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B M ES3 7Et 
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LIBRARY 

OF TIIK 
UNIVERSITY OF CALIFORNIA. 

GIKT OK 

Mrs. SARAH P. WALSWORTH. 

Received October, 1894. 
Accessions No.S+fif-l, 5~~" Class M). ... 

4 




The frontispiece consists of two copper-plate engravings, 
one printed from an engraved plate, and the 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. 



D A V I S'S 

MANUAL OE MAGNETISM. 

INCLUDING ALSO 

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



WITH A DESCRIPTION OF THE ELECTROTYPE PROCESS. 

FOR THE USE OF STUDENTS AND LITERARY INSTITUTIONS. 

WITH 100 ORIGINAL ILLUSTRATIONS,  (free download in WWW)





BOSTON: 

PUBLISHED BY DANIEL DAVIS, J 

MAGNETICAL INSTRUMENT MAKER, 
No. 11 Cornhill. 

1842. 






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. DAMHELL, 

PRINTER, 
NO. 11 CORNHILL. 






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 manufactured 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. 



VI 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. 



BOSTON, AUGUST, 1842. 



CONTENTS, 



INTRODUCTORY CHAPTER. 

Page 



DEFINITIONS AND EXPLANATIONS, 



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, 52 



Vlll CONTENTS. 

CHAPTER II. 

INDUCTION OF MAGNETISM. 

Page. 
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 INFLUENCE 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. 
1 



2 DANIEL DAVIS, JR.'s MANUAL. 

ARTIFICIAL MAGNETS. Bodies of whatever form or 
composition, in which magnetism is artificially induced, 
are called artificial 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 considerable 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 be 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. 3 

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 explained 
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 acquires 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. 

1. BAR MAGNET. An artificial permanent magnet in 
the form of a straight bar, is called a bar magnet. 

lg ' ' n Fig.l represents a 

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




DANIEL DAVIS, J R. S MANUAL. 



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

COMPOUND BAR MAGNET. A magnet composed of 
several straight bars joined together, side by side, with, 
their similar poles in contact, for the purpose of increasing 
the magnetic power, is called a compound bar magnet. 

- 2. 8. HORSE-SHOE OR U MAGNET. 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 a horse-shoe 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. 

COMPOUND HORSE-SHOE 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 horse-shoe magnet or magnetic 
battery. These magnets are charged 
separately, and are put together with all 
the similar poles in the same direction. 
9. ARMATURE. A piece of soft iron, adapted to, and 
intended to connect the poles of a magnet, is called an 
armature, or keeper. Horse-shoe magnets are usually 
provided with an armature, consisting of a straight bar 
of iron, for the purpose of preserving their magnetic 




EXPL ANATI ONS . 5 

power : this should be kept constantly 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. 



Fig. 




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 
their power to attract iron, and their tendency, when 
freely 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 small pieces of iron seems to have 
been known from the remotest ajitiquity ; but its polarity 
with regard to the earth does not appear to have been 
observed until the eleventh or twelfth century of the 
Christian era. 



1* 



DANIEL DAVIS. J R/S MANUAL. 



PRODUCTION OF ELECTRICITY. 

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 frictional, 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 magnetical effects. 
Mechanical electricity is also developed, though not in 
so striking a manner, 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 from 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 

Fig. 5. 
H a B 



I 



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 OF ELECTRICITY. i 

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 G. 

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 as to issue from the 
valve as a transparent vapor. 

