Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 23 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 23 of 45)
Font size
QR-code for this ebook

the positive fluid expressed by m, and retaining on the side
next to B an equal quantity of the negative fluid. Now, this
negative fluid m acting on the natural electricity of B at the
same distance will produce a proportionate decomposition, and
will develope on the side of B next to A an additional quantity
of the positive fluid, just so much less than m as m is less than
1. This quantity will therefore be m x m, or m*.

This quantity m 2 of positive fluid again acting by induc-
tion on A, will develope, as before, a quantity of negative
fluid expressed by m 2 x m, or m 3 . And in the same manner


this will develope on B an additional quantity of positive fluid
expressed by m 3 x m, or m 4 . These inductive reactions being
indefinitely repeated, let the total quantity of positive electricity
developed on B be expressed by P, and the total quantity of
negative electricity developed on A by N, we shall have

&c. ad inf.
&c. ad inf.

Each of these is a geometrical series ; and, since m is less than
1, they are decreasing series. Now, it is proved in arithmetic,
that although the number of terms in such series be unlimited,
their sum is finite, and that the sum of the unlimited number

of terms composing the first series is ^ ^ an( ^ tnat f tne

second -. We shall therefore have

In this case we have supposed the original charge of the
conductor B to be the unit. If it consist of the number of
units expressed by E, we shall have

It follows, therefore, that the original charge E of the con-
ductor B has been augmented in the ratio of 1 to 1 m' 2 by the
proximity of the conductor A.

The less is the distance between the conductors A and B, the
more nearly m will be equal to 1, and therefore the greater
will be the ratio of 1 to 1 m 2 , and consequently the greater
will be the augmentation of the electrical charge of B produced
by the presence of A.

For example, suppose that A be brought so near B, that the
positive fluid on B will develope nine-tenths of its own quantity
of negative fluid on A. In that case m=^=0'9. Hence it
appears, that 1 m 2 =l 0-81=0-19 ; and, consequently, the
charge of B will be augmented in the ratio of 0' 19 to 1, or of
19 to 100.

1746. Principle of the condenser. In such cases the elec-
tricity is said to be CONDENSED on the conductor B by the in-


ductive action of the conductor A, and apparatus constructed for
producing this effect are called CONDENSERS.

1747. Dissimulated or latent electricity. The electricity
developed in such cases on the conductor A is subject to the
anomalous condition of being incapable of passing away, though
a conductor be applied to it. In fact, the conductor A in the
preceding experiment is supposed to be connected with the
earth by conducting matter, such as a chain, metallic column, or
wire. Yet the charge of electricity N does not pass to the earth
as it would immediately do if the conductor B were removed.

In like manner, all that portion of the positive fluid p which
is developed on B by the inductive action of A, is held there by
the influence of A, and cannot escape even if the conductors be
applied in contact with it.

Electricity thus developed upon conductors and retained there
by the inductive action of other conductors, is said to be latent
or dissimulated. It can always be set free by the removal of
the conductors by whose induction it is dissimulated.

1748. Free electricity. Electricity, therefore, which is
developed independently of induction, or which, being first
developed by induction, is afterwards liberated from the in-
ductive action, is distinguished as free electricity.

In the process above described, that part of the charge P of
the conductor B which is expressed by E, and which was im-
parted to B before the approach of the conductor A, is free, and
continues to be free after the approach of E. If a conductor
connected with the earth be brought into contact with B, this
electricity E will escape by it ; but all the remaining charge of
B will remain, so long as the conductor A is maintained in its

If, however, E be discharged from B, the charge which
remains will not be capable of retaining in the dissimulated
state so great a quantity of negative fluid on A as before. A
part will be accordingly set free, and if A be maintained in con-
nection with the ground it will escape. If A be insulated, it
will be charged with it still, but in a free state.

If this free electricity be discharged from A, the remaining
charge will not be capable of retaining in the latent state so
large a quantity of positive fluid on B as previously, and a part
of what was dissimulated will accordingly be set free, and may
be discharged.


