituted for 
the iron bar, it will acquire magnetism much-leiBS readily, but wJQl 
retain it after removal ; becoming in fact a permanent magnet 

6 



62 DANIEL DAVIS, Jiu's MANUAL. 

106. It was for a long time supposed that the at- 
tractive force of the loadstone or any other magnet was 
exerted upon iron simply as iron; whereas it is now 
known to be the attraction of one pole of a magnet for 
the opposite pole of another magnet. In all cases, when 
a magnet is brought near to or in contact with any 
magnetizable bodies, as pieces of iron, iron filings, or 
ferruginous sand, all such bodies, whether large or 
small, coming thus within the influence of a magnetic 
pole, become magnetized; the part which is nearest 
acquiring a polarity opposite to that of the pole of the 
magnet ; while the remote extremity becomes a pole of 
the same name. 

Exp. 13. — ^If several pieces of iron wire of the same length be 
suspended from a magnetic pole, they will not hang parallel ; but 
the lower ends will diverge from each other, in consequence of 
their all receiving the same polarity by induction, while the 
upper ends will be retained in their places by the attraction of 
the magnet. 

Exp. 14. — Suspend two short pieces of iron wire by threads of 
equal length, fastened to one end of each piece so that the wires 
may hang in contact If now the south pole of a magnet be 
placed below the wires, the lower ends of both will become north 
poles, and their upper ends south poles ; and the wires will recede 
from each other. This divergence will increase as the magnet 
is brought nearer, until it reaches a certain limit, when its at- 
traction for the lower poles will overpower their mutual repulsion 
and cause them to approach each other ; while the repulsion of 
the upper ends will remain as before. 

107. In former times artificial magnets were always 
made by induction from strong magnets previously pre- 
pared ; the original source of the power being provided 
by natural magnets. When this was the case, it became 



INDUCTION OF MAGNETISM. 63 

important to ascertain what arrangements and what 
modes of applying a magnet to a bar or needle, were 
most efficacious in communicating or developing the 
magnetic virtue ; and accordingly various and compli- 
cated arrangements and manipulations for this purpose, 
are detailed in old treatises on this science. Recently, 
however, other and far more powerful means have been 
discovered for magnetizing bars of iron or steel, as will 
be hereafter described ; so that all those methods have 
been in a great measure superseded. The induction of 
magnetism by the means above referred to, is now only 
employed for magnetizing needles or small bars. 

108. It may however be convenient to know a good 
process for magnetizing (or touching^ as it is technically 
called) by the aid of steel magnets. One of the simplest 
and best will here be given. A small bar of steel may 
be magnetized by drawing it across the poles of a mag- 
net in the following manner ; place the middle of the 
bar on one of the poles and draw one end of it over the 
pole a number of times ; the direction of the motion 
being always from the middle to the end. Then turn 
the bar in the hand, and pass the other half over the 
other pole of the magnet in the same way. If the bar 
is thick, the process may be repeated with its different 
sides. The end which has been drawn over the south 
pole of the magnet will now possess north polarity, and 
the other extremity south polarity. 

109. The magnet which is used to induce magnetism 
loses none of its own power in the process, but often 
receives a permanent increase by the reaction of the 
polarities it has induced upon its own. 



64 DANIEL DATIS, J R. S UkJUVkli. 

ExF. 15. — That a. mftgnet possesKs greater power while exeit- 
ing ita inductive action, may be ahown bj auapending from one 
pole of a bar magnet as much iron as it can bold. If now a bw 
of iron be applied to the other pole, the first will be found capa- 
ble of lustaioing a greater weight than before. 

110. When the arrangement of the experiment is 
such that while one extremity of an iron bar is exposed 
to the influence of one poie of a magnet the other ex- 
tremity may be acted upon by the other pole, there will 
be a sort of double induction, and the effect will be 
increased. 
Eip. 16.— Let M, fig, 40, be a eompoond horee-ahoe magnet, 
n armature, of such a length that while one extremis 



Fig.H). 



Is applied to one pole of the magnet the otbsr 
extremity may be applied to the other. In thi* 
case both poles of the magnet will act, each 
inducing a polarity oppoaite to its own in that 
extremiQ' of the armature wliich is under its 
influence, as is indicated by the letters in the 
cut The force with which the armature ad- 
heres will consequently be greatly increased, 
for there will be a strong attraction between 
g each pole of the magnet and the corresponding 
extremity of the armature, that is, correspond- 
ing in position ; for the polarity of the parts in 
contact will evidently be of opposite denominations. If a bar of 
iron be placed between the north poles of two magnets, both 
extremities will become south poles, while a north pole will be 
developed at the middle of the bar. 

111. Y Ahhatcre. This consists of a piece of soft 
iron in the shape of the letter Y. If one of the branches 
of the fork be applied to the north pole of a horse-shoe 
magnet, as seen in fig. 41, the lower end of the arma- 
ture, and also the other branch of the foric acquire north 




mDUOTION 



or MAGNSTISH 



€5 



Fig.il. 



polarity, and will sustain small pieces of 
iron. If both branches of the fork be 
I applied, one to each pole of the magnet, 
I as shown by the dotted lines in the cut, 
I the polanty of the lower end immediately 
, disappears. Tliis is because the two 
poles tend to induce opposite polarities 
of equal intensity in the extremity of 
■^ the armature, which of course neutralize 
each other. If the branches of the fork 
are applied to the aimlar poles of two 
magnets, th^ influence will conspire in 
inducing the same polarity in the lower 
end,and a greater weight will be support- 
ed by it, than when one branch is applied to a single pole. 




J%.42. 







Exp. 17.— Place the 
north pole of a bar 
magnet M (fig. 42) on 
the centre of a circu- 
lar pl&te of iron ; it will 
now induce south po- 
larity in the part im- 
meilintely beneath it, 
and a weak north po- 
larity in the whole cir- 
^ cumference, bo that it 
will sustain iron filings 
' BB shown in the cut 



Exp. 18.— IfaniwHi 
|F*pkte be cut into the 
form of a star, as in fig. 
I 43, each point will ac- 
qniie a Rtxonger aoith polarity than the edge of the immd plate 
6* 



66 



DANIEL BATIS9 JIU'S MANUAL. 




Fig. 44. in the last experiment, and may be able to 

lift several iron acrews or nails; the let- 
ters in the cut indicate the position of 
the poles. 

Exp. 19.— Place the north pole cm the 

middle of a bar of iron ; both extremities of 

the bar will become north poles and the 

middle a south pole, as indicated by the 

V^ letters in the cut (fig. 44) where M repre- 

ents the magnet 

112 Fig. 45 represents the successive development 
of magnetism in several bars of iron. The bar a being 
M Fig. 45. g b e placed near to (mt in 
SIMP ^ nB — M ft Hft ri^ contact with the north 

pole of a magnet M, becomes itself temporarily mag- 
netic, and is able to induce magnetism in a second bar 
b ; this again in c, and so on, each succeeding bar being 
less and less strongly magnetized. The same thing 
occurs with the iron nails represented in fig. 43, hanging 
from the points of the star. If the magnet M be re- 
moved from the bar a, the magnetism of the whole series 
disappears. This successive development of magnetism 
is well shown by plunging one of the poles of a strong 
bar magnet in a mass of small iron bodies, such as 
screws, nails, &z;c. 

1 13. It is not easy to magnetize a bar whose length 
considerably exceeds its diameter, in such a manner that 
its two poles may be developed along two opposite sides 
mstead of at its extremities ; for the opposite polarities 
tend to keep as far from each other as possible. The 
points of greatest intensity in a permanent magnet are 
not however situated precisely at its ends, but at a little 
distance from them. 



INDUCTION OF MAGNETISM. 67 

114. The inductive action of a magnet is not impeded 
by the interposition of any unmagnetizable body what- 
ever. Thus, if a plate of glass be placed between the 
magnet and a piece of iron, the iron will be as much 
influenced, and will be attracted as strongly, as it would 
be at the same distance with no glass interposed. 

115.. Fracture of Magnets. A close analogy 
exists between the phenomena of magnetism and elec- 
tricity in many important points. But in some respects 
it altogether fails. Electricity, whether positive or 
negative, can be actually transferred fix)m one body to 
another, so that/ a body may be charged with an excess 
of electricity of either kind. It is not so with magnet- 
ism. Every magnet possesses both polarities to an equal 
extent, though each may be difilised through different 
portions of its mass. A long conductor exposed to the 
inductive influence of an electrified body, has opposite 
electricities developed at its two ends. If now it be 
divided in the middle, we obtain the two electricities 
separate ; one half of the conductor possessing an excess 
of positive, the other of negative electricity. The con- 
dition of a magnet in regard to the distribution of its 
polarities appears to be exactly analogous to that of the 
conductor ; the north polarity seeming to be collected 
in one half of its length, and the south in the other. 
We might therefore naturally expect that by breaking 
the magnet in halves we should obtain the two polarities 
separate, one in each portion of the bar. But such is 
not the case ; each half at once becomes a perfect mag- 
net. The original north pole still remains north, but 
the other extremity of the magnet, that is, the broken 




68 D.ANIEL DAVIS, J R.'s MANUAIj. 

end, has acquired a south pole. The converse of this 
occurs with respect to the other portion in which the 
south pole wa| situated, as shown in fig. 46. These 
halves may be again broken with the same result ; and 

Fig, 46. in fact into however 

^small fragments a 
■■■■■■■■■■^■■■magnet may be sub- 
divided, each will possess a north and a south pole. 

Exp. 20. — Suspend a piece of iron from one pole of a magnet 
and bring up to this pole the opposite pole of another magoet 
The iron will immediately fall : the poles when in contact repre- 
senting the middle or neutral portion of a maguet. If the piece 
of iron is nearly as heavy as the pole can sustain, it will !idl on 
the mere approach of the other magnet to the pole and before it 
touches it 



II. BY THE INFLUENCE OF A CURRENT OF 

ELECTRICITY. 

116. It has already been stated, ynder the head of 
the directive tendency of a magnet in reference to a 
current of electricity, that a magnetized body, freely 
suspended within the influence of such a current, tends 
to assume a position at right angles to it. It is also 
found that if any magnetizable body be placed in this 
position with regard to an electrical current, it acquires 
magnetism by its influence. This phenomenon is termed 
ekctro-magnetic induction. The subject of this section, 
with the one referred to above, form the department of 
ekctro-magnetism, 

117. A short copper wire connecting the poles of a 
battery will attract iron filmgs, as represented in fig. 47. 



INDUCTION or HAONBTtlK. 



69 




It will b« obMired that th« linet of filing! have not that 
brittled, divergent armngomant, whicb they exhibit 
upd«r the influence of a iteel magnet, but adhere equally 
Fig.47. all Bround the circumference 

of the wire; forming cir- 
cular bandi, the parttclei of 
which mutually cohere in 
coniequence of each particle 
becoming a magiiet with iti 
poliw tranvorse to the wire. 
The attraction ii also equal 
at every part of the length of 
t)io wire : hence theiie tran»' 
veno band<, lying in contact 
with each otht^r, present tho appearance of a eloiely- 
eomptcied layer. WhaUiver form the metal conducting 
the electricity may have, the filings will always arrange 
themselves in lines encircling it at right angles to the 
course of the current. The iron filings will of course 
fall oft when the current ceases to flow ; but if steel 
filings be employed, they will remain attached, in con- 
aequence of the adhesion of the magnetized particles 
among themselves. ' 

Eir. 91.— A lawlDK-needle nmj be iTitgnetix4d bjr jilteing it 
uroM tho wira and >t risht ucIm to It. If plscsd pusllol to 
lb* win, It la^iuirM fttbia polsritj on its opposite sidst Instettl 
of In tha dirsctloa of iu length, and probibl j will not retain It after 
removal) It bein|{ nry difficult to roaintaln this tnnsvene dl^ 
Uibutlon of TMgnMjmm in magnets wboH length coniidorably 
•xeaeds their dismeter. 

Exp. 33.— Place ■ short iron red or a piece of iron wire at right 
aaglas tothawlraoonveylDgtbeeurronL On bringing a doUoata 



70 DANIEL DAVIS, J R .'s MANUAL. 

magnetic needle near to its extremities, they will be foand to 
possess a sensible polarity ; which however they will lose when 
removed from the influence of the current 

118. Though the relation between the current and 
the direction of the polarity which it induces is fixed 
and determinate, yet it is very difficult to express. The 
action of the current in inducing magnetism ibilows the 
same law which we have already seen to determine its 
influence in moving a magnetic pole placed near it. 
See <5. 79. 

119. The following mode of fixing the rule in the 
memory is perhaps the best that has been contrived. 
First, it is more natural to fix our attention on the cur- 
rent ofposiiivey than of negative electricity. Secondly, 
in a vertical wire, a descending current will occur to us 
more readily than an ascendmg one ; or, if we imagine 
ourselves borne along by the current, it would be more 
natural to conceive ourselves moving with our^ec^ fore- 
most ; but if, on the contrary, we suppose ourselves to 
be at rest, we should conceive the current to be passing 
from our head to our feet. Our face wmld, of course, 
be turned towards the body to be magnetized ; we should 
attend to the north pole in preference to the south ; and 
to our right hand rather than to our left. Combining 
these conditions, then, we may always recollect, that if 
we conceive ourselves lying in the direction of the currenty 
the stream of positive electricity flowing through our head 
towards our feet, with the bar to he magnetized bef<yre 
us, the north pole of that bar will always he towards our 
right hand. If any one of these conditions be reversed, 
the result is reversed likewise. 




IITDUCTIOH or UAaNETISH. 71 

f^ ^ 120. Helix, on stand. The 

1 magnetizing power will be greatly 

jl increased if the wire be coiled in 

the manoeF of a cork-^crew, so as 

/ to form a hollow cylinder into 

which the body to be magnetized 

can be inserted. Such a coil b 

denominated a £re&! ; and Is repre- 

^sented at d, Gg. 48, mounted upon 

a stand. 

ISl. In using the coil, the following rule will indicate 
the extremity at which the north pole will be found. 
If the helix be placed before the observer with one of 
Its ends towards him, and the current of electricity in 
passing from the positive to the negative pole of the 
battery, circulates in the coil in a direction similar to 
that of the hands of a watch or the threads of a common 
screw ; then the north pole will be from the observer, 
and the south pole towards him. If it passes round in 
the contrary direction, the poles will be reversed. Or 
the formula ma^ he stated thus : the loutk pole will 
always be found at that end of the helix where the posi- 
tive current circulates in the direction of the bands of 
a watch. 

123. Thus, in fig. 48 Ae current flows from the cup 
C, up the wire a, to the coil ; and then down again 
by the wire b, to the cup Z, producing north polarity 
at N, and south polarity at S. This rule is strictly de- 
ducible from that given in 4' 119 for finding the direc- 
tioa of the polarity induced by a current flowing in a 
straight wire. 



72 DANIEL DAVIS, JR.'8 MANUAL. 

EzF. 23. — ^Place a bar of soft iron within the coil, and connect 
it with the battery by means of the two caps attached to the stand. 
Then the two extremities of the bar will be found to be strongly 
magnetic, as will be seen by bringing a key or other piece of iron 
in contact with them. On separating one of the wires commoni- 
eating with the battery, the magnetic power of the iron bar will 
be immediately destroyed, and the key will drop. If iron filingi 
or small nails are held near one of the extremities of the iron, thej 
will be taken up and dropped alternately, as the connection with 
the battery is made or broken. 

Exp. 24. — ^If two soft iron bars are inserted in the helix, at the 
opposite ends, in such a manner as to have their extremities in 
contact in the middle of the helix, they will be held in conjunc- 
tion by a strong force. 

Exp. 25. — ^The coil being connected with the battery and a bir 
of iron placed within it, bring a magnetic needle near the two 
extremities of the bar, in succession. One of the extremities win 
be found to have north and the other south polarity, and they will 
attract and repel the poles of the needle accordingly. 

Exp. 25. — ^Place a steel bar, instead of an iron one, within the 
helix. It will acquire polarity somewhat less readily, but the 
polarity will continue alter the connection with the battery is 
broken, and after it is removed from the helix ; and thus a perma- 
nent magnet be made. Any small rods or ban of steel, needles, 
&C., will answer for this experiment ^ 

Exp. 27. — ^Bars of iron or steel brought near the outside of the 
helix will not acquire any appreciable degree of magnetism. An 
iron tube will not become perceptibly magnetic when a current 
is passed through a helix placed within it, though when enclosed 
in a larger helix it will become strongly so. 

Exp. 28. — ^If a needle or a small bar of steel previously mag- 
netized, is placed within the helix, in such a position as to bring 
the north pole at the south pole of the helix, as indicated by the 
preceding rule, the polarity of the needle or bar will be destroyed, 
and perhaps a new and contrary polarity communicated. 

Exp. 29. — ^If a small magnetic needle be suspended by a thread 
near the helix, the mutual action between them will cause the 



INDUCTION OF MAGNETISM. 73 

iieedle to enter the helix, its north pole entering the south end of 
the helix, or its south pole the north end. When the needle reaches 
the middle, its north pole will be within that end of the coil which 
exhibits north polarity. If the magnet be placed within the helix, 
in a contrary direction, its north pole entering the north end, it 
will be repelled, and then revolving without the helix, will return 
and enter by the other pole. This effect will take place unless 
the electro-magnetic power of the coil is sufficient to reverse the 
poles. When the needle has entered with its poles correspond- 
ing in direction with those of the helix, the action of the helix will 
tend to keep it in the middle of its length, though not in the line 
of its axis. 

ExF. 30. — ^Place the helix with its axis vertical, and a small 
rod of iron or steel within it. If it be now connected with the 
battery, it may be raised from the table without the bar falling 
out: ^e tendency of the helix to keep the bar within it over- 
powering its gravitation. 

Exp. 31. — ^The power of the helix to induce magnetism may 
be shown by holding it vertically, as in the last experiment, 
while the current is flowing. A small steel bar, merely allowed 
to fall through the helix, will acquire a considerable degree of 
magnetism. 

123. Flat Spiral. Fig. 49 represeDts a ribbon of 
sheet copper, coiled into a spiral. This instrument is 
described here in consequence of its possessing consider- 
able magnetizing power, though its principal uses will not 
Fig. 49. ^ be mentioned till the inductive 

action of electrical currents 
comes under consideration, in 
chap. Ill, section 1. The cop- 
per ribbon may be an inch wide 
and one hundred feet long, the 
strips being cut from a sheet, 
and soldered together. Being then wound with strips of 
thin cotton, it is coiled upon itself, like the mainspring of 

7 




74 DANIEL DAVIS, JR.'s MANUAL. 

a watch ; intead of covering it with cotton, it may be 
coiled with a strip either of cotton or list intervening. 
Two binding screw cups are soldered to the ends of the 
ribbon; the internal end, for convenience, is brought 
from the centre, underneath the spiral, to its o^ide, 
care being taken to insure insulation where it passes the 
coils. The whole may be firmly cemented together, if 
desired, by a solution of shellac in alcohol. The spiral 
being connected with the battery, its two faces will 
exhibit strong polarity : a dipping needle placed on any 
part of its surface or near it will always direct one of its 
poles towards the centre, as seen in fig. 49, where a 
dipping needle N S is represented on the spiral. On re- 
versing the battery current, the other pole of the needle 
will turn towards the centre. If the spiral be fixed in a 
vertical position, a horizontal magnetic needle may be 
used with the same result. When brought near to one 
side of the coil, it will be found to direct its north pole 
constantly towards the centre ; when on the other side, 
its south pole. When either the horizontal or dipping 
needle is placed near the outside, with its axis of motion 
in the same plane as the spiral, neither pole will be 
directed towards the centre, but the magnet will place 
itself at right angles to the plane of the spiral. 

Exp, 32. — The magnetizing power of the spiral may be Bhfiiwn 
by connecting it with the battery, and placing a rod of iron or 
steel in the central opening, or upon it in the direction of a radios, 
when the iron will become temporarily magnetic, and the steel per- 
manently so. If the bar, when laid upon the coil, extends across 
the central opening, both ends will become similar poles, and the 
part over tlie centre, a pole of the opposite denomination. 

124. If the spiral be of considerable diameter, it will 
exert a feeble magnetizing power on its outside, and a 



INDUCTION OF MAGNETISM. 



75 



short rod of soft iron placed near it will become able 
to sustain a few iron filings ; its polarity will be in the 
reverse direction to that which it would acquire were it 
placed within. The influence of the earth in inducing 
magnetism in the iron must not be overlooked ; it may be 
allowed for by observing whether the transmission of the 
cunrent through the coil causes more or fewer filings to 
be sustained by the bar, or avoided by placing the spiral 
in a vertical position with its axis east and west, and the 
rod horizontally east and west. 

125. When the spiral is in the form of a ring, having 
a large central opening, it will be found that the magnet- 
ism communicated to a bar placed in the centre will be 
somewhat less than when it is near the side, though very 
much greater than that acquired by one on the outside« 
Fig. 50. 126. Magic Circle. This is a helia- 

cal coil of wire, shown at R in fig. 50| 
about two inches in diameter, with the 
extremities a and b of the wire left free, 
in order to be inserted into the cups of 
the battery. If two semicircular arma- 
~ tures of three quarter inch iron, provided 
ith handles, are passed partly within the 
ring, as represented in the cut, they will 
adhere together so strongly as to support 
a weight of fifty-six pounds or more, when 
the current from even a small battery is 
transmitted through the coil. The attrac- 
tive power manifested by the armatures 
when near each other, but not in actual 
contact, is comparatively very feeble. 




76 DANIEL DAVIS, JR.'s MANUAI.. 

127. If a ring and armatures of larg^ size are em- 
ployed, as represented in fig. 51, wfaere A A are the 

armatures, and C the coil, great force will be required 

Fig. 51. 




to separate them. The handles are attached to the arm- 
atures by ball and socket joints, to prevent them firom 
being twisted or wrenched by irregular pulling. The 
inductbn of magnetism in these armatures by means of 
the current from a thermo-electric battery has already 
been mentioned in <^ 56. 

128. If the coil while conveying the current be 
plunged in a mass of small iron nails, a large quantity 
of them will be sustained by it. An iron bar introduced 
within it will become strongly magaetic. If the flow of 
the current in the coil is stopped while the armatures 
are applied to each other, as shown in figures 50 and 
51, they will still contmue firmly attached ; but if once 
separated, will not adhere again. 

129. Page's Double Helix. This instrument con- 
sists of two helices fixed side by side, into which two 
bars of iron of the U form, fitted with handles, can be 
inserted so as to bring their extremities in contact in the 
centre. A very strong force will be required to separate 



INDUCTION OF MAGNETISM. 



77 



them, when the electrical circuit is completed throu^ 
the helices. The attractive force manifested by the 
bars when their extremities meet in the centres of the 
helices is much greater than when the ends of one of 
the bars project beyond the coils. It is also greater witfi 
short bars than with long ones. 

130. De la Rivers Ring. A coil of wire while 
transmitting the electric current is not only capable 

IXg. 52. of communicating 

magnetism to iron 
or steel placed 
within it, but itself 
[possesses magnetic 
polarity. This fact 
may be shown by 
means of the appa- 
ratus figured in the 
adjoining cut. One 
end of the wire 
forming the coil C 
is soldered to a very 
small plate of cop- 
per c, and the other to a similar plate of zinc z. These 
plates are fastened to a small piece of wood, in order 
to keep them apart, and placed in a little glass cup D. 
To put the instrument in action, a sufficient quantity of 
water, acidulated by a few drops of sulphuric or nitric 
acid, is poured into the glass cup to cover the plates, 
and the whole apparatus is floated in a basin of water. 
The coil will now be found to place itself with its axis 
north and south ; its polarity bemg in the same direction 




78 DAHIZL DAT IB, JB.'s HA IT UAL. 

as that iducb would be exhibited by an iron rod placed 
within it. The arrow indicates the course of the gal- 
vanic current in the coil, from the copper to the one. 

Exp. 33.->-Take a bu ma^et H, and hoiding it homontBllr, 
bring its ix»th pole near to tha south pole of tbe ring. Tbe ring 
will move towards the magnet, and pass over it until it reBctw 
its middle, where it will rest in b state of equilibrinm; retonung 
to this position, if moved towards either pole and then left at lib- 
erty. Now, holding the ring in its pomtion, withdraw the nwgMt, 
and pass it again half way through tbe coil, bat with its polee n- 
veiBed. Tbe ring when set at liber^, will, unless placed enotly 
at the centre, move towards the pole which is nearest; and paosing 
on till clear of tbe magnet, will turn round and pnaent ks other 
face. It will then be attracted, and pass agun over the pole till 
it rests in eqailibrium at the middle of the magnet 

131. Electro-Magnets. Bars of iron wound with 
insulated wire so as to be enclosed in a permanent belis, 
are termed Electiv-MagTteti. During the passage of an 
electric current along the wire, ih^ exhibit a remarkable 
degree of magnetic power, indeed far superior to that 
of steel magnets of the same size. They are usually 

J%- 51 made in the U form, as shown in 

Gg. 53, the bar being from six to 
eighteen mches in length before 
being b«it. These, when con- 
nected with a medium size cylindri- 
\ cal battery, will sustain from a few 
pounds to fifty or a hundred pounds. 
A cunent from the thermo-electric 
I battery (fig. 15), when transimtted 
through the wires of an electro-magnet, induces a con- 
siderable charge of magnetism. 

132. Frof. Henry, late of the Albany Academy, ap- 




INDUCTIOH OF MAGNETISM. 79 

pears to have been the Grst to construct electro-magnets of 
any great lifting power. In one instance, he employed a 
soft iron bar, two inches square and twenty inches long^ 
bent into the horse-shoe form ; its weight was twenty^ne 
pounds. This was wound with five hundred and forty 
feet of copper bell-wire, not in one continuous length, 
but in nine separate coils of sixty feet each, each strand 
of wire occupying about two inches of the bar, and being 
coiled several times backward and forward upon itself. 
By this arrangement the different coils could be com- 
bmed in a number of ways ; thus, if the second end of 
the first wire was soldered to the first end of the second, 
and so on through the series, the whole would form a 
single coil of five hundred and forty feet. Or they might 
be united so as to form a double coil of two hundred and 
seventy feet, or a triple one of one hundred and eighty 
feet, and so on. A small battery was used, consisting 
of two concentric cylmders of copper, with a zinc cylm- 
der between them. The battery required only half a 
pint of diluted acid for its charge, and the surface of zinc 
exposed to the acid was but two-fifths of a square foot. 
Each strand of the wire being soldered in succession to 
this battery, one at a time, the magnetism was just suf- 
ficient to sustain the armature, which weighed seven 
pounds. When the first end of each of the nine strands 
was soldered to the zinc cylinder and the second end to 
the copper cylinder, so that the current circulated in 
nine channels of sixty feet each, the magnet supported 
the extraordinary weight of six hundred and fifty pounds. 
With a larger battery it sustained seven hundred and 
fifty pounds. Each pole, separately, could lift but five 



80 



DANIEL DATIB, JB.B MANUAL. 



or six pounds. On uniting the ends of the tnres, so as to 
forrn a continuous length of five hundred and forty feet, 
the weight raised was onlf one hundred and forty-five 
pounds. He afterwards constructed another electro-mag- 
net on a similar plan, which was wound with twenty^ox 
strands of copper wire, covered with cotton thread, dn 
aggregate length of the wires being seven hundred and 
twenty-eight feet. With a battery of 47.9 square feet, 
this magnet supported two thousand and sixty-three 
pounds, or nearly a ton. Others hare since been made 
with a lifUng power of three thousand pounds. 
Fig. 54. 




133. Fig. 54 represents an electro-magnet fixed in a 
frame, for the purpose of supporting heavy weights. A 



INDUCTION OF MAGNKTISlf. 81 

semicircular armature A is adapted to its poles, as this 
form gives the greatest lifting power. It will be observed 
that if the iron of the magnet is soft and pure, its mag- 
netic power will be immediately communicated and lost, 
according as the connection with the battery is made or 
broken. If, however, the armature is applied to the 
poles, and the flow of the current is stopped while it is 
attached, it will continue to adhere for weeks or months 
with great force, so as to be able to sustain one third or 
one half as much weight as while the current was circu- 
lating. But if the keeper be once removed, nearly the 
whole magnetism will disappear, and the magnet, if of 
good iron, will not even be able to lift an ounce. The 
polarity of the magnet will of course be reversed by 
changing the direction of the current. 

