Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
lectrical from A to C, at the same time that the tail K connects
pparatus. t]ie ball with the plate C. In this situation the electrici-
ty in B acts upon that in C, and produces the contrary
state by virtue of the communication between C and the
ball; which last must therefore acquire an electricity of
the same kind with that of the revolving plate. But the
rotation again destroys the contact, and restores B to its
first situation opposite A. Here, if we attend to the effect
of the whole revolution, we shall find that the electric
states of the respective masses have been greatly in¬
creased ; for the ninety-nine parts in A and in B remain,
and the one part of electricity in C has been increased so
as nearly to compensate ninety-nine parts of the opposite
electricity in the revolving plate B, while the communi¬
cation produced an equal mutation in the electricity of
the ball. A second rotation will of course produce a
proportional augmentation of these increased quantities,
and a continuance of turning will soon bring the intensi¬
ties to their maximum, which is limited by an explosion
between the plates.”
onalds’ An ingenious instrument, called ^.pendulum doubler, has
ndulum been recently constructed and described by Mr Ronalds.
i ppyv Hav^nS found it necessary to keep a telegraphic wire con-
j'; j8 ’stantly electrified with a very small source of electricity,
lfl ' he converted the bob of a pendulum into the centre plate
of a doubler, and he found the instrument thus modified
not only useful for that purpose, but also for that class of
experiments, such as those on vegetation and animal life,
which require a constant supply of small quantities of elec¬
tricity to supply the loss occasioned by unavoidable de¬
fective insulation, either in the glass wliich is used, or in
the surrounding atmosphere. This improvement on the
doubler is shown in fig. 18, where A and B are the two
fixed plates, about four inches in diameter, supported by
glass pillars; C is the bob carried by the pendulum rod
D, and insulated by the piece of glass e. The form of the
bob C is that of a plano-convex lens, with its interior filled
with lead ; e is a small cylinder connected to C with screws,
which also adjust the plane of C parallel to the plane of
vibration; g is another insulating glass rod, carrying the
bent wire /*, the left end of which lies nearly in the same
vertical plane as the end of the wire m, the right end be¬
ing nearly in the same plane as the end of the wire n. A
wire, i, rises perpendicularly from C ; and another, h, per¬
pendicular to the plane of vibration, is fixed into the brass
cup at the end of the pendulum rod. A wire, l, is screwed
into the edge of the plate B, and the long wire m is fixed
on the lower edge of B, so as to approach within a small
distance of A, where it is bent at right angles, and then
projects in a plane perpendicular to that of vibration.
Another wire, n, is fixed into the edge of A, so as to bend
and project similarly; but n projects farther than m, that
the right side of the bow k may pass the end of m with¬
out touching it. A wire, o, is fixed'at right angles into
the base of the instrument.
When the bob C is exactly opposite A, the insulated
wire h touches simultaneously the ends of the wires mand
n, and establishes a communication between A and B,
while at the same time the wire i, by touching o, forms
a communication between C and the ground. Now, if
a quantity of positive electricity, for example = 1, is
given to A or B when the centres of A and C are oppo¬
site to each other, that quantity will be nearly all con¬
densed on A, and C will acquire negative electricity near¬
ly = 1.
“ If C,” says Mr Ronalds, “ be now allowed to begin its
vibrations, the connection of A and Bwith each other will
be instantly broken, as also that of C with the earth, and
they will be all insulated, and all retaining the electric
states which they possessed before the connections were
653
broken (i. e. A will be positive nearly = 1, B negative Electrical
nearly = 1, and C positive almost 0). Apparatus.
“ When C has arrived opposite B, the uninsulated wire
k will touch the wire L, and thus place B in connection
with the earth ; therefore C, by virtue of its negative
charge, will induce a positive charge in it nearly — 1.
“ When C arrives a second time opposite to A, all the
former connections will be re-established, and the charge
of B will (bj' means of the wire m) be nearly all con¬
densed on and added to the original charge of A, making
a tension nearly = 2 of positive electricity, which tension
will induce a tension of nearly == 2 of negative electricity
on C.
“ And so the charges in A and C would go on, nearly
doubling at each vibration of the pendulum, until their
tensions would arrive at such a point as to cause a spark
to pass between them.
“ But P is a Leyden jar furnished with a Lane’s dis¬
charging electrometer g; a connection is established by
means of a small chain between it and A ; and the distance
between the tw^o balls r and s is considerably less than
that between A and C ; therefore the spark will be given
to the jar, and a spark will be continued to be given at the
completion of almost every second vibration, until it is
charged almost as highly as A is capable of being charged,
or the sparks will continually supply the loss of electricity
by any defect of insulation, either of the jar, or of any con¬
ducting body in connection with its interior coating with¬
in certain limits.
“ The contacts of the wires do not impede the velocity
of the vibrations, because they are made small enough to
act as springs of a required force ; but the electric attrac¬
tions of the plates and bob do tend to do so. The pen¬
dulum is suspended by two springs, placed one at each
extremity of a cross piece, to which the rod is attached,
for the purpose of preventing the bob from being drawn,
by their attractions, out of its assigned plane of vibration
as much as possible.”
Sect. III.—Description of Instruments for Multiplying
Electricity.
The electrical multiplier invented byM. Cavallo is shown Multipli-
in fig. 19, on a scale about one third of its real size, and ers °f elec-
is chiefly useful in ascertaining the presence of a consider- ,
able quantity of electricity occupying an extended space. j^qL'lier
Its principal parts are four plates of brass A, B, C, D.
The plates A, C are supported by two glass rods G, H,CCXV.
fixed in the wooden base R, S, Q. A similar plate B is Fig. 19.
supported by another glass rod I, cemented into the wood¬
en lever LK, moving round a pivot K. The fourth plate
D is supported by a metallic rod. By the lever KL the
plate B can be moved from its position on the figure into
the dotted position KX. The plate D is screwed at P
into a piece of brass FP, which slides in a groove, so that
D can be pulled out to any distance from C. At the cor¬
ner Q is fixed a brass rod N, and 0;m is a small bent wire
fixed to the brass socket O on the back of B. When B
is as near as possible to A, their distance being one twen¬
tieth of an inch, this wire m touches the rod N, and forms
a communication with the earth ; when FP is pushed in as
far as possible, the surfaces of C and D are one twentieth
of an inch distant. As the lever KL moves towards X,
the end m of the wire mO quits N and insulates the plate
B; and when the lever has the position KX, the wire m
will touch the plate C, so as to put the insulated plates B
into communication with each other.
If a body weakly electrified positively is now made to
touch A, when A and B are placed together as in the figure,