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ing them.

If a conductor be terminated, not by sharp angular edges,
but by rounded sides or ends, then the distribution will become
more uniform. Thus, if a cylindrical conductor of considerable
diameter have hemispherical ends, the distribution of the elec-
tricity upon it will be nearly uniform ; but if its ends be flat,
with sharp angular edges, then an accumulation of the fluid
will be produced contiguous to them. If the sides of a flat
plate of sufficient thickness be rounded, the accumulation of
fluid at the edges will be diminished.

The depth of the fluid is still more augmented at corners
where the increase of depth due to two or more edges meet
and are combined ; and this effect is pushed to its extreme
limit if any part of a conductor have the form of a POINT.

The pressure of the surrounding air being the chief, if not
the only force, which retains the electric fluid on a conductor,
it is evident that if at the edges, corners, or angular points,
the depth be so much increased that the elasticity of the fluid
exceeds the restraining pressure of the atmosphere, the elec-
tricity must escape, and in that case will issue from the edge,
corner, or point, exactly as a liquid under strong pressure
would issue from &jet d'eau.

1 1 77. Experimental illustration of the effect of a point. Let
T,fig> 509., be a metallic point attached to a conductor c, and let



LAWS OF ELECTRICAL FORCES.



239



1




Fig. 509.

perpendicular n"p.
of the atmosphere



the perpendicular n express the
thickness or density of the electric

i fluid at that place ; this thickness
will increase in approaching the
point P, so as to be represented
by perpendiculars drawn from
the respective points of the curve
n, n', n" to AF, so that its density
at P will be expressed by the
Experience shows that in ordinary states
very moderate charge of electricity given
to the conductor c will produce such a density of the electric
fluid at the point P as to overcome the pressure of the atmo-
sphere, and to cause the spontaneous discharge of the electricity.
The following experiments will serve to illustrate this escape
of electricity from points.

Let a metallic point, such as AP,Jig 509., be attached to a
conductor, and let a metallic ball of two or three inches in dia-
meter, having a hole in it corresponding to the point P, be stuck
upon the point. If the conductor be now electrified, the elec-
tricity will be diffused over it, and over the ball which has
been stuck upon the point P. The electric state of the con-
ductor may be shown by a quadrant electrometer being attached
to it. Let the ball now be drawn off the point P by a silk
thread attached to it for the purpose, and let it be held sus-
pended by that thread. The electricity of the conductor c will
now escape by the point P, as will be indicated by the electro-
meter, but the ball suspended by the silk thread will be elec-
trified as before.

1778. dotation produced by the reaction of points. Let two
wires AB and cv,Jig. 510., placed at right angles, be supported
by a cap E upon a fine point at the top of an
insulating stand, and let them communicate by
a chain F with a conductor kept constantly elec-
trified by a machine. Let each of the four
arms of the wires be terminated by a point in a
horizontal direction at right angles to the wire,
each point being turned in the same direction,
as represented in the figure. When the elec-
tricity comes from the conductor to the wires,
Fig. 5io. it will escape from the wires at these four points




240



ELECTRICITY.




Fig. 511.



respectively ; and the force with which it leaves them will be
attended with a proportionate recoil, which will cause the wire
to spin rapidly on the centre E.

1779. Another experimental illustration of this principle.
An apparatus supplying another illustration of this principle is

represented \nfig. 511. : a square
wooden stand T has four rods of
glass inserted in its corners, the
rods at one end being less in
height than those at the other.
The tops of these rods having
metal wires A B and c D stretched
between them, across these wires
another wire E F is placed, having
attached to it at right angles another wire G H, having two
points turned in opposite directions at its extremities, so that
when G H is horizontal these two points shall be vertical, one
being presented upwards, and the other downwards. A chain
from A communicates with a conductor kept constantly elec-
trified by a machine.

The electricity coming from the conductor by the chain,
passes along the system of wires, and escapes at the points G
and H. The consequent recoil causes the wire G H to revolve
round E F as an axis, and thereby causes E F to roll up the in-
clined plane.