Exp. 84. ^A small electro-magnet will sustain a large mass of 
iron nails or filings about its poles, which will fall when the flow 
of the current is stopped. A very small electro-magnet has been 
made to lift four hundred and twenty times its own weight 

134. An electro-magnet, like the steed magnet, exerts 
its attractive force through intervening substances ; and 
the phenomena are more striking with the former, in 
consequence of its greater power. Thus, it will often be 
able to lift its armature, with a plate of glass interposed ; 
and when a few thicknesses of paper only intervene, a 
considerable additional weight will be supported. 

135. Electbo-Magnet, with three poles. This 
consists of an iron rod wound with wire, which is carried 
in one direction around half the length of the rod, and 
then turns and is wound in the other direction. The 
effect of this arrangement is, that when the connection 




82 DANIZL DAT IB, ) lu's MAHDAL. 

is made with the battery by means of the brass cups on 
the stand, tbe two extremities of the bar, c and d, fig. 55, 
become ^milar poles, while the middle a acquires a 
Fig.SS. polarity opposite to that of 

bllie ends. By reretsing the 
direction of the current, all 
the poles wiU be reversed. 
The arrangement of tbe poles 
may be shown by passing i 
magnetic needle along tbe 
bar, or by small iron tacks, 
a large number of which will adhere to its extremities 
and to its middle. 

136. COMMTNICATION OF MaGNETIBM TO StEKL BT 

THE Electbo-Magnet. The great power possessed 
by the electro-magnei, renders it peculiarly fitted ibr 
inducing magnetism in steel ; hence it is very coDvenient 
for charging permanent magnets. A short steel bar, if 
applied like an armature to the poles of a U shaped 
electro-magnet, will become strongly magnetic, tbe end 
which was in contact with the nortli pole acquiring, of 
course, south polarity. A longer bar may be charged, 
by employing the same process that has been described 
in ^ lOa, for touching by steel magnets, 

137. Bare of the U form are most readily magnetized 
by drawing them from the bend to the eztremiues across 
the poles of the U electro-magnet, in such a way that 
both halves of the bar may pass at the same time over 
the poles to which they are applied. This should be 
repeated several times, recollecting always to draw tbe 
bar in the same direction. ' Then, if it has a conddersble 



INDUCTION OF, MAGNETISM. 



83 




thickness, turn it in the hand and repeat the process 

^' 56. with its opposite 

surface, keepmg 
each half applied 
to the same pole 
as before. Of 
course, the result 
will be the same, 
zy if the steel bar is 

kept stationary and the poles of the electro-magnet 
passed over it in the proper direction, that is, in the re- 
verse direction of the arrow in fig. 56. 

138. In order to remove the magnetism of a steel 
magnet of the U form, it is only necessary to reverse 
the process just described ; that is, placing one pole of 
the electro-magnet on each of its poles, to draw the 
electro-magnet over it, towards its bend, in the direction 
of the arrow in fig. 56. In this way, a steel magnet 
may often be so completely discharged as to be unable 
to lift more than a few iron filings. A bar magnet may 
also be deprived of its magnetism in a great degree by 
passing the north pole of an electro-magnet over it, fix)m 
its south pole to its middle, and then lifting it oflF per- 
pendicularly ; if, then, the south pole be passed in the 
same manner over the other extremity of the steel bar, 
it will be found to have lost the greater part of its 
polarity. If necessary, this process may be repeated 
several times. A still more effectual mode is to make 
use of two electro-magnets ; place the north pole of one 
on one end of the bar, and the south pole of the other on 
its other extremity, and draw the poles along the bar till 



\ 



84 DANIKL DAVIS, JR.'s MANUAL. 

they meet at its middle ; then lift them off. If the sted 
bar whose polarity is to be removed is of small size, steel 
magnets may be substituted for the electro-magnets in 
the above processes, though with less effect. 

MOTIONS PRODUCED BY THE MUTUAL ACTION OF MAGNETS 

AND CONDUCTORS. 

139. When a wire conveying a current of electricity 
is brought near to a magnetic pole, the pole tends to re- 
volve around it, as has been explained in <^ 79. If the 
current acts equally upon both poles, no rotation occurs, 
because they tend to move in opposite directions ; and 
the magnet rests across the wire in a position of equilib- 
rium between the two forces. But if the action of the 
current b limited to one pole (which was first effected 
by Prof. Faraday), a continued revolution is produced. 
If the magnet has liberty of motion, it will revolve around 
the wire ; if the wire only is free to move, it will rotate 
around the pole. When both the wire and the magnet 
are at liberty to move, they will revolve in the same 
direction round a comnon centre of motion. A number 
of instruments have been contrived for exhibiting these 
movements. 

140. Magnet revolving round a Conducting 
Wire. In the instrument represented in fig. 57, the 
magnet N S has a double bend in the middle, so that 
this part is horizontal, while the extremities are verticaL 
At its north pole N is attached a piece of brass at a right 
angle, and bears a pivot which rests in an agate cup fixed 
on the stand. A wire loop attached to the upper pole S 
encircles a vertical wire fixed in the axis of motion, and 



BLEOTBO-KAeNXTIC 



ROTATIONS. 



thns keeps the magnet upright. The galvanic cunent b 
CODTeyed by this vertical wire : it is sunUounted by a brass 
cup A, and its lower end dips into a small mercury cup 
on the horizontal portion of the 
magnet. From this part projects 
a bent wire, which dips into a 
circular cistern of mercury, open 
in the centre, to allow the mag- 
net to pass through, and sup- 
ported independency of it. A 
wire, terminated by a brass cup 
B, for connection with the bat- 
tery, proceeds outwardly from 
the cbtem. This arrangement 
allows the current to flow down 
by the side of the upper pole of 
the magnet, until it reaches iti 
middle, whence it is conveyed off 
k in such a direction as not to act 
f upon the lower pole. On making 
connection with the battery, the 
magnet will revolve rapidly around the wire ; the direc- 
tion of the rotation dependmg upon that of the current 
141. Magnet revolving bound its own axis. Hie 
instrument represented in fig. 58 is designed to show- 
that the action between the current and the magnet 
takes place equally well when the magnet itself forms 
the conductbr of the electricity. The lower end N of 
tiie magnet, being pointed, b suppc»ted on an agate at 
the bottom of a brass cup connected under the base- 
board with the binding screw cup P. The upper end 
8 




86 



DANIEL DATI8, JE-'s UANUAI^ 




S IS hollowed out to receive the end of the wire fixed 
to the cup A ; the brass arm supporting this cup is insu- 
^'^ lated firom the brass 

pillar at 1 1, by some 
non-conductor of elec- 
tricity. To the middle 
of the magnet is fixed 
a small ivory cistern 

C, for containing mer- 
cury, into which dips 
the end of the wire 

D. Thus the mag- 
net is supported with 
its north pole dowD- 
wards, and is fifee to 

rotate round its vertical axis. A little mercury should 
be put bto the cavity at S, and mto the brass cup at 
N^ and the ivory cistern be filled sufficiently to establish 
a connection between the magnet and the wire D. 

142. On connecting the cups A and B with the bat- 
tery, the current will flow through the upper half of the 
magnet, causing it to rotate rapidly. If the cups B and 
P form the connection, the current will traverse the 
lower half, equally producing revolution of the magnet. 
Now connect A and P with the battery, and no moticMi 
will result, because the electricity passes through the 
whole length of the magnet in such a manner that the 
tendency of one pole to rotate is counteracted by that of 
the other to move in the opposite direction. Connect B 
with one pole of the battery, and A and P both with the 
other pole. The magnet will now revolve ; since the 



ELBCTRO-H 



LGNETIC SOTATI 



87 



conent will ascend in one half of its length and descend 
in the o^er. 

143. Revolting Wire Frame. The revolution of 
a conductor round a magnet b shown by the instrument 
represented in fig. 59. Two light liiimes of copper 

' B R R are supported by pivots resting on the poles N 
and S of a steel magnet of the 
U form ; a small cavity being 
drilled in each pole to receive 
an agate for the bearing of the 
pivot. The lower extremities 
of the wires dip into mercury 
contained in two circular cis- 
terns sliding on the arms of the 
magnet. Bent wires passing 
from the interior of the cells sup- 
port the cups A and D ; and the 
cisterns themselves are fixed at 
any required height by means 
of binding screws attached to 
them. Each of the ^n^ frames 
is surmounted by a mercury cup; into these dip the 
wires projecUng downwards from the cups B and C. 

144. The cisterns being partly filled with mercury, fix 
them at such a height that the lower extremities of the 
wire frames may just touch its surface. The cups siu^ 
mounting the frames should also contain a little mercury. 
On connec^ng the cups A and B with the battery, the 
left hand frame will revolve, in consequence of the action 
of the north pole of the magnet upon the current flowing 
in the vertical portions of the frame. By uniting C and 




fM DAKIBL DATI8, JB.'S HAlTirjLL. 

D with the battery, the other frame will route. On 
traDsmiltiDg the current from A to D, it will ascend m 
ooe frame, and passing along the brass ann which sup- 
ports B and C, will descend in the other, causiog than 
both to revolre in the same direction. Instead of the 
frame, a »ogle wire may be employed, having the form of 
a loose helix sunounding the pole, its conTc4utions htaag 
a quarter of an inch or more apart. , 

_^/%.6IX^ 145. Revoltino Ctlimdeb. 

This instrument is on the same 
principle as that last described, 
and the motion takes place 
in the same manner: the only 
Bodifierence being that two li{^ 
copper cylinders c e, fig. 60, ue 
subsdtuted for the wire frames. 
These cylinders are serrated at 
th^ lower edge, as shown in 
the figure, to lessen the friction 
which they experience in mor- 
ing through the mercury. The 
cups for battery connecdc»is are 
lettered in correspondence with 
those in the preceding cut, fig. 59. 

146. In the case of a conducing wire revolving lomd 
a magnet, the cu-cumstance of the two b^g joined to- 
gether does not affect the result, the wire moving mlb 
sufficient power to cause the magnet to turn on its axis 
with considerable rapidity, when delicately supported ; 
a bar magnet is of coutse employed. A figure and de- 
scription of an instrument designed to ^ow this revo- 




BLECTBO-HAaiTETIC BOTATIOKS. 



lution will be found in Silliman's American Journal of 
Science and Arts, Vol. XL, No. 1, p. 111. 

147. The current passing within the voltaic battery 
itself exhibits the same electro-magnetic properties that 
it does while flowing along a conducting wire connecting 
the poles. Hence the battery, if made small and light, 
will revolve by the influence of a magnet. Thb is 
efiected in the following manner. 

148. Amp ebb's Rotatinq Battebt. A small 
doable cylinder of copper, closed at the bottom, is sup- 
pCHted upon the pole of a magnet, by means of an arch 

f% 61, of copper passing across the inner 

cylinder, and iiaving a pivot pro- 
jecting downwards from its under 
I surface, which rests in an agate 
cup on the pole. The inner cyl- 
inder of course has no bottom. A 
cylinder of zinc is supported by a 
pivot in a similar manner upon the 
copper arch, and being intermediate 
in size between the two copper 
cylinders, hangs freely in the cell. . 
This arrangement allows each plate 
to revolve independently of the 
other. In fig. 61 two batteries are 
represented, one on each pole of a 
V magnet, the one on the south pole b^ng shown la 
section ; in thb the zinc plate z is seen suspended within 
the copper vessel C. 

149. On introducing diluted acid into the copper 
ressel, an electric current-immediately begins to circu- 

8* 




90 



DARIEL DATIB, >B.'8 HAHUAI- 



late, which passes irom the mc to the copper, thcoo^ 
the acid, and, ascendiog from the copper through the 
pivot, descends again to the unc. Hrace the zinc plate 
is in the condition of a conductor conTeying a strena 
of electricity^ dofrnwards, and will consequently revohe 
undn the mfluence of the pole which it surrounds. The 
copper cylinder, on the contrary, is in the mtuatioii of i 
conductor conreying a current upwards, and will rotate 
in the opposite directicHi. When there b a battety on 
each pole of a U magnet, the two copper vessels will 
be seen to reToIre in contrary directioiu, and the two 
vac cylinders in directions opposite to these, and of 
course also cimtrary to each other. 

150. Mabsec's Vibbatino Wire. A copper wife W, 
^■^ inGg.63,Usiispeaded 

over a small bawi fir 
conlainbg mercury ex> 
cavated in the stand, 
by means of a biasB 
arm supporting a mer- 
cury cup, in which the 
upper end of the wire 
rests : thb mode of 
suspension allows it to 
vibrate freely, if its 
upper end is propwly 
bent. Two cups for connection with the battery com- 
municate, one with the mercury ia the excavation, the 
other with the cup which sustains the wire. 

151. The basin being supplied with a sufficient quan- 
tity of mercury to cover the lower end of the suspended 




XLJBCTRO-MAGNSTIC MOTIONS. 91 

wire, lay a horse-shoe magnet in a horizontal position on 
the stand, with one of its legs on each side of the wire. 
On establishing communication with the battery, the 
poles of the magnet will conspire in urgmg the wire 
either backwards or f<xwards between' them, according 
to the direction in which the current flows through it, 
and the positi(»i of the magnetic poles. In either case, 
the motion will carry it out of the mercury, as shown 
by the dotted lines in the cut ; and the circuit being 
thus broken, the wire will fall back by its own weight : 
when the current being re-established, it will again quit 
the mercury as before, and a rapid vibration will be 
produced. 

152. The vibration may be made somewhat more 
active by rabing the magnet a little from the stand, and 
nearly to the height of the middle of the wire. Or the 
magnet may be held in a vertical position with one of 
its poles on each side of the wire. The wire will also 
vibrate by the side of a single pole placed either in 
a horizontal or vertical position, but its motion is less 
active. The wire tends to revolve round the pole pre- 
sented to it, as has been explained in <^ 79 ; and when 
suspended between a north and south pole, as in fig. 62, 
simultaneously around both. 

153. Gold Leaf Galvanoscope. A glass tube 
fixed in a vertical position between the poles of a steel 
magnet of the U form, as shown in fig. 63, contains a 
narrow slip of gold leaf c, suspended loosely from for- 
ceps connected with a brass cup B, surmounting the 
tube. The lower end of the slip is held by another 
forceps communicating with the cup E on the stand. 



98 DAHIEL DITIS, Ja.1 MARVAL. 

When a rery feeble cnrrent of electriciq^ i* t 
fV-63. throagb the gold leaf, it will 

curved forwards ot backwards acccr&g 
to the course of the cimmt : in ehber case 
tending to move awaj from between ibe 
magnetic poles in a lateral ^rectiDii ; (or 
the same reason that causes Ae modon of 
the wire in the last described apparabu. 
I D The instrument does not in^cate the 
quantitjr of the electrical current, as other 
galvanometers do, but is an exceedb^j 
delicate test of its existence and direction. 
I A powerful current would of course destroy 
the gold leaf. 

154. Vibrating Maoic Circle. An electro-magnet 
M, fig. 64, is supported upon a stand, in a horizontal po- 
sition ; and a circular coil of wure c is suspended from the 
f%.64. 




arm of the upright post S iu such a manner as to allow it 
to pass along one of the poles of the magnet, the ring en- 
circling the pole. On making communication with the bat- 
tery, the coil will move over the pole towards the middle 



ELECTRO- UAOITXTIO HOTIOITS, 



of the magDet, in the same macner as De la Rive's ring 
already described. When it has passed some distance, 
the electrical circuit is broken by means of the bent wire 
a, which leaves the mercury cup e. The ring then falls 
. back to its previous vertical position by the side of the 
post S, and the connection with the battery is restored. 
It is then again attracted by the pole of the magnet, and 
thus a continued ^bratory motion is produced. The 
flow of the current through the wires of the electro- 
magnet is not interrupted by the breaking of the circuit 
in the coil c. 

155. BoimLE ViBBATiNO Maoic Circle. In the 
instrument represented in 
fig. 65 two coils A and B 
are employed, with a steel 
magnet. One end of the 
^re fonniog each c(m1 
is 90 bent m to dip into 
merciuy ccmttuned in the 
cup C, when the ring 
hangs Ireely ; and to be 
raised out of the mercury 
when it moves over the 
pole. The double wire, 
by which one of the coils 
is suspended, is somewhat 
longer than that which 
[sustains the other, its axis 
of motion being higher in 
proportion. This inequality of length occasions the 
vibrations of the two rings to be irregularly alternating. 




94 



DAVIS, JB.'S MAirtTAL. 



156. Bablow's Retolthiq Sphb-Wheel. The re- 
ciprocating movemeDt id Marsh's apparatoa described in 
^ 150, may be conrerted ioto one of rotatiOD by making 
1^.66. 




-cr 



use of a copper wheel the ctrcamfereace of which is cut 
into rays, instead of the wire. The points of the wheel R, 
fig. 66, dip ioto mercury contained in a groove hollowed 
out in the stand. A more rapid revoluHon will be obtained 
if a small electnymagnet be substituted f«r a steel magnet, 
as is shown in the cut. The electro-magnet is fixed to 
the stand, and included in the circuit with the spur-wheel, 
so that the current flows through them in succession. 
Hence the direction of the rotation will not be changed 
by revering that of the current ; ance the polarity of 
the electro-magnet will also be reversed. 

157. The course of the current is as follows. Sap- 
pose the cup A to be connected with the posidve pole 
of the batteiy, and B with the negative : the electricity 
will Sow from A through the wire of the electro-magnet 
N S, and thence to the mercury contained in the groove, 



KLECTBO-MAONETIC ROTATIONS. 95 

which is connected with one end of this wire. It 
will then pass along the wheel R, through any point 
which happens to touch the mercury, to its axis, whence 
it will be conveyed by the wire W, to the cup B. 
Under these circumstances, the ray through which the 
current is flowing passes forward between the poles of 
the magnet, like the vibratbg wire in Marsh's instrument, 
until it rises out of the mercury. At this moment the 
next succeeding ray enters the mercury, and goes through 
the same process ; and so on. 

158. If the quantity of mercury is so adjusted that 
one ray shall quit its surface just before the next one 
touches it, a spark will be seen at each rupture of con- 
tact. When the machine is set in motion in the dark, 
so that it may be illuminated by the rapid succession of 
these sparks, the revolving wheel will appear to be 
nearly at rest ; exhibiting only a quick vibratory move- 
ment, in consequence of the sparks not succeeding 
each other precisely at the same point. This optical 
illusion arises from the fact, that the electric light is so 
extremely transient in its duration that the wheel has 
not time to move to any appreciable extent during the 
electrical discharge ; and therefore each spark shows it 
in an apparently stationary position. If the sparks occur 
at one place more frequently than at the rate of eight 
in a second of time, the eye cannot appreciate them 
separately, and the impression of a continuous light is 
received. For this reason the wheel is seen constantly, 
as if it were illuminated by a steady light, instead of an 
intermitting one. 

159. At the bottom of the groove in the stand, the 



96 DAHISL DATia, Ja.'t MAKUJLL. 

extremity of a wire projects dighdj to form the con- 
aectioQ betweeo the meroury and the dectriMnagneL 
In using the mstmment, care should be taken that the 
end of this wire and also the points of Uie spur-wheel 
are clean and bright, so that they may come into good 
metallic contact with the mercuiy. 

160. Double Spor-Wheei.. In this instiumeDt 
there are two spur-wheels and two electnMnagneta ; 
and their arrangement is such that the current risei 
through the radius of one wheel, and passing almg tbe 
axis descends by the other wheel. 

161. Stubocon's Revoltiitg Disc. It is not ee- 
sential to divide the wheel into rays, in order to obtain 
rotation. A circular metallic disc will revolve equally 
well between the poles of a magnet. In this case, the 
electric circuit remains uninterrupted during the entin 
revolution, and no sparks appear as with the spur-wheel. 

^■<^- 162. Pace's Revolving Rdio. 

This instrument consists of a U 
shaped steel magnet, fixed upon a 
stand, in a vertical position, and a 
circular coil of insulated copper 
wire C, fig. 67, so arranged as to 
revolve ou a vertical axis between 
the magaetic poles. The rotation 
is effected in a different manner 
from any previously mentioned. ■ 
The polarity of the ring is reversed 
twice in each revolution, by means 
of a contrivance of Dr. Page's called 
■ a. poh-chajtgtr, which is employed 





ELECTRO-MAGNETIC ROTATIONS. 91 

in many of the instruments to be hereafter described. 
f%. 68. The pole-changer attached to the 

"ring is seen at P, and a horizontal 
*fBs section of it is shown in fig. 68. It 

consists of two small semi-cylindrical 
pieces of silver s s fixed on opposite sides of the axis of 
motion A, but insulated fix)m that and from each other ; 
to each of these segments is soldered one end of the 
wire composing the ring. The battery current is con- 
veyed to the coil by means of two wires terminated by 
horizontal portions of flattened silver wire W W which 
press slightly on opposite sides of the pole-changer, 
whose segments must be so arranged that the direction of 
the current in the ring may be reversed at the moment 
when its axis is passing between the poles of the magnet. 
163. On placing the ring with Its axis at right angles 
to the plane of the poles, and making connection with a 
battery, one extremity of the axis, or in other words, one 
face of the coil, will acquire north polarity, and the other 
south polarity, in the same manner as De la Rive's ring ; 
the action of the magnet will now cause it to move round 
a quarter of a circle in one direction or the other accord- 
ing to the course of the current, so as to bring its poles 
between those of the magnet. In this position it would 
remain, were it not that as soon as it reaches it, the pole- 
changer, which is carried round with it, presents each of 
its segments to that stationary silver spring which was 
before in contact with the opposite segment. By this 
movement the current in the ring is first interrupted for 
a moment, and as the ring passes on is immediately 
renewed in the contrary direction, thus reversing the 

9 



100 DANIEL DATIS9 JB.'S MANUAL. 

contact and connected by a strip of brass. The circle 
thus formed is a little larger than the coil^ and revolves 
freely around it on a vertical axis. A peculiar arrange- 
ment is required in order to transmit the voltaic current 
to the pole-changer belonging to the ring. The springs 
which press upon it are connected with two small cylin- 
ders of silver fixed on the axis of motion of the magnet 
and insulated from it, one being a little below the other; 
or a part of the axis itself being made cylindrical may 
answer for one of them : the wires proceeding from the 
brass cups on the stand press upon these cylinders* In 
this manner the current is conveyed to the springs of the 
pole-changer in a constant direction notwithstanding that 
they are carried round with the magnet in its revolutions. 
When the current is transmitted through the coil, the 
mutual action between it and the magnet causes them 
both to revolve, but in contrary directions ; on the well 
known mechanical prmciple that action and reaction are 
always equal and opposite to each other. 

167. The arched flame obtained between two char- 
coal points attached to the poles of a powerful battery, 
as repesented in fig. 11, will be thrown into a rapid 
rotary motion when a magnetic pole is placed near it. 
This efiect may also be very satisfactorily shown by 
pressing one of the battery wires firmly upon a steel 
magnet, and bringing the other wire up to one of its 
poles. The flame which may now be obtained by 
withdrawing this wire a little, will rotate in one direction 
if drawn from the north pole, and in the opposite direc- 
tion if from the south. When the magnet is connected 
with the negative end of the voltaic series, the flame 



ELECTRO-MAGNETIC ROTATIONS. 101 

drawn from its north pole revolves from left to right, in 
the direction of the hands of a watch. 

MOTIONS PRODUCED BY THE REVERSAL OF THE FOLARTTY OF 

AN ELECTRaMAGNET. 

168. Ritchie's Revolving Magnet. A steel mag- 
net of the U form is supported upon a stand m a vertical 
position, its poles bemg uppermost. The revolving piece 
is a small straight bar of soft iron wound with insulated 
wire ; it has a pivot projecting downwards from its under 
surface, which enters a deep pivot-hole on the top of an 
upright rod so fixed that the iron bar may rotate hori- 
zontally between the poles of the U magnet. The two 
extremities of the wire surrounding this electro-magnet 
descend into a circular basin of ivory for containing 
mercury, attached to the upright rod. a little below the 
revolving bar. This basin is divided into two separate 
cells by two low partitions of ivory, so arranged that 
when the electro-magnet is passing between the poles of 
the steel magnet the ends of the wire may be movmg 
across the partitions and just above them. On supplying 
the cells with a proper quantity of mercury, its surface 
will be found to curve downwards on every side towards 
the ivory, so that its general level will be higher than 
the partitions ; thus allowing the extremities of the wire 
to be immersed in it except when passing across them. 
A wire connected ^th a brass cup, for making com- 
munication with the battery, projects into the mercury 
in each compartment of the basin. 

169. On transmitting the voltaic current, when the 

9* 



102 DANIEL DAVIS^ JR.'s MANUAL. 

bar is at right angles to the plane of the magnet, it will 
immediately acquire a strong polarity. Its north pde 
will then be attracted by the south pole of the steel 
magnet and repelled by its north pole. The south pde 
of the bar, on the contrary, will be repelled by the 
similar pole of the upright magnet, and attracted by its 
opposite pole. These four forces will conspire in 
bringing the electro-magnet between the poles of the U 
magnet ; as soon as it reaches this position, the ends of 
the wire will quit their respective mercury cells, and by 
the momentum of the bar, which at this moment loses 
its magnetism, will be carried across the partitions, so 
that each will dip into that portion of the mercury which 
the other has just left. This will renew the circuit and 
restore the polarity of the electro-magnet, but in the 
reverse direction. Each pole of the bar will now be 
repelled by that pole of the permanent magnet which it 
has just passed, and attracted by the opposite one ; it 
will thus continue to move on, its polarity being, reversed 
twice in each revolution. 

170. At the moment when the wires quit the mercury 
to pass across the partitions, a spark is seen. When the 
machine b put in motion in a dark room, these sparks 
give rise to an optical illusion of the same character as 
that mentioned under the head of Barlow's Revolving 
Spur- Wheel, causing the bar to appear at rest in the 
position it is in when the sparks are emitted. The 
points of the wires which dip into the mercury should 
be kept clean and well amalgamated. The tendency 
of the mercury to be drawn over the partitions may be 



BLtCTBO-HAGHETIC BOTATIONS. 103 

partially prevented by a little water on its surface, which 
however diminishes the bnlliancy of the sparks. 

171. Page's Revolving Magmet. In this instru- 
ment, represented in fig. 71, the polarity of the electro- 
^- ''i- magnet is reversed, not by 
means of mercury ,as in the 
Vone last described, but by 
• Dr. Page's pole-changer, 
r§ 162, the segments of 
which are so arranged that 
the poles of the revolvbg 
bar may be changed at 
the moment when it is 
passing the poles of the 
fixed magnet. The silver 
springs which press upon 
the pole-changer are at- 
tached to two stout brass 
wires which pass through 
the brass arch surmount- 
ing the U magnet, but are 

insulated from it by the 

intervention of ivory or horn ; each of these wires sup- 
ports a brass cup for connection with the battery. In 
this way a more rapid revolution is obtained than with 
Prof. Ritchie's arrangement, but the fine sparks afibrded 
by that do not make their appearance. A still more rapid 
rotation may be produced, both in this and in Ritchie's 
instrument, by employing a U shaped electro-magnet in 
place of the stationary steel magnet. In this case, the 




104 



KIEL DA7Ig, JB.8' HAKBAL. 



revolution is DOt reversed by changiag the direction of 
tbe current, as it is when a steel magnet is used, since 
the poles of both electro-raagnets are reversed at the 
same time, and their relative polarity remains tbe same. 
173. RoTATiNo Bell Engine. The general con- 
struction of this iastniment is similar to the preceding, 
the U magnet, however, being inverted, so that the 
revolving electro-magnet A, fig. 
72, is near to tbe stand ; tbe 
pole-changer being attached to 
the axis below it. There is, in 
\ addition, an arrangement for 
striking a bell fixed above tbe 
magnet. To tbe axis of the 
revolving bar is attached an 
endless screw S ; this acts upon a 
toothed wheel, which b provided 
with a pin projecting laterally, 
for the purpose of moving tbe 
hammer of the bell. As the 
wheel turns, the pm presses 
upon the handle of tbe hammer, 
raising it from tbe bell until it is 
^released by the pin at a certain 
P'point of the revolution ; when a 
spiral spring fixed to tbe handle 
impeb tbe hammer against the bell. 