In this manner, by alternate discharges from the one and the
other conductor, the dissimulated charges may be gradually
liberated and dismissed, without removing the conductors from
one another or suspending their inductive action.

1749. Forms of condensers. Condensers are constructed in
various forms, according to the strength of the electric charges
they are intended to receive. Those which are designed for
strong charges require to have the two conductors separated
by a non-conducting medium of some considerable thickness,
since, otherwise, the attraction of the opposite fluids diffused on
A and B would take effect ; and they would rush to each other
across the separating space, breaking their way through the
insulating medium which divides them. In this case, the
distance between A and B being considerable, the condensing
power will not be great, nor is it necessary to be so, since the
charges of electricity are by the supposition not small or feeble.

In case of feeble charges, the space separating the conductors
may be proportionally small, and, consequently, the condensing
power will be greater.

Condensers are usually constructed with two equal circular
plates, either of solid metal or having a metallic coating.

1750. Collecting and condensing plates. The plate corre-
sponding to the conductor A in the preceding paragraphs is
called the CONDENSING PLATE, and that which corresponds to B
the COLLECTING PLATE. The collecting plate is put in com-
munication with the body whose electrical state it is required
to examine by the agency of the condenser, and the condensing
plate is put in communication with the ground.

1751. Cuthberfsons condenser. A form of condenser con-
trived by Cuthbertson is represented \nfig. 491. The collecting
plate B is supported on a glass pillar, and communicates by a

chain attached to the hook D with the source
of electricity under examination. The con-
// densing plate A is supported on a brass pillar,
movable on a hinge, and communicating
with the ground. By means of the hinge the
disk A may be moved to or from B. The
space between the plates in this case may be
merely air, or, if strong charges are used, a
plate of glass may be interposed.
Fig. 491. When used for feeble charges, it is usual to


cover the condensing plate with a thin coating of varnished
silk, or simply with a coating of resinous varnish. An instru-
ment thus arranged is represented in fig. 492., where b b f , the
condensing plate, is a disk of wood coated
with varnished silk tt'. The collecting
plate c c' has a glass handle m, by which
it may be raised, and a rod of metal a b
by which it may be put in communication
with the source of electricity under ex-

The condensing plate in this case has generally sufficient
conducting power when formed of wood, but may be also made
of metal, and, instead of varnished silk, it may be coated with
gum-lac, resin, or any other insulator.

When the plate cc' has received its accumulated charge, its
connection with the source of electricity is broken by removing
the rod ab ; and the plate cc' being raised from the condensing
plate, the entire charge upon it becomes free, and may be sub-
mitted to any electroscopic test.

1752. The electrophorous. A small charge of free elec-
tricity may by the agency of induction be made to produce a
charge of indefinite amount, which may be imparted to any in-
sulated conductor. This is effected by the electrophorous, an
instrument consisting of a circular cake, composed of a mixture
of shell-lac, resin, and Venice turpentine,
cast in a tin mould AB, fig. 493. Upon
this is laid a circular metallic disk c, rather
less in diameter than AB, having a glass

As* 3B Before applying the disk c, the resinous

Fig. 493. surface is electrified negatively by striking

it several times with the fur of a cat. The disk c being then
applied to the cake AB, and the finger being at the same time
pressed upon the disk c to establish a communication with the
ground through the body of the operator, a decomposition takes
place by the inductive action of the negative fluid on the resin.
The negative fluid escapes from the disk c through the body
of the operator to the ground, and a positive charge remains,
which is prevented from passing to the resin partly by the thin
film of air which will always remain between them even when


the plate C rests upon the resin, and partly by the non-conduct-
ing virtue of the resin.

When the disk c is thus charged with positive electricity kept
latent on it by the influence of the negative fluid on AB, the
finger being previously removed from the disk c, let it be raised
from the resin and the electricity upon it, before dissimulated,
will become free, and may be imparted to any insulated con-
ductor adapted to receive it.