1780. Electrical orrery. An apparatus called the electrical

orrery is represented in fig- 512. A
metallic ball A rests upon an insu-
lating stand by means of cap within
it, placed upon a fine metallic point
forming the top of the stand.

From the ball A an arm D A pro-
ceeds, the extremity of which is
turned up at E, and formed into a
fine point.

A small ball B rests by means of
a cap on this point, and attached to
it are two arms extended in opposite directions, one terminated
with a small ball c, and the other by a point P presented in the
horizontal direction at right angles to the arm. Another point
p', attached at right angles to the arm D A, is likewise presented




Fig. 512.



MECHANICAL EFFECTS OF ELECTRICITY. 241

in the horizontal direction. By this arrangement the ball A
together with the arm D A is capable of revolving round the
insulating stand, by which motion the ball B will be carried in
a circle round the ball A. The ball B is also capable at the same
time of revolving tin the point which supports it, by which
motion the ball C will revolve round the ball B in a circle. If
electricity be supplied by the chain to the apparatus, the balls A
and B and the metallic rods will be electrified, and the electricity
will escape at the points r and P'. The recoil produced by this
escape will cause the rod D A to revolve round the insulating
pillar, and at the same time the rod P c together with the ball
B to revolve on the extremity of the arm D A. Thus, while the
ball B revolves in a circular orbit round the ball A, the ball C
revolves in a smaller circle round the ball B, the motion re-
sembling that of the moon and earth with respect to the sun.



CHAP. IX.

MECHANICAL EFFECTS OF ELECTRICITY.

1781. Attractions and repulsions of electrified bodies. If a
body charged with electricity be placed near another body, it
will impress upon such body certain motions, which will vary
according as the body thus affected is a conductor or a non-con-
ductor ; according as it is in its natural state or charged with
electricity; and in fine, if charged with
electricity, according as the electricity
is similar or opposite to that with which
the body acting upon it is charged.

Let &,f,g. 513., be the body charged
with electricity, which we shall sup-
pose to be a metallic ball supported on
an insulating column. Let B be the
body upon which it acts, which we
shall suppose to be a small ball sus-
pended by a fine silken thread. We
shall consider successively the cases
above mentioned.

1782. Action of an electrified body




Fig. 513.



242 ELECTRICITY.

on a non-conductor not electrified. 1. Let B be a non-conductor
in its natural state.

In this case no motion will be impressed on B. The elec-
tricity with which A is charged will act by attraction and re-
pulsion on the two opposite fluids which compose the natural
electricity of B, attracting each molecule of one by exactly the
same force as it repels the molecule of the other. No decom-
position of the fluid will take place, because the insulating
property of B will prevent any motion of the fluids upon it,
and will therefore prevent their separation. Each compound
molecule therefore being at once attracted and repelled by equal
forces, no motion will take place.

1783. Action of an electrified body on a non-conductor
charged with like electricity. 2. Let B be charged with elec-
tricity similar to that with which A is charged.

In this case B will be repelled from A. For, according to
what has been explained above, the forces exerted on the natural
electricity of B will be in equilibrium, but the electricity of A
will repel the similar electricity with which B is charged; and
since this fluid cannot move upon the surface of B because of
its insulating virtue, and cannot quit the surface because of the
restraining pressure of the surrounding air, it must adhere
to the surface, and, being repelled by the electricity of A, must
carry with it the ball B in the direction of such repulsion. The
ball B therefore will incline from A, and will rest in such a
position that its weight will balance the repulsive force.

1784. Its action on a non-conductor charged with opposite
electricity. 3. Let B be charged with electricity opposite to
that with which A is charged.

In this case B will be attracted towards A, the distribution of
the fluid upon it not being changed, for the same reasons as in
the last case.

1785. Its action on a conductor not electrified. 4. Let B
be a conductor in its natural state.

In this case the action of the fluid on A attracting one con-
stituent of the natural electricity of B, and repelling the other,
will tend to decompose and separate them ; and since the con-
ducting virtue of B leaves free play to the movement of the
fluids upon it, this attraction and repulsion will take effect, the
attracted fluid moving to the side of B nearest to A, and the
repelled fluid to the opposite side.