173. If tbe wheel has sixty-four teeth, tbe electro- 
magnet must revolve sixty-four rimes in order to pro- 
duce one revolution of the wheel, and consequently 




ELECT&O-UAGRETIC SOTATIORS. 105 

one stroke upon the bell. By counting the number 
of strokes in a given time, the velocity of the rotating 
bar may be measured : it often makes one hundred or 
more revoluUons in a second. In order that the motion 
of the' wheel may raise the hammer, it is nec^sary to 
transmit the battery current so that the bar may rotate 
in the proper direction. 

174. Electbo-Magnetic Seasons Machine. In 
the instrument shown in 6g. 73, the revolving magnet a 
imparts moUon to an astronomical machine, representing 
the rotation of the earth and moon round the sun. The 
earth and sun revolve round a common centre of motion 
near the latter, which is represented by a gilt ball S ; 
the earth also rotates on its axis. The axis of the earth 
has its proper obliquity with respect to the ecliptic, and 
preserves its parallelism, poinUng in the same direction 
during the whole revolution. These circumstances oc- 

Fig. 7a casion the north pole to be in- 
clined towards the sun in one 
half of the orbit, and the south 
pole in the other, the degree of 
inclination constantly varying. 
This, in the case of the real 
earth, is the cause of the varia- 
tion of the seasons and of the 
unequal length of the day and 
night. The moon is also seen to 
([revolve around the earth, attend- 
ing it in its course round the sun. 

175. Double Revolving Magnet. In this instru- 
ment, represented in fig. 74, there are two semicircular 




106 



DAVIS, J B. S 



electro-magnets of the same «ze, both of which have 



F!g.7i. 




Fig. 75. 




Ireedom of motion. 
The lower semicircle 
is supported by a pirol 
entering the upright 
pillar below it ; its 
own axis is hollowed 
to receive the pivot 
on which the upper 
semicircle revolves. 
At D, m the figure, 
is seen a contrivance 
for con veying the cur- 
rent in 

direction, of the s 
to the Revolving 



kind as that ap] 

Ring and Magnet, ^ 166, and which 

therefore need not be again described. 

176, Fig. 75 represents another form 
of the instrument, in which the upper 
electro-magnet is supported on the lower 
one without the aid of the brass arm and 
pillar, seen in the preceding cut ; thus 
admitting of the use of a small circular 
stand. This figure is lettered in cor- 
b respondence with the above. 
177. The cups C C being connected with the batleiy, 
the current will flow along one of the wires W W, to 
one of the silver rings secured to the axis at D, thence 
through the wire enveloping half of tlie lower electro- 
magnet, to one of the springs playing on the pole-changer 



ELECTRO-MAONETIC ROTATIONS. 107 



at P ; It then traverses the wire surrounding the upper 
electro-magnetjwith which the pole-changer is connected. 
Descending now to the opposite spring at P, it circulates 
around the .other half of the lower semicircle, and thence 
back to the battery. By this means the poles of the 
upper semicircle are reversed twice in each revolution, 
while the polarity of the lower one remains unchanged. 
The upper electro-magnet will consequently rotate in 
the same manner as those in the instruments we have 
just described, while the lower one will move in the 
opposite direction, on the principle of reaction ; its own 
poles being of necessity attracted and repelled with 
epqual force while they are attracting and repelling those 
of the upper one. It would revolve as rapidly as the 
other, were it not that the friction of its axis is doubled 
in consequence of sustaining the weight of both electro- 
magnets. By holding the other stationary, however, 
the lower one will acquire a considerable velocity, which 
it will retain for a while when its fellow is released ; their 
rapid motion causes them to present the appearance of 

a hollow sphere. 

~ ~~ 178. Magnet revolving 

BY THE Earth's action. As 
the earth itself exhibits mag- 
rietic polarity, an electro-mag- 
net may be made to revolve 
by its influence; though, in 
consequence of the feebleness 
of the action, the instrument 
y \VrQ — ~ )iA must be constructed with some 
/_ j y Wj ffl delicacy, A small electro- 



s Fig. 76. 




106 DANIEL DAYIS, JR-'s MANUAL. 

supported as to have freedom of motion in a verdcal ' 
plane like the dipping needle, a pole-changer being I 
secured on its axis of motion. The springs which 
press upon the pole-changer should be dbposed in snch 
a manner that the polarity of the bar may be reversed 
when in the course of its revolution it reaches the line 
of the dip. 

179. On placing the electro-magnet horizontally io 
the magnetic meridian, that is to say, with its extremities 
directed north and south, and transmitting the voltaic 
current, its north pole Qn this hemisphere) immediately 
inclines downwards towards the earth, in the same 
manner as that of the dipping needle. As soon as it 
arrives at the line of the dip, its poles are reversed, and 
it continues to move on in the same direction as long as 
the battery c(Hinections are maintaiped, revolving with 
a moderate velocity. In high latitudes it will be suf- 
ficient to arrange the pole-changer so as to reverse the « 
poles of the bar when it becomes vertical. 

180. By placing a steel magnet in a proper position 
near the revolving bar, it will rotate with much greats 
speed than by the action of terrestrial magnetism alone ; 
its motion may be reversed, notwithstanding the oppodng 
influence of the earth, by disposing the permanent mag- 
net in a suitable manner. 

181. The electro-magnet may be so fitted as to re- 
volve horizontally instead of vertically. In this case 
the springs of the pole-changer must be arranged in such 
a manner as to reverse its polarity when it assumes the 
position of the compass-needle, pointing north and 
south. 



ELECTRO-MAGNETIC ROTATIONS. 109 



HOnONS PRODUCED BY THE ALTERNATE DESTRUCTION AND 
RENEWAL OF THE POLARITY OF AN ELECTRO-MAGNET. 

182. Page's Revolving Armature. A small bar 
of iron, not wound with wire, is fitted to revolve hori- 
zontally just above the poles of an electro-magnet of the 
U form, fixed in a vertical position ; as seen in fig. 77, 
where A is the iron bar, and M the electro-magnet. 
The rotation is efiected by means of the following ar- 
rangement. To the axis of motion of the iron bar is 
affixed what is called a breakpiece, made by filing away 
two opposite sides of a small solid cylinder of silver. 
Upon the narrow prominent portions thus left, play two 
Fig. 77. %^ silver springs, shown at W in the 

cut, opposite to each other. One 
of these springs is connected with 
a brass cup on the stand ; the other 
communicates with one extremity 
of the wire enveloping the electro- 
magnet, the other end of this wiro 
being fixed to a second cup on the 
stand. The breakpiece is so ar- 
ranged as to release the springs 
from their bearing just as the 
armature passes over the poles; 
and to restore them to it again 
when it has moved on somewhat 
more than a quarter of a circle, so 
as to be a little inclined from a 
position at right angles to the plane of the magnet. 

10 




110 DANIEL DAVIS, JB^'s MANFAL. 

183. On placing the bar in this position and connect- 
ing the cups on the stand with a battery, the electro- 
magnet will become charged, and consequently will 
attract the armature towards its poles; a» soon as it 
reaches their plane, the springs leave the projecting 
parts of the breakpiece, and the current is cut off. The 
polarity of the magnet is now destroyed, and it ceases to 
attract the armature ; which moves on by the momentum 
it has acquired, until it passes a little beyond a position 
at right angles to the plane of the magnet. At this 
point the springs again come in contact with the break- 
piece, and the flow of the current is renewed* The 
atti*action now exerted by the poles gives a new impulse 
to the armature, and the circuit being again broken 
when it reaches their plane, it continues its motion in 
the same direction, revolving with great speed. 

184. In the original form of the breakpiece, one of 
the springs pressed constantly upon a portion which was 
left cylindrical ; but this is disadvantageous where only 
one electro-magnet is to be charged, as it increases the 
friction. Care should be taken that the springs are m 
such a state of tension as to open and close the circuit 
at the proper points, as indicated in the above descrip- 
tion. The motion of the bar will not be reveised by 
changing the direction of the current. 

185. Horizontal Revolving Armatures. In this 
instrument there are several armatures fixed to the cir- 
cumferepce of a vertical brass wheel, and parallel to its 
axis ; in fig. 78, three are represented, each of them 
marked A. On the poles of the electro-magnet M 
is secured a brass plate, from which rise two brass pillars 



ELECTRO-MAGNETIC ROTATIONS. Ill 



support the axis of the wheel ; as the wheel turns, the 
iron bars pass in succession over the poles with their 
^•78. extremities very near to them. 

At B, on the shaft of the wheel, 
but not insulated from it, is the 
breakpieeey consisting of a small 
metallic disc, from which project 
m a lateral direction, several pins, 
equal in number to the iron bars ; 
or the disc may be furnished with 
a corresponding number of teeth. 
A silver spring connected with 
one end of the wire surrounding 
the electro-magnet plays upon 
these pins or teeth ; the other end 
of this wh-e is soldered to the iron 
of the magnet, which brings it 
into metallic communication with 
the shaft by means of the brass 
plate and pillars. Or the wire may be terminated by a 
second spring pressing upon a cylindrical part of the axis. 
186. The breakpiece is arranged in such a manner 
that the electro-magnet will be charged when any one 
of the iron bars is brought near it by the motion of the 
wheel. The approaching armature is then attracted 
towards the poles ; when it arrives at the plane of the 
magnet the current is cut off, in consequence of the 
corresponding pin or tooth releasing the silver spring 
from Its bearing. The armature being no longer at- 
tracted, the wheel moves on by its momentum till the 
next bar comes into the same position, causing the 




113 DANIEL DATIS, JB.'s MANUAL. 

magnet to be recharged ; it is then attracted in its tam, 
and passes on like the preceding one. 

187. The spring playing on the breakpiece must be 
K) disposed that the circuit shall be broken when each 
bar reaches the poles, and not be renewed again imdl 
it has passed to a greater distance from them than that 
between the next succeeding bar and the poles, or it 
will be attracted back again, preventing &e continuance 
of the motion. 

188. In this and many of the instruments of the same 
class, an electro-magnet of a peculiar constructioa may 
be employed with advantage. Instead of a solid bar 
within the belix, there is an iron tube filled with wires 
of the same metal ; the tube is sawed open on one side 
throughout its whole length. By this airangemeat the 
magnetism is acquired and lost with greater rapidity 
than by a solid bar. 

189. Page's Recipbocatino Encinb. Two U 
Fig.79. 




shaped electro-magnets, M M, Gg. 79, are firmly secured 
in a vertical position on a stand, the four poles appearing 
just above a small wooden table. The two armatures, 



ELECTSO-HAOITETIO HOTIQNS. 



113 



A A, conoected together by a brass bar, move upon a 
horizontal axis io sucb a manner tbat while one is 
approaching the poles of the magnet over which it is 
placed, the other b receding from those of the other 
magnet. The brass bar is connected with one extrem- 
ity of a borizoDtal beam, the other end of which com- 
nnmicates mo^on by the intervention of a crank to 
the fly-wheel W,. To the axis of the dy-wheel at B 
is fixed the silver breakpiece, by means of which the 
magneta are alternately charged. It is similar to the 
one described under Page's Revolvmg Armature, •^i 182; 
there are, however, three springs, one playing upon a 
cylindrical portion, the others upon two dissected por- 
tions of the breakpiece. Each magnet being charged 
in succession, the armatures are attracted alternately, 
communicating a rapid reciprocating motion to the beam 
and consequently a rotatory one to the fly-wheel. 
190. Upright Recipbocatino ErtoiNE. In this in- 
strument, represented in 
fig. 80, the armatures A 
A, which are semicircular 
instead of being straight 
as in the one last de- 
scribed, are each affixed 
to one extremity of a 
vibrating beam, which 
nriparts motion to a bal- 
.nce wheel placed above 
I the magnets. At W are 
seen the -three springs 
which play upon abreak- 
10* 




114 VANIKL DAT 1 8, JB.*8 KAHrAU 

piece fixed to the axis of the wheel. The moUon a 
produced in the Eiame maimer as in Page's Engine. 

191. Fig.81 represeata anotherfonnoftheinstrummt 
^' '^^' A which is more compact. The 

electro-magnets, MM, are seoiov 
ed to a circular stand ; and the 
straight armatures, A A, afe con* 
nected by a short beam, which 
communicates modoa by mdans 
of a bent lever and crank to die 
fly-wheel. In other respects 
its constiuctioD b similar to that 
of the preceding instrumei^ 
At B is the breakpiece with the 
three silver spnngs, marked W, 
pressing upon it. 

192. Recipbocatins Bell Enoine. Two electro- 
magnets of the U form, M M, fig. 82, are supported in 
a horizontal position, with a single armature A fitted to 





vibrate horizontally between them. This armature im- 
parts motion by means of a crank to the fly-wheel W, 
and at the same time to machinery by which a hammer 



THERMO-ELECTBIC BOTATIONS. 115 



is made to strike the bell placed over one of the magnets. 
The breakpiece is the same as in the three precedbg 
instruments. 

193. When the battery connections are made with 
the eups on the stand, one of the magnets will be 
charged, provided the breakpiece is in such a position 
with regard to the springs as to complete the circuit. 
The armature will now be attracted towards the charged 
poles. Just befere it reaches them, the movement of 
the breakpiece will interrupt the current in the magnet, 
destroying its polarity, and then cause the current to be 
transmitted through the opposite one ; this will become 
charged in its turn, and attract the iron bar A, which 
will thus vibrate backwards and fOTwards between the 
two magnets. 

THEEMO-ELECTRIC REVOLUTIONS. 

194. Thermo-Electric Revolving Arch. It has 
been shown that when a galvanic current flows through 



Fig. 83. 




a helix, such as De la Rive's 
ring, <^ 130, its faces acquire 
polarity, and if free to move, 
arrange themselves north and 
south. In fig. 83 there is a 
stand supporting an upright 
brass pillar with an agate cup 
at the top. On this is bal- 
anced by a pivot at A an arch 
of brass wire, the two ends of 
which are connected by a 
German silver wire encircling 
the pillar. 



116 DANIEL DAVIS, JB.'s MANUAL* 

195. If the stand be arranged according to the points 
of the compass, and one of the junctions of the brass 
and German silver be heated by a spirit lamp on the 
east side of the stand at E, a thermo-electric current 
will be set in motion from the German silver through 
the heated junction to the brass, and back through the 
arch to the German silver. The current thus established 
gives polarity to the faces of the arch, as if it were an 
heliacal ring ; circulating in such a direction that the 
face which is turned towards the north exhibits south 
polarity. Since the magnetic pole of the earth there 
situated is itself a south pole, as has-been stated in 
^ 102, similar poles will be presented towards each 
other, and the arch will be obliged to make a semi- 
revolution on its axis in order to present its northern &ce 
to this pole. This movement will bring the other junc- 
tion into the flame, and a current will be produced op- 
posite to the former one, which will change the polarity 
of the arch and oblige it to move on through another 
semi-revolution. Thus the currents are reversed, and 
slow rotation ensues. This is probably the most deli- 
cate reaction between the magnetism of the earth and a 
current of electricity which has ever been observed. 

196. If the lamp be put to the south of east, the heated 
junction of the arch will move round by the south ; if it 
be put to the north of east, the heated junction will 
move round by the north ; just as a compass-needle, if 
its north pole is made to point south, will return to its 
natural position either by the east or west, if it is inclined 
to the one or the other. If the spirit lamp be placed 
exactly wpst, or at W in the figure, the current which 
is excited will tend to keep the arch stationary, by 



THSRUO-ELECTSIO HOT AT IONS. 117 

causing the face which exhibits Dorth polarity to be 
directed towards the touth magnetic pok of the earth, 
197. Thebmo-Electbic REvoLvuua Arch on U 
. *■ Magnet. If a thermo-electric arch, 

Tj A B, 6g. 84, similar to the ooe just 
described, be balanced on one of the 
poles of a U magnet, the reaction 
between the polarity induced in it, 
by heating one of its junctions, and 
the magnetism of the opposite -pole 
of the magnet, will be much more 
energetic than in the former case 
with the earth. It resembles, in 
principle, Page's Revolying Ring, 
•^ 162, only that it is attracted and 
repelled by a single pole instead of 
two, the pole on which it is sup> 
ported having no influence upon it. 
^In this and other instruments of the 
same kind, the upper part of the arch may, with equal 
ftdrantage, he of silver instead of brass. 

196. The most favorable position for the lamp is 
DOt that represented in the Ggure, but at a right angle 
with the line connecting the two poles, and in a line 
with the pole on which the frame is mounted ; or in a 
situation analogous to the east side of the stand of the 
last described instrument. By varying the lamp to one 
side or the other of this position, the arch will revolve 
in either direction, as before. On the opposite side of 
the pole the lamp would have no tendency to produce 
revolution; though if the arch were mounted on the 




lie 



DANIEL DATIS, JB.'l KASHAL. 



south pole, the lamp should be on the farther sde of the 
magnet, and in a line with that pole, in order to caue 
rotation. 

199. Thebho-Electric Retoltino Wire Pbames. 
This instrument, represented in fig. 85, consists of two 
^' ' frames mounted upon the poles of a U 
I magnet. These frames are formed of 
twoarches, or rather rectangles, similar 
I in construction to that in the last in- 
strument, crossing each other at right 
angles; and they act on the same 
principle as that, the second rectangle 
only contributing to the rotation pro- 
duced by the Gist. In each individual 
n rectangle the cuirent is reveised erery 
half revolu^oD. These were fonneriy 
made of ^rer and platinum, but ance 
the recent ohserratioa of the superioi^ 
ity of German silver in combination 
with brass or alver, these substances are emplc^^ 
The lower horizontal portions of the frames, mark^ 6 
O in the cut, are ccnnposed of German silver, and the 
other parts, s s, of silver. A frame is usually mounted 
on each pole ; the attrac^ons and repulsions of each 
frame proceeding altogether from the opposite pole. In 
order to heat the junctions of both frames at once, the 
lamp is placed between the two poles, by which there b 
a loss of attraction and repulsion to each frame through 
the distance of 90°, ui which the heat would act, if two 
lamps were employed at right angles to the line of 
junction of the poles. 




thkbmo-electaic botations. 119 

200. ThebmoElkctbic Aboh botatinq betwees 
THE POLES OF A U Magnet. Fig. 86 represeots a 
tbermo-elecuic arch mouoted upon a brass pillar between 
the poles of a. horse-shoe magnet ; the cfrcular part G- 
JV- ^ 19 of German silver, and the upper 

part A of silver. Id this case, both 
poles conspire in producing revolu- 
tion, the motion of the arch depend- 
ing upon the same principle as that 
of Page's Revolving Ring; the dif- 
ferent mode of reversing the current 
in this instrument, however, causes 
the arch to rotate in either direction 
when the lamp is in front of the mag- 
net, and to remam at rest when the 
lamp is on the other side. A stand 
to support the lamp slides on the 
brass pillar, and is fixed at any re- 
quired height by means of a binding 
h screw. The lamp should be placed 
in the position represented in the cut, in front of the 
magnet, its north pole being on the left. 

201. When either of the junctions is in the flame, a 
current will flow from the German silver to the silver, 
ascending bj the heated side of the arch and descending 
by the other. That face which is presented towards 
the north pole will possess north polarity, and the other 
&ce south polarity, according to the rule given in -^ 121. 
The influence of the magnet will now cause the arch to 
turn half way round, so as to present its southern face 
to the north pole. This movement brings the other 




120 DANIEL BAYIS, JB^'s MAHUAL. 

junction mto the flame ; the polarity of the arch is re- 
versed, and it moves on as before. 

202. If the lamp be placed m the corresponding po»* 
tion on the other side of the magnet, the direction of the 
current will be such that the southern face of the arch 
will be presented towards the north pole. In this por- 
tion the arch tends to remain, returning to it when moved 
to either side ; and consequently no revolution can be ob- 
tained. Care should be taken not to allow the junction 
to remain so long in the flame as to melt the hard solder. 

203. Double Thermo-Electric Revolving Arch. 
In this mstrument, two arches, a and J, fig. 87, are so 

i%. 87. mounted as to revolve 

between the poles of a 
U magnet fixed in a 
horizontal position. The 
horizontal portion of the 
arch a is of German sil- 
Lver, and the upper part 
of silver ; while in i the 
lower portion is of silver, and the upper part of German 
silver. A single lamp is so placed as to heat both arches ; 
the current excited in each will ascend on its right side 
and descend on its left side, because the heat is applied 
IQ the right junction of a and to the left of J. Each of 
them now presents a north pole towards the north pole 
of the magnet, the currents circulating in the opposite 
direction to that of the hands of a watch. They will 
consequently both revolve, either in the same or in op- 
posite directions. If the arches be transposed, so that 
b occupies the place of a, neither of them will move as 
long as the lamp is in the position represented in the cut. 




INDUCTION OF MAGNETISM. 121 

204. Electro-Magnetism as a motive power. 
The strong attractive force and the great velocity of 
motion exhibited by many of the small electro-magnetic 
instruments naturally suggested the application of this 
power to the purposes of the arts as a mechanical agent ; 
and numerous experiments have been made with this 
view, but hitherto without success. Prof. Henry was 
the contriver of the first instrument whose motion de- 
pended upon magnetic attraction and repulsion : in his 
little machine, an electro-magnet, whose polarity was 
alternately reversed, was made to vibrate above the 
north poles of two straight steel magnets. He, how- 
ever, made no attempt to apply this power to practical 
purposes. There are many obstacles of a purely me- 
chanical character in the way of its employment; these, 
though important, are not perhaps insurmountable. But 
the most serious difficulties are those which seem to be 
inherent in the very nature of the power. The motion 
of the attracting poles of two electro-magnets towards 
each other, actually lessens the attractive force in pro- 
portion to the velocity with which they approach : the 
same thing occurs in the recession of mutually repelling 
poles. These phenomena are due to the influence of 
secondary electric currents produced by the motion, as 
will be explained hereafter, which flow against the 
battery current, and of course partially neutralize its 
magnetizing power. The secondary currents present 
a very formidable obstacle, as their opposing influence 
increases with the size of the machine in a rapid ratio. 
To their action and that of some other causes, is owing 
the fact, which was early discovered by those engaged 

11 



122 DANIEL DAYIS, JR.'s MANtTAI.. 

in these investigations, tliat the smallest machines pos- 
sess by far the greatest proportional power. 

III. BY THE INFLUENCE OF THE EARTH. 

205. It has already been stated (^ 92) that a mag- 
net freely suspended assumes a certain direction with 
respect to the earth. Now if an unmagnetic bar of iron 
or steel be placed in this position, that is, in the line of 
the dip, it will be found to a,cquire magnetism by induc- 
tion from the earth. That extremity which is directed 
towards the north pole of the earth will hav6 north po- 
larity, and the other end south polarity. 

Exp. 35.— Take a rod of soft iron, and hoMing it horizontally, 
bring it near to a magnetic needle. In this position the earth 
exerts very little inductive action upon it, and each end will at- 
tract indiscriminately either pole of the needle ; showing that it 
possesses no perceptible magnetism except that induced in it by 

Fig.SS. ^ 




the needle, and which is the cause of its attraction. In fig. 88, 
A B represents an iron bar presented in this manner to the north 
pole of the needle. Now keeping the end B in the same place, 
raise the end A so as to bring the bar into the position C D. The 
north pole N will recede from C, as the bar is raised, as indicated 
by the dotted lines in the cut The upper end of the bar D, on 
the coDtmry, will be found to altxacl 1^, «xi^ x^'^X ^. *^\!k»«^ 



INDUCTION OF MAGNETISM. 133 

facts show that C D has hecome magnetic, C heing the north 
pole. On reversing the har, so as to bring the end D downwards, 
C will immediately become the south pole : thus the polarity of 
the rod may be changed at pleasure, the induced magnetism 
being only temporary. If the bar be brought very near to the 
pole of the needle, the inductive action of the earth will be over- 
powered by that of the needle, causing attraction to be exhibited 
in every position of the bar. 

206. Except in places near to the equator, it is 
sufficient to hold the bar vertically, as the line of dip 
approaches to the perpendicular in high latitudes. In 
consequence of this inductive action of the earth, all 
large bars of iron standing in an upright position are 
more or less magnetic, their lower ends, in this hemi- 
sphere, being north poles. Where they have remained 
for a long time in this situation, the polarity does not 
disappear on changing their position. 

207. The induction of magnetism by the eartli is 
greatly facilitated by causing a motion among the par- 
ticles of the bar, as by percussion or twisting. 

Exp. 36. — ^Place a rod of iron or steel in the propw position, 
with its lower end near the north pole of a magnetic needle, but 
at a sufficient distance to avoid the repulsion of the pole by the 
bar in consequence of the magnetism induced in it under these 
F^, 89. circumstances. Now strike 

the end of the bar with a 
hammer, as represented in 
fig. 89, and the pole will be 
instantly repelled. The po- 
larity thus induced will not 
be reversed by merely in- 
verting the rod, but the aid 
of percussion will also be re- 
quired, in order to remove or 
reverse the magnetism. 




134 DANIEL DAYIS, JR.'s MANUAL. 



ExF. 37^ — ^Take a piece of iron wire, and placing it in a vertical 
position, twist it powerfully. It will then be found to have ac- 
quired the power to sustain iron filings at its extremities, and to 

Fig. 90. g 





turn itself north and south, when balanced upon a pivot, as shown 
in fig. 90; the end which was downwards being its north pole. 

208. The magnetism in these cases is not due directly 
to the percussion or twisting, which merely favors the 
action of the earth. A considerable degree of perma- 
nent magnetism may be communicated to a steel bar, 
by placing it vertically on a large mass of iron and 
striking its upper end repeatedly with a hammer : it will 
acquire much greater power if struck while resting on 
iron than on any other substance. 

209. Percussion may be used to facilitate the removal 
of magnetism. Thus the polarity of a steel magnet may 
be lessenedjOr even entirely destroyed, by repeated blows 
of a hammer, while held horizontally east and west. 
This process is very convenient for removing slight 
degrees of magnetism from iron or steel bars. Merely 
falling upon the floor will often injure the power of a 
magnet considerably, in consequence of the vibration 
excited among the particles of the steel. 



MAGNETISM. 



HI. 



INDUCTION OF ELECTRICITY. 

I. BY THE INFLUENCE OF A CURRENT OF 

ELECTRICITY. 

210. That branch of the science of electricity which 
treats of the phenomena presented by it when at rest, 
is termed Electro-statics : the branch which relates to 
electricity in motion, is called Ekciro-dynamics. The 
phenomena which characterize the latter state are classi- 
fied by Faraday as follows : " The effects of electricity 
in motion or electrical currents may be considered as 
1st, Evolution of heat ; 2d, Magnetism ; 3d, Chemical 
decomposition ; 4th, Physiological phenomena ; 5th, 
Spark.'' 

21 1. Many of the phenomena presented by electricity 
in motion being closely related to magnetism, are usually 
treated of in connection with that subject^ as in the 
present case, rather than with electricity. 

212. Before entering upon the particular subject of 
the present section, that is, the inductive action of cur- 
rents, it will be advisable to occupy a few pages with 
a comparison of the phenomena exhibited by electricity 
in the two states of motion and rest, as induction is ex- 

11* 



126 DANIEL BAYIS, J R.'s MAHUAL. 



erted in them both ; it has already been intimated (^ 3) 
that the inductive action is different in the two cases. 

