Encyclopaedia Britannica > Volume 8, DIA-England

642
ELECTRICITY.
Electrical for placing it horizontally. By these means the machine
Apparatus. may be so adjusted and fixed that the axis of the plate of
glass, which has a free motion backwards and forwards in
the holes in which it turns, may not tend more to one
side than to the other, and occasion an equal action on
the rubbers. The rubbers, which are four in number,
are insulated on pillars of glass A, B, one placed at each
extremity of the horizontal diameter A, B of the plate.
The positive conductor C, B, D projects in a vertical
position in front of the plate Z Z, while the negative con¬
ductor passes in a curvilineal direction behind, and con¬
nects the rubbers of each side.
The plate of glass is turned by an insulated handle,
immediately in front of which is placed a short index,
which is fixed to the axis, and which moves over a gra¬
duated circle L, attached to the horizontal part of the
frame, and through the centre of which the axis passes. In
this manner the number of revolutions of the plate may
be accurately registered.
In order to strengthen the centre of the plate, two small¬
er plates are cemented to each side by varnish and a
small stop is inserted into the axis, to prevent the pres¬
sure from increasing beyond a certain point.
When the machine is used for ordinary purposes, the
conductors shown in fig. 7 are employed; but when it is
employed to accumulate electricity, the conductors should
have the smallest extent possible, and, excepting at the
receiving points, where they collect the electricity from
the edge of the silk flaps about H, H, they should be
covered with sealing-wax. In this case the positive con¬
ductor is formed of small straight tubes, as shown in fig.
Fig. 8. 8, and its extremities terminate in balls of varnished
wood, through the substance of which the metallic com¬
munications pass.
2. Description of Van Marum's Electrifying Machine.
Van Ma- This machine, to which we have made a brief reference
rum’s elec-in Sect. III. Chap. II. Part I., is represented in elevation
trical ma- and in section in fig. 1 and 2 of Plate CCX. The plate of
F^'cCX ^ass which is thirty-one inches in diameter, is sus-
j 5 ' tained byn single pillar E, at the upper extremity of which
’ are two similar brass collars I, I, one of which is shown sepa¬
rately in fig. 4. The horizontal axis MN rests upon these
collars, and this axis carries a counterweight L, in order to
balance the plate of glass and its appendages, and thus
equalize the friction on the collars. The rubbers, which
cannot be seen in the section, fig. 2, are shown at m, n, fig. 1.
The pair at m is attached to the ball O, and supported by
the glass pillar e; and in like manner the pair at n is at¬
tached to the ball P, and supported by the glass pillar^/! A
horizontal section of the rubbers and balls is shown sepa¬
rately in fig. 3. A semicircle of brass CD is attached to
an axis g that turns on the ball G, resting on the pillar
F, so as to give the conductor CGD a motion round that
axis. Collectors six inches long and two and a half in
diameter are placed at C and D, to collect the electricity
from the revolving plate AB. At the outer end of the
axis ^ is a copper tube H/<, terminating at its lower end
in a ball H, and its upper end in a smaller ball It, two
inches in diameter, which, screwing into G, will fix the
tube H/t in any position round g. An arch of brass wire
c\d, half an inch in diameter, is fixed to the end of the
bearing piece K, and moves round I into any given azi¬
muth, so as to be placed, as in fig. 1, opposite the rubbers
m, n, or at right angles to them. In like manner, the con¬
ductor CGD can be placed either horizontally, so that the
collectors C and D may be opposite the rubbers m and n,
or vertically, as shown in fig. 1. By this apparatus it is
easy to produce either positive or negative electricity. In
the position of the conductor shown in fig. 1, where CGD Electri
is at right angles to the rubbers, and where the rubbers Appara
are connected with the ground by the arch cld, and by
the wire KK, fig. 2, the conductor G will give positive
electricity; but when we wish negative electricity, the con¬
ductor CGD is placed horizontally, with its collectors C, D
opposite the rubbers, and the arch cld is placed vertically,
so as to insulate the rubbers.
A mahogany cap T covers the metallic caps of the sup¬
ports, in order to insulate them more perfectly. A hollow
ring of mahogany, VX, is, for the same reason, made to
cover the metallic socket into which the support is in¬
serted. In fig. 3, a, h, a, b, are four pieces of gum-lac. In
fig. I and 2, W is the jiandle by which the machine is
wrought.
3. Description of Hares Electrical Machine.
This machine, which we have previously noticed, differs Hare’s
from those generally made, in having its glass plate hori-electric
zontal; and it is considered by its inventor, Professor Hare
of Philadelphia, as giving negative electricity in a way
preferable to that in which it is obtained in Van Marum’s ‘‘
machine. The glass plate MN, thirty-four inches in dia¬
meter, is supported on an upright iron bar PR, about an
inch in diameter, and covered by a stout glass cylinder,
sixteen inches high and four and a half inches in diame¬
ter, open only at the base, through which the bar is in¬
troduced so as to form its axis. At the top of the bar
PR is a block of wood turned to fit the cavity at the apex
of the cylinder, and cemented therein. The external
apex of the cylinder is fixed by cement into the brass cap
which carries the plate. The glass cylinder, which is
liable to no strain, effectually insulates the plate from the
iron axis PR. The brass cap seen at P is surmounted by
a screw and flange, which, with the aid of a correspond¬
ing nut and discs of cork, keeps the plate firm. The wheel
W, driven by a handle, communicates by means of a band
with another wheel about twenty inches in diameter,
placed on the iron axis RS.
“ Nearly the same mode of insulation and support,”
saj's Dr Hare, “ which is used for the plate is used in
the case of the conductors. They consist severally of
arched tubes of brass (ABC, DEF), of about an inch and
a quarter in diameter, which pass over the plate from one
side of it to the other, so as to be at right angles to, and
at a due distance from, each other. They are terminated
by brass balls and caps, which last are cemented on glass
cylinders of the same dimensions nearly as that which
supports the plate. The glass cylinders are suspended
upon wooden axes, surmounted by plugs of cork turned
accurately to fit the space which they occupy. The cy¬
linders are kept steady below by bosses of wood which
surround them. In this way the conductors are effectual¬
ly insulated, while the principal strain is borne by the
wooden axes.”
The collectors are shown at MN in connection with the
positive conductor ABC, and the rubbers are shown be¬
tween P and the balls D and F in connection with the
negative conductor DEF. The advantage of this form of
the machine over that of Van Marum is, that the two con¬
ductors are permanently fixed in their places, and that
positive and negative electricity can be at any time ob¬
tained without any change in the machine. Dr Hare
considers the band as of advantage in preventing the
plate from being cracked by any hasty effort to put it in
motion when it adheres to the cushions, as it often does.
Dr Hare uses a winch on the other side of the wheel, so
that two persons, or one with both hands, may drive it.
The great expense of large cylinders and plates of