Encyclopaedia Britannica > Volume 8, DIA-England

652
ELECTRICITY.
Electrical stand H; a circular disc of glass D, one and a half inch
Apparatus, in diameter and two tenths thick, is fixed to the vase A,
v—and revolves along with it, while a similar plate E is fixed
on the top of the stand H. These two discs are shown
Fig. 15. separately in fig. 15. In the edge of the plate E are
drilled two holes to receive metallic hooks F, G, and
into the edge of the upper plate D are cemented two
small tails of the flattened wire used in making silver lace.
These tails are bent down so as to strike the hooks F, G
during their revolution, without touching the rest of the
apparatus. The two adjacent faces of the glass discs are
coated with segments of tinfoil, as shown in fig. 15; and
they may be set at any distance by means of the screw
C. Each tail communicates with the tinfoil coating of
D; the hook F communicates with that of E, but the
hook G is insulated so as to communicate only with the
electrified body. The coating of E communicates with
the earth by means of the stand H.
If the vase A, the plate D, and the axis EK, are now
set a spinning by the action of the finger and thumb
applied at T, one of the tails will strike the hook G, and
receive through it from the electrified body some of its
electricity, which it will convey to D, which will thus as¬
sume the electric state of the body. The tail which has
struck G proceeding onwards, will after half a revolution
touch F, and will convey the free electricity received at
G to the two coatings, which with the hook F constitute
one insulated mass. The tail advances, acquires more
electricity from G, deposits it at F, and thus condenses it,
on the principle of the common condenser, till it is capa¬
ble of affecting the pith balls at F. The instrument con¬
structed by Mr Nicholson was five inches high, and con¬
densed very small degrees of electricity.
Sect. III.— On the Construction of Electrical Doublers.
Electrical This class of instruments operate by continually doub-
doublers. ling small quantities of electricity till the common elec¬
trometer is capable of indicating its presence and qua¬
lities.
Bennet’s The doubler invented by Mr Bennet consists of three
doubler, plates, A, B, and C, fig. 16. The plate A, which is of brass,
Pl.CCXY. has an insulating handle rising from its centre; the plate B,
ijg. 16. which is also of brass, has a similar handle fixed in its cir¬
cumference. The third plate, C, also of brass, is placed on
Bennet’s gold-leaf electrometer. The under side of A,
the upper side of C, and both sides of B, are varnished.
The body whose electricity it is required to double is
brought into contact with the under side of C, which rests
on the cap of the electrometer, while B is touched with
the finger of the other hand. The communication with
the electrified body being broken off, B is lifted up by its
glass handle. If the electrometer leaves do not diverge,
A is placed by its handle upon B, thus lifted up; and" A
being now touched by stretching a finger over the junc¬
ture of its insulating handle and immediately withdraw¬
ing it, A is separated from B. In this situation two of
the plates have obviously nearly equal quantities of one
kind of electricity, while the third plate has the opposite
kind. The plate A is then made to touch the under sur¬
face of C, resting on the electrometer, and at the same
time C is covered with B. The plate B is now touched
by the finger as A was; and removing A, and withdraw¬
ing the finger from B, and lifting it up from C, the elec¬
tricity is doubled. By repeating this operation ten or
twenty times, which may be done in forty seconds, the
electricity will, by continual duplication, be augmented
500,000 times. When sparks are required, C must rest
on an insulating stand in place of the electrometer.
It was found by Mr Bennet, Cavallo, and others, that Electric;
the doubler became strongly electrified even wdien no elec-Apparati
tricity was communicated to it. To remove this evil, M.
Cavallo used three plates without varnish, and he placed
them on insulating stands, so as to have a vertical direction,
and to stand within one eighth of an inch of each other,
the plates of air being a substitute for the varnish. The
method of doubling is exactly the same as before. Dr
Robison adopted the same idea, but he kept his plates
horizontal, making them rest on each other by three small
spherules of glass or sealing-wax. Notwithstanding these
precautions, however, electricity w'as still produced.
In order to perform the operation of doubling with more
rapidity, Dr Darwin proposed the moveable doubler, or one
in which the plates could be moved by wheel-work into
their proper positions. Dr Nicholson improved upon this
idea by producing the whole effect with the simple revo¬
lution of a winch.
This revolving doubler, as it has been called, is repre- Nichol-
sented in fig. 17. It consists of two fixed plates of brass A,C, son’s re-
two inches in diameter, insulated separately, and placed involving
the same plane, so that a revolving plate B may pass near d°ubler-
them without touching. A brass ball D is fixed on the endFlg'l7'
of the axis which carries B, and is loaded within at one side
so as to counterpoise the plate B, and allow it to rest in any
position. The axis PN, and the axes that join the three
plates with the brass axis NO, which passes through the
brass piece M, by which the plates A and C are sup¬
ported, are made of varnished glass. One end of this axis
carries the ball D, and the other is connected with a rod
of glass NP, upon which the handle L is fixed, and also
the piece GH insulated separately. The pins E, F rise
from the back of the plates A, C, at equal distances from
the axis. The arm K is parallel to GH, and the ends of
both are armed with pieces of harpsichord wire, so as to
touch the pins E, F in certain points of their revolution.
A pin I is fixed on M to intercept a small wire proceed¬
ing from the revolving plate B. These wires are so bent
that, when B is opposite to D, GH connects the two fixed
plates A, C, while the wire and pin at I connect the ball
D and plate B. On the other hand, when B is opposite
C, D is connected with C by the contact of F with the
wire at K, the plates A, B being then entirely unconnect¬
ed with any other part of the instrument. In all other
positions the three plates and the ball D will have no con¬
nection with each other. The operation of this instru¬
ment is thus described by Mr Nicholson: “ When the
plates A and B are opposite to each other, the two fixed
plates A and C may be considered as one mass, and the
revolving plate B, together with the ball D, will constitute
another mass. All the experiments yet made concur to
prove that these two masses will not possess the same
electric state ; but that, with respect to each other, their
electricities will be plus and minus. These plates would
be simple, and without any compensation, if the masses
were remote from each other ; but as that is not the case,
a part of the redundant electricity , will take the form of a
charge in the opposed plates A and B. From other ex¬
periments, I find that the effect of the compensation on
plates opposed to each other at the distance of one for¬
tieth part of an inch is such that they require to produce
a given intensity, at least a hundred times the quantity of
electricity that would have produced it in either singly
and apart. The redundant electricities in the masses un¬
der consideration will therefore be unequally distributed;
the plate A will have about ninety-nine parts, and the
plate C one; and for the same reason the revolving plate
B will have ninety-nine parts of the opposite electricity,
and the ball D one. The rotation, by destroying the con¬
tacts, preserves this unequal distribution, and carries B