213. In the case of electricity at rest, two bodies, 
charged either positively or negatively, repel each oth»; 
while if one is charged with positive and the other with 
negative electricity, they exert a mutual attractbn. 
Electrical currents, on the contrary, attract each other 
when flowing parallel in the same direction, and repd 
each other when flowing in opposite directions. The 
result is the same whether two different currents or two 
portions of one current be experimented upon. 

214. The mstrument represented in fig. 91 is designed 
to exhibit the attractions and repulsicms of curr^ts. 
Two wooden troughs for containing mercury are sup- 
ported opposite to one another, each bebg divided into 



Fig. 9L 



B 




two oblong cells by a 
paitition in the middle. 
Each of the four por- 
tions of mercury thus 
insulated, is connected 
by means of a wire 
projecting into the 
cell, with one of the 
binding screw cups 
.c fixed at the ends of 
the troughs. ThQ 
points of two rec- 
tangular wires A and 
B rest in the opposite 
compartments of the 
troughs ; this mode of 



INDUCTION OF ELECTRICITY. 127 

support allows the wires to be placed nearer to or farther 
from each other at pleasure, still remaining parallel. 
These wires are balanced by two brass balls b b^ attached 
to them below, which are capable of being raised or 
depressed by means of a screw cut in the wire ; they 
may thus be so adjusted that the wires will be moved 
fbom their vertical position by a very slight force, their 
upper portions rocking towards or away from each other 
without requiring any motion of the points of support. 

215. Cups C and E being united by a copper wire, 
connect cups D and F with the galvanic battery. The 
current will now traverse A and B in succession, flowing 
in the same direction in both, and they will be seen to 
incline towards each other. The motion is slight, but 
may be made considerable by breaking and renewing 
the circuit in correspondence with their oscillations. The 
same effect will be produced by uniting D with F, and 
connecting C and E with the battery. If a powerful 
current is employed, the wires will still attract each other 
when separated to a considerable distance, by moving 
the points which rest in the mercury to the farther ends 
of the cells ; with a feeble battery, the wires should be 
placed near to one another. 

216. Now unite C with D, and connect E and F with 
the battery. This will cause the current to flow in op- 
posite directions in the two wires, and they will recede 
from each other ; the extent of the motion may be in- 
creased as before by alternately opening and closing the 
circuit. Cups C and D may be connected with the 
battery with the same result, E and F being united by 
a wire. 



128 DANIEL BAYIS, J R.'s MAN17AL. . 

217. The current, instead of traversing the wires in 
succession, may be divided into two portions by umting 
C with D, and E with F, by two wires, and then con- 
necting the battery either with C and F or D and £. 
In this case the two portions of the current will flow m 
the same direction in A and B, causing them to attract 
each other. By uniting C with E, and D with F, the 
currents in A and B will be in contrary directions, and 
the wires will exhibit a mutual repulsion. The move- 
ments produced by a divided current will be feebler than 
when it traverses the wires in succession, unless the 
battery employed is so powerful that one of the wires 
singly is not able to convey the whole of the electricity 
supplied by it. 

218. These attractions and repulsions are sometimes 
called magnetiCf the two currents when flowing side by 
side, acting upon each other like two magnets presented 
end to end. In fact, if two short pieces of iron wire 
be suspended end to end, and at right angles to the 
conducting wires, the magnetism induced in them by 
the cun^nts (see Exp. 22) will cause them to exhibit 
similar attractions and repulsions to those of the wires 
themselves. It is, however, preferable to regard this pe- 
culiar action as a primary one ; it being -highly probable, 
though not as yet certain, that the polarity of even a steel 
magnet is due to electric currents circulating within its 
substance. The mutual actions of two magnets or of 
a magnet and a current would thus be secondary 
effects, depending upon the attractions and ^repulsions 
just described. 

219. It is not essential that the current should tra- 



INDUCTION OF ELECTRICITY. 129 

verse metallic wires in order to produce these effects. 
Two streams of electricity flowing through a vacuum, 
or even through the air, will exhibit the phenomena in 
a very satisfactory manner. 

Exp. 38. — The attraction of currents moving in the same di- 
rection may be shown by means of frictional electricity, in the 
following manner. Connect the inner coatings of two Leyden 
jars with either the positive or negative conductor of a common 
electric machine, their outer coatings being insulated sufficiently 
from each other to prevent the passage of a spark between them 
when tl^ jars are discharged in the mode about to be described* 
With the exterior coating of each jar is connected a wire having 
one end free. These ends are left free for the purpose of being 
placed on a card over which the charge is to be passed. The 
conmion enamelled cards should be used, as they receive a dark 
colored and permanent mark from the passage of the spark over 
their surface. A third wire, attached to the discharging rod, is 
also to rest on the card, at such a distance from the two other 
wires that the sparks from the jars may be able to pass. The 
ends of the wires proceeding from the outside of the jars should 
be placed a quarter or a half of an inch apart, and nearer to one 
another than, to the third wire, which is to be equally distant from 
both, so that if two straight lines were drawn from it to them they 
would form the letter V. The jars being charged (during which 
process the exterior coatings should, of course, be uninsulated), 
arrange the points as directed, and bring up the ball of the dis- 
charging rod to the conductor. The inner coatings being con- 
nected, and the outer ones insulated, the current is obliged 
to divide into two portions as it proceeds from the point attached 
to the discharger to those in connection with the outside of the 
jars. The two sparks will thus pass simultaneously over the sur- 
face of the card, and were they unafl^cted by each other, would 
leave a mark in the shape of the letter V. It will be found, on 
the contrary, that the tracks left on the card will be more or 
less in the form of the letter Y, the two currents coalescing in 
their passage over its surface. The result will be the same 
whether the jars be charged positively or negatively on the inside. 



130 DANIEL DAYIS, JR«'s MANUAL. 



If the wire connected with the discharger be placed under the 
card while the others are on the upper side, it will be perfo- 
rated in one or more places by the passage of the electricity. 

Exp. 39. — ^The experiment may be varied, by connecting with 
the discharging rod a wire whose ends may both rest on the card 
at the same distance from each other as that between the two 
wires attached to the exterior coatings of the jars. The two sell 
of points being arranged parallel to each other, and their dis- 
tances properly adjusted, the two currents will remain separate 
during the whole of their passage over the card ; and it wiH be 
seen by the marks which Uiey leave, that instead of proceeding 
in straight and parallel lines, they form curves whose cpnvezity 
is turned towards each other. The curvature of the lines is 
greater in proportion to their proximity : if the points are placed 
too near together, both currents will flow in one track, not seps^ 
rating until they reach one of the wires connected with the out- 
side of the jars. The resistance of the air and other causes often 
occasion a stream of electricity to follow a very crooked path in 
passing over a card. Hence the lines traced by the two currents 
/ in these experiments may be very irregular, though the tendency 
to converge is perfectly evident. 

2^. Electro-Dynamic Revolving Ring. The 
mutually attractive and repulsive action of currents may 



Fig. 92. 




be made to produce a revolution 
analogous to some of those strictly 
called electro-magnetic ; as in the 
instrument represented in fig. 92^ 
which consists of a coil of insulated 
wire B fitted to rotate on a vertical 
axis within a larger one A, mounted 
on a brass pillar. The inner coil 
has a pole-changer fixed to its axis 
of motion for the purpose of reversing 
the current twice in each revolution. 



INDUCTION OF ELECTRICITY. 131 

The current may traverse the two coils in succession, 
of be divided between them, but its direction must be 
changed only in B. 

221. The coil B being placed at right angles to A, 
and the cups t)n the stand connected with the galvanic 
battery, the faces of each coil immediately exhibit north 
and south polarity, like those of De la Rive's Ring 
(<^ 130); and B is obliged to make a quarter of a revolu- 
tion in order to bring its north pole within the north pole 
of A, the two coils corresponding in direction. As soon 
as B reaches this position, the current is reversed by 
means of the pole-changer, and its south pole now being 
within the north pole of A, it continues to move on in 
the same direction. The motions in this case depend 
upon the same principle as those of the wires in the in- 
strument represented in fig. 91 ; but it is more convenient 
to refer them to the polarity exhibited by a current 
flowing in a circle, as was done in describing Page's 
Revolving Ring. 

222. It is, however, easy to explain the revolution 
with direct reference to the mutual action of the currents. 
As these circulate in the same direction in every convo- 
lution of each coil, they may be regarded as two single 
circular currents. Now suppose the current in A to be 
ascending by the left side and descending by the right 
side of this coil. If B be placed at right angles to A, 
with its current ascending by the side towards the spec- 
tator, this side will be attracted by the left side of A 
and repelled by the right. The farther side of B, on 
the contrary, will be repelled by the left side of A, 
and attracted by the other. These forces will conspire 



132 DANIEL DAYIS, JR.'s MAKtJAL. 

in bringing B into the same direction with A ; when the 
current being reversed, each side of B is repelled by the 
corresponding one of A, and it is obliged .to continue its 
motion, revolving from left to right. 

223. Portions of the same or of different currents 
moving in a continuous line repel one another. Hence 
a short wire whose ends rest in two mercury cups inter- 
posed in the circuit of a galvanic battery, consisting of 
a few pairs of very large plates, and in vigorous action, 
will be thrown out of the mercury at the moment of 
completing the circuit. The repulsion is here exerted 
between the immediately succeeding portions of the 
current, as it passes from the mercury to the wire, and 
also as it leaves the wire to enter the other portion of 
mercury ; the forces thus acting at each end of the wire 
will conspire in raising it out of the cUps. 

224. An electrified body attracts light substances in 
its neighborhood, having previously induced in their 
nearest ends the opposite electricity to its own ; and on 
their approach communicates to them a part of its 
charge, when, if insulated, they are instantly repelled 
by It. A wire conveying a current exerts no such in- 
fluence upon light bodies, although placed in the imme- 
diate vicinity. 

225. We now proceed to consider the inductive 
action of currents, taking first in order those phenomena 
which are referred to the induction of a current on itself. 
When the poles of a small galvanic battery, consisting 
of a single pair of plates, are connected by a copper wire 
of a few inches in. length, no spark is perceived when 
the connection is either formed or broken, or at most a 



INDUCTION OF ELECTRICITY. 133 

very faint spark at the moment of opening the circuit; 
but if a wire forty or fifty feet long be employed, though 
no spark is seen when contact is made, a bright one 
appears whenever the connection is broken l>y lifting 
one end of the wire out of the cup in which it rests. 
By coiling the wire into a helix, the spark becomes more 
vivid ; and a still greater effect is produced by making use 
of the wire surrounding an electro-magnet. 

226. The most advantageous length for producing 
the spark depends upon the diameter of the wire, and 
also upon the number of pairs in the battery and the 
size of its plates ; the larger the wire, the greater is the 
length required to produce the maximum result. With 
a single battery whose zinc plate exposes about a square 
foot of surface to the solution, and a wire of one sixteenth 
of an inch in diameter, a length of sixty or seventy feet 
will probably give the brightest spark, though much will 
depend upon the degree of vigor with which the battery 
is acting. This peculiar action of a long conductor, 
either extended, or coiled into a helix, in increasing the 
intensity of the current from a single galvanic pair, at 
the moment when it ceases to flow, was discovered by 
Prof. Henry (now of New Jersey College) in 1831, 
while at the Albany Academy. 

227. With a wire two or three hundred feet long, a 
slight shock may be felt at the moment of opening the 
circuit, if its ends near their connections with the poles 
are grasped with moistened hands ; with a shorter wire, 
shocks may be obtained through the tongue ; their in- 
tensity increases until a length of five or six hundred 
feet is attained. A single pair of plates can, of course,. 

12 



134 DANIEL DATIS, JR^'fl MANUAL. 

give no shocks directly : the peculiar and coDtinuoas 
sensation excited in the tongue when the current from 
a single pair is made to pass through it, is not called a 
shock. With a battery of smaller size or consisting of a 
number of pairs, greater lengths may be used with ad- 
vantage both for the spark and shock. The maximum 
effects of a small battery are, as might be expected, 
much inferior to those of a large one. If the requisite 
lengths of wire are exceeded, the effects are lessened. 

228. The brilliancy of the spark is much increased 
by employing a ribbon of sheet copper coiled into a flat 
spiral, instead of a wire. A description and 6gure of th'is 
instrument has been given in <^ 123. The spiral being 
connected with the battery, a brilliant spark will be seen, 
accompanied by a pretty loud snap, whenever contact 
is broken ; and if two metallic handles be attached by 
wires to the cups of the coil, and held in the hands, a 
slight shock will be felt ; if the battery is in feeble action, 
the shocks may be perceptible only when passed through 
the tongue. No shocks can be obtained by interposing 
the body in the direct circuit with the coil, so that the 
battery current may traverse them in succession ; as the 
electricity supplied by a single pair of plates is of too 
low intensity to be transmitted, to any considerable 
extent, by so poor a conductor as the human body. 
Prof. Henry was the first to employ coils of metallic 
ribbon for obtaining sparks and shocks from a single 
pair of plates. 

229. For the purpose of rapidly breaking the circuit, 
the Contact Breaker, represented in fig. 93, is very 
convenient. It consists of a bent copper wire W W, 



IHDCOTION OF ELZCTBICITT. 



135 




which by means of clock-work set in motion by a spring, 
is made to vibrate rapidly, dipping its ends alternately 
into the glass cups G G, intended to contain mercury. 
The spring is wound up by turning the milled head A. 
I^. 93. The glass cups are 

open at the bottom 
to allow the mercury 
to come in contact 
with the brass pillars 
into which they are 
cemented. These pil- 
:^ lars are both connect- 
^ed with one of the 
binding screw cups C 
C ; the other cup communicates with a brass mercury 
cup P, into winch dips a short wire connected with the 
vibrating wire. Sufficient mercury must be put into the 
cup P, to keep the end of the vertical wire covered, 
and enough into the glass cups to allow one end of W W 
to leave the mercury in its cup a little before the other 
end dips into its portion. 

230. The Contact Breaker may he advantageously 
used in connection with many of the instruments for 
afibrding sparks and shocks, which will be described 
under the following bead. The current must be trans- 
mitted through the two instruments in succession, by 
connecting one of the cups C C with one pole of the 
battery, and the other cup with one of those atuched 
to the spiral or other piece of apparatus, the remaining 
cup of which is to communicate with the other pole of 
the battery. It is better to break the circuit mechan- 



136 OANIEL DATIS^ JR^'s 1CAK17AL. 

icallj in this way, than hj means of any interraptmg 
apparatus worked by the battery itself, as a considerable 
part of the power of the current is then expended in 
giving motion to the intemiptor. 

231. On making connection in this manner with a 
flat spiral (6g. 49), and turning the milled head A to 
put the vibrating wire in motion, a brilliant spark will 
be seen at each rupture of contact, accompanied by a 
loud snap, and producing considerable combustion of the 
mercury. With a battery consisting of a few pairs of 
plates of large size, such as Dr. Hare's Calorimotor^ the 
size of the spark will be greatly increased and the snap 
become as loud as the report of a Leyden jar. The 
shock will also be pretty strong, and may be increased 
by covering the mercury in the glass cups with a stratum 
of oil. A shock may be obtained, especially when oil 
is used, on closing the circuit as well as on opening it, 
though inferior to that given in the latter case ; a faint 
spark is also sometimes seen when the wire dips into 
the mercury. 

2S2. The requisite length and thickness of the copper 
ribbon to give a maximum result depend upon the size 
of the battery employed. With spirals of considerable 
length, even if the copper be pretty thick, two or three 
pairs of plates are better than one, as the metal opposes 
some resistance to the passage of a current of low inten- 
sity. A ribbon spiral of moderate length interposed in 
the circuit of a compound battery, consisting of a con- 
siderable number of small pairs, produces scarcely any 
peculiar effect : while a coil containing three or four 
thousand feet of fine insulated wire will give an intense 



INDUCTION OP ELECTRICITY. 137 

shock, though not a very hrilliant spark, under the same 
circumstances. The higher the intensity of the elec- 
tricity and the smaller its quantity, the less is the size 
requisite in the metallic conductor and the greater may 
be its length. 

233. The sparks and shocks given by long wires and 
by spirals are due to secondary currents induced in the 
metallic conductor at the moment of opening and closing 
the circuit ; their intensity being higher than that of the 
current which produces them. The phenomena belong 
to the same class as those presented by the secondaries 
induced in another conductor placed in the vicinity of 
the one which is conveying the battery current. 

234. The secondary currents just referred to may be 
obtained by placing a second spiral of copper ribbon 
upon the one through which the battery current is 
transmitted. If the edges of the copper strips are ex- 
posed, some insulating substance, such as glass or paper, 
must be interposed between the two spirals. 

Exp. 40. — ^Two wires being connected with the cups belonging 
to the upper ^piral,rub their ends together while the circuit through 
the lower one is rapidly broken. Sparks will be seen, and slight 
shocks may be felt through the fingers or by placing the wires 
in the mouth. When the ends of the wires are joined, the sparks 
and snaps given by tlie spiral connected with the battery are 
considerably diminished and no shocks can be obtained from it. 

Exp. 41. — Connect the cups of the upper coil with a delicate 
galvanometer such as that represented in fig. 13. Whenever the 
battery circuit is completed through the lower spiral, the mag- 
netic needle will be deflected to a considerable extent, but will 
immediately return to the meridian, indicating the flow of a 
momentary current through the wire of the galvanometer. On 
opening the circuit a similar transient deflection will occur in 

12* 



136 OAHIKL BATlSi J- ; jr^/fUAL. 



./*' 



ically in this way, tb- .'i'-oec" *l"le ^^ '>«tte^ 

annaratus worke ' • ' . ;>i*°»" >»• *»''«' ^"^t *^« B^- 

' ^ ' ' ',. ;,^ce from the lower spiral, that 

part of the pc .r .^^^^jr-j^ 

giving modon ^ '' vi^'i^ill be magnetized if placed within 
231. On ' l-»Zffitl diameter connected with the upper 

flat spiral > -l^'J^tf^^^ ^7 ^® current which attends the 

th« ' ■-«■ '«***!ii^i^^ ^® *^® reverse of that communicated 

" . ' * '^iVJji rupture. If both currents are allowed to 

be seen ^^^^it^^*-^ ^.|2 acquire little or no magnetism. 

if^^ Ife purpose of determining the direction of 

jf^^ntSj the Magnetizing Helix represented in 

• iit'^'f be employed. Its construction is similar to 

£(' ''^he'b^^^ described in <^ 120: it should, however, 

^ ik'^ consist of a single length of wire, 

but wound so as to form six or eight 

layers of coils, to enable it to be 

used for examining currents of con- 

1 \ A / siderable intensity. Its power will 

be greater if its internal diameter is 
very small. In the cut, the helix 
Q is mounted upon a stand, with a small piece of steel 
^ire within it. 

236. The momentary waves of electricity excited by 
electro-dynamic induction in a conductor conveying a 
current, or in a neighboring one, are termed secondary 
currents, the battery current itself being called in this 
connoction the jyrimary one. The wave which accom- 
panies the closing of the circuit is termed the initial 
secondary, and flows in the opposite direction to that of 
the current which induces it. The other, which follows 
the opening of the circuit, is called the terminal second- 




INDUCTION OF ELXCTRICITT. 



139 



\ 

jwind flows in the same direction as the inducmg 
JBnt. These currents were discovered by Prof. Fara- 
way, in 1831. 

237. In fig. 95 a coil of fine insulated wire W is 
represented placed over a ribbon spiral A, which is 
connected by one cup with the cup C attached to the 

Fig. 95. 




copper plate of a sustaining battery. A wire from the 
cup Z, belonging to the zinc plate, is drawn over a steel 
rasp resting on the other cup. of the flat spiral, for the 
purpose of breaking the circuit rapidly. 

238. The ends of the wire coil W being fixed in the 
binding screw cups of the metallic handles, powerful 
shocks will be felt when these are grasped in the hands 
and the wire connected with Z drawn over the rasp. 
In order to obtain the initial and terminal shocks sepa- 
rately, the circuit should be broken, not by means of the 
rasp, but by a cup containing mercury into which one 
of the battery wires can be dipped at pleasure. The 
mercury should, of course, be connected by a wire with 
one of the cups attached to the ribbon spiral. 

239. When a battery of a single pair of plates is em- 
ployed, the initial secondary is much inferior in intensity 
to the terminal, and consequently gives a feebler shock. 
Prof. Henry discovered that the intensity of the terminal 



140 DANIEL DATIS, J R.'s MANUAL. 

current is very liltle increased by adding to the numb^ 
of pairs ; the slight increase which occurs is due to the 
greater quantity of electricity transmitted by the ribbon 
spiral, when the intensity of the battery current is in- 
creased. With the initial secondary it is different ; eveiy 
additional pair was found to raise its intensity, so that 
with about ten pairs it equalled, in this respect, the 
terminal, and with a larger number excelled it. The 
initial shock may also be increased, though not in any 
great degree, by employing a shorter ribbon spiral, as 
for instance, one fifteen or twenty feet in length, with a 
single pair of {)lates. In quantity^ as indicated by the 
galvanometer, the two secondaries are equal ; those of 
the wire coil being inferior in this respect to the currents 
afforded by a ribbon coil. 

240. The coil represented at W contains three thou- 
sand feet of copper wire, about one-fiftieth of an bch 
in diameter, wound with thread ; the layers are firmly 
cemented together by shellac, careful insulation being 
requisite in consequence of the length of the wire and 
the high intensity of the current obtained. Where a 
small battery is used, this length of wire is unnecessary, 
as the shock given by it is scarcely greater than that 
from a coil of one thousand feet ; with a larger battery 
the longer one will be much superior. A sewing needle 
may be magnetized by the currents from a long wire 
coil, as well as by those from a ribbon spiral (see Exp. 
42) : if the wire is fine and very long this effect will be 
diminished. 

241. The sustaining battery shown in section in fig. 
95 is of similar construction to the cylindrical battery 



INDUCTION OP ELECTRICITY. 141 

described in ^ 20, except that the zinc plate is placed 
within a double cylinder of leather L, closed at the 
bottom ; the space between this and the copper cylinder 
on each sidens alone occupied by the solution of sulphate 
of copper, which may be a saturated one ; while within 
the leather case is a rather weak solution of Glauber's 
salt (sulphate of soda) or of table salt. The leather 
should be free from oil, or the power of the battery will 
be greatly reduced. Other porous or membranous sub- 
stances, such as thick brown paper, or bladder, will 
answer the same purpose as leather, in preventing the 
ready admixture of the solutions, and allowing a free 
passage to the electrical current. When the partition 
b sufficiently thin or permeable, the battery is as pow- 
erful as if charged in the usual mode with a solution of 
blue vitriol. 

242. The action within the battery is as follows: 
the zinc is oxidized as usual, at the expense of the water 
of the solution which surrounds it, while the hydrogen, 
instead of being given off at the surface of the negative 
plate, as in most batteries, decomposes the sulphate of 
copper, forming water with the oxygen of the oxide of 
copper, and liberating the sulphuric acid, which passes 
through the porous partition into the other cell. A 
gradual and steady supply of acid is thus furnished to 
dissolve the constantly forming oxide of zinc. 

243. This form of battery will maintain a nearly 
unvarying power for several days in succession, if the 
solution of sulphate of copper is kept saturated by occa- 
sionally adding a little of the pulverized salt and stirring 
the liquid to make it of uniform strength. With weaker 



142 DANIEL DATIS, J R .'s MANUAL. 

solutions, or a less permeable partition, an action of suf- 
ficient energy for many purposes may be sustained for a 
week or more ; and when it declines, may be renewed 
by cleaning the zinc plate and removing any loose de- 
posit from the cells. This constancy of action renders 
the battery of great value in the electrotype process, 
which will be described hereafter. The deposition of 
metallic copper on the negative plate is the principal 
inconvenience attending it : this deposit sometimes ad- 
heres so firmly as to be difficult of removal, which, how- 
ever, is only necessary when it interferes mechanically 
with the working of the battery. The adhesion may 
be partially prevented by slightly oiling or greasing 
the copper cylinders previous to the introduction of the 
solutions. 

244. Instead of flat coils, long helices of insulated 
wire may be employed for obtammg the secondary 
currents, though with less effect when not aided by 
magneto-electric induction. Several of the magneto«elec- 
tric instruments which will be described under the next 
head may be used for this purpose, the iron bar or 
bundle of wires being withdrawn from the helices. A 
description of one of them (the Double Helix and Elec- 
trotome) may properly be introduced in this connection. 

245. Double Helix and Electrotome. In this 
instrument, represented in fig. 96, the double helix a a 
is confined to the base-board by three brass bands. The 
inner helix is composed of several strands of large in- 
sulated copper wire. The similar ends of these strands 
at one extremity of the helix are connected with the 
binding screw cup c. Their other ends are soldered 



IND1TCTI01T or KLKCTBICITT. 143 

to the middle brass band, which is surmounted by a 
brass mercury cup e. Into this cup descends a copper 
wire attached to the wire w v>, which by means of clock- 
work set in motion by a concealed spring, is made to 




dip its ends alternately into the glass cups G G for 
containing mercury. The cups being open at the bot- 
tom, the mercury b brought in connection with the outer 
brass bands, upon which they are fixed. Both these 
bands are connected with a binding screw cup c', corre- 
sponding to c, but not seen in the cut. A second helix, 
consisting of about two thousand feet of fine insulated 
wire, encloses the one just described, but is insulated 
from it : its ends are soldered to the binding screw cups 
to which the bandies, seen at H, are attached. 

246. The cups c and & being connected with the 
galvanic battery, the current will pass through the inner 



144 DANIEL PATIS, J R.'s MANUAL. 

helix whenever either end of the wire w w dips into the 
mercury ; which should stand at such a height in the 
cups that both extremities of the wire shall not be im- 
mersed at the same time. By turning the milled head i 
the spring is wound up and the wure is made to vibrate 
rapidly. When either end leaves the mercury, the flow 
of the current is interrupted, and a bright spark is seen 
in the cup. If the handles be grasped with moistened 
hands, strong shocks will be felt whenever the circuit is 
broken. Introduce into the helix a brass tube, and the 
spark becomes small and the shock feeble ; if the tube 
be sawed open in the direction of its length, it no longer 
produces these effects. 

247. When an iron bar or a bundle of soft iron wires 
b introduced into the helix, the brass tube being with- 
drawn, the brilliancy of the sparks and the intensity of 
the shocks are greatly increased, the instrument being 
under these circumstances one of the most powerful be- 
longing to the department of magneto-electricity. 

248. We have seen that a battery current of con- 
siderable quantity and low intensity can induce either a 
quantity or an intensity current. By substituting for 
the ribbon spiral through which the battery current is 
transmitted, a coil consisting of one thousand feet or 
more of fine insulated wire, and connected with a battery 
of a number of pairs, it will be found that an intensity 
current is able to induce secondaries of intensity in a 
wire coil, and of quantity in a ribbon coil. 

249. The shocks obtained when the body is intro- 
duced into the circuit of a voltaic battery of a consider- 
able number of pairs, without a coil, appear to be due 



INDUCTION OF ELECTRICITT. 145 

to secondary currents induced in the battery itself. 
During the uninterrupted circulation of 'the galvanic 
current through the body, little or no effect is perceived ; 
but^at the moment of either opening or closing the cir- 
cuit, a shock is experienced. When the series is very 
extensive, a dull pain is felt during the continuance of 
contact. The primary current has sufficient intensity 
to traverse the body, though not to give shocks, and 
doubtless induces initial and terminal secondaries when 
it commences and ceases to flow. 

250. A flat spiral being in connection with the bat- 
tery, let a fine wire coil be placed at a little distance 
above it ; shocks may now be obtained from the wire, 
but their intensity diminishes in a rapid ratio as the 
distance between the coils is increased. With the ar- 
rangement represented in fig. 95, shocks through the 
tongue are readily obtained when the wire coil is a foot 
or two above the other ; and the distance may be still 
farther increased by using a longer ribbon coil or a more 
powerful battery. This furnishes a convenient mode of 
regulating the intensity of the shock at pleasure : the 
same effect is produced when one coil lies upon the 
other, by sliding the wire coil from its central position 
more or less beyond the edge of the flat spiral. The 
shocks are in any case much increased by wetting the 
bands, especially with salt water. 

