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

See other formats

Google

This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project

to make the world's books discoverable online.

It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject

to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books

are our gateways to the past, representing a wealth of history, culture and knowledge that's often difficult to discover.

Marks, notations and other maiginalia present in the original volume will appear in this file - a reminder of this book's long journey from the

publisher to a library and finally to you.

Usage guidelines

Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the
public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing tliis resource, we liave taken steps to
prevent abuse by commercial parties, including placing technical restrictions on automated querying.
We also ask that you:

+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for
personal, non-commercial purposes.

+ Refrain fivm automated querying Do not send automated queries of any sort to Google's system: If you are conducting research on machine
translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the
use of public domain materials for these purposes and may be able to help.

+ Maintain attributionTht GoogXt "watermark" you see on each file is essential for in forming people about this project and helping them find
additional materials through Google Book Search. Please do not remove it.

+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just
because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other
countries. Whether a book is still in copyright varies from country to country, and we can't offer guidance on whether any specific use of
any specific book is allowed. Please do not assume that a book's appearance in Google Book Search means it can be used in any manner
anywhere in the world. Copyright infringement liabili^ can be quite severe.

About Google Book Search

Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers
discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web

at |http: //books .google .com/I

T .

").^-:

.:. u •. w. -.M^ y!^ U-* •. 4>.t,».' » > .■

« '

V- \..

. • J. • ^ . •

• *.^,.

%> • .*A.

/ » / .

A-^,

^.^X^.U^vi/t'

1-5^ . fiu^ ^f^i,-

ft_c6c.Oi

ffHl-V

£.7«

^o^c^

D^o^

DAVIS'S MANUAL

MAGNETISM.

..iviviYiyiy^viV

♦ ♦♦♦♦♦♦♦I

The frontispiece consists of two copper-plate engn^arings, one printed from an
engrared plate, and tlie other from an electrotype copy taken from it in the mode
described on page 204. The engravings are introduced for the purpose of showing
the accuracy of the copies obtained by this process.

I "

•^*-. >

Oi{l41I>\\l,.

KLKCTUOTVI'K <'(ll'\'.

0.„„./'y.„,, y,.

"^vvA.1>9TrM'»\

'"-^

D A V I S'S

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.

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

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.

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.

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.

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

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

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

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

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.

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,

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

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.

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.

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,

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;

fTBF

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

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-

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

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.

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

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-

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

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>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.

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

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*

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.

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

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.

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.

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

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.

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

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

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-

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.

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

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

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.

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

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

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,.

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.

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-

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

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

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.

^ ^

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.

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

((

« .

((

redi

xvlii

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

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

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

See other formats