MECHANICAL EFFECTS OF ELECTRICITY. 243

To render the explanation more clear, let us suppose that A is
charged with positive electricity.

In that case, the negative fluid of B will accumulate on the
side next A, and the positive fluid on the opposite side. The
negative fluid will therefore be nearer to A than the positive
fluid ; and since the force of the attraction and repulsion in-
creases as theequare of the distance is diminished (1771), and
since the quantity of the negative fluid on the side next A is
equal to the quantity of positive fluid on the opposite side, the
attraction exerted on the former will be greater than the repul-
sion exerted on the latter; and since the fluids are prevented
from leaving B by the restraining pressure of the air, the fluids
carrying with them the ball B will be moved towards A and
will rest in equilibrium, when the inclination of the string is
such that the weight of B balances and neutralizes the attractioa.

If A were charged with negative electricity, the same effects
would be produced, the only difference being that, in that case,
the positive fluid on B would accumulate on the side next A, and
the negative fluid on the opposite side.

Thus it appears that a conducting body in its natural state
is alway attracted by an electrified body, with whichever species
of electricity it be charged.

1786. Its action upon a conductor charged with like electri-
city. 5. Let B be a conductor charged with electricity similar
to that with which A is charged.

In this case the effect produced on B will depend on the re-
lative strength of the charges of electricity of A and B.

The electricity of A will repel the free electricity of B, and
cause it to accumulate on the side of B most remote from A.
But it will also decompose the natural electricity of B, attracting
the fluid of the contrary kind to the side near A, and repelling
the fluid of the same kind to the opposite side. It will follow
from this, that the quantity of the fluid of the same name accu-
mulated at the opposite side of B will be greater than the quan-
tity of fluid of the contrary name collected at the side near A.
While, therefore, the latter is more attracted than the former, by
reason of its greater proximity, it is less attracted by reason of
its lesser quantity. If these opposite effects neutralize each
other, if it lose as much force by its inferior quantity as it
gains by its greater proximity, the attractions and repulsions
of A on B will neutralize each other, and the ball B will not

M 2



244 ELECTRICITY.

move. But if the quantity of electricity with which B is charged
be so small that more attraction is gained by proximity than is
lost by quantity, then the ball B will move towards A. If, how-
ever, the quantity of electricity with which B is charged be so
great that the effect prevail over that of distance, the ball B
will be repelled.

It follows, therefore, from this, that in order* to ensure the
repulsion of the ball B in this case, the charge of electricity
must be so strong as to prevail over that attraction which
would operate on the ball B if it were in its natural state. A very
small electrical charge is, however, generally sufficient for this.

1787. Its action upon a conductor charged icith opposite
electricity. 6. Let B be charged with electricity of a contrary
name to that with which A is charged.

In this case B will always be attracted towards A, for the at-
traction exerted on the fluid with which it is charged will be
added to that which would be exerted on it if it were in its
natural state.

The free electricity on B will be attracted to the side next A,
and the natural fluid will be decomposed, the fluid of the same
name accumulating on the side most remote from A, and the fluid
of the contrary name collecting on the side nearest to A, and
there uniting with the free fluid with which B is charged.
There is therefore a greater quantity of fluid of the contrary
name on that side, than of the same name on the opposite side.
The attraction of the former prevails over the repulsion of the
latter therefore at once by greater quantity and greater proxi-
mity, and is consequently effective.

1788. Attractions and repulsions of pith balls explained.
What has been explained above will render more clearly under-
stood the attractions and repulsions manifested by pith balls
before and after their contact with electrified bodies (1697).
Before contact, the balls, being in their natural state, and being
composed of a conducting material, are always attracted, what-
ever be the electricity with which the body to which they are
presented is charged (1785); but after contact, being charged
with the like electricity, they are repelled (1786).

When touched by the hand, or any conductor which com-
municates with the ground, they are discharged and restored to
their natural state, when they will be again attracted.

If they be suspended by wire or any other conducting thread,



MECHANICAL EFFECTS OF ELECTRICITY. 245

and the stand be a conductor communicating with the ground,
they will lose their electricity the moment they receive it,

The electric fluid in passing through bodies, especially if they
be imperfect conductors, or if the space they present to the fluid
bear a small proportion to its quantity, produces various and
remarkable mechanical effects, displacing the conductors some-
times with great violence.