251. The intensity of the shock also diminishes rap- 
idly as the wire coil is raised from a horizontal position 
into an inclined one ; and when it reaches a vertical 
position, its edge resting on the ribbon coil, they are no 
longer felt. Similar phenomena are presented when. 

13 



146 DANIEL DATIS^ J R.'s MANUAL. 

the flat spiral has a sufficiently large central opening to 
allow the wire coil to pass within it ; no shocks being 
obtained when their axes are at right angles to each 
other. If the diameter of the wire coil be considerably 
less than that of the ring, and it be placed in a hori- 
zontal position within it, the shocks will be somewhat 
stronger when it is near the side than when in the centre. 

252. The interposition of any good conductor of 

electricity between - the fine wire coil and the one 

connected with the battery will nearly neutralize the 

shocks. 

Exp. 43. — The coils being arranged as in fig. 95, interpose a slip 
of wood or a plate of glass between A and W, and the shock will 
be the same as if air only intervened. This will be the case with 
any non-conductor of electricity. Now interpose a plate of metal, 
for instance, lead or zinc, one-tenth of an inch thick and as 
broad as the coils. The shock will be so much reduced as to 
be scarcely perceptible. The magnetizing power of the current 
is also lessened, in respect to hard steel, so that a sewing needle 
placed within a helix, as in Exp. 42, will be but feebly charged. 
A certain thickness of metal is required to produce these efiects^ 
as several sheets of tinfoil may be interposed without diminishing 
the shocks in any appreciable degree. 

253. The interposition of a metallic plate does not 
prevent the occurrence of the secondary currents, but 
merely causes their intensity to be greatly reduced. 
That the quantity of the current is not affected may be 

f shown by connecting the ends of the upper coil, espe- 
cially if it be a ribbon coil instead of a wire one, with a 
galvanometer ; when the deflections will be the same 
whether the plate is interposed or not, provided the 
distance between the two coils is not altered ; except 
the plate is of iron, when they are somewhat diminished. 



INDUCTION OF ELECTRICITT. 147 

If a slip be cut out of the interposed plate in the direc- 
tion of a radius, the cut extending to the centre, it no 
longer lessens the shocks. 

Exp. 44. — ^Instead of a metallic plate, interpose a flat spiral 
between the battery coil and the wire one. No diminution of 
the shocks will be perceived. Now connect tlie cups of the in- 
terposed coil by a wire, and the intensity of the shocks will be 
even more reduced than in the last experiment Whenever the 
shocks are diminished, the brilliancy of the sparks given by the 
battery spiral will also be lessened to some extent 

254. Secondary currents may also be obtained, with- 
out breaking the primary circuit, by altering the quantity 
of the battery current or the distance between the coils, 
as in the following experiments. 

Exp. 45. — Connect a ribbon coil with the battery, and place 
a second spiral of the same kind upon it, with its cups in con- 
nection with a galvanometer. While the current is flowing 
steadily through the lower spiral, no secondary will be excited, 
and the needle of the galvanometer will be unafiected. Now 
lift the zinc plate of the battery partly out of the liquid. The 
moment the plate begins to be raised, the needle moves in the 
same direction as if the circuit were broken ; the deflection, how- 
ever, is not momentary as in that case, but continues during the 
movement of the plate. Then, without taking the zinc out of the 
solution, which would break the circuit, depress it again. The 
galvanometer will now indicate a current in the opposite direc- 
tion to the former one. 

Exp. 46. — Similar currents are produced by raising the upper 
coil froQ) the lower one, through which the galvanic current is 
steadily flowing. As the coil recedes, a secondary flows through 
it in the same direction as that of the battery current in the other 
spiral ; as it again approaches, a current in the reverse direction 
is induced. Instead of raising the upper spiral, it may be moved 
laterally from its central position on the lower one, with the 
same result 



148 DANIEL DATIS^ JB.'s MANUAL. 

255. These currents produce a greater efiect upon the 
galvanometer than those excited by closing and opening 
the circuit, as they are not momentary, but last as long 
as the motion continues. The more rapid the nK>yemeDt 
of the zinc plate or of the spiral, the more powerful are 
the secondary currents, as they depend upon the sud- 
denness of the change, in the quantity of the primary 
current in one case, and in the distance between the 
coils in the other. ' They are, however, of low intensity, 
and are unable to afford shocks. The interposition of 
metallic plates or coils produces no efiect upon them. 

256. The neutralizing action described in <5> 252 and 
253 is due to a secondary current excited in the inter- 
posed metallic plate or spiral, which itself induces a 
tertiary current in the wire coil, flowing in the opposite 
direction to the secondary induced in it by the battery 
current, and therefore retarding its development. A 
tertiary current is also induced in the battery coil, which 
occasions the reduction in the spark and shock noticed 
in Exp. 40 and 44. When the interposed plate of 
metal is divided to its centre, no secondary is induced 
in it, and it exerts no neutralizing action ; the same is 
the case with the ribbon spiral in Exp. 44, when its 
cups are disconnected. Similar phenomena are pro- 
duced by the introduction of a metallic tube into a wire 
helix, as described in <5> 246. 

257. This tertiary current can be separated fix)m 
the secondary, and obtained by itself, in the following 
manner. 

Exp. 47. — A ribbon coil B, fig. 97, being laid upon the coil A, 
through which the battery current is transmitted, connect its 



IRDUOTION OF ELSCTBIOITT. 149 

cups with those of a third spiral C, of the some kind, removed to 
K Uttle dlatuice, so as to be beyond the inBuence of the curreot in 

Fig. a 




A> The aecondarjr current induced in B will nov flow through 
C, and if a fine wire coll W is laid on C, Htrong Hhocks may be 
obtained. If W be raiaed up, the ehocks will atill be felt when 
it ifl at a ccnsiderable height above C. 

Exp. 46.— Place a fourth ribbon coil on C instead of the vrire 
coil, and a quantity current will be obtained, capable of sfiecting 
the galvanometer slightly, and of magnetizing a eewing needle 
placed in a helix of small internal diameter, such aa that repre- 
sented in fig. 94. 

ExF. 49.--Sabatitute for the flat apirala B and C in fig. 97 
two fine wire coila. A secondary intensi^ current wilt now be 
obtained, which will induce a tertiary of intenaity in a third wire 
coil laid on the second, enabling' it to afford etrong ahocks ; and 
ft tertiary of quantity in a ribbon coil. 

258. If the second spiral B is alone replaced by a 
wire coil, little or no shock can be obtained from W, 
the quantity of the secondary current furnished by the 
wire coil not being sufficient for the production of a 
powerful tertiary, unless it is passed through a conduc- 
tor of many convolutions. So, on the other hand, if 
a fine wire coil be substituted for C only, no tertiary is 
induced by it, or at most a feeble one, the secondary 
current from B not having sufficient intensity to enable 
it to overcome the resistance of the long wire. The 
13* 



150 DANIEL DAVIS, JR.'s MANUAL. 

tertiary current, like the secondary, can be induced at a 
distance ; and has its intensity greatly reduced by the 
interposition of metal between the flat spiral C and the 
wire coil. 

259. The tertiary currents may be conveniently 
obtained by causing the secondary from a ribbon spiral 
to flow through the inner helix of the instrument repre- 
sented rn fig. 96 or of almost any of the magneto-electric 
instruments to be described under the next head. Thus 
if the wires attached to B, in fig. 97, are fixed in the 
cups c and cf of the Double Helix and Electrotome, 
strong shocks may be obtained from the tertiary current 
induced in the fine wire helix. The circuit through the 
inner coil should not be broken by the electrotome, as 
the only interruption wanted is that in the battery cur- 
rent. The shocks will be increased by placing a bundle 
of iron wires within the helix, as the inductive action of 
the current will then be assisted by that of the electro- 
magnet. 

260. Tertiary currents, like secondaries, are induced 
both when the primary circuit is opened and when it is 
closed. The initial and terminal tertiaries both flow in 
the opposite directions to the corresponding secondaries. 
In fact, each secondary must produce two tertiaries, one 
when it commences, and another when it ceases to flow : 
but in consequence of the exceedingly short duration of 
the secondary itself, they cannot be separated as the 
initial and terminal secondaries can; and the current 
which is obtained, whether initial or terminal, is only 
the difference between the two. This accounts for the 
slight efiect it produces upon the galvanometer, while 



INDUCTION OF ELECTRICITY. 151 

capable of affording strong shocks. The two parts may 
differ very much in intensity, but being equal in quantity 
would not affect the galvanometer, did they occur pre- 
cisely at the same instant : the needle, however, is first 
deflected by the momentary wave induced by the com- 
mencement of the secondary, and as soon as it has moved 
a degree or two is arrested by the opposite wave due 
to its cessation. 

261. The effects of the interpositions described in 
<5> 252 and 253, may now be more clearly explained. 
The secondary induced in the interposed conductor, on 
opening the primary circuit, for instance, itself induces 
a tertiary in the wire coil at the instant of its commence- 
ment, which flows against the secondary induced in it 
by the battery current. When the secondary in the 
interposed body ceases, another tertiary is excited in the 
wire coil flowing in the same direction as the secondary. 
The total amount of the current will not be altered, 
since the same quantity is added at its ending as was 
subtracted at its beginning; but its intensity will be 
greatly reduced, probably in consequence of the dimin- 
ished rapidity of its development. 

262. Currents op higher orders. It has been 
shown that a secondary current, though only momentary 
in its duration, can induce a tertiary of considerable 
energy. It might therefore be expected that the ter- 
tiary would produce a current of the fourth order ; this 
another, and so on ; and such is found to be the case. 
It is only necessary to remove the tertiary out of the 
influence of the secondaiy in the same manner as the 
secondary is removed from that of the primary (see 



152 DANIEL DAVIS, Jr/s MANUAL. 

Exp. 47) in order to obtain a current of th^ fourth order. 
The currents of the third, fourth and fifth orders were 
first obtained by Prof. Henry, and two other orders have 
been since added. These currents progressively dimin- 
ish in energy, but the phenomena presented by them 
are similar to those of the tertiary. With a larger 
number of coils and a powerful battery, the series might 
doubtless be extended much farther. 

263. In the following table the directions of the cur- 
rents produced both at the beginning and ending of the 
battery current are given, those which flow in the same 
direction as the primary being indicated by the sign +, 

« 

and those in the opposite direction by the sign — • 

At the beginning. At the endiog. 

Primary current, + -j" 

Secondary current, — -{- 

Tertiary current, + ,. — 

Current of the fourth order,. ... — -|" 

Current of the fifth ^order, .... -}- — 

Current of the sixth order, .... — + 

Current of the seventh order, . . -j- — 

If the induction at the ending of the battery current be 
regarded as opposite to that at the beginning, the second 
column rnay commence with minus instead of plus, and 
the second series will then alternate like the first. 

264. Induced currents of the different orders may be 
obtained from frictional electricity, though in conse- 
quence of its high intensity the conductors require better 
insulation than is necessary when they are used with 
the galvanic battery. The flat spirals and wire coils 
may, however, be employed, if their layers are carefully 
insulated by means of shellac, or if covered with silk 
instead of cotton. 



INDUCTION OF £LECTRIOITT. 158 

ExF. 50. — ^Place a fine wire coil over a ribbon spiral, with a 
plate of glass interposed ; a secondary shock may now be obtained 
from the wire when the charge of a Leyden jar is passed through 
the spiral. A still better mode is to employ a second wire coil, 
instead of the flat spiral ; if the ends of one coil be held in the 
hands, a strong shock will be felt at the moment of discharging 
the jar through the other. The secondary current flows in the 
same direction as the one which induces it ; as may be shown by 
passing it through the helix described in § 235, when it will 
magnetize a sewing needle placed within it. 



II. BY THE INFLUENCE OF A MAGNET. 

265. Currents of electricity may be excited in metal- 
lic wires by means of magnetic changes taking place in 
their vicinity. This is in fact the converse of the prin- 
ciple explained in chap. 11^ sect. 2. It was there shown 
that a current of electricity passing in the vicinity of a 
bar of iron or steel produces a magnetic change in that 
bar. The branch of science which treats of the devel- 
opment of electricity in this way is called Magneto- 
Eieciridty. 

266. There are several modes in which these mag- 
netic changes may be produced in the vicinity of the 
wire in which the current of electricity is to be excited. 
The movement of a magnet near a wire, or of a wire 
near a magnet, is one method. The approach of a 
magnet to a bar of soft iron surrounded with wire, or in 
general, a change in the relative position of the magnet 
and the bar, is a second. The passage of a galvanic 
current round an iron bar wound with wire is a third : 
in this case an induced current may be obtained either 
from the wire conveying the primary current, or from a 



154 .DANIEL DATIS, Jtiu's HAITITAL. 

seconil wire also surrounding the iron ; but the current 
excited by the iuAuenca of the magnetized bar cannot 
be separated from that which is the result of electro- 
dynatntc induction, at least with Uie usual arrangement 
of ihe wires. 

267, If the cups of the helix on stand, described in 
1^ 130, be connected with a delicate galvanometer, and 
a bar magnet be introduced into the helix, as in fig. 98, 
the needle will be deflected while the magnet is passing 

■fVg:. 98. in, hut will return to its Ibrmer 

2 position as soon as the magnet is 
'' at rest within the coil. On draw- 
ing the magnet out, the needle wiS 
be deflected in the opposite direc- 
tion. By moving the magnet m 
and out so as to keep time with 
hhthe oscillations of the needle, the^ 
will be greatly increased. Reven- 
ing the direction of the magnet so as to cause H to enter 
by the contrary pole, will reverse the indications of die 
galvanometer. If the magnet be carried through the 
helix so as to bring it out at the opposite end to that by 
which it entered, the effect is the same as if it had been 
drawn out as before. No current b excited while the 
magnet and coll are both at rest. 

268. Connect the cups of a flat spiral, such as that 
described in ■^ 123, with the galvanometer; and pass a 
U magnet over it, towards the centre, with one of its 
poles above and the other below. The needle will be 
deflected in opposite directions as it passes on and off. 
A less effect will be produced by moving a bar magnet 




NDOCTION OF - 



1 L £ C T R I C 1 T T . 



155 



r the spiral, or bjr passing 



in the direction of a radius o 
it iolo the ceotral opening, 

269. Let the ends of the coil of insulated wire A, 
6g. 99) be connected with the gold leaf galvanoscope, 
destdbed in ^ 153. Then pass the ring down over one 

■^' 99- of the poles, say the 

south pole, of a U 
magnet. The gold 
leaf will be sensibly 
deflected. Take the 
ring from the south 
pole and pass it over 
Iji'the north pole. It 
will be found that 
the gold leaf is de- 
flected the same way 
5 by both these motions 
of the ring, but in the 
opposite direction to what it was previously. Thus 
drawing it off of one pole and putting it over the oppo- 
site pole produce deflections in the same direction, but 
similar motions, such as putting It over either pole or 
drawing it off of either, produce opposite deflections. 
The wire coil A is the same as that described in '^ 126. 

270. Place a bar of soft iron within the helix on 
stand, fig. 98, its cups being connected with the gal- 
vanometer as before. Then bring the opposite poles of 
two bar magnets in contact with the extremities of the 
iron. The bar will suddenly be magnetized by induc- 
tion, and the needle will be deflected. It will, how- 
ever, immediately return to its former position, the setlled 




156 DANIEL DAVIS, JR.'s MANUAL. 

magnetic condition of the bar having no power to af^ 
it. On withdrawing the magnetic poles, the bar loses its 
magnetism, and the needle is deflected in the opposite 
direction. 

271. By bringing the poles in contact with the iron, 
and withdrawing them alternately, in such a manner as 
to keep time with the vibrations of the needle, they 
may be greatly increased as before. If the two other 
poles of the bar magnets touch each other so as to form 
a letter V, the inductive power is much increased. TThen 
by opening apd shutting the magnets as if joined by a 
hinge at the vertex, the bar within the helix may be 
magnetized at pleasure. 

272. When an armature or any piece of soft iron is 

brought in contact with one or both of the poles of a 

magnet, it becomes itself magnetic by induction^ and by 

its reaction adds to the power of the magnet. On the 

contrary, when it is taken away it diminishes the power 

of the magnet. The approach and departure of iron 

therefore from the poles of a magnet alters its magnetic 

state and tends to induce a current of electricity iq a coil 

surrounding it, as may be shown experimentally thus. 

Exp. 51. — Pass a wire coil, whose ends are connected with a 
galvanometer, over one of the poles of a U magnet, as in fig. 99, 
and keep the magnet and coil stationary. The needle will now 
be deflected in one direction when an armature is applied to the 
poles, and in the opposite direction when it is removed. 

273. When a galvanometer is used in these experi- 
ments, it must be placed at such a distance from the 
instrument where the magnetic movements and changes 
are made, that the needle will not be deflected by any 
influence but that which reaches it through the connect- 



IKDUCTION or ELECTRICITY. 



157 



ing wires. With the gold leaf galvanoscope this pra- 
cautioa is not needed. 

374. Magneto-Electric Armature. This instru- 
ment consists of an armature of the U form, wound 
with fine msulated wire and enclosed in a metallic case ; 
Fig. 100. the armature itself is not 

a solid har, but a bundle 
of iron wires. It is seen 
at A, in 6g. 100, and a 
sectional view of it is 
n separately. In the 
ion, B D is the ar- 
\ mature, having several 
ijcrs of wire wound on 
" each of its legs, and con- 
tained within the case C, 
from which its ends pro- 
tject slightly. One end 
^of the wire which en- 
velops the armature is connected with the case, and also 
with the armature itself. The other end is soldered to 
the brass cup E, which is attached lo the exterior of the 
case, but is insulated from it by means of an ivory collar. 
By this instrument (be current of electricity produced 
by a sudden change in the magnetic state of the arma- 
ture may be rendered sensible by a strong shock. Let 
the experimenter bring the ends of the armature in con- 
tact with the poles of a powerful compound U magnet, 
in the manner represented in ihe figure. In his left 
hand he holds a metallic handle H, from which two 
wires proceed ; one to the cup M, attached lo the U 
14 




158 DANIEL DAYIS^ JJu's MANUAL. 

magnet N S, and the other to the cup E, upon the case 
of the armature, which he holds in his right band. It 
is not essential to have a cup fixed on the steel magnet, 
as the end of the wire may simply be pressed against it. 

275. The apparatus being thus arranged, the experi- 
menter suddenly separates the armature from the magnet 
by slipping it upwards or downwards from the position 
represented in the figure. As the armature leaves the 
magnet, it loses the magnetism which had been induced 
in it (see <5> 110), and a current of electricity is in con- 
sequence excited in the wire coiled around it within the 
case. This current passes from the cup E, connected 
with one end of the enclosed wire, round to the handle 
H, and thence to the cup M ; it then flows through the 
magnet to the armature itself which is connected with 
the other end of the wire. The current is excited at 
the very moment of separation, and passes from the 
magnet to the armature as a spark of inappreciable 
length, but at the same time very perceptible. This 
primary induced current passes only through the metallic 
conductors or through the short interval of air between 
the armature and magnet. It is not sufficiently intense 
to produce the shock which occurs at the moment when 
the spark passes. This shock is due to a secondary 
current of higher intensity induced by the primary cur- 
rent in the same wire, at the moment when the circuit 
through the metallic conductors is broken. This sec- 
ondary current then passes from the cup E, by the 
wire E H, through the body and back to the armature, 
this being the only circuit which is left for its passage. 

276, When the armature is first brought in contact 



INDUCTION OF ELECTRICITT. 159 

with the magnet, there is of course a change in the 
magnetic state of the former, and a current of electricity 
is consequently induced in the wire surrounding it. This 
current passes through the metallic circuit which is com- 
pleted at the same time, and induces a secondary current 
capable of giving a shock were it to pass through the 
body. But as the metallic circuit remains complete, 
the secondary current passes through that in preference 
to the body of the experimenter. It is only therefore 
when the circuit is broken at the same moment that the 
primary current is excited, that the shock is obtamed 
fix>m the secondary current induced almost at the same 
instant by the primary, and which is then obliged, in the 
absence of some other circuit, to pass through the body. 

277. K the wire H M be taken away, no circuit is 
left for the primary induced current except through the 
body of the experimenter. If the armature be slipped 
on and off the magnet under these circumstances, the 
primary current will pass through the body so as to give 
a slight shock to the tongue or even to the hands. The 
shock will be felt both when the armature is brought in 
contact with and separated from the magnet, though the 
former will be much the stronger. This is probably 
owing to the greafer suddenness of the change in the 
magnetic state of the armature when it first touches the 
magnet, than when it leaves it. In proportion to the 
quickness of the magnetic change is the intensity of the 
induced current and the consequent shock. 

278. If one of the wires of a galvanometer be con- 
nected with the cup E, and the other wire with the 
case of the armature, the needle will be deflected 



160 



DANIEL DATIS, JR.'8 HANUAL. 



Strongly in opposite directions whenever the armature 
is brought in contact with or separated from the mag- 
net. If one of the wires of the galiranometer be con- 
nected with the cup M and the other with the cup 
E, the same result will ensue, although in this case 
the current flows through the magnet, and has to pass 
as a spark when the armature and magnet are separated. 
279. Magneto-Electric Machine, for shocks. 
In this instrument a powerful compound U magnet is 
mounted on a stand. Before its poles is the armature 
A, resembling a U armature, although for convenience 

IHg. 101. 




the iron instead of being curved is bent at right angles. 
It is a solid bar and not a bundle of iron wires as in the 
last described instrument. Around each pole of this 
armature is wound a coil of fine insulated wire j the two 
coils are connected so as to act as a single one. The 
armature does not quite touch the magnetic poles, and 
is mounted on an axis of rotation extending from the 
post P to the central support of the magnet. The upper 



INDUCTION OP ELECTRICITY. 161 

part of the post P is made to slide over the lower part, 
and by means of a screw can be fastened in any po- 
sition. In this way the band connecting the two wheels 
may be tightened at pleasure by increasing the distance 
between them. This arrangement also renders the in- 
strument much more portable than it would otherwise be. 
By means of the multiplying wheel W, which is con- 
nected by the band with a small wheel on the axis, the 
armature may be made to revolve rapidly, so that the 
end of the armature which was one instant opposite to 
the north. pole of the magnet, will be the next instant 
opposite the south pole, and the one that was opposite 
the south magnetic pole will be opposite the north pole. 
A rapid reversal of the magnetism of the armature thus 
takes place, and electric currents are excited in the sur- 
rounding wire. One extremity of tlie coil ,of wire is 
connected with a ferrule or cylindrical piece of silver a 
on the axis of motion, but insulated from it by ivory. 
The other extremity is attached to the axis and thus 
connected with the toothed wheel or breakpiece fixed on 
the axis near the post P. A silver wire 6, flattened at 
the bearing part, presses constantly against the ferrule «, 
and is connected under the base-board with the wire e, 
which touches from time to time the teeth of the break- 
piece during its revolution, thus closing and opening the 
circuit of the coil in rapid succession. 

280. It is evident, on the principle of the Magneto- 
Electric Armature last described, that if two handles 
held by an experimenter were connected, one with the 
ferrule and the other with the breakpiece, that a shock 
would be experienced whenever the metallic circuit of 

14* 



*. 



162 DANIEL DAVIS, JR.'s MANUAL. 

the coil was broken. It was shown that at the moment 
when the induced primary current was interrupted, a 
secondary current would pass through the body, if that 
was included in a circuit between the ends of the wire. 
This is accomplished here by connecting one of the 
binding screw cups C C with the wire b under the base- 
board, and the other cup with the post P, which has 
metallic connection with the axis and breakpiece. The 
body therefore will complete the circuit if interposed 
.between the handles H H, and will receive a shock 
whenever the primary current is interrupted. A spark 
is seen when the wire e leaves each tooth of the break- 
piece ; if the wire is of iron, beautiful scintillations are 
produced. 

281. In the Magneto-Electric Armature the shock 
is obtained only when the armature separates from 
absolute contact with the magnet, but if 'the primary 
induced current is broken when an armature is movmg 
towards or away from a magnet, a shock will be felt, 
though it will be most powerful when the magnet and 
armature are nearest, as the magnetic action is then 
greatest between the two. In this instrument the toothed 
wheel breaks the circuit when the armature is in all 
possible positions in reference to the magnet. Yet a 
shock is always obtained, except when the armature is 
nearly at right angles to the poles. When the armature 
is made to revolve rapidly by means of the crank and 
wheel W, the torrent of shocks which results is insup- 
portable. The. muscles of the hands which grasp the 
handles are involuntarily contracted, so that it is impos- 
sible to loosen the bold or escape from the infliction. 



induction op xlectricitt. 163 

282. Magneto-Electric Machine, for decompo- 
sitions. In the magneto-electric machine just described, 
the induced currents flow in opposite directions during 

Fig. 102. 




each half revolution of the armature. One pole of the 
armature, in leaving the north pole of the magnet and 
approaching the south pole, induces electricity in one 
direction, but when it passes the south pole and again 
approaches the north pole, it induces a current in the 
opposite direction. The principle of the machine now 
to be described is the same as the last. The modes 
of connection are modified. Instead of the cylinder 
a, Dr. Page's pole-changer, <§> 162, consisting oLtwo 
semi-cylinders insulated from the axis, is substituted. 
The two extremities of the coil surrounding the armature 
are soldered to these. Two flattened silver wires b b 
press against tlie opposite sides of the pole-changer, and 
are connected under the base-board with the cups C C. 
These are the only connections used in producing de- 
compositions, the circuit not being broken. The efiect 



164 DANIEL DAVIS) Jlu's MANUAL. 

of the pole-changer is to change the end of the coil 
which communicates with either cup eveiy half revolu- 
tion. But as the current itself flows in opposite direc- 
tions in the coil each half revolution, the result is that 
one of the cups is constantly positive and. the other 
negative. The current flows between them, if they are 
connected, always in one direction, unless the revolution 
of the armature is reversed. 

283. To enable this machine to aflTord strong shocks, 
one of the wires 6 6 is also connected with the post P, 
and thereby with the axis and breakpiece. The other 
wire is connected with the pillar jp, which has a binding 
screw at the top, in which a wire can be fastened to 
play against the breakpiece and break the circuit, as in 
the last described instrument. All the efiects produced 
with the other machine may be equally well shown with 
this. The flow of the current in a constant direction 
also allows of the performance of many additional ex- 
periments. 

284. When the metallic handles attached to C C are 
held in the hands, the arm connected with the negative 
cup will be found to be most aflTected by the shocks. 
This is a physiological phenomenon, the current pro- 
ducing a greater efiect upon the arm in which it flows 
in the direction of the ramification of the nerves, than 
upon the one in which it ascends. The initial second- 
ary is too feeble to afford shocks, so that only the 
terminal secondary need be taken into account. The 
intensity of the terminal shock is however constantly 
varying, according to the position of the armature m 
respect to the magnet, and the difference vsl the effect 



INDUCTION OF ELECTRICITY. 165 

upon the two arms is not so well marked as with some 
of the instruments which will be described hereafter. 

285. Slight shocks may be obtained from the primary 
current, as in the case of the Magneto-Electric Arma- 
ture, by grasping the metallic handles connected with the 
cups C C. The wire which rests on the breakpiece must 
be removed so that the circuit may not be broken. If 
the cups be connected with those belonging to the inner 
coil of the Double Helix and Electrotome (*§» 245), and 
the central opening of that instrument be filled with iron 
wires, secondary shocks of considerable strength will be 
obtained from the exterior helix whenever the armature 
is made to revolve. The vibrating wire should be put 
in motion to break the primary circu|tf Bright spad^ 
are also seen in the mercury cups. The sparks are 
conveniently shown by passing the primary cufient of 
the machine through the Contact Breaker, ^ 229, the 
wire W W being made to vibrate. 