II. 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 powerfully 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 powerful 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 
Fig. 6.^ the zinc, through the liquid between 
them ; as is shown in the adjoining 
cut, fig. 6, which represents a vessel 
f some non-conducting substance, 
as gl ass ? P art ly 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 within the series, and from 
copper to zinc without it. C is therefore called the 
positive or delivering pole of the arrangement, and Z 
the negative 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 from 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 



PRODUCTION OF ELECTRICITY. 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, from 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, 

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 increased 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 
fractional 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, it 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 intensity, 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 current of electricity de- 
pends upon an increase of its quantity, mainly. Increas- 
ing 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 single 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- 
,fcd Q, p 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 




ty 



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, according 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 r 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 dissolving 
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 o 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 little gas is 
given off during the action of a battery charged by sul- 
phate of copper, as the hydrogen, which usually escapes, 
is 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 after 
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 



PRODUCTION OF ELECTRICITY. 15 

of wire, a battery of a considerable 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 
intensity 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, copper 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, burns with beautiful scin- 
tillations. The shorter and finer the wire, within certain limits, 
the greater is the effect produced. 




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 into the cup C, and then connecting W 
and W' with the poles of the battery, the platinum will 
become heated, 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 

*ffi ^=*=, & passes up through a brass 

piece which screws into 
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. This object is 
effected in the following manner : Connect 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 muzzle 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 




PRODUCTION OF ELECTRICITY. 17 

is the best proportion. If too much hydrogen is in- 
troduced, no explosion will occur ; it is 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 from 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 (<> 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 

2* 




18 DANIEL DAVIS, 

cases, open only at top and bottom ; each zinc plate 
10> being insulated from 

the surrounding copper 
by slips of wood at the 
edges, and connected 
by a strip of copper 
soldered to it, with the 
case belonging to the 
next pair. The whole series is firmly fixed in a wooden 
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, containing the exciting 
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 copper : 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 are 
small hand-vices, connected with the poles, for the pur- 
pose of holding wires, &tc. 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 this 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 ; 



PRODUCTION OF ELECTRICITY. 19 
Fig. 11. 




each having a separate trough to contain the acid. The 
plates are stationary, 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 platform 
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. 



20 DANIEL DAVIS, J R.'s MANUAL. 

33. In the batteries described in <> 30 and 31 in which 
the plates are fixed permanently 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 is put, so that one solution 
comes in contact with the copper, and the other with 
the zinc plate, the battery is called a sustaining 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, in 
addition, covered with varnish. This is sufficient 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 connections are made by means of 
mercury cups, as they then become amalgamated when 
dipped into the mercury, and thus form a perfect metallic 
contact. 



III. THERMO-ELECTRICITY. 

36. The term Thermo- Electricity expresses the de- 
velopment of electricity by the agency of heat. It was 
discovered by Prof. Seebeck, of Berlin, in 1822, that 
if the junction of two dissimilar metals was heated, an 
electrical current would flow from one to the other. 
Thus, if the ends of two wires, or strips of German silver 
and brass are made to touch each other, or are brazed 
together, and the junction heated, a current will flow 
from the German silver to the brass, if the free extremi- 
ties of the wires are connected by any conductor of 
electricity, and an electrical circuit will be established, 
as the galvanic circuit is established by connecting the 
Fig. 12. 
G 




B 

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

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



22 DANIEL DAVIS, J R.'s M A N U A L. 

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, a'nd 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- 
ever, have not yet been fully 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. 
Some of the peculiarities in the combinations of the more 
useful metals are given in <> 43. It is necessary, however, 
to say a few words with regard to the galvanometer, an 
instrument to indicate or measure electrical currents, 
and which is more fully 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 will be frequently 



24 DANIEL DAVIS, J R.' S 31 A N U A L. 

alluded to hereafter. In the figure, a thermo-electric 
. pair, of bismuth and antimony, heated by a spirit lamp, 
is shown in connection with the galvanometer. The 
arrows indicate the course of the 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 current 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. 



PRODUCTION OF ELECTRICITY. 25 

43. German Silver and Antimony. 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 metals 
have been heretofore generally used in large thermo- 
electric arrangements. The current excited by heating 
their junctions is greater than from many other metals, 
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 to 
the antimony. 