The charge of negative electricity remaining undiminished on
the resin AB, the operation may be indefinitely repeated ; so that
an insulated conductor may then be charged to any extent, by
giving to it the electric fluid drop by drop thus evolved on the
disk c by the inductive action of A B.

This is the origin of the name of the apparatus.



1753. General principle of electroscopes. Electroscopes in
general consist of two light conducting bodies freely suspended,
which hang vertically and in contact, in their natural state.
When electricity is imparted to them they repel each other,, the
angle of their divergence being greater or less according to the
intensity of the electricity diffused on them. These electroscopic
substances may be charged with electricity either by direct com-
munication with the electrified body, in which case their elec-
tricity will be similar to that of the body ; or they may be acted
upon inductively by the body under examination, in which case
their electricity may be either similar or different from that of
the body, according to the position in which the body is pre-
sented to them. In some cases, the electroscope consists of a
single light conductor to which electricity of a known species is
first imparted, and which will be attracted or repelled by the
body under examination when presented to it, according as
the electricities are like or unlike.

These instruments vary infinitely in form, arrangement, mode


of application, and sensitiveness, according to the circumstances
under which they are placed, and the intensities of the electri-
cities of which they are expected to detect the presence, measure
the intensity, or indicate the quality. In electroscopes, as in all
other instruments of physical inquiry, the most delicate and
sensitive is only the most advantageous in those cases in which
much delicacy and precision are required. A razor would be
an ineffectual instrument for felling timber.

1754. Pith ball electroscope. One of the most simple and
generally useful electroscopic instruments is the pendulous pith
ball already mentioned (1697), the action of which may now be
more fully explained. When an electrified body is presented
to such a ball suspended by a silken thread, it acts by induc-
tion upon it, decomposing its natural fluid, attracting the consti-
tuent of the contrary name to the side of the ball nearest to it,
and repelling the fluid of the same name to the side most remote
from it. The body will thus act at once by attraction and re-
pulsion upon the two fluids ; but since that of a contrary name
which it attracts is nearer to it than that of the same name
which it repels, and equal in quantity, the attraction will prevail
over the repulsion, and the ball will move towards the electri-
fied body. "When it touches it, the fluid of a contrary name, which
is diffused round the point of contact, combining with the fluid
diffused upon the body, will be neutralized, and the ball will re-
main charged with the fluid of the same name as that with
which the body is electrified, and will consequently be repelled
by it. Hence it will be understood why, as already mentioned,
the pith ball in its neutral state is first attracted to an electrified
body, and after contact with it repelled by it.

1755. The needle electroscope. The electric needle is an
electroscopic apparatus, somewhat less simple, but more sensitive
than the pendulum. It consists of a rod of copper terminated

^ by two metallic balls B and B', jig. 494.,

B/ which are formed hollow in order to render
them more light and sensitive. At the
middle point of the rod which connects them
is a conical cup, formed of steel or agate,
suspended upon a fine point, so that the
Fig. 494. needle is exactly balanced, and capable of

turning freely round the point of support in a horizontal plane,



like a magnetic needle. A very feeble electrical action ex-
erted upon either of the balls B or B' will be sufficient to put
the needle in motion.

1 756. Coulomb's electroscope. The electroscope of Coulomb,
better known as the balance of torsion, is an apparatus still
more sensitive and delicate for indicating the existence and in-
tensity of electrical force. A needle gg', fig. 495., formed of
gum-lac, is suspended by a fibre of raw silk/ At one extremity
it carries a small disk e, coated with metallic foil, and is so
balanced at the point of suspension, that the needle resting

horizontally is free to turn in either direction
round the point of suspension. When it turns,
it produces a degree of torsion or twist of
the fibre which suspends it, the reaction of
which measures the force which turns the
needle. The thread is fixed at the top to a
small windlass t, by which the needle can be
raised or lowered, and the whole is included
in a glass cage, to preserve the apparatus
from the disturbance of the air. Upon this
glass cage, which is cylindrical, is a graduated
circle d d', which measures the angle through
which the needle is deflected. In the cover of
the cage an aperture o is made, through which may be intro-
duced the electrified body whose force it is desired to indicate
and measure by the apparatus.