286. When the primary magneto-electric current is 
made to pass through water in a constant direction, the 
water is resolved into its elements, and the gases hydro- 
gen and oxygen are given off separately, by the two 
wires which convey the current. If the direction of the 
current alternates, the water is still decomposed, but the 
gases cannot be obtained separately as both are given 
off fnom each wire. The other machine is able to 
decompose water, though very feebly, because the con- 
nections are such that only the secondary current can 
be used. 

287. Two platinum wires being connected with the 
cups C C, and their ends immersed in water, a slender 



166 DAHIEI. DAVIS, JB.'a HANITAI^ 

Stream of gas will be seen to escape fioni each wire 
when the armature is made to revolve. If the wires are 
of iron or copper, the oxygen will unite with the one 
connected with the positive cup to form oxide of iron <a 
of copper, and hydrogen alone will he given off. Pla- 
tinum wires are not attacked hy the oxygen, and are 
therefore hest for conveying the current. The decom- 
position of water is greatly facilitated by dissolving in it 
some salt, as for instance, Glauber's salt, or what is still 
more effectual, by the addition of one part of sulphuiic 
acid to ten or fifteen of the water. These substances in- 
crease its- conducting power. 

288. fig. 103 represents a DeccHDposing Cell mount- 
Fifr.W3. ed on a stand, and designed to 

be used with this machine. Two 
platinum wires connected with the 
cups A and B on the stand pass 
up into the cell, which is of glass. 
A glass tube G may be inverted 
over these wires to collect any gas 
which is evolved ; it passes through 
a cork GtUng the mouth of the cell 
with sufficient tightness to allow 
the tube to be filled with the liquid 
by merely inverting the instrument. 
The cell being partly filled with 
acidulated water, and the tube 
wholly full, connect the cups A and 
B with those of the machine. As 
the wheel W is turned, bubbles of 
■gas will be seen to escape from 




INDUCTION OF ELECTRICITY. 167 

each wire, and to rise into the tube, displacing the liquid 
from it. When the tube is full, it may be removed and 
the mixed gases exploded by holding its mouth to a 
flame. 

289. By having two gfass tubes O and H passed 
through a cork so that one of them may be inverted 
over each wire, as shown in the cut, where p p are the 
platinum wires, the gases may be obtained separately ; 
oxygen only being collected in the tube placed over the 
positive wire, and hydrogen alone in the other. The 
volume bf the latter gas is twice that of the former, as 
indicated in the figure by the relative height of the liquid 
in the tubes O and H, occupied respectively by the 
oxygen and hydrogen. On removbg the tubes when 
lull, the hydrogen will bum if a flame be applied, and 
the oxygen will increase the brilliancy of the combustion 
of any ignited body put into it. 

290. With a good machine, one cubic inch of the 
mixed gases will be liberated in from five to ten minutes. 
If the conducting power of the liquid be made too great 
the evolution of gas will be lessened. Strips of platinum 
foil, which are superior to wires in decomposing by a 
compound galvanic battery, do not answer so well with 
the magneto-electric current, especially when the wire 
coiled upon the armature is fine. 

291. The primary current is able to decompose va- 
rious saline solutions. For this purpose some porous 
partition should be interposed between the portions of 
liquid in which the wires are placed, in order to prevent 
their too ready admixture. These experiments may be 
performed in the Decomposing Cell, fig. 103, by placing 



168 DANIEL DATIS, J R.'s MANUAL. 

a piece of unsized paper across it, between the wires ; it 

need not fit closely the sides of the cell. A better in- 

Fig, 104. strument for the purpose is a glass tube bent 

r\ r\ into the form of the letter U, as shown in 

I B fig- ^^^' ^ loosely crumpled piece of un- 

I I sized paper, or of cotton cloth, may be thrus^ 

^^ into the bend of the tube as a partition, thus 

^ >^ dividing it into two cells. 

292. The tube being partly filled with a solution of 

some neutral salt to which has been added enough of 

the infusion of red cabbage to give it a blue color, let 

two platinum wires connected with the cups of the 

machine be immersed, one in each portion of the liquid. 

When the armature is made to revolve, the blue color 

will soon be changed to red in the cell containing the 

positive wire ; and to green in the other, provided the 

salt has an alkaline base. By reversidg the motion of 

the armature, the original color will be first restored in 

each leg of the tube, and then the opposite change will 

occur. If the solution be colored blue by the infusion or 

tincture of litmus, it will become red in the cell in which 

the acid is developed, but will suffer no change in the 

other. When the yellow infusion of turmeric is used, it 

is turned brown by the alkali evolved in the negative 

cell, but is not affected by the acid in the other. 

Exp. 52. — Let the tube contain a weak solution of Glauber's 
salt (sulphate tof soda), colored blue by the infusion of red cab- 
bage. On transmitting the current, sulphuric acid will be liber- 
ated in one cell, changing the blue to red ; and soda in the other, 
changing it to green. Similar phenomena present themselves 
with a large number of salts, but in some cases different effects 
are produced. 



INDUCTION OF ELECTRICITY. 169 

Ezp. 53. — ^If a solution of muriate of ammonia, colored by some 
vegetable infusion, be employed, chlorine gas will be given off 
fipih the positive wire ; this may be recognized by its peculiar 
odor and by the bleaching effect it produces upon the liquid in 
the positive cell, which quickly becomes colorless. In this case 
ammonia and hydrogen are set free in the negative cell, and mu- 
riatic acid and oxygen should have been liberated in the other. 
The chlorine appears therefore to be a secondary product, set 
ftee by the combination of the hydrogen of the muriatic acid 
with oxygen, to form water. 

ExF. 54. — ^Let the tube be filled with a weak solution of hy- 
driodate of potash, without any coloring liquid. By causing the 
armature to revolve, iodine will be abundantly liberated round 
the positive wire ; this being slightly soluble gives a brown color 
to the liquid, but most of it remains in suspension, forming a 
dense cloud. If a few drops of a weak solution of starch had 
been previously added, an intense blue color will be developed. 
The hy driodate of potash is more easy of decomposition than any 
other salt ; even the current of a single galvanic pair will liberate 
iodine from it. 

Exp. 55. — ^When a solution of sulphate of copper is employed, 
sulphuric acid and oxygen are set free in the positive cell, and 
metallic copper is precipitated upon the negative wire. If the 
current is powerful, it is deposited as a slightly adherent black, 
powder ; but if of moderate strength, a thin coating is formed, 
' ix>8sessing the proper color and appearance of the metal. In 
this case little or no hydrogen escapes from the coated wire, 
though oxygen is given off by the positive one. On reversing 
the current, the copper will be gradually dissolved from off the 
coated wire, and a similar deposit will occur on the other. No 
oxygen escapes from the wire which Is now positive until its 
coating has nearly disappeared. When the experiment is con- 
cluded, the deposited copper may be removed from the platinum 
wires by a little diluted nitric acid. If two copper wires be im- 
mersed in the solution, as much copper will be dissolved off of one 
as is deposited upon the other. Sulphuric acid does not act upon 
copper in the cold unless aided in this way by an electric current 
Exp. 56. — ^Let the tube contain a diluted solution of muriata- 

15 



170 DANIEL DAVIS, J R.'s MAVUAIi. 

of gold, the conducting wires being of platinam. The negative 
wire will soon become covered with a coating of gold, which in- 
creases in thickness as the current is continued. Other metak, 
as for instance, silver, copper and brass, may be thus gilt ; the 
coating does not adhere very firmly unless the metallic surface 
on which it is to be deposited has been perfectly cleaned by acid. 
The positive wire should always bo of platinum or gold. The 
ethereal solution of gold may be employed in this experiment 
It is made by mixing ether with a strong solution of the muriate; 
the ether containing the gold rises to the surface and may be 
poured off from the acid. 

293. Many other metallic salts may be decomposed 
in the same manner, and the metals precipitated, but in 
most cases the deposit is of a black color. In precipi- 
tating metals, both wires may be in the same portion of 
liquid, no partition being required ; in fact, if the tube 
is of considerable length it will not be necessary in the 
other experiments. The deposition of the metals from 
their solutions in these cases depends upon the same 
principles which are concerned in the electrotype pro- 
cess to be described hereafter. 

294. The galvanometer is strongly affected by the 
primary current. Even a large and heavy needle sur- 
rounded by a single wire, as in the instrument repre- 
sented in fig. 29, may readily be deflected. A sewing 
needle or a piece of steel wire placed in the magnetizing 
helix (fig, 94), will be fully charged. If an iron wire 
be introduced into the helix, its ends will sustain a con- 
siderable quantity of iron filings during the flow of the 
current. 

295. When the extremities of the wire surrounding a 
small electro-magnet, such as is represented in fig. 53, 
are fixed in the cups C C, it will be able to sustain a 
weight of some ounces while the primary current is 



ON OF £LECTItI 



171 



flowing. If the electro-magnet be covered with four or 
five layers of coils, the wire being in a single length, it 
will lift several pounds. 

296. The prinaary magneto-electric current resembles 
a galvanic current excited by a number of small pairs. 
Its quantity and intensity are, however, both greatly 
influenced by the size and length of the wire enveloping 

. the armature. A short wire of large diameter gives a 
current of moderate iotensity but of considerable quan- 
tity, and is therefore best for producing sparks, decom- 
positions and magnetism. A long and fine wire affords 
a current of small quantity and high intensity, and is 
most suitable for giving shocks. 

297. Page's Revolving Magnet, as a magneto- 
electric MACHINE. This instrument esJiibits tbe mag- 

F!g. 105. 




neto-electric machme in its most simple form. A soft 
iron bar capable of revolving between tbe poles N S of 



172 DANIEL DAVIS, J B.'8 MANUAL. 

the U magnet is wound with wire, and its extrenuties 
connected with the cylinders of a pole-changer fixed mi 
the axis of motion. Two silver springs pressing on this 
convey the induced current to the cups A and B. The 
instrument has been described in ^ 171. 

298. The cups A and B being connected with those 
of a galvanometer with an astatic needle (^ 87), as 
represented in the figure, the needle will be powerfully 
deflected when the bar is made to rotate rapidly by 
drawing the hand over the axis. By reversing the mo- 
tion of the bar several times in correspondence with the 
oscillations of the needle, it may be made to revolve 
rapidly. With a sufficiently delicate galvanometer, any 
of the electro-magnetic instruments in which motion is 
produced by the mutual action between a galvanic 
current and a steel magnet, may be made to afford 
a magneto-electric current by producing the motion 
mechanically. In all cases the current excited flows 
in the opposite direction to the galvanic current which 
would be required to produce the same motion. 

299. If the cups A and B be connected with the 
cups C and C' of the Double Helix and Electrotome, 
slight secondary shocks, which may sometimes be felt 
in the bands, will be obtained from the fine wire helix 
by rotating the iron bar, as in the figure. The hollow 
of the double helix should be filled with iron wires, and 
the vibrating wire be put in motion so as to break the 
circuit rapidly. 

300. In the magneto-electric instruments which have 
been described, steel magnets are employed, and me- 
chanical motion is made use of to excite the electrical 
current; in those which remain, the current is induced 



INDUCTION OF ELECTEICITT. 173 

by an electro-magnet whose magnetism is alt^natelj 
acquired and lost. The instruments consist essentially 
of double helices containing bars or wires of soft iron. 
The magneto-electric current is thus obtained in con- 
junction with that excited by electro-dynamic induction, 
and the combined current is called a secondary^ though 
only^ in part such. 

301. Separable Helices. This instrument is rep- 
resented in fig. 106. The external helix is of fine wire 

1^.106. 




Jiom one to three thousand feet long. It is made wholly 
separate from the interior helix and can be lifted directly 
off, as is shown in fig. 107, where a is the exterior coil, 
and b the interior one. The ends of this helix are en- 
closed in two brass caps to which the extremities of the 
fine wire are attached, and from which proceed the bind- 
ing screw cups C and D. The inner belvsL^'^V^Rjck ^ 

15* 



174 



DANIEL DAVIS, JE.S MANUAL. 




fixed in a vertical position on the stand, consists of three 
or more strands of coarse copper wire each about twenty- 

iVff.107. ^^ five feet long. The 

similar extremities of 
these wires are con- 
nected at one end 
with the cup A, and 
at the other with the 
steel Vasp or break- 
piece B. If one wire 
of a galvanic battery- 
be fixed in the cup 
A, and the other be 
drawn over the rasp, 
sparks will be seen ; 
and if handles be connected with the cups C D, slight 
shocks will be felt when the circuit is completed, and 
strong ones when it is broken. The instrument thus 
resembles the double helix described in '5> 245. 

302. If a rod of soft iron be introduced into the centre 
of the helix the sparik is very much increased, brilliant 
scintillations are produced, and the shock when the circuit 
is broken becomes intolerable. The iron acquires and 
loses magnetisrti whenever the circuit is made and broken 
and induces a secondary current in both of the coils 
which surround it. In the coarse wire coil, which also 
conveys the battery current, this appears in the increased 
sparks and scintillations. In the fine wire coil it is felt 
in the violent shock which results. 

303. Slight shocks may be obtained from the inner 
coil itself by connecting one of the handles with the cup 



, INDUCTION OF ELECtBICITY. 175 

A, and the otlier with the rasp B, The bundle of 
iron wires seen at d should be within the helix. The 
shocks are somewhat stronger when one handle is in 
connection with the rasp and the other with the battery 
wire which is drawn over it ; in this case the battery is 
included in the circuit of the secondary current. 

304. If the bundle of annealed iron wires seen at d 
be removed from the brass tube c, and substituted for the 
soft iron rod, the spark and shock are much increased. 
If the rod or bundle of wires be introduced gradually 
into the helix, the spark and shock increase as it enters. 
The intensity of the shock may also be varied at pleas- 
ure, by altering the number of iron wires, the addition 
of a single wire producing a manifest effect. If a glass 
tube be slipped over the iron wires in the helix, it will 
not interfere with their inductive action on the surround- 
ing coils. But if a brass tube be passed over them, 
their influence will be entirely suspended, as far as the 
shock and the spark are concerned. If the tube be 
slijpped partly over them, their influence will be partially 
suspended. Here also is a means of regulating the shock 
with the same battery current. 

305. The cause of the neutralizing action of the tube 
is thus explained. The magnet induces in the tube, 
as well as in the two coils, a secondary current of 
electricity, which flows round it when the cu'cuit is 
made or broken. This secondary induces a tertiary 
current in both the coils, which flows at the first mstant 
in an opposite direction to the secondaries induced in 
the coils by the magnet, and therefore retards them. 
As the secondary current in the tube is, however, instan-p 



176 daniel'^datis, jr.'s mahvai.. 

taneous, it induces another t^arjr in the same direction 
with itself when it ceases to flow. The consequence is 
that the quantity of the current in either helix is not 
altered, but its intensity is reduced, owing to the slow- 
ness of its development. This is always the eflfect of 
any closed circuit in the neighborhood of an inducing 
magnet or current, on other circuits near it. 

306. If the cups of the fine wire coil be joined by a 
wire, it will form a closed circuit around the magnet, 
and will impair the spark when the circuit of the coarse 
wire coil b broken, though not to so great an extent 
as the brass tube, since the latter offers a freer and 
shorter circuit for the induced current. The spark is 
but slightly lessened when shocks are taken from the 
fine wire coil, because the human body is too poor a 
conductor to allow of the ready flow of the secondary 
through it. A metallic cylinder siifix)unding the helices 
will neutralize the sparks and shocks as well as an en- 
closed tube. 

307. When a bar of iron is placed in the centre of 
the coil, a secondary is induced in it as in the tube, 
which somewhat retards the secondary currents in the 
coils. Hence the greater shock obtained from a bun- 
dle of wires, where this secondary current cannot 
circulate. To this cause is probably added another, 
the more sudden change in the magnetism of the wires, 
when the battery current ceases, firom the neutralizing 
influence of the similar poles of the wires on each other. 

308. If the secondary current can be hindered from 
circulating in the brass tjube, its retarding influence will be 
prevented. Thus, if the tube be longitudinally divided 



INDUCTION OF ELECTRIGITT* 177 

on one side, it no longer diminishes the shock or spark. 
In the same manner if the bar of soft iron be sawed 
through to its centre, longitudinally, the shock and 
spark will be increased. If a soft iron tube be divided 
like the brass tube and placed in the helix, the shock 
will be still stronger, though never so great as with the 
wires. The two brass caps at the ends of the fine wire 
coil would exert a considerable neutralizing influence if 
they were not divided on one side, as is represented in 
fig. 106. The ends of the caps are also cut through fpr 
the same reason. 

309. In this instrument there are some peculiarities 
in the shock occasioned by the motion of the battery wire 
over the rasp. If it is moved slowly, powerful, distinct 
shocks are experienced ; if the motion b quickened, the 
arms are much convulsed ; and if it is drawn over rapidly, 
the succession of shocks become intolerably painful. 
This however can be easily regulated. The shock firom 
the secondary coil increases within certain limits in pro- 
portion to the length and fineness of the wire of which 
it is composed. There is, however, no advantage in 
^nploying a very long wire, unless the battery is pow- 
erful. The shock will also be lessened if a very fine 
wire is used, except its length be moderate. 
. 310. The strength of the shock depends greatly upon 
the extent of the surface of contact between the hands 
and tlie metallic conductors. Thus, if two wires be 
fixed in the cups of the outer coil and grasped in the 
hands, the shocks will be slight in comparison with 
those given by the bstndles, .and still more so if the 
wires are held lightly in the fiiigers. These effects, as 



178 DANIEL DAVIS^ JR.' S MANUAL.. 

well as the increase of the shock by wetting the hands, 
are due to the comparatively low intensity of the secon- 
dary current, which causes it to be transmitted very 
imperfectly by poor conductors. With irictional elec- 
tricity it is well known that no diflFerence in the shock 
is thus occasioned. 

311. When the quantity of the secondary current is 
very small, an imperfect conductor or a surface of 
limited extent may be able to convey the whole of it, 
even if its intensity be not very high ; in which case 
the sensation and muscular contractions produced by it 
will not be increased, but often lessened, by any farther 
increase of the conducting power. Thus, if the shocks 
are received by placing the hands in two vessels of 
water connected with the cups of the outer coil, and the 
current be rather feeble, it will produce the strongest 
sensation when the ends of the fingers only are immersed. 
When the current is powerful, the shock is intolerable, 
whether the surface of contact with the water be large or 
small ; in the latter case it extends to a less distance up 
the arms, though it may be felt very strongly in the fingers. 

312. The shocks have sufficient intensity to pass 
without much diminution through a circuit formed by 
several persons with their hands joined, especially if 
their hands are moistened. Different individuals will 
be found to manifest remarkable differences in regard to 

"susceptibility to the shocks; some being but slightly 
affected, perhaps feeling the shocks only in the hands or 
arms ; while others will feel them as far as the shoulders 
or across the breast, and will experience strong muscu- 
lar contractions in the arms. 



INDUCTION OF ELECTRICITY. 179 

313. The difference in the strength of the shock in 
the two arms, which has been described in the case of 
the Magneto-Electric Machine (see § 284), is exhibited 
more satisfactorily by the separable helices, as a rapid 
succession of shocks may be obtained of very nearly the 
same intensity. Suppose -the handle connected with 
the positive cup of the exterior helix to be held in the 
right hand, and the one connected with the negative 
cup in the left hand. The left hand* and arm will then 
experience the strongest sensations and be the most 
convulsed. In determining the positive or negative 
character of the cups, regard should be had only to the 
terminal secondary current, it being found that the initial 
secondary, whether induced by means of a voltaic battery 
or a permanent steel magnet, produces comparatively 
feeble physiological effects, and consequently need not, 
in this case, be taken into account. This singular dif- 
ference in the intensity of the shocks is regarded as a 
purely physiological phenomenon, the greatest effect 
both as respects sensation and muscular contractions 
being produced by the electric current when it proceeds 
in the direction of the ramification of the nerves. 

314. If the ends of the secondary wire are put into 
vessels of water, a peculiar shock may be taken by 
putting the fingers or hands into the vessels, so as to 
make a communication between them through the body. 
If both wires be put into a trough, at some distance 
apart, and two fingers of^ the operator be placed in the 
water in a line between 'the two wires, a shock will be 
felt. Here the current prefers a passage through the 
body to that through the water which intervenes be- 



180 "DANIEL DATIS^ JR^'s KAKUAL. 

tween the fingers. The conducting power of the water 
may be made better than that of the human body by the 
addition of a sufficient quantity of common salt ; in which 
case little or no shock can be perceived. If the fingers 
be placed at right angles to the line between the wires, 
no shock will be felt. The trough should not be of 
metal^ but of some poor conductor of electricity. 

315. If a delicate galvanometer be connected with 
the ends of the fine wire coil, the needle will be deflected 
in opposite directions and equally far when the battery 
circuit is closed and opened. The same effect is pro- 
duced when the brass tube is slipped over the iron wires. 
In this case, though the shock may have been prevented, 
the induced current still evidently passes. 

316. When a flat coil of fine wire, such as that rep- 
resented at W in fig. 95, is passed over the interior helix 
(the exterior one being removed), the shocks will "be 
found to be strongest when the coil surrounds the middle 
of the helix, and to decline considerably in strength as 
it is either raised or depressed from this position. Now 
the magnetism of the enclosed iron wires, which induces 
the principal part of the current, manifests itself chiefly 
at the ends of the bundle; it might therefore have 
been expected that the flat coil would give the strongest 
shock when surrounding one of these ends. The shocks 
from the exterior helix are also lessened when it is raised 
from the stand so as to enclose only the upper part of 
the inner helix. 

317. This instrument is convenient for illustrating 
some of the most important principles of magneto-elec- 
tric and electro-dynamic induction, in consequence of 



INDUCTION or XLECTRICTTT. 181 

the facility with which the powers and uses of its several 
parts can be separately exhihited. The ohservations 
which have been made with regard to this iDstrument 
apply equally well to the two foUowmg, which are modi- 
fications of it. 

313. Separable Helices and Electrotome. In 

the instnimeDt represented in fig. 103, the ioner helix 

is connected with an Electrotome or Contact Breaker, 

w J%. 108. 




similar to that described in '^ SS9, fixed on the same 
stand, in addition to the steel rasp. There are two 
cups A and D for the battery wires ; these are con- 
nected through the electrotome with ihe inner coil. 
When the electrotome is made to vibrate, the curved 
wire dips its ends alternately into the cups of mercury, 
and rapidly breaks the circuit. One end of the coarse 
wire coil is also connected with the steel rasp, so that 
16 



182 DANIEL DAVIS^ J S.'s MANUAL. 

this may be used as in the last instrument, when the 
current is not made to pass through the electrotome. 
At W is seen the end of the bundle of wires, and at 
T the brass tube, which may be slipped over them at 
pleasure. 

319. This instrument, and others resembling it in 
being provided with a mechanical contrivance for break- 
ing the battery circuit, may be used with a very small 
battery, although its effects are of course most striking 
with a powerful one. If a voltaic pair, consisting of a 
silver dollar and a piece of zinc of the same size be 
used, and the helix be filled with soft iron wires, the 
shock is quite severe. 

320. When the circuit is broken at the surface of the 
mercury, an intensely brilliant spark is seen, and the mer- 
cury is consumed or deflagrated^ passing off in a white 
vapor. If the quantity of mercury be properly adjusted, 
the sparks occur alternately in the two cups, and in such 
rapid succession as to appear simultaneous. A little 
water or oil poured upon the surface of the mercury 
diminishes the brilliancy of the sparks, but increases the 
intensity of the shocks. 

321. These sparks are of so short duration that mov- 
ing objects appear stationary by their light. One of 
Page's Revolving Armatures, although rotating many 
hundred times a minute, appears at rest when viewed 
in this way ; and where the sparks succeed each other 
rapidly, it appears to leap from place to place as jtheir 
light falls on it. Many optical illusions of this kmd 
may be observed, as in moving the fingers rapidly, when 
their number seems increased, or rapidly turning over 



INDUCTION OP ELECTRICITT. 183 

the leaves of a book, when they seem to leap in the 
same manner as the armature. 

322. If the ends of the secondary wire be separated 
from each other at the same moment that the battery 
circuit is broken, a spark will be seen from the passagq 
of the induced current. A beautiful light is produce4 
if prepared charcoal points are attached to the ends of 
the secondary wire and held almost in contact. 

323. Water may be decomposed by connecting the 
ends of the fine wire coil with an instrument for that 
purpose, having very small platinum wires guarded with 
glass, as originally used by Wollaston. These are pre^- 
pared by inserting the wires into capillary glass tubes, 
which are heated till the glass melts and adheres to their 
ends so as to cover them completely. The platinum 
points are then exposed by filing away the glass. Or 
the wires may be thickly coated with sealing-wax which 
is afterwards to be removed in the same way from their 
points. It is of course only necessary to coat those 
parts of the wires intended to be immersed in the fluid. 

324. The extremities of the platinum wires, while 
the decomposition is going on, appear in a dark room, 
one constantly and brightly, and the other intermittingly 
and feebly luminous. If the apparatus for decomposition 
is removed out of the noise of the electrotome, rapid 
discharges are heard in the water, producing sharp 
ticking sounds, audible at the distance of eighty or one 
hundred feet, and synchronous with the ruptures of the 
voltaic circuit. Decomposition is effected both by the 
initial and terminal secondary currents, that is to say, 
by the currents induced both on completing and on 



184 DANIEL BJLYlSy J R^'s MANUAL. 

breaking the battery circuit ; but the ticking noise and 
sparks accompanying the rapid discbarges in the water, 
are produced only by the terminal secondary current. 
Both gases, hydrogen and oxygen, are given off in small 
quantities at each wire. The secondary current of the 
magneto-electric machine presents the same phenomena 
with the guarded points. 

325. A Leyden jar, the knob of which is connected 
with its inside coating by a continuous wire, may be 
feebly charged, and slight shocks be rapidly received 
from it, by bringing the knob in contact with one of the 
cups of the outer helix, and grasping with the two hands 
respectively the outer coating of the jar and a handle 
connected with the other cup. A gold leaf electroscope 
is readily affected by touching its cap with a wire fixed 
in either cup of the exterior helix. If the contact, 
which should only be momentary, is made at the instant 
of the rupture of the primary circuit, the gold leaves 
will exhibit a considerable divergence without the aid 
of a condenser. Or the knob of a Leyden jar may be 
touched for a moment with the wire, when it will be 
found to retain a feeble charge, capable of diverging the 
gold leaves and of giving a slight shock. The wire 
must be well insulated from the hand in which it is held, 
or the electricity will be conveyed off, and no accumu- 
lation be obtained. 

326. If the cups of the large Thermo-Electric Battery 
(fig. 15) be connected with A and D, and the vibrating 
wire be put in motion, faint sparks will be seen in the 
mercury cups, attended by audible snaps ; and strong 
shocks may be obtained by grasping the handles 



INDUCTION or ELSCTRICITY. 185 

attached to the fine wire coil, especially if both heat and 
cold are applied to the battery. A single thermo-electric 
pair of antimony and bismuth, or of German silver and 
brass, will give a slight shock to the tongue when heated 
by a spirit lamp : it will be more perceptible when the 
ends of two wires fixed in the cups are made to touch 
the tongue than with a more extended surface of contact. 
This is probably due to the small quantity of the in- 
duced current, as has been mentioned in <5>311. These 
sparks and shocks are, of course, not strictly thermo- 
electric but magneto-electric. 

327. When a bar of iron is contained within a horizon- 
tal helix, such as is represented in fig. 96, where the cir- 
cuit can be rapidly broken, and a small key or some nails 
are applied to one end of the bar, notwithstanding its 
magnetic attraction is intermitted every time the voltaic 
circuit is interrupted, yet, it being almost instantaneously 
renewed, they do not cease to be sustained. This ex- 
periment succeeds best when the iron bar is enclosed in 
a brass tube previously to being introduced into the 
helix, the closed circuits of the tube tending to prolong 
its magnetism. 

328. If an iron tube of sufficient diameter to admit a 
long helix of fine wire within it be itself passed within a 
coil of coarse wire, no shocks can be obtained from the 
enclosed helix, even when the tube is divided longi- 
tudinally on one side, to prevent the flow of a current 
in its substance which might neutralize that of the fine 
wire. It has been stated in Exp. 27, that a galvanic 
current passed through a coarse wire helix, enclosed in 
an iron tube, induces no magnetism in it. 

