Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
643
ectrical glass, and their liability to injury, have induced artists to
paratus. construct electrical machines of different substances. M.
Walckiere de St Amand of Brussels constructed a machine
ichines 0f extraordinary power, which consisted of a web of var-
^ nished silk twenty-five feet long and five feet wide, re-
s’ volving upon two wooden cylinders covered with woollen
serge. During the revolutions of the cylinders, the silk
moves between two cushions, each seven feet long and two
inches in diameter, covered by cat’s skin or hare’s skin, and
moveable so as to vary the friction. The machine was
driven by four men, and it had so great power that it
gave sparks fifteen inches long, and nobody durst take a
spark from it but with the shoulder and elbow,
varnish- Dr Ingenhousz constructed machines with discs of paste-
paste- board four feet in diameter, and soaked in copal or am-
ard; ber varnish dissolved in linseed oil. They were covered
with the same varnish, and were mounted upon an axis
or flat board, three inches broad, and covered with flan¬
nel or hare’s skin, being placed between each two discs, so
as to act as a rubber. Sparks one and even two feet
long were given out by the front disc when the knuckle
was presented to it.
wooden Wooden discs, and cylinder discs of gum-lac partly im-
iCS' mersed in mercury, which acted on the rubber, and
stretched varnished ribbons, have been all used in the con¬
struction of electrifying machines, but it would be an un¬
profitable task to describe them.
4. General Observations on the Construction and Use of
the Electrical Machine.
bsorva- Although in fine dry weather, and in a warm and dry
ms on place, a good electrical machine may be brought into an
celectri.excepent state of action, merely by wiping it with a warm
.ma’ linen cloth, and afterwards with a silk handkerchief, yet
111C’ in a different state of the atmosphere, and in humid apart¬
ments, every precaution is necessary to insure the vigo¬
rous and steady action of the machihe. By turning the
machine before a fire, or placing it in a current of heated
air, or, as Dr Faraday suggests, by placing it over a
sand-bath or a hot iron plate, whose temperature does not
exceed 212 degrees, the different parts of the machine
will be thoroughly dried and heated without affecting the
cements.
lethods W’e have already described (see page 576) the improve-
hming ment of Mr Ronalds, who heats the inside of the machine,
Iema' &c. by a spirit-lamp. Dr Faraday recommends the heat¬
ing of a cylinder-machine by placing a chemical Argand
lamp with a low flame beneath the cylinder, and to sup¬
port a plate of metal nearly six inches square, about an
inch above the chimney of the lamp. This plate, by
being heated, varies the air above it, and produces a large
moderately heated current, which encircles the cylinder,
and thoroughly warms it. Care must be taken not to
heat the cylinder in spots, but to bring it, and especially
the insulating parts, to an uniform temperature, which
shall never be sufficient to melt the cement which is used
in any part of it.
The state of the rubbers requires particular attention.
They must be carefully freed from dust, and supplied
with a soft and uniform coating of amalgam, which should
always be rubbed in a mortar with tallow previous to being
used. Large spots of amalgam should be removed from
the cylinder or plate, either by the nail or a piece of
wood. Dr Faraday remarks, that a few spots of amalgam
rather increase than diminish the activity of the machine,
and that the silk which proceeds from the rubber is better
when impregnated with amalgam than when free from it.
Dr Faraday adds, that it is often useful to hold a piece
of silk, with some amalgam adhering to it, against the re¬
volving plate or cylinder, and also to rub the surface of Electrical
amalgam on the rubber with the same amalgamated silk. Apparatus.
When the machine is thus put into good action, and the
prime conductor removed, it should discharge a continued
series of brushes from the edge of the silk, and abundance
of sparks flying round the glass.
Sect. II.—Description of the Electrophones.
This ingenious instrument, which was invented by the Electro-
celebrated Volta, is shown in Plate CCXIV. fig. 9. It con- phorus of
sists of a circular metallic disc A, or a plate of w0°d
covered with tinfoil, having an insulating handle of glass CCXIV.
screwed into a nut E, made of wood or brass. The plate A pig. 9.
is called the upper conductor, or cover. The next plate B,
called the resinous plate, consists of a plate half an inch
thick, composed of equal parts of shell-lac, common resin,
and Venice turpentine, poured when hot upon a marble or
stone table. The next plate is a metallic one C, called
the lower conductor, or sole, which may be either separate
or not from the resinous plate which rests upon it. The
edge of the first plate A must be pretty thick, and made
smooth and round. The following is the method of gene¬
rating electricity with this apparatus.
The cover A being held in the left hand, rub the upper
surface of the resinous plate B with a piece of dry fur, or
whip it with a fox’s tail or stripe of cat’s skin. It will
thus be excited negatively. Place the upper conductor
above the resinous plate, and while it is there touch it
with the finger, and then raise it by its glass handle. It
will exhibit signs of positive electricity, and will yield a
spark either to the knuckle or to the knob of a Leyden
phial. If the cover A is again placed upon B, and, after
being touched, again raised, it wall give another spark, and
twenty of these sparks will charge a Leyden jar of a mo¬
derate size. If the upper conductor A is not touched by
the finger when placed upon B, it will exhibit, when raised,
very faint, if any, traces of electricity. Now, as the resi¬
nous plate B continues, without any new excitation, to
charge the upper conductor A, it is manifest that its elec¬
tric condition is not destroyed by the contact and removal
of A ; and as it is necessary to connect the upper conduc¬
tor with the ground, by touching it previous to its being
raised, it is obvious that the electricity acquired by A is
derived from its contact with B.
In order to explain the theory of the electrophorus, let
us insulate the lower conductor C, by placing it on a glass
stand, as in fig. 10, and let this conductor communicate Fig. 10.
with the pith balls of an electroscope. As soon as the
upper surface of the cake B is excited, the pith balls will
diverge with negative electricity. The negative electrici¬
ty developed by the excitation of the upper surface has
decomposed the natural electricity of C, by attracting the
positive part and repelling the resinous part into the elec¬
troscope where it is indicated. If we now touch the con¬
ductor C, its negative electricity is carried off, and the
positive undergoes no diminution ; but, owing to the escape
of the negative portion, the balls will collapse. II wTe now
make the upper conductor A approach to 13, and rest upon
it, touching it at the same time with the finger, so as to con- •
nect it with the ground, the positive electricity of the cake
B will decompose the natural electricity ol A, repelling its
negative electricity to the earth through the finger, and at¬
tracting its positive portion to its lower surface. 1 his posi¬
tive electricity of A attracting the negative electricity ot
the surface of B, and repelling the vitreous electricity of C,
thus doubly tends to diminish the force by which this po¬
sitive electricity is rendered latent or detained. Some ot
it, therefore, will be set free, and the pith balls will di-