1757. Quadrant electrometer. This instrument, which is

generally used as an indicator on the conductors
of electrical machines, consists of a pillar AB,
fig. 496., of any conducting substance, termi-
nated at the lower extremity by a ball B. A rod,
also a conductor, of about half the length, termi-
nated by a small pith ball D, plays on a centre C
in a vertical plane, having behind it an ivory
semicircle graduated. When the ball B is
charged with electricity, it repels the pith ball D,
Fig. 496. and the angle of repulsion measured on the gra-
duated arc supplies a rough estimate of the intensity of the

1758. Gold leaf electroscope. A glass cylinder ABC D, fig.

Fig. 495.



Fig. 497.

497., is connected to a brass stand E, and closed at
the top by a circular plate AB. The brass top G is
connected by a metallic rod with two slips of gold
leafy, two or three inches in length, and half an inch
in breadth. In their natural state they hang in
contact, but when electricity is imparted to the plate
G, the leaves becoming charged with it indicate its
presence, and in some degree its intensity, by their
divergence. On the sides of the glass cylinder
opposite the gold leaves are attached strips of tin-
foil, communicating with the ground. When the
leaves diverge so much as to touch the sides of the
cylinder, they give up their electricity to the tinfoil, and are
discharged. This instrument may also be affected inductively.
If the electrified body be brought near to the plate G, its natural
electricity will be decomposed ; the fluid of the same name as
that with which the body is charged will be repelled, will
accumulate in the gold leaves, and will cause them to diverge.

1759. Condensing electroscope. The condenser applied to
the electroscope supplies an instrument which has the same
fl analogy to the common electroscope as the com-

pound has to the simple microscope. An electro-
scope with such an appendage is represented in
fig. 498. The condenser is screwed on the top,
the condensing plate communicating with the
electroscope, and the collecting plate being laid
over it. When the collecting plate is put into
communication with the source of electricity to be
examined, a charge is produced by induction in
Fig. 498. ^0 con( j ens j n g plate under it, and a charge of
a contrary name is collected in the electroscope, the leaves of
which will diverge in this case with an electricity similar in
name to that of the body under examination.

In the use of instruments of such extreme sensitiveness, many
precautions are necessary to guard against disturbances, which
would interfere with their indications, and expose the observer to
errors. The plates of the condenser in some experiments may
be exposed to chemical action, which, as will hereafter appear,
is always combined with the development of electricity. In such
eases, the condenser of the electroscope should be composed of


gilt plates. The apparatus is sometimes included in a glass
case, to protect it from atmospheric vicissitudes; and to preserve
it from hygrometric effects, a cup of quicklime is placed in the
case to absorb the humidity. The plates of the condenser at-
tached to electroscopes vary from four to ten inches in diameter.
When greater dimensions are given to them, it is difficult to
make them with such precision as to ensure the exact contact of
their surfaces.

Becquerel used plates of glass twenty inches in diameter, accu-
rately ground together with emery, and coated with thin tin-
foil. This apparatus had great sensibility, but as the metal was
very oxidable, the results were disturbed by chemical effects not
easily avoided. A coating of platinum or gold would have been
more free from disturbing action.



THE inductive principle which has supplied the means in
the case of the condenser of detecting and examining quan-
tities of electricity so minute and so feeble as to escape all
common tests, has placed, in the Leyden jar, an instrument
at the disposal of the electrician by which artificial electricity
may be accumulated in quantities so unlimited as to enable him
to copy in some of its most conspicuous effects the lightning of
the clouds.

To understand the principle of the Leyden jar, which at one
time excited the astonishment of all Europe, it is only neces-
sary to investigate the effect of a condenser of considerable mag-
nitude placed in connection, not with feeble, but with energetic
sources of electricity, such as the prime conductor of an electrical
machine. In such case it would be evidently necessary that the
collecting and condensing plates should be separated by a non-
conducting medium of sufficient resistance to prevent the union
of the powerful charges with which they would be invested.