16* 



HAHUAL. 



339. Separable Helices and Retoltiho Abiu- 

TUKE. Aootber form of the separable helices ia rep- 




resented in fig. 109. When the battery wires are 
connected wilh the cups A and C, the cuirent flows 
through the coarse wiie coil, &11& ^Wi \\ii(»i^ P«.%e'a 



INDUCTION OF ELECTRICITT. 187 

Revolving Armature, which is attached to the stand. 
This is a modified form of the instrument described in 
^ 182. The armature revolves rapidly, and breaks the 
circuit each half revolution. The rasp R R is also 
connected with one end of the battery coil, so that if 
the battery wire be removed from the cup C and drawn 
over the rasp, the current will be interrupted and scin- 
tillations produced. In this instrument and those 
which immediately follow, the apparatus for breaking 
the circuit is self-acting, — a very interesting feature. 
The motions are readily produced by the smallest bat- 
tery ordinarily employed for these purposes. 

330. With a battery of even moderate power, the 
shocks may be made to follow each other with exceed- 
ing rapidity. When their strength is lessened consider- 
ably by removing nearly all the iron wires from the 
centre of the helices, it will be found that with this 
rapid succession instead of distinct shocks a peculiar 
sensation of numbness is experienced, extending a 
greater or less distance up the arms, and attended by 
loss of power over the muscles as far as it reaches. 

331. The shocks are never so powerful with this 
instrument as with the one last described, supposing the 
length of the coils to be the same ; because the battery 
current is obliged to maintain the motion of the armature 
as well as to traverse a circuit of greater length. This 
reduction, which is not however very considerable, may 
be avoided by uniting the cup B with one of the binding 
screw cups of the Contact Breaker (<5> 229), and fixing 
one of the battery wires in the remaining cup of that 
instrument and the other in the cup A. 

332. When the cups S S are united by a wire^ tha 



168 



ANIEL SATIS, 



JB. fl KABOAI,. 



Speed of the revolving armatun b altered to scxne ex- 
tent, in consequence of the prerenlioii of the secooduy 
current which would otherwise be excited in the inner 
helix, and which prolongs the magnetism of the U 
magnet after the breakiog of the circuit. It will depoid 
upon the position and pressure of the springs upon the 
breakpiece whether the motion is accelerated or retarded 
by thb circumstance. 

333. Page's Retoltino Abhatdre for shocxs. 
The instrument represented Jn Gg. 110 consists of aU 
electro-magnet wound with a coil of fine wire for shocks, 
in addition to the coarse wire coil for the battery current. 

Fig. 110. This is enclosed in a cylindrical brass 
case C resting on a wooden base. The 
iron of the electro-magnet is not a solid 
bar but a bundle of wires ; its poles pass 
■' up through the upper board, and an 
armature A is fitted to revolve above 
them When the cups c c are c(m- 
nected with the battery, the current 
circulates through the inner coil, and 
passes through the springs which are 
L seen m the figure bearing on the break- 
piece. The fine wire coil is connected 
with tlie cups seen at S, from which shocks may be 
obtained by handles as usual. 

334. It might be expected that the brass cylinder C 
would exert a neutralizing action upon the shocks, as it 
is not divided longitudinally. But it is found that a me- 
tallic casing which thus entirely envelops a U magnet 
cannot act as a closed circuit, because each magnetic 

pole tends (o induce a current In \\ \n \V« o^^oslte 




INDUCTION OF ELECTRICITT* 189 

direction to that which the other pole would excite ; 
and consequently the secondaries of the coils are not in 
the least impaired by this arrangement. 

335. If one of the battery wires is brought firmly in 
contact with one of the small pillars in which the silver 
springs are fastened, and the other put into one of the 
cups c c, so that the electro-magnet may be charged 
without the circuit being inteiTupted by the revolution 
of the armature, the fine wire coil will aflford shocks 
perceptible to the tongue when the armature is made to 
revolve by drawing the finger over the axis. These 
shocks are due to the disturbance in the magnetic state 
of the electro-magnet by the approach and recession of 
the armature. They are very slight, because the inner 
coil affords a closed circuit for a secondary current whose 
neutralizing influence reduces the intensity of the one 
excited in the outer coil. 

336. When an armature is brought suddenly up to 
the poles of a charged electro-magnet, an electric cur- 
rent is excited in its wires flowing against the battery 
current When it is withdrawn, a current flows in the 
same direction as that from the battery. The phenome- 
na belong to the same class as those described in <^ 272. 
The same effect3 are produced by bringing up a steel 
magnet or a second electro-magnet, if the attracting 
poles are presented to each other. When the repelling 
poles are presented, the two currents excited by their 
approach and recession flow in the reverse directions to 
those just described. 

337. It has been mentioned in <5> 204 that these cur- 
rents excited by motion present some of the most formi- 
dable obstacles to the employment of electro-magpetisca 



t 



190 DANIEL DATIS, JR.' B ■fAHITl.L. 

as a mechanical power. The independent motion of an 
electro-magnetic machine lessens the magneUiing powei 
of the battery in proportion to its velocity, because the 
currents thus excited in the wires flow against the gal- 
ranic current; white the application of mechanical 
power to drive the machine against its own motion assists 
the battery current in producing magnetism. 

338. Page's Compound Magnet aNd Electrotoke. 
In fig. Ill a double helix, is seen attached horizontally 
to the base-board by two brass bands. In the centre a 
bundle of soft iron wires is permanently fixed. There 

■Pfe- IIL are two cups for 

the battery wires 
at one aid of the 
stand : one of these 
b connected with 
%A the band which 
^ sustains the glass 
> cup C for contain- 
ing mercury. To the second cup is soldered one end 
of the coarse wire coil, the other extremity of which is 
connected with the band upon which the brass cup B, 
also intended to hold mercury, is fixed. A bent wire 
W, moving on a horizontal axis supported by two pillars, 
dips its ends into the two mercury cups. To the oppo- 
site side of the axis is attached a curved piece of iron 
P, the lower extremity of which approaches neariy the 
end of the enclosed bundle of iron wires. 

339. When the connections are made with the bat- 
tery, the current will traverse the wire W and the inner 
helix, causing the iron wires to become magnetic. They 
Will now Attract the end of tbe iioiv to4 P ■, whose rao- 




INDUCTION or XLECTRICITX. 191 

tioD raises the bent wire out of the mercury in the cup 
C and breaks the circuit. This destroys the magnetism 
of the iron wires, and P ceases to be attracted. The 
wire W then falls back by its own weight, and the cir- 
cuit is renewed. A thin slip of brass is soldered to the 
^tremity of P, to prevent it from being retained by the 
electro-magnet after the rupture of the circuit. 

340. In this manner a rapid vibration of the wire is 
produced, and brilliant sparks and deflagration of the 
mercury take place Iq the cup C. The proper balance 
of the vibrating apparatus is ensured by means of a 
brass ball screwing on a bent wire above the axis. The 
ends of the fine wire coil are connected with the other 
two cups on the stand, one of which b seen at A, 
whence shocks may be taken. 

341. Disguised Helix, roK sparks and shocks. 
This consists of a metallic cylinder, fig. 112, enclosing 
a double helix and bundle of iron wires. It is divided 

f%. 112. Fig. 113. into three bands, insulated 

from each other by rings of 
ivory. At each end there 
is a circular rasp of steel 
attached to the metallic 
Dband nearest it. Fig. 113 
represents a section of the 
instrument: A is the bundle 
of wires, B the battery coil, 
'H C the secondary coil, and 
D D the insulating rings of 
ivory. The similar strands 
of ths battery coil are con- 




193 DANIEL DATIS, JK.*fl MANVAL. 

rasp 6xed to the band K, and at the other end with the 
rasp and band I. One extremity of the fine wire coit b 
also coniiected at E with the band I, and of couise 
through the battery coil with the band K. The other 
extremity is soldered to the insulated band J. 

342. If now the instrument be grasped by the imddle 
band, as in Gg. 112, and one end being rested on the 
pole of a rolt^c battery, the wire W from the other 
pole be drawn orer the rasp, the circuit in the helix 
will be alternately completed and broketi in rapid suc- 
cession, and briDiani scintillations will be seen. So long 
as the operator confines his hand to the central band be 
will feel nothing, but if bis fingers touch at the same time 
ather of the outer bands, he will receive a strong shock 
through the hand from the fine wire c(»l. If the wire W 
is not insulated from the right hand by being wound 
with cotton, shocks will be felt in the arms when the 
other hand touches only the middle band. 

343. Magneto-Electbic Appabatds for hedical 
USE. The instrument most convenient, perhaps, for this 

Fig. 114. 




purpose is that represented in fig. 114. It consists of a 
double helix into which a bundle of iron wires can be 
inserted. The inner belts is composed of two or more 



INDUCTION OP ELECTRICITY. 193 

Strands of coarse insulated copper wire. Their similar 
terminations at one end of the coil are soldered to a 
binding screw cup standing singly near one extremity 
of the base-board ; their other ends are connected with 
both of the brass bands which confine the double helix 
to the stand, and by means of these with. the steel rasp 
fixed above it. The outer helix is completely insulated 
from the other, and consists of fine insulated copper or 
iron wire. Its ends are connected with two binding 
screw cups at one extremity of the stand. In the figure, 
two metallic handles for shocks are seen at H, connected 
by wires with these cups. 

344. The galvanic battery represented in the cut is 
the small cylindrical battery (^23), which is to be 
charged with a solution of blue vitriol, as directed in 
<5> 21. This will keep in good action for fifteen or thirty 
minutes at a time. When a more enduring power is 
wanted, it may be converted into a sustaining battery 
as described in <§> 241. This will maintain a steady 
current for several days in succession. The ends of the 
connecting wires should be kept clean and bright. 

345. The battery being charged, unite one of its 
cups by means of a copper wire, with the cup belong- 
ing to the inner coil. Then draw over the steel rasp 
another wire W, whose end is fixed ia the remaining 
cup of the battery. If the hollow of the helix is filled 
with iron wires, bright sparks will be seen as the wire 
leaves each tooth of the rasp, and strong shocks will 
be felt by grasping one of the handles seen at H in each 
hand. When the iron wires are withdrawn, the spark 
becomes faint and the shock feeble^ These effects are 
produced by secondary currents excited in the coils in 

17 



194 DANIEL DAYIS, J R.'s MANUAL. 

consequence of the alternate closing and opening of the 
circuit of the galvanic cun*ent in the inner helix, by the 
movement of the wire W over the rasp : see «5> 236, 302. 

346. The strength of the shock may be regulated at 
pleasure by varying the number of iron wires which 
are placed within the helix, or the distance which the 
bundle is allowed to enter it. The addition of a single 
wire produces a very perceptible increase in the shock, 
especially when only a few are already within. The 
intensity of the shock may be considerably increased by 
wetting the hands or other parts to which the handles 
are applied, especially with salt water. It may, on the 
contrary, be lessened in some degree by diminishing the 
extent of contact between the handles and the surface 
of the body. If, however, the current is powerful and 
the contact too slight, a disagreeable burning sensation 
will be experienced at the. part touched by the metal. 

347. The shocks may be passed through any portion 
of the body by placing the handles so as to include that 
part in the path of the secondary current ; their inten- 
sity is greater when the handles are near each other. 
The influence does not extend beyond the direct course 
of the current unless the shocks are severe. When, 
however, one of the handles is placed directly over a 
large and tolerably superficial nerve, the shock will be 
felt not only in the parts intervening between the han- 
dles, but through those to which the ramifications of the 
nerve are distributed. Thus, if one handle be held in 
the right hand and the other pressed upon the inside of 
the left arm over the median nerve, the sensation will 
be experienced even to the ends of the fingers, attended 
hy convulsive motions of their muscles. This iSiiAiqiues 



INQCCTION or ELECTBICITT. 195 

tionably a physiological pbenomenon, and not a conse- 
quence of the flow of the current below the position of 
the handle. The difference in the intensity of the 
sBock in the two arms, described in i^dlS, may be ob- 
served with this instrument. 

348. When it is inconvenient to break the circuit 
mechanically, some self-acting interruptor may be added 
to the arrangement last described. In fig. 115, Page's 
Revolving Armature {'^ 182), which is probably the 

iV- II''- 




best instrument for the purpose, is seen in connection 
with the Double Helix. The galvanic current is trans- 
mitted through the two instnunents in succession, by 
uniting one of the battery caps with one of those be- 
longing to the Revolving Armature, whose olher cup is 
connected with a cup h surmounting the Double Helix. 
Tbe cup a on the stand is to be connected with the 
other plate of the battery. 

349. A convenient form of the sustaining battery is 
shown in the 6gure, Tbe copper vessel C, which is a 



196 DANIEL DAYIS, J R.'s MANUAL. 

single cylinder provided with a bottom, has on one side 
a projecting mouth communicating by a nunaber of per- 
forations with the interior of the cylinder. This is j 
designed to hold solid sulphate of copper for the purpose 
of keeping the solution saturated. The zinc cylinder is 
surmounted by two binding screw cups Z Z ; its internal 
surface is painted or varnished, to protect it from the 
action of the solution. Between the zinc and copper 
plates is a cylinder of leather closed at the bottom. The 
management of this battery is the same as of the one 
described in <§>241. The zinc plate might remain in 
the solution several days at a time without the battery 
materially declining in power, but it is better to remove 
it when not in actual use, as it would be needlessly cor- 
roded if kept constantly immersed. 

350. When the connections are made as shown in 
the cut, the armature will rotate with great speed, 
breaking the circuit twice in each revolution. The 
shocks will consequently succeed each other very rapid- 
ly. In the figure, the handles are seen applied to the 
arm for the purpose of confining the shock to the parts 
between them. A single thickness of wetted linen or 
cotton cloth may be interposed between the metal and 
the skin, if desired, without producing much diminution 
in the shock. 

351. If the apparatus is in use for half an hour only 
at a time, the battery represented in fig. 114 is better 
adapted than a sustaining one. When very powerful 
shocks are wanted, the Separable Helices (fig. 106) 
may be employed with a medium size or large cylindri- 
cal battery (<§> 23), instead of the Double Helix ; the 
Revolving Armature, seen m fig. 115, c^\x be connected 



INDUCTION OF ELECTRICITY. 



197 



with the inner coil. The shocks are stronger when one 
of the battery wires is drawn over the steel rasp than 
when the armature is included in the circuit. The 
galvanic battery may be dispensed with altogether by 
employing the Magneto-Electric Machine represented 
in fig. 101 ; the shock is regulated by the speed of the 
armature, but is never very powerful. 



III. BY THE INFLUENCE OF THE EARTH. 

352. Currents of electricity may be induced by the 
influence of terrestrial magnetism, but in consequence of 
the feebleness of the action it is not easy to render it 
sensible by the aid of wire coils alone. Deflections may, 
however, be obtained by connecting with a very delicate 
galvanometer a helix of coarse wire, such as is repre- 
sented in fig. 48, or a flat spiral, fig. 49, and having placed 
its axis in the line of the dip, suddenly inverting it. 

353. A very evident effect may be produced by 
employing the instrument represented in fig. 116. It 



A Fi^. lie. 





HHIlllllllllHliiiiilllllliltlllM 




consists of a small rod of soft 
iron wound with wire, and 
fitted to revolve on a horizon- 
tal axis, which is provided 
with a pole-changer. Upon 
the segments of the pole- 
changer press two wires con- 
nected with the cups c c. The 
instrument is the same as that 
described in <§> 178, though for 
this purpose it is an advantage 



• < 



to have several layers of wire wouad u^jovv v.\\ft vtosk, 

J7* 



198 DANIXL DAVIS^ J R^'s HANITAL. 

354. The instrument being placed in such a directioll '^ 
that the current may be reversed when the bar A B 
arrives at the line of the dip, connect the cups c (/with 
those of a delicate galvanometer. Now on causing the 
bar to revolve by hand in one direction, each end of 
the iron will become alternately a north and a south 
pole, as has been explained in ^ 205, and a current of 
electricity, whose direction changes twice in each revo- 
lution, will be induced in the surrounding wire. The 
two currents are turned into one direction by the pole- 
changer, and the needle of the galvanometer will be 
strongly and steadily deflected. By reversing the 
motion of the bar, a deflection in the opposite direction 
will be obtained. With this instrument, the current is 
slightly augmented by the feeble one excited in the 
wire coil by the direct magneto-electric induction of the 
earth. 

355. In this and all other cases where electricity pro- 
duces motion, and motion reciprocally electricity, the 
motion must be the reverse of that which would be 
produced by a galvanic current flowing in a certain 
direction, in order to cause a current in the same direc- 
tion to be induced ; the same motion as that produced 
by the battery current exciting an opposite current. A 

'similar reciprocal relation exists in the case of electricity 
and heat (see <§> 60), and of electricity and magnetism. 



c 



^ ^ 






THE ELECTROTYPE PROCESS. 



356. It has been stated in Exps. 55 and 56, that 
metallic solutions may be decomposed by the magneto- 
electric cuiTent, and the metals deposited on the negative 
wire with their proper characters. The same effect is 
produced by the galvanic current ; the precipitation of 
copper on the negative plate of the sustaining battery 
has been noticed in <§> 243. When the deposited sheet 
of copper is stripped off, it is found to have copied with 
accuracy every scratch and irregularity on the surface 
of the battery plate, 

357. The idea of applying this fact to practical pur- 
poses appears to have occurred nearly at the same time 
to Prof. Jacobi, of St. Petersburgh, and to Mr. Spencer, 
of Liverpool. Jacobi's first results were published in 
1838, and Mr. Spencer's the following year, but he 
had made some experiments as early as 1837. Th6^ 
principal uses to which the process has been applied 
are the copying of medals, engraved copper plates, , 
plaster casts, &z;c., in copper : the name of electrotypes 
is given to the copies thus obtained, and sometimes the 
process or art itself is called simply The Electrotype. 
This mode of working the metals promises to be of 
some value to the arts, though full success has as yet 
been attained with but a few o{ them. 



300 DANIXL DAYIS, J B .'s HANUAL. 

358. The readiest mode of obtaining a copy of a 
coin or medal is to make a cast of it in the fusible metal, 
which consists of eight ounces of bi^imuth, five of tin, 
and three of lead to the pound : this alloy melts at or 
near the temperature of boiling . water, A little of it 
being melted in a clean iron ladle, is poured on a flat 
board, and the oxide skimmed from its surface by a card. 
Then the medal, which may be fixed with wax to the 
end of a stick, is to be suddenly and forpibly pressed 

■ 

upon it. By one or two trials a mould may be made, 
presenting a perfect reverse of one face of the medal. 

359. A clean copper wire is then soldered to the 
projecting edge of the mould by heating it iii a lamp 
near one end, on which a little rosin should be put. 
When the wire is hot enough to melt the fusible metal, 
it is removed from the flame and its end pressed on- the 
mould, which will adhere to it. The back of the mould 
and any other part which is not intended to receive a 
deposit are to be varnished once or twice with a solution 
of shellac or sealing-wax in alcohol. This will dry in a 
few minutes, and the mould is then ready for the solution. 

360. A piece of thick rolled zinc may be soldered to 
the other end of the wire, which is bent in such a man- 
ner as to allow the mould to be immersed in a saturated 
solution of sulphate of copper, separated by^ome porous 
partition from a weak solution of sulphate of soda, in 
which the zinc is placed so as to be opposite the face 
of the mould. The solution of blue vitriol must be kept 
saturated by suspending in it a muslin bag containing 
some of the salt. 






THE ELECTROTYPE PROCESS. 201 

361. A better •mode is to connect the wire attached 
to the mould with the zinc plate of a small sustaining 
battery such as fs described in <§> 241 or 349. With 
the copper plate of the battery is connected a piece of 
copper which is to be immersed with the mould in an 
acidulated solution of blue vitriol contained in a glass or 
well -glazed earthenware vessel. No partition is used, 
but the piece of copper and the mould must not be 
allowed to touch each other. They should both be 
Connected with the battery, and the copper placed in 
the solution, before the mould is introduced ; in this way 
the chemical action, which would otherwise be exerted 
on the fusible metal, is prevented, and the deposition 
of copper commences immediately. Any air bubbles 
which adhere to the mould must be dispersed. 

362. The solution is prepared by diluting a saturated 
one of blue vitriol with one half or one third of its bulk 
of a mixture of one part of sulphuric acid with eight of 
water by measure. As the copper is deposited on the 
moiild, an equal quantity is dissolved off of the immersed 
plate, so that the original strength of the solution is 
maintained except for the loss of water by evaporation. 
The wire which connects the piece of copper with the 
battery must be defended from the solution in the satne 
manner as the back of the mould, or it will soon be- 
dissolved off. 

363. During tlje solution of the positive plate a con- 
siderable quantity of black matter is left, which would 
injure the copy if allowed to fall on the mould. It is 
therefore best to place both in a vertical position, the 
face of the mould being opposite the piece of copper. 



202 DANIEL DAYIS, J R.'s MANUAL. 

The solution must be stirred occasionally to keep its 
upper and lower parts of equal strength. If the copper 
is entirely dissolved before the deposit is sufficiently 
thick, a new piece may be soldered to the wire. 

364. When the process is going on well, the deposited 
metal will be of a very light copper color. The rapidity 
of the deposition depends greatly upon the temperature; 
the process proceeds much faster in warm weather than 
in cold, and still more so if the solution be kept hot. 
A thickness of one tenth of an inch may require from 
three days to a week for its formation, when arti6cial 
heat is not used. When a sufficient thickness has been 
attained, the copy may generally be removed from the 
mould without difficulty, care being taken to cut away 
any copper which embraces the mould at the edges. 

365. The cast will be found to be a perfectly accu- 
rate and sharp copy of the original ; its surface is- usually 
of a bright copper color, but sometimes it presents a 
brilliant silvery lustre. If it is discolored, it may be 
cleansed by immersion for a few moments in nitric acid 
and then washed with water. It may be bronzed by 
brushing it over with black lead immediately upon 
its removal from the solution, and having heated it 
moderately over a clear fire, rubbing it smartly with a 
brush, the slightest moisture being used at the same 
time, in order to remove the black lead. 

366. A mould may be formed by placing the medal 
or coin itself in the solution and depositing copper upon 
it. A fine copper wire should be passed i-ound the rim 
to connect it with the wire attached to the zinc plate of 
the battery ; and as one face only can be advantageously 



THE ELECTROTYPE PBOtESS. 203 

copied at a time, the other side should be coated with 
wax or varnish. The deposit is apt to adhere very 
firmly, sometimes so much so that its removal is impos- 
sible. This may be avoided by covering the medal 
with melted wax, and while warm wiping off the wax 
as far as possible with a cloth. Or advantage may be 
taken of the very thin film of air which adheres to bodies 
exposed to the atmosphere, by not placing the medal in 
the solution until the connections have been made with 
the battery, and the copper plate introduced. This film 
is soon removed by immersion in the liquid ; and imme- 
diately by strong nitric acid, or a solution of potash, or 
by the application of heat. 

367. The mould thus obtained may have a wire sol- 
dered to it and be placed in the solution like the^ fusible 
metal one ; but after being heated by the soldering, and 
particularly if cleaned by nitric acid, it should be ex- 
posed to the atmosphere for twenty-four hours to gain a 
film of air, or be treated with wax like the original 
medal. It is so easy to take a copy by the fusible 
metal, by white wax, &c., that a valuable medal should 
never be trusted in the solution. 

368. Every ounce of copper deposited requires the 
solution of somewhat more than an ounce of zinc from 
the zinc plate of the battery. Five or six electrotypes 
may be made at once, without increasing this expense, by 
arranging in succession several vessels, each containing 
a mould and a copper plate connected by a wire with 
the mould in the next one. The plates of copper and 
the moulds should all be nearly of the same size, and 
he solution should contain less blue vitriol and more 



204 DANIKL DAYIS, JR^'s IfANUAL. 

sulphuric acid than directed in «5> 362, particularly if the 
series extend beyond two or three. When the moulds 
are small, glass tumblers form the most convenient ves- 
sels. In this way several ounces of copper are obtained 
with but a slight increase in the quantity of blue vitriol 
required for working the battery, and a little more cor- 
rosion of the zinc plate. 

369. An engraved copper plate may be copied by 
taking an impression on clean and bright sheet lead with 
a powerful press ; or if the plate is small, it may be 
pressed by hand on the melted fusible metal. Or a 
mould may be made by depositing copper on the plate 
itself, but care must be taken to prevent adhesion both 
of the mould to the original, and of the copy to the 
mould, as directed in <§> 366 and 367. The duplicate 
thus obtained will furnish engravings which cannot be 
distinguished from those printed from the original plate, 
however elaborate the design and delicate the workman- 
ship may be. 

370. An engraving printed from an electrotype plate 
is given, as a specimen of the art, in the frontispiece to 
this Manual, in company with one from the original 
copper plate. No difference can be detected between 
the impressions except that arising from the greater or 
less quantity of ink left in the work, as occurs in different 
engravings printed from a single plate. This appears 
to be the most important application of the art yet made, 
as in cases where a large number of impressions are 
required, two or more plates have been obliged to be 
engraved, while now it is only necessary to engrave one, 
which will not be injured in the slightest degree by 



THE ELECTROTYPE PROCESS. 205 

taking copies from it. Steel plates may be copied by 
means of lead or fusible metal> but they must not them- 
selves b^ placed in the solution. 

371. Wood cuts may be copied by taking impressions 
from the blocks in the fusible metal ; this has been done 
in the present work where it was desirable to introduce 
a single instrument in one figure and afterwards to show 
it in connection with some other. Thus fig. 63 is print- 
ed from an electrotype taken from the block of fig. 99. 
This is not, however, an important application, as the 
blocks can easily be stereotyped. The electrotypes 
may thus be obtained either with the design in relief, 
like wood blocks, or in intaglio, like copper plates. 

372. Moulds are obtained from plaster medallions by 
placing them in hot water with the face upwards until 
the water (which should not be deep enough to reacb 
the face) has thoroughly penetrated the plaster in every 
part ; but none should remain on the surface. The cast 
being then removed and a slip of paper wrapped round 
the rim, melted white wax is immediately poured into 
the cup thus formed. Any air bubbles which are seea 
must be dispersed. The wax will be completely cold 
and hard in two or three hours, when it may be takei^ 
off of the cast with perfect facility, if the latter has beea 
wetted sufficiently. The medallion will not be injured 
by the process except perhaps discolored. 

373. It is now necessary to render the surface of the 
wax mould a conductor of electricity. This is done by 
giving it a coating of good black lead, which should be 
rubbed over its face with a soft brush until it acquires a 
shining black appearance ; a very thin film is sufficient. 

18 



206 DANIEL DAYIS, JuJs HANUAL. 

A copper wire is then to be heated in a lamp, and its 
end pressed upon the edge of the mould, when it will 
become imbedded in the wax. Communication between 
the wire and the face of the mould is to be ensured hj 
rubbing a little black lead on the parts around the wire. 
Great differences exist between one sample and another 
of plumbago, some being very poor conductors. Per- 
haps the best test of good black lead is its caking to- 
gether and adhering when pressed between the thumb 
and finger. 

374. The mould when thus prepared may be put into 
the solution, care being taken to remove air bubbles. 
The deposit commences upon the wire and extends 
gradually over the black leaded portions. It is better 
that the copper connected with the copper plate of the 
battery and placed in the solution should be a wire 
rather than a large piece, until the deposit has extended 
some distance over the black lead. When the copy is 
taken from the mould its surface will usually be found 
discolored, though if the layer of black lead was thin it 
may be perfectly bright. The mould may be employed 
again, if a new coating of black lead is given to it; the 
fusible metal moulds can also be used several times if 
uninjured. 