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

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

3 



26 DANIEL DAVIS, JR.'s MANUAL. 

value in thermo-electric combinations has only recently 
been observed. It will be used in many of the thermo- 
electric instruments, to be hereafter described. German 
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, or Iron. In these combinations, 
and also with antimony, the carbon is positive, the cur- 
rent being rather feeble. 

46. The deflections given in the following table admit 
of comparison with each other to a considerable extent, 
though not so strictly as if wires of the same size 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 far- 
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 80, but considerably less. Nor 
can momentary deflections be compared with permanent 
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 deflection. 

47. The wires were not soldered together, but their 
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 fusibility. 
The object was to produce the greatest current that 
could easily be obtained from each combination. It 



PRODUCTION OF ELECTRICITY 



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



CURRENT FLOWS THROUGH HEATED JUNCTION. 
From positive. To negative. 



DEFLECTION OF 
THE NEEDLE. 



German Silver,. ...... 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

German Silver, 

Silver, 

Bismuth, 

Bismuth, 

Bismuth, 

Bismuth, 

Bismuth, 

Platinum, 

Carbon, 



Antimony, 

Silver, 

Brass, 

Iron, 

Palladium, 

Copper, 

Cadmium,. 

Zinc, 

Platinum, 

Carbon, 

Antimony, 

Antimony, 

Silver, 

Palladium, 

Carbon, 

German Silver, 

Carbon, 



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



Antimony, 

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 heating 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 is, however, often uncertain, and 
the few experiments which have been made, afford no 
explanation of the cause of the changes. 



28 DANIEL DAVIS, 

49. Iron and Platinum. When heat is 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 iron, 
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 is applied 
to the copper near it. 

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

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

Zinc and Iron. Current moderate, and on heating 
the zinc near the junction to its melting point, changes 
its direction. 

51. Platinum and Silver. Deflection 70. On heat- 
ing the platinum a strong current flows in the opposite 
direction. 

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

52. In quantity, the thermo-electric current much 
resembles a feeble galvanic current. In intensity, it is 



PRODUCTION OF ELECTRICITY. 29 

somewhat 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, from which it proceeded, 
without passing through the fluid between the plates, 
which is a poor conductor. It is, therefore, partially, 
though very imperfectly, insulated. In a thermo-electric 
pair, the electricity is set in motion from 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 effort to 
return to equilibrium. It is probably for this reason 
that the intensity of thermo-electricity is less than that 
of galvanism. 

EXP. 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 
numbers 100 and 14. 

53. In soldering the wires or plates together, they 
are not usually connected in a straight line, but at an 
acute angle with each other. If several of these single 
pairs be associated together consecutively, that is, by 
connecting the German silver of the one to the brass of 
the next, or the bismuth of one to the antimony of the 
next, and so on, we have a thermo-electric battery, in 
3* 



30 DANIEL DAVIS, J R.'s MANUAL. 

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

Fig.U. 




54. Fig. 14 represents a series, consisting of eleven 
pairs of German silver and brass wire, arranged in 
two rows, one behind the other. When several pairs 
are connected in this 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 heating the junctions of the 
wires on one side of the series with a spirit lamp, a cur- 
rent is produced which increases or diminishes as the heat 
is applied, depending altogether for its existence on the 
difference of temperature in the opposite junctions of 
the wires. By grasping the junctions on one side in the 



PRODUCTION OF ELECTRICITY. 31 

fingers, even the warmth of the hand produces a sensible 
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 
inch 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 
junctions 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 mixture of snow or pounded ice with half its weight of 
common 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 ; difference 
of heat at the different ends, however produced, being 



32 DANIEL DAVIS, 

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 II, sect. 2, 
by which the magnetizing power of the current is shown. 
The ends of the coil of insulated wire C being fixed in 
the cups, the current is obliged to traverse the coil, 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 in sect. 55. 