1789. Strong electric charges rupture imperfect conductors.
Card pierced by discharge of jar. The current of elec-
tricity discharged from a Ley den jar will penetrate several
leaves of paper or card.

A method of exhibiting this effect is represented in fig. 514.
The chain A communicates with the outside coating
of the jar. The card c is placed in such a position
that two metallic points touch it on opposite sides,
terminating near each other. The pillar G, being
glass, intercepts the electricity. The ball of the dis-
charger being put in communication with the inside
coating of the jar, is brought into contact with the
ball B, so that the two points which are on opposite
sides of the card, being in connection with the two
coatings of the jar, are charged with contrary fluids,
which exert on each other such an attraction that they
rush to each other, penetrating the card, which is
Fig. 514. f oun( j } n t ki s case pi erce (j by a hole larger than that
produced by a common pin.

It is remarkable that the burr produced on the surface of the
card is in this case convex on both sides, as if the matter pro-
ducing the hole, instead of passing through the card from one
side to the other, had either issued from the middle of its thick-
ness, emerging at each surface, or as if there were two distinct
prevailing substances passing in contrary directions, each
elevating the edges of the orifice in issuing from it.

The accordance of this effect with the hypothesis of two
fluids is apparent.

1790. Curious fact observed by M. Tr emery. A fact has
been noticed by M. Tremery for which no explanation has
yet been given. That observer found that when the two
points on opposite sides of the card are placed at a certain
distance, one above the other, the hole will not be midway
between them. When the experiment is made in the

M 3




246



ELECTRICITY.



atmosphere, the hole will always be nearer to the negative
fluid. When the apparatus is placed under the receiver of an
air-pump, the hole approaches the positive fluid as the rare-
faction proceeds.

If several cards be placed between the knobs of the universal
discharger (1744), they may be pierced by a strong charge of
ajar or battery, having more than one square foot of coated
surface.

1791. Wood and glass broken by discharge. A rod of wood
half an inch thick may be split by a strong charge transmitted
in the direction of its fibres, and other imperfect conductors
pierced in the same manner.

If a leaf of writing-paper be placed on the stage of the dis-
charger, the electricity passed through it will tear it.

The charge of a jar will penetrate glass. An apparatus for
exhibiting this effect is shown in Jig. 515. It may also be
exhibited by transmitting the charge through the side of a
phial, fig. 516.

A strong charge passed through water scatters the liquid in
all directions around the points of discharge, fig. 517.

1792. Electrical bells. The alternate attraction and re-
pulsion of electrified conductors is prettily illustrated by the
electrical bells.





Fig. 517.

AB and CD, Jig. 518., are two metal rods supported on a glass
pillar. From the ends of these rods four bells A' B' c' D' are



MECHANICAL EFFECTS OF ELECTRICITY. 247

suspended by metallic chains. A central bell G is supported on
the wooden stand which sustains the glass pillar EF, and this
central bell communicates by a chain GK with the ground.
From the transverse rods are also suspended, by silken threads,
four small brass balls H. The transverse rods being put in
communication with the conductor of an electrical machine,
the four bells A' B' c' D' become charged with electricity.
They attract and then repel the balls H, which when repelled
strike the bell G, to which they give up the electricity they
received by contact with the bells A'B'C'D', and this electricity
passes to the ground by the chain G. The bells will thus
continue to be tolled as long as any electricity is supplied by
the conductor to the bells A' B' c' D'.

1793. Repulsion of electrified threads. Let a skein of linen
thread be tied in a knot at each end, and let one end of it be
attached to some part of the conductor of the machine. When
the machine is worked the threads will become electrified and
will repel each other, so that the skein will swell out into a
form resembling the meridians drawn upon a globe.

1 794. Curious effect of repulsion of pith ball. Let a metallic
point be inserted into one of the holes of the prime conductor,
so that, in accordance with what has explained, a jet of electri-
city may escape from it when the conductor is electrified. Let
this jet, while the machine is worked, be received on the
interior of a glass tumbler, by which the surface of the glass
will become charged with electricity.