375 Seals may be copied by a very simple process. 
They are to be covered with a thin film of black lead 
rubbed on with a hard brush. If this does not adhere 
readily, the seal may be very slightly wet with alcohol, 
care being taken not to roughen the surface. A wire is 
then melted into the sealing-wax and the seal placed in 
the solution. The operation is similar in all respects to 
that required for the white wax moulds. 



THE ELECTROTYPE PROCESS. 207 

376. The copper may be deposited in three different 
states ; as a black, spongy or pulverulent mass, or in a 
crystalline form, or lastly, as a ductile and malleable 
plate. The black deposit is obtained when the quantity 
of electricity is too great in relation to the strength of 
the solution. This can be remedied in several ways ; 
as by using a weaker charge for the battery, or by in- 
creasing the proportion of blue vitriol and lessening that 
of the sulphuric acid in the solution. Or the mould 
may be removed to a greater distance from the opposed 
plate of copper, or lastly, the size of this plate may be 
diminished. 

377. The crystalline deposit is obtained when the 
quantity of electricity is too small in proportion to the 
strength of the solution. In this case, the crystals are 
minute and the copper is veiy brittle. The quantity of 
electricity which passes through the solution may be in- 
creased by adopting the opposite measures to those just 
indicated for avoiding the black deposit. 

378. Another variety of the crystalline deposit occurs 
when the quantity of electricity is large, and at the same 
time the solution is very strong and but slightly acidu- 
lated, especially if the mould is small and the opposed 
copper plate of considerable size. The deposited metal 
is then very hard and is composed of large crystals. 

379. For most purposes the metal is w^anted in a 
ductile and malleable state. To effect this, both of the 
extremes above indicated must be avoided. It is better 
that the metal should be somewhat hard and elastic 
rather than very soft and flexible. When the current 
is of proper strength, the outer surface of the deposit 



308 DANIEL DAVIS; JB.'s MANUAL. 

remains nearly smooth until it attains a considerable 
thickness, if the solution is kept of uniform strength 
throughout by stirring it up occasionally. The soft, 
flexible deposit is obtained in the greatest perfection 
when a current is maintained of such a power that hy- 
drogen is just on the point of evolution from the negative 
plate or mould ; if bubbles of the gas are seen to rise 
from it, the current is too strong, and the deposit will 
partake more or less of the spongy character. 

380. When the copper plate which is opposed to the 
mould in the solution is coated with wax, in which lines 
are drawn reaching the metal, it will be etched by the 
acid, and may afterwards be printed from like a plate 
etched in the usual way by nitric acid. The sulphuric 
acid dissolves the copper just in proportion to the quan- 
tity of electricity passing. The negative plate should 
be of the same size as the positive one, and be placed 
parallel to it in the solution. 

381. The action which takes place is as follows: the 
sulphate of copper and the water of the solution are 
both decomposed ; sulphuric acid and oxygen are deter- 
mined towards the plate connected with the positive 
pole of the battery, and oxide of copper and hydrogen 
towards the other. The oxygen and the acid combine 
with the positive copper plate, again forming blue vitriol; 
while at the negative plate, the hydrogen forms water 
with the oxygen of the oxide of copper, and the pure 
metal is deposited. 

382. The precipitation of the other metals is regu- 
lated by the same laws, but it is more difficult to obtain 
them in a useful state. Those which it is most impor* 



THE ELECTROTYPE PR0CS9S. ${09 

tant to be able'to work in this way are gold, silver^ and 
platinum. The solutions of all the noble metals are 
good conductors of electricity, and very easily decom- 
posed ; hence there is a great tendency to the evolution 
of hydrogen and the formation of a black deposit. 

383. A battery consisting of three or four pairs of 
plates of small size and very weakly charged is bes|: 
adapted for the noble metals, as the current should be 
of considerable intensity but small quantity. We have 
seen in Exp. 56 that gold is readily deposited with its 
proper characters by the magneto-electric current. 
The face of a medal may be made of gold or silver 
by depositing a thin layer of either of these metals, and 
afterwards filling up the back with copper ; but the face 
of the mould must be itself of gold or silver. A more 
important application is to cover the oxidizable metals 
with a thin and permanent coating of the noble ones. 

384. Silver, copper, and brass may be gilt by em- 
ploying a very dilute solution of the nitro-muriate of 
gold. The article should be previously clewed by 
diluted nitric acid or by a solution of potish, and ifter 
washing in water, immediately connected with the zinc 
end of the battery series and placed in the solution. Its 
immersion must be the last thing needed to complete 
the circuit, or the gold will not adhere firmly. The 
smoother and larger its surface, the more favorably the 
deposit will take place upon it. A very fine gold or 
platinum wire is to be used as a positive pole, being 
immersed to a greater or less depth in the ^solution. 
Whenever during the process the deposit becomes 

18* 



310 DANIEL DAVIS, JB.' 8 MANUAIi.' 

blackened, the negative plate should be taken out and 
rubbed with a little whiting. 

385. When the surface is completely covered with 
gold, the strength of the solution may be increased. 
The coating can be made of any desired thickness, and 
may be limited to any portion of the article, by cover- 
ing the remaining parts with wax, or varnishing them. 
Silver spoons may be gilded, after being cleaned as above, 
by pressing a wire connected with the zinc pole of the 
battery upon the handle by a small forceps and then 
immersing the rest of the spoon in the solution. In 
gilding copper, the point only of the positive wire must 
be immersed, and tlie solution must be very weak, or 
the deposited gold will be of a red color, in consequence 
of the solution of some of the copper. 

386. Silver may be deposited on copper by employ- 
ing a solution of the sulphate or acetate of silver, but it 
is difficult to prevent the formation of the black powder. 
The article should be rubbed with whiting before being 
placed in the solution, and frequently during the process. 
A very fine silver wire is used as a positive pole. 

387. Platinum may be thrown down on silver, cop- 
per, &tc., from its solution in nitro-rauriatic acid, but the 
process is difficult. The solution must be very weak, 
and the object to be coated smooth and well cleansed 
by potash. The positive pole should be a fine platinum 
wire. Any powder which may be deposited on the 
article is removed by rubbing it occasionally with 
whiting. The coating thus obtained has almost pre- 
cisely the color of polished steel. 



INDEX. 



Section. 

Ampere's Rotating Battery, - - - - 148 

Animal electricity, - ----- gX 

Arch of flame between charcoal points, - - - 32 

" ** revolution of, ----- - 167 

Armature, --------- 9 

" and magnet, action between, - - - - 110 

" Magneto-Electric, ----- 274 

Artificial magnets, - - - - - -- -2 

Astatic needles, - - - - - - - -81, 82 

Attraction of currents, shown by frictional electricity, - 219 

Attractions and repulsions of currents, . . - 213-222 

" " magnetic poles, - - 62-64 

Aurora borealis affects magnetic needle, - - 103 

Bar magnet, ---------7 

Barlow's Revolving Spur- Wheel, - - - - 156 

Batteries, compound galvanic, ----- 30-33 

" thermo-electric, ------ 53-58 

" single, how connected, ----- 29 

Battery, cylindrical galvanic, ----- 20 

" magnetic, --------8 

« sustaining, 241-243,349 

Black lead, used in the electrotype process, - - - 373 

Calori motors, -------- 17 

Carbon, kind used in thermo-electric experiments, - - 44 
Charcoal points, arch of flame between, - - - 32 
Coins and medals, mode of copying, - - - 358-368 

Cold produced by galvanic current, - - - - 59, 60 

Compound bar magnet, -------7 

" horse-shoe magnet, - - - - - 8 

• " Magnet and Electrotome, Page's, - - - 338 
Conducting power of metals, ----- 25 

Connecting wires, - - - - - - - 25,35 

.Contact Breaker, . ------- 229 



212 INDEX. 

Section. 

Copper, deposited in three different states, - - 376-379 

** plates, how copied, ----- 360, 370 

Cylindrical battery, -------20 

De la Rive's Ring, 130 

Decomposing Cell, ------- 288 

Decomposition produced by Magneto-Electric Machine, 286-293 
Deflection of galvanometer needle, - - - - 46 

Difference of shock in the arms, - - - - 284, 313 

Dip of the magnetic needle, ------ 94 

Dipping needle, - - - - - - -92,93 

" " deflected by electric current, - - 77 

Directive tendency of magnet, defined, - - - 69 

Disguised Helix, for sparks and shocks, - - - - 341 

Double « . . - - 1 . . 343 

« " Page's, 129 

" " and Electrotome, - - - - 245 

" « u Revolving Armature, - - - 348 

" Revolving Magnet, - - - - 175-177 

« Spur-Wheel, 160 

" Thermo-Electric Revolving Arch, - - 203 
" Vibrating Magic Circle, - - - - 155 

Earth, induction of electricity by the - - -. 252-254 

« ** magnetism by the - - - 205-208 

Electric current, tangential action of the - - - 78, 79 

^ currents, mutual attractions and repulsions of, 213-219 

Electricity, animal, ------- 61 

" frictional or mechanical, - - - - 12 

" galvanic or voltaic, ----- 14-35 

^ induced by movement of armature, - 272,336 
" ** " ** magnet near a wire, 267 

« a « « u a flat spiral, 268 

" " " " wire near a magnet, 269 

" " " temporary magnetism of iron, 270, 271 

" " " the magnetism of the earth, - 352, 353 

" obtained from steam, ----- 13 

Electrodes, -------- 15 

Electro-Dynamic Revolving Ring, - - - - 220 

Electro-Dynamics and Electro-Statics, phenomena of, 210-224 
^ Magnet, attracts its armature at a little distance, 134 
" " compound, ----- 188 

" « in frame, 133 

" " retains its power while armature is applied, 133 

" " sustains iron when magnetism is lost, - 327 

" " with three poles, - - - - 135 

^ Magnetic induction, definition of, - - - 116 



INDEX. 213 

Section. 

Electro-Magnetic Seasons Machine, . - - 174 

" Magnets, 131 

" " Prof. Henry's, 132 

Electro-Magnetism as a motive power, - - - 204, 337 
" " definition of, - - - - 1 

Electrotype medals, 358-368 

" " bronzed and cleansed, - - 365 

" " several made at once, - - - 368 

" " time required for making, - - 364 

" " with gold or silver faces, - - 383 

« origin of the, 356, 357 

" process, nature of the - - - - 381 

Engraved copper plates, how copied, - - - - 369, 370 

I* steel " . " " - - - - 370 

Etching by the galvanic battery, - - - - 380 

Explanations and Definitions, - - - - - 1-11 

Filings sustained by wire conveying a current, - - - 117 

Flat coil of fine wire, ------- 240 

« Spiral, - 123 

" " exerts slight magnetizing power on outside, - 124 

Fracture of magnets, ------ 115 

Frictional electricity, -------12 

Fusible metal, used in the electrotype process, - - 358 

Galvanic batteries, cylindrical, - - - - 20, 23 

*♦ " directions for using, - - - 21, 22 

** " exciting liquid used in, - - - 21 

" « compound, 30-33 

" « sustaining, - - - 241-243,349 

" current produces heat and cold, - - - 59, 60 
" " direction of, ----- 14 

" " quantity and intensity of, - - - 17-19 

« etching, 380 

Galvanometer deflected by magneto-electric current, 278, 294 
" " secondary current, - - 234, 305 

" measures quantity but not intensity, - 88 

" Upright, 86 

" with astatic needle, - - - - 87 

Galvanometers, ------- 83-88 

German silver, composition of - - - - - 45 

Gilding by the electrotype process, - - - 384, 385 

Gold leaf Electroscope affected by magneto-electricity, 325 

" Galvanoscope, ------ 153 

Heating of metallic wires by electricity, - - - 24-28 
Heliacal Ring or Magic Circle, ' " " " . 126-18 
Helix, exerts no perceptible magnetizing power on outside, 122 



214 IKBEX. 

Section' 

Helix, Magnetizing, -.---- 235 
« on stand, 120 

Horizontal Revolving Armatares, - - - - 185 

Horse-shoe magnet, -------8 

ftiduced correntB from frictional electricity, - - 264 

** " table of 963 

Induction of a current on itself, - - - - 225-233 

" electricity, -------3 

" magnetism, ------ 3 

" secondaries at a distance, - - - - 250 

Inductive action of magnet not affected by interposed bodies, 114 

Initial and terminal secondaries, . - - - . 236 

« " tertiaries, - - - - - 260 

* secondary of lower intensity than terminal, - - 239 
Instruments for illustrating the magnetism of the earth, 99, 101 
Instrument for showing the mutual action of currents, - 214 

" ^ production of cold and heat, 60 

Intensity and quantity in electricity, . - . 17, 18 

Iron, cause of its being attracted by a magnet, - - 106 

** filings, arrangement of round wire conveying current, 117 

* " " " the poles of a magnet, 73,74 
^ increases sparks and shocks, - - - . 302, 304 
** small piece of, scarcely attracted by magnet, - 65 
^ wires superior to bar in increasing sparks and shocks, 304,307 

Leyden jar charged with magneto-electricity, - . - 395 
Line of no variation, ------- 95 

Loadstones, ---------2 

Magic Circle or Heliacal Ring, - - - - 126-128 

Magnet, ---2 

« artificial, 2 

"bar, 7 

" horse-shoe orU, - -.- - - - 8 

" natural, ------- -2 

•* permanent, ------- 6 

" revolving by the earth's action, - - - - 178 

" " round a conducting wire, - - 140 

" " " its own axis, - - - - 141 

Magnetic attractions and repulsions, - - . - 62-65 
" curves, --------74 

" needle, 10 

« " half brass, 80 

" observations, ------ 104 

«* poles, --------5 

« « of the earth, 95,102 

toys, 66 



INDEX. 315 

Section. 

Magnetism, definition of ......^ 

" of the earth, theories concerning, - - 96, 97 
" induction of -,---.3 

" " by the earth, - - - 205, 206 

« " " « how aided, - - 207 

" " " magneto-electric current, 295 

Magnetism, induction of, by the secondary currents, - 234 

** probably due to electric currents, - - 218 

"^ modes of communicating, - - - 108, 208 
" " " by electro-magnets, 136,137 

" « removing, - - - -138,209 

Magnetizing Helix, ------- 235 

Magneto-Electric Apparatus for medical use, - . 343-351 
" Armature, ----- 274 

*' current, different modes of exciting, - 266 

" . " quantity and intensity of, - 296 

^ Machine for decompositions, - • 282 

" " shocks, - - - 279 

Magneto-Electricity, definition of, ----- 1 

Magnets, fracture of, - - - - - - - 115 

** modes of charging, ----- 108,208 

« « ♦* by an electro-magnet, 136, 137 

Marsh's Vibrating Wire, ------ 150 

Medals copied by the electrotype process, - - 358-368 
Medical use, apparatus for, ----- 343-351 

Metallic solutions decomposed by magneto-electricity, 292, 293 

Metals, precipitation of by galvanism, - . - - 382 

Motions produced by attraction of armature, - 182-193 

" " changing poles of electro-magnet, 168-191 

** " magnets and conductors, - - 139-167 

Natural magnets, ------- 2 

Needle, astatic, -------81, 82 

« dipping,- 92,93 

« floating, 67 

" horizontal magnetic, - - - - - 90, 91 
" magnetic, --------10 

(Ersted's experiment, ------- 75 

Optical illusions, transient duration of electric light, 158, 170, 321 
Oxygen and hydrogen obtained separately from water, - 289 

Page's Compound Magnet and Electrotome, - - - 338 

" Double Helix, - 129 

" Reciprocating Engine, - - - - - lfc9 

" Revolving Armature, ----- 182 

« " " for shocks, - - - - 333 



216 INDEX. 

Section. 
Page's Revolving Magnet, ------ 171 

•* " " as a magneto-electric machine, 297 

" " Ring, 162 

« Rotating Multiplier, 165 

Percussion, development of magnetism by - - 207, 208 
Permanent magnets, -------6 

Plaster casta copied by the electrotype, - - 372-374 
Platinating by the electrotype process, - - - - 3c7 

Plating by the electrotype, - - - - - 386* 

Platinum, a poor conductor of electricity, - - - 25 

Pole-changer, Dr. Page's, ------ 162 

Poles of a galvanic battery, - - - - - 14, 15 

" ** magnet, - - - - - - -4,5 

" " " situated near its extremities, - - 113 

" magnetic, of the earth, - - - - - 95, 102 

Powder Cup, --------26 

Primary magneto-electric current, effects produced by, 285-295 
Properties of the magnet, little known till recently, - 11, 89 

Quantity and intensity in electricity, - - - - 17, 18 
** " of the magneto-electric current, - 296 

** « a thermo-electric current, - 52 

Reciprocating Bell Engine, ------ 192 

Repulsion of successive portions of current, - - 223 

Revolution of arch of electric flame, - - - - 167 

Revolving Armature, - - - - - - - 182 

" " for shocks, 333 

« Cylinder, 145 

" Disc, - - - 161 

" Rectangle, 164 

" Ring, 162 

« " and Magnet, ----- 166 

" " Electro-dynamic, 220 

« Spur-Wheel, ------ 156 

« Wire Frame, 143 

Ritchie's Revolving Magnet, ----- 168 
Rolling Armature, - - -- - - - -68 

Rotating Battery, - - 148 

« Bell Engine, - - - - - , - - 172 

« Multiplier, - ' - 165 

Rule for determining polarity produced by straight current, 1 19 

" " " « current in helix, 121 

Salts decomposed by the Magneto-Electric Machine, 291, 292 
Seals copied by the electrotype, ----- 375 

Secondary currents, ------ 234-256 

" " from a primary intensity current, - 248 



INDEX. 217 

Section. 

Secondary currents from wire helices, .... 244 

" " induced at a distance, - ' - - 250 

" " " in different positions of coils, 251 

" « produced by motion, - - 254, 255 

Separable Helices, - - -"- - - - 301 

« « and Electrotome, - - - - 318 

" « « Revolving Armature, - - - 329 

Shock from galvanic battery due to a secondary current, 249 

" primary coil, ------ 303 

" " magneto-electric current, - 277, 285 

Shock given to several persons at once, - - - - 312 

*' strength of, depends upon extent of contaQt, 310, 311 
« " regulated, - - - 250,251,304,346 

*' strongest from coil surrounding middle of helix, 316 

'* cannot be obtained from helix in an iron tube, - 328 
" difference of in arms, - - - - 284, 313 

** from secondary of fine wire coil, - - - - 238 

" passed through any part of the body, - - 347 

Shocks produce numbness when very rapid, - - - 330 

" produced by movement of armature, - - - 335 

" taken from water, ------ 314 

Silvering by the electrotype process, - - - - 386 

Spark from long wire and helix, - - - - 225, 226 

" " secondary helix, - - - " . - 322 
" and shock from flat spiral, - - - - 228, 231 

fine wire coil, with compound battery, 232 
long wires and spin 's, cause of, - 233 
Thermo-Electric Battery, - - 326 
wire coils increased by iron, 302, 304 
reduction of - - - 252,253,304-308. 
why reduced, - - - 25C, 261, 305, 306 

Star Plate, Ill 

Sturgeon's Revolving Disc, ----- 161 

Successive induction of magnetism, - - - - 112 

Sustaining Battery, - 241, 349 

*« " action within, 242 

" " precipitation of copper in, - - 243 

Tangential action of electric current, - - - 78, 79 
Terminal secondary of higher intensity than initial, - 239 

Tertiary currents, - 257-260 

" " obtained from wire helices, - » - 259 

Therrao-Electric Arch rotating between poles of U magnet, 200 

« Batteries, - - - - - 53-58 

" " sparks and shocks from, - 326 

** combinations, - - - - 43-^1 

19 



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818 IITBKX* 

Section. 

Thenno-Electric combinations, table o^ - - - 47 

** current, cause of, - - - - - 39 

«* «* direction of, - - . . 42 

* ** excited in a single metal, - 37, 38 

« ** quantity and intensity o^ - 53 

^ ** reversed in some cases, - 48-^1 

" Revolving Arcb, - - - - 194 

" a tt on U magnet^ - - 197 

« « Wire Frames, - - - 199 

Thermo-Electricity, discovery of, - - - - - 36 

U magnet, ----.-.. 8 

Upright Reciprocating Engine, ... - 190, 191 

Variation of the magnetic needle, .... 93 

u u u u how found, - - - 100 

Vibrating Magic Circle, ------ 154 

" Wire, 150 

Voltaic batteries, 20-34 

« Gas Pistol, -27 

« electricity, 14-35 

Water decomposed by the magneto-electric current, ^3, 324 
" " •• Magneto-Electric Machine, 286-290 

White wax used in the electrotype process, - - - 372 

Wire conveying current does not attract light bodies, - 224 
" for conveying electric currents, - - - - 35 

Wood cuts copied by the electrotype, - - - - 371 

Y Armature, ---Ill 



DANIEL DAVIS, Jr. 

MANUFACTtmSS AND KEEPS FOR SALE 

Instrnments to illnstrate the Prfneiplea of 

GALVANISM, ELECTRO-MAGNETISM, 

ELECTRO-DYNAMICS, MAGNETO-ELECTRICITY, 

MAGNETISM, THERMO-ELECTRICITY. 

ICT' IMPROVED MEDICAL APPARATUS. 
ELECTROTYPE APPARATUS MADE TO ORDER. 



Figure.* Price. 

1. Case of Bar Maarnets, from $2.50 to 5.00 

2. Horse-slioe or V JUag^nety .12^0 .60 

3. Compound Horte-shpe Maenety - • $3to 10.00 

4. Jllaji^iietlc Needle on brass stana, 75 to . 1.00 

5. Rod for collecting Electricity from the Steam Enginj, - 5.00 

6. Plates of zinc and copper for elenientary experi- 

ments, .2510 .75 

7. Cylindrical Battery, - small, $2, medium, $5, large, 7.00 

8. Powder Cup, -..-:... from .25 to .50 

9. Voltaic Gas Pistol, from 1.60 to 3.00 

10. Galvanic Battery, 2.5 pairs of double plates, > - - $20 to 25.00 

Galvanic Battery, 100 pairs of double plates, - - • . 85.00 

12. Tliern»0"Ejlectric pairs, of various metals^ • - from .12} to .50 

13. Galvanometers, for Therms- Electric and other experiments, 3 to 8.00 

14. Tliermo-Electrio Battery, 10 pairs, - > 1. to 2.00 

15. « <* *« 60 large plates, - - - $15. to 25.00 

16. Apparatus for the production of lieat and cold by 

tlie galvanic current, 3.00 

17. Bar DIaguct and Magnetic Needle, 1.00 

19. Oblong and circular pieces of iron, for Exp. 5, • .12 
Set of Ma{;netie Toys, swans, ships, fishes, &c., - - • 2.00 

20. Rolling Armature and Magnet, - - • $3to 6.00 

21. Bar 9Iagnet and iron bar, 1.26 

2i, Iron Filings, for experiments, per box, .26 

27. Astatic Needle, on stand, from 1 to 2.00 

29. Galvanometer, for (Ersted's experiments, • • - 3.50 to 5,00 

30. *• on tripod stand, with leveling screws, - - - 6.00 

31. UprigHt Galvanometer, on tripod stand, - -from 5 to 6.00 

33. Dipping Needle, with universal joint, 3 to 6.00 

31. Terrestrial Globe, with bar magnet within it, - - - 3 to 5.00 

35. Instrument, f(>r explaining variation and theory of magneti8m,4 to 8.00 

37. Terrestrial Globe, withcoil, needle, &c., - - ■ - 3 to 6.00 

41. Y Armature, .50 

42. Circular Iron plate, .50 to .76 

43. Star Plate, - .50 to .76 

46. Bar Magnets, for breaking, &c. each, .10 

47. Galvanic Battery, largess ize, $5 to 7.00 

4S. Helix, on stand, with iron rod, from 1 to 3.00 

* The figures in the first column refer to DaTis's Manual of Magnetism. 



Price. 
49. Flat Spiral, 1.60 to 10.00 

60. Heliaoal Ring and ArmatureSy . . c . 2.50 to 3.00 

61. <• •< « *< bail and socket joints, $5 to 8^ 

62. De la Rlve*s Rlna^f 100 

63. Small Elcciro-Maf^eUy • .60 tn liO 

64. Kleeiro-Masn^ty in frame, 95, 910, and 30,00 

65. •« " with three poles, 3.00 

66. « « foi charging ma^ets, • • • from .75 to liO 

67. Magnet revolving round conducting wire, • - • brass frame, 6J0O 

68. «» •' •♦ its own axis, .... from #5 to 6.00 

69. Double Re'rolvlnar "Wire Fraufce^brass stand,leyeling screws, 8.00 

60. « « Cylinders, .< <« . - 5 to 6.00 

61. « Rotating Batteries on steel magnet, 6 to 8.00 

62. Marsli's Vibrating 'Wire, without magnet, 1 50, with magnet, 6 00 

63. Gold leaf Gal vanosoopey brass stand, • 4 to 6.00 

64. Vibrating Magle Circle, 4 to 6.00 

65. Double Vibrating Magie Circle, .... 5 to /7.00 

66. Barlow*s Rerolvlnfr Spnr-^Wbeel 3.50 to 6XX) 

67. Pagers Rerolrlng Ring, from 5 to 6.00 

69. Revolving Rectangle, 5 to 6.00 

70. •< Ring and Magnet, 8 to 10.00 

Rltcliie*e Revolving <• see S16S, - 4 to 6.00 

71. Page's •< " 6 to 6.00 

72. Rotating Bell Engine, • 10 to 12.00 

73. Blectro-.tlagnetlo Seasons Machilnc, • 12 to 15.00 

74. Double Revolving Magnet, 6 to 10.00 

76. Magnet revulvin!! by the Earth's action. • • • - 3 to 6.00 

77. Page's Revolving Armature, 3 to 4.00 

78. Horizontal do. Armatures, > • 4 to 6.00 

79. Page's Reciprocating JBIngine, 10 to 12.00 

80. VpHgbt do. do 10 to 20.00 

81. do. do. do* round base, ... 15.00 

82. Reciprocating Bell Bnslne, IS to 20.00 

83. Tliermo-£lectrlc Revolving Axclkf . 1.50 to 2.00 

84. ** « " " on U magnet, 3 to 4.00 

85. Double Therm o-Electrie IVire Frames, 6 to 8,00 

86. Tliermo-JBlectric Arob rotating between poles of U magnet, 3 to 5.00 

87. Double « Revolving Arch, 4 to 5.00 

88. Bar of soft Iron and snuOl Magnetic Needle, • 1 to 2.00 

89. Steel Bar, iinmagnetized, .25 to .60 

91. Movable Wires, for showing attraction and repulsion, • 3 to 4.00 

92. Electro- Dynamic Revolving Ring, ... 6 to 8.00 

93. Klectrotome, or Contact Breal(er, 5 to 8.00 

94. Magnetizing Helix, for induced currents, .... 3.00 

95. Flat Spiral and rasp, 5 to 8.00 

96. Double Helix and Electrotonte, .... $20 to 25.00 

97. Fine IVire Spiral, 8 to 10.00 

99. Gold Leaf Galvanoscope, with magnet and coil, - > 9.00 

100. Magneto-JBllectric Armature, witliout magnet, $6, with do., 12.00 

101. « •< Machine for shoclcs, .... 30.00 

102. « " « for decompositions, from #30 to 35.00 

103. Decomposing Cell, 3 to 5.00 

104. U Tube for deciMnposiiions, .50 to 1.00 

105. Pagers Revolving Magnet and Galvanometer, 9.00 

106. Separable Helices, with handles for shoclcs, • . - $12 to 18.00 
103. *< Helices and Electrotome, - 20 to 25.00 

109. « «< and Revolving Armature, • 15 to 20.00 

110. Fage*s Revolving Armature for shocks, - • 5 to 6.00 

111. ** Compound Magnet and Electrotome, • 8 to 12.00 

112. Disguised Helix for sparKs and shocks, • 6 to 8.00 

114. Magneto-lSlertric Apparatus for medical iiite. • from S 10 to 12.00 

115. Magneto-Klectrio Apparatus for medical use, -^vith 

Page s Revolving Armature, 15.00 

Fusible metal, for the eiecuotyi>e, per lb. .65 



s 

in