58. By forming a bundle or small battery, consisting 



PRODUCTION OF ELECTRICITY. 33 

of many pairs of wires, the slightest increase of heat at 
one end produces a sensible current of electricity. This 
forms an instrument for measuring heat far more delicate 
than any other which has been contrived. It has been 
used in ascertaining 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 

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 x ; 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 arrows, from the bismuth 
B', through the antimony, to the other bar of bismuth, 




34 DANIEL DAVIS, J R.'s MANUAL. 

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 reversing 
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 distinct from the heating 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 application of heat 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 gyrn- 
notus, or electrical eel, inhabiting the fresh waters of 
South America, and the silurus electricus, living in the 
rivers of Africa, have been celebrated for their powers 
of producing 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 attracting 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. 

",'.';. ".". :'-'" ">.;'; ! -'','""' ,~'"-'- r *"; V" ' 

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

T\C S the south pole. Then bring near 
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 repelled by its south pole. 




36 DANIEL DAVIS, J R.'s MANUAL. 

63. The intensity of the attraction or repulsion exerted 
between two magnetic poles, varies in the inverse ratio 
of the square of their distance ; that is, if the distance of 
the poles is doubled, the force with which they attract or 
repel each other is reduced to one quarter of its previous 
amount ; if their distance is 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 magnetism. 

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- 
NJJ duction,aswill 
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 off 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 this, that it is only that part of a frag- 
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 length in proportion to its breadth, the end 
which is repelled will be at such a distance from the 
influence of the magnet that its repulsion will be over- 
powered by the attraction of the extremity which is near 
it. If, however, the fragment is very short, so that the 




DIRECTIVE TENDENCY OF MAGNET. 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. 

EXP. 5. Let M (fig. 19) be the south pole of a bar or horse- 
shoe magnet, and A a piece of sheet iron, somewhat smaller 
than the end of the magnet. When this 
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 
"1 P 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 
surfaces, 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 
repulsions, described in 62 such as swans, ships, fishes, 
4 




38 DANIEL DAVIS, J R.'s MANUAL. 

and other figures, with magnets concealed within them, 
and intended to float upon the water. When thus 
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 very 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 direction, 
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. 

68. ROLLING ARMATURE. This apparatus consists 
of a compound horse-shoe magnet and an armature con- 
sisting of an iron wire whose length is a little 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. 



DIRECTIVE 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 from B to A, at pleasure. 

69. It results from 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 
Fig. 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,as has already been ex- 
plained. Both these forces 
will conspire to retain the 
body in the direction rep- 
T resented in the drawing; 

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







40 



DANIEL DAVIS, J R.'s MANUAL. 



account of the inclined position of the bar, the attractive force 
between the south extremity of the mag-net and the north ex- 
tremity >f 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 direc- 
tive tendency. 

70. This effect 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 

pole: for example; near- 
ly opposite the centre of 
the magnet, as in fig. 22, 
where M represents the 
magnet as before. Now 
in 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 with a 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. 
Fig. 23. 




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- 




42 DANIEL DAVIS, JR.'s MANUAL. 

tions and repulsions of the magnetical poles, may be 
also rendered sensible in a very satisfactory manner by 
the following experiment. 

EXP. 7. Spread a thin covering of iron filings or ferruginous 
Fig. 24. sand over a sheet of paper, and 

place a powerful horse-shoe 
magnet vertically beneath it, 
with the poles very near to the 
paper. The dotted lines in the 
cut (fig. 24) show the arrange- 
ment which the particles of 
iron will assume. Each one 
becomes a magnet with its two 
poles, and connects itself with those adjoining it so as to form 
curved lines of a peculiar character. This experiment may be 
performed in a still more satisfactory manner, by supporting the 
paper, with the magnet in contact with its under surface, and 
then showering down iron sand or iron filings from a sand-box 
held some inches above. The particles of iron, as they strike 
the paper can thus more readily assume the positions to which 
they tend under the magnetic influence. 