Let A B, fig. 499., represent the collecting plate of such a con-


denser, connected by a chain K
with the conductor E of an electric
machine ; and let A' B' be the con-
densing plate connected by a chain
K' with the ground. Let c D be a
plate of glass interposed between
A' B' and A B.

Let e express the quantity of
electricity with which a superficial
unit of the conductor E is charged.
It follows that e will also express

the free electricity on every super-

Fi-.~499 ficial unit of the collecting plate

A B ; and if the total charge on each
superficial unit of A B, free and dissimulated, be expressed by a,
we shall, according to what has been already explained, have

The charge on the superficial unit of the condensing plate
A' B' being expressed by a', we shall have

1-w 2

which will be wholly dissimulated.

If s express the common magnitude of the two plates A' B'
and A B, and E express the entire quantity of electricity accu-
mulated on A B, and E' that accumulated on A 7 B', we shall have

1-m 2

It is evident, therefore, that the quantity of electricity with
which the plates A B and A' B' will be charged, will be aug-
mented, firstly, with the magnitude (s) of the plates ; secondly,
with the intensity (e) of the electricity produced by the machine
upon the conductor E ; and thirdly, with the thinness of the glass
plate c D which separates the plates A' B' and A B. The thinner
this plate is, the more nearly equal to 1 will be the number m,
and consequently the less will be 1 m 2 , and the greater the
quantity E.


When the machine has been worked until e ceases to increase,
the charge of the plates will have attained its maximum. Let
the chains K and K' be then removed, so that the plates A B and
A' B' shall be insulated, being charged with the quantities of
electricity of contrary names expressed by E and E'.

If a metallic wire w, or any other conductor, be now placed
so as to connect the plate A B with the plate A' B', the free elec-
tricity on the former passing along the conductor w will flow to
the plate A' B', where it will combine with or neutralize a part
of the dissimulated fluid. This last being thus diminished in
quantity, will retain by its attraction a less quantity of the fluid
on A B, a corresponding quantity of which will be liberated, and
will therefore pass along the conductor w to the plate A' B',
where it will neutralize another portion of the dissimulated fluid;
and this process of reciprocal neutralization, liberation, and con-
duction will go on until the entire charge E' upon the plate A' B'
has been neutralized by a corresponding part of the fluid E ori-
ginally diffused on the plate AB.

Although these effects are strictly progressive, they are prac-
tically instantaneous. The current of free electricity flows
through the conductor w, neutralizes the charge E', and liberates
all the dissimulated part of E in an interval so small as to be
quite inappreciable. In whatever point of view the power of
conduction may be regarded, a sudden and violent change in
the electrical condition of the conductor w must attend the phe-
nomenon. If the conductor w be regarded merely as a channel
of communication, a sort of pipe or conduit through which the
electric fluid passes from A B to A' B', as some consider it, so
large an afflux of electricity may be expected to be attended
with some violent effects. If, on the other hand, the opposite
fluids are reduced to their natural state, by decomposing suc-
cessively the natural electricity of the parts of the conductor w,
and taking from the elements of the decomposed fluid the elec-
tricities necessary to satisfy their respective attractions, a still
more powerful effect may be anticipated from so great and
sudden a change.

Such phenomena are accordingly found to be attended with
some of the most remarkable effects presented in the whole
domain of physical research. If the charge E be sufficiently
strong, and the intermediate conductor w be thin metallic wire,
it will be instantly rendered incandescent, and may even be


fused. If the human body be made the conducting medium,
however inconsiderable the charge may be, an effect is produced
on the nerves which is to most persons extremely disagreeable,
and if the charge be considerable, it may even have the effect
of destroying animal life.

In order to divest these principles of whatever is adventitious,
and to bring their general character more clearly into view, we
have here presented them in a form somewhat different from
that in which they are commonly exhibited in electrical experi-
ments. The phenomenon which has just been explained, consist-
ing merely in the communication of powerful charges of elec-

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 23 of 45)