The following are selected from the very able series of lectures
delivered by Professor Faraday at the Royal Institution:
_The Two Electricities._--After having shown by various experiments
the attractions and repulsions of light substances from excited
glass and from an excited tube of gutta-percha, Professor Faraday
proceeds to point out the difference in the character of the
electricity produced by the friction of the two substances. The
opposite characters of the electricity evolved by the friction
of glass and of that excited by the friction of gutta-percha
and shellac are exhibited by several experiments, in which the
attraction of the positive and negative electricities to each other
and the neutralisation of electrical action on the combination
of the two forces are distinctly observable. Though adopting the
terms “positive” and “negative” in distinguishing the electricity
excited by glass from that excited by gutta-percha and resinous
bodies, Professor Faraday is strongly opposed to the Franklinian
theory from which these terms are derived. According to Franklin’s
view of the nature of electrical excitement, it arises from the
disturbance, by friction or other means, of the natural quantity
of one electric fluid which is possessed by all bodies; an excited
piece of glass having more than its natural share, which has
been taken from the rubber, the latter being consequently in a
minus or negative state. This theory Professor Faraday considers
to be opposed to the distinct characteristic actions of the two
forces; and, in his opinion, it is impossible to deprive any body
of electricity, and reduce it to the minus state of Franklin’s
hypothesis. Taking a Zamboni’s pile, he applies its two ends
separately to an electrometer, to show that each end produces
opposite kinds of electricity, and that the zero, or absence of
electrical excitement, only exists in the centre of the pile. To
prove how completely the two electricities neutralise each other,
an excited rod of gutta-percha and the piece of flannel with which
it has been rubbed are laid on the top of the electrometer without
any sign of electricity whilst they are together; but when either
is removed, the gold leaves diverge with positive and negative
electricity alternately. The Professor dwells strongly on the
peculiarity of the dual force of electricity, which, in respect
of its duality, is unlike any other force in nature. He then
contrasts its phenomena of instantaneous conduction with those of
the somewhat analogous force of heat; and he illustrates by several
striking experiments the peculiar property which static electricity
possesses of being spread only over the surfaces of bodies. A metal
ice-pail is placed on an insulated stand and electrified, and a
metal ball suspended by a string is introduced, and touches the
bottom and sides without having any electricity imparted to it,
but on touching the outside it becomes strongly electrical. The
experiment is repeated with a wooden tub with the same result;
and Professor Faraday mentions the still more remarkable manner
in which he has proved the surface distribution of electricity
by having a small chamber constructed and covered with tinfoil,
which can be insulated; and whilst torrents of electricity are
being evolved from the external surface, he enters it with a
galvanometer, and cannot perceive the slightest manifestation of
electricity within.
_The Two Threads._--A curious experiment is made with two kinds
of thread used as the conducting force. From the electric machine
on the table a silk thread is first carried to the indicator a
yard or two off, and is shown to be a non-conductor when the glass
tube is rubbed and applied to the machine (although the silk, when
wetted, conducted); while a metallic thread of the same thickness,
when treated in the same way, conducts the force so much as to
vehemently agitate the gold leaves within the indicator.
_Non-conducting Bodies._--The action that occurs in bodies which
cannot conduct is the most important part of electrical science.
The principle is illustrated by the attraction and repulsion of
an electrified ball of gilt paper by a glass tube, between which
and the ball a sheet of shellac is suspended. The nearer a ball of
another description--an unelectrical insulated body--is brought
to the Leyden jar when charged, the greater influence it is seen
to possess over the gold leaf within the indicator, by induction,
not by conduction. The questions, how electricities attract each
other, what kind of electricity is drawn from the machine to the
hand, how the hand was electric, are thus illustrated. To show the
divers operations of this wonderful force, a tub (a bad conductor)
is placed by the electric machine. When the latter is charged, a
ball, having been electrified from it, is held in the tub, and
rattles against its sides and bottom. On the application of the
ball to the indicator, the gold leaf is shown not to move, whereas
it is agitated manifestly when the same process is gone through
with the exception that the ball is made to touch the outside only
of the tub. Similar experiments with a ball in an ice-pail and
a vessel of wire-gauze, into the latter of which is introduced a
mouse, which is shown to receive no shock, and not to be frightened
at all; while from the outside of the vessel electric sparks are
rapidly produced. This latter demonstration proves that, as the
mouse, so men and women, might be safe inside a building with
proper conductors while lightning played about the exterior. The
wire-gauze being turned inside out, the principle is shown to be
irreversible in spite of the change--what has been the unelectrical
inside of the vessel being now, when made the outside portion,
capable of receiving and transmitting the power, while the original
outside is now unelectrical.
_Repulsion of Bodies._--A remarkable and playful experiment, by
which the repulsion of bodies similarly electrified is illustrated,
consists in placing a basket containing a heap of small pieces