74. The lines formed by the filings afford a good ex- 
perimental illustration of what are called magnetic curves, 
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 attrac- 
tive force exerted upon them by the magnet, being the 
difference of its action upon the two poles of each 
particle, is exceedingly slight ; while the directive force 
is very considerable. 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 TENDENCY 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. 8. Let N S, fig. 25, 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 TO:r --*- 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, the needle N S will 
be deflected and will turn towards the position aborcd, according 
to the direction of the current of positive electricity, whether from 




44 DANIEL DAVIS, JR.'s MANUAL. 

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. The 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 



DIRECTIVE 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 y 
occur ; but they are all referable to a force acting ta* 
gentially upon the magnetic poles, and in a plan^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). 
Fig. 26. Now suppose the north pole of 

a magnet N to be brought near 
the wire, and to be perpen- 
dicular to any point C. If free 

> ^ to move, the pole will revolve 

c jy around C as a centre in the di- 

Af\> rection indicated by the arrows 
in the cut ; that is, in the same 
direction as the hands of a 
watch, when its face is 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 pote 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 in a similar manner, and in the same 



f 



46 DANIEL DAVIS, 

directions. Thus, a wire conveying a descending 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 motion 
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 this 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 
upon 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 
solely by the action upon the other pole ; no rotation, 
however, can be obtained. The object of the instru- 
ment is to show the directive tendency of a single pole 
with reference to the electrical current. 

81. ASTATIC NEEDLE. A needle so contrived that 
its directive tendency in respect to the earth is neutral- 
ized, so that it shall remain at rest in any position, is 
called an astatic needle. It is constructed as represented 
in the following cut, fig. 27, consisting essentially of two 
needles, one above the other, placed in positions the 
reverse of each other in respect to their poles. Such a 



DIRECTIVE TENDENCY OF 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. 
Fig. 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 sup- 
ported upon an agate cup. These 
needles need not be perfectly astatic, 
nor is it easy to make them so. 



48 



DANIEL DAVIS, JR. S MANUAL. 




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 from 
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 beloiv the needle, 
will produce an effect 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. 



DIRECTIVE TENDENCY OF MAGNET. 49 

The electric current flows, it is true, in a contrary direc- 
tion, below the needle, but then it is on the opposite side 
of it, and therefore the effect produced by the lower 
portion 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 unaffected ; 
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 on GALVANOMETER. In- 
struments of a variety of forms are constructed on the 
above principles, and are called galvanoscopes or gal- 
vanometers, as they serve to indicate the presence of a 
current of electricity and in some degree to measure its 
quantity. If the wire is carried many times around the 
needle, as in fig. 30, the power of the instrument is much 
increased, as each turn of the wire adds its influence ; 
provided the wire is not so long or of so small a size as 

5 



50 



DANIEL DAVIS, J R. S MANUAL. 



to be unable to convey the whole of the current. The 
instrument thus becomes a delicate test of the presence 
of a current of electricity. The coil of wire is supported 
Fig. 30. 




Fig. 31. 



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

86. UPRIGHT GALVANOMETER. 
In this instrument, represented in 
fig. 31, both the coil of wire and the 
needle are placed in a vertical po- 
sition, the north pole being made a 
little heavier, in order to keep the 
magnet perpendicular. When a cur- 
rent is passed through the coil, the 
deflection is towards a horizontal po- 
sition. The needle is made of large 
size, for the purpose of exhibiting 
the deflections before an audience. 




DIRECTIVE TENDENCY OF MAGNET. 51 

87. GALVANOMETER WITH ASTATIC NEEDLE. This 
instrument is similar in construction to the preceding, 
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 affected. 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 from 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 difference 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 
pair, and the current is strongly indicated by the galva- 



52 DANIEL DAVIS, J R.'s MANUAL. 

nometer. The amount 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. 

III. 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 with 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 in the 
second century or earlier. 




DIRECTIVE TENDENCY OF MAGNET. 53 



S Fig. 33. 