If a number of pith balls be laid upon a metallic plate com-
municating with the ground, and the tumbler be placed with
its mouth upon the plate, including the balls within it, the balls
will begin immediately leaping violently from the metal and
striking the glass, and this action will continue till all the
electricity with which the glass was charged has been carried
away.

This is explained on the same principle as the former ex-
periments. The balls are attracted by the electricity of the
glass, and when electrified by contact, are repelled. They give
up their electricity to the metallic plate from which it passes
to the ground ; and this process continues until no electricity
remains on the glass of sufficient strength to attract the balls.

1795. Electrical dance. Let a disk of pasteboard or wood,
coated with metallic foil, be suspended by wires or threads of



248 ELECTRICITY.

linen from the prime conductor of an electrical machine, and
let a similar disk be placed upon a stand capable of being
adjusted to any required height. Let this latter disk be placed
immediately under the former, and let it have a metallic com-
munication with the ground. Upon it place small coloured
representations in paper, of dancing figures, which are prepared
for the purpose. When the machine is worked, the electricity
with which the upper disk will be charged will attract the
light figures placed on the lower disk, which will leap upwards ;
and after touching the upper disk and being electrified, will be
repelled to the lower disk, and this jumping action of the figures
will continue so long as the machine is worked. An electrical
dance is thus exhibited for the amusement of young persons.

1796. Curious experiments on electrified water. Let a small
metallic bucket B, Jig. 519., be suspended from the prime con-
ductor of a machine, and let it have a capillary tube c D of the
siphon form immersed in it ; or let
it have a capillary tube inserted in
the bottom ; the bore of the tube
being so small, that water cannot
escape from it by its own pressure.
When the machine is put in opera-
tion, the particles of water becom-
ing electrified, will repel each
other, and immediately an abun-
dant stream will issue from the
tube ; and as the particles of water
F>g- 5 '9- after leaving the tube still exercise

a reciprocal repulsion, the stream will diverge in the form of a
brush.

If a sponge saturated with water be suspended from the
prime conductor of the machine, the water, when the machine
is first worked, will drop slowly from it ; but when the con-
ductor becomes strongly electrified, it will descend abundantly,
and in the dark will exhibit the appearance of a shower of
luminous rain.

1797. Experiment with electrified sealing-wax. Let a piece
of sealing-wax be attached to the pointed end of a metallic rod ;
set fire to the wax, and when it is in a state of fusion blow out
the flame, and present the wax within a few inches of the
prime conductor of the machine. Strongly electrified myriads




THERMAL EFFECTS OF ELECTRICITY. 249

of fine filaments will issue from the wax towards the conductor,
to which they will adhere, forming a sort of net-work resem-
bling wool. This effect is produced by the positive electricity
of the conductor decomposing the natural electricity of the wax ;
and the latter being a conductor when in a state of fusion, the
negative electricity is accumulated in the soft part of the wax
near the conductor, while the positive electricity escapes along
the metallic rod. The particles of wax thus negatively elec-
trified being attracted by the conductor, are drawn into the
filaments above mentioned.

1798. Electrical see-saw. The electrical see-saw a b, Jig.
520., is a small strip of wood covered over with silver leaf
or tinfoil, insulated on c like a balance. A slight prepon-
derance is given to it at a, so that it rests
on a wire having a knob m at its top ; p
is a similar metal ball insulated. Connect
p with the interior, and m with the ex-
Fig. 520. terior coating of the jar, charge it, and
the see-saw motion of a b will commence from causes similar
to those which excited the movements of the pith balls.




CHAP. X.

THERMAL EFFECTS OF ELECTRICITY.

1799. A current of electricity passing over a conductor raises
its temperature. If a current of electricity pass over a con-
ductor, as would happen when the conductor of an electrical
machine is connected by a metallic rod with the earth, no change
in the thermal condition of the conductor will be observed, so
long as its transverse section is so considerable as to leave
sufficient space for the free passage of the fluid. But, if its



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 25 of 45)