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

r 

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 from 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 dipping needle. 

93. Fig. 33 represents a dipping needle whose mode 
of suspension allows of its turning freely in any direction. 
It is fixed by means of a universal joint to a brass cap 
containing an agate, which rests upon the pivot. The 
usual arrangement allows only of motion in a vertical 
plane, the needle having an axis passing through its 
middle at right angles to its length, which axis is sup- 
ported horizontally. The small needles shown in fig. 34 
are suspended in this manner. Sometimes a vertical 
graduated circle is added, to measure the angle which 
the needle makes with the horizon. In using a needle 
5* 




54 



DANIEL DAVIS, JR.S MANUAL. 



whose motion is confined to a single plane, it must be so 
placed that this plane may be directed north and south, 
coinciding with the plane of the magnetic meridian. A 
dipping needle, before being magnetized, should be as 
equally balanced as possible, so as to remain at rest in 
any direction 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 

latitudes the directions 
exhibited in the annexed 
diagram, fig. 34, where 
the point of the arrow 
indicates the north pole 

i] < Jcui^d the Bather the 
r\ ^^^^l^rl south pole of the needles 

placed around the globe. 
The angle which the 
needle makes with the 
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 ends. 

95. In fig. 34, M represents the North American 
magnetic pole near S the north pole of the earth. The 
line L V is 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. 




DIRECTIVE TENDENCY OF MAGNET. 55 

96. By comparing 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 in sect. 72, 
it will be found that there is a great analogy between 
them. This analogy led to the opinion, which 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 forms 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 different 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 position. 



56 DANIEL DAVIS, 

Then magnetize it, by touching it with any 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 that its 
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 
pole 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 extremity of 
the needle used, reverse its poles, as described under the last ex- 
periment, 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 by 
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 




DIRECTIVE TENDENCY OF MAGNET. 57 

magnet, n s, is placed 
in the magnetic axis of 
the earth, not coinciding 
exactly with the axis of 
rotation, N S. A small 
^magnetic needle placed at 
fB *B on the magnetic meri- 
dian, wili 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 s are not fastened 
together, but only fixed on one axis. This allows their 
poles to be separated a little, to imitate more closely the 
distribution of terrestrial magnetism : 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 from 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 



DANIEL, DAVIS, JR. S MANUAL. 




whose true position at the time is known. It is imme- 
diately obtained by comparing the direction of the needle 
V- 36. w ith the north star 

when it crosses the 
meridian or by cal- 
culation when the 
north star is at its 
E greatest elongation. 
An observation of 
the sun, however, 
is usually preferred. 
The latitude of a 
place A (fig. 36)being known, the exact bearing of the 
sun S, east or west, can be obtained by calculation,* for 
any given moment of time 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 time 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 bearing 
and the true bearing, represented by the angle M A N, 
is the variation of the needle, and equals 9 30' .f 

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 axis. N S is merely a wooden axis to 
the globe. When a galvanic current is sent through the 

* See Bowditch's Navigator. 

t The present variation at Boston is 9 deg. 30 min. west. The westerly variation 
appears to be increasing. The present dip is 74 deg. 20 min. north. 



DIRECTIVE TENDENCY OF MAGNET. 59 

coil of wire about the equatorial regions, small needles 

placed in different situations will arrange themselves as 

Fig. 37. 




they would in similar terrestrial latitudes. By compar- 
ing this figure with fig. 35, representing the globe with 
the included magnet, a comparison 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 
both, and those assumed by it on the earth's surface are 
very striking. 

102. It will be observed that, in fig. 35, the south 
pole of the included magnet is represented at the north 
geographical pole of the earth. So also, in fig. 37, the 
wooden rod N S, passed through the axis of the globe, 
shows the direction of the polarity induced by the cur- 
rent to be 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 DAVIS, JR.'s MANUAL. 

considered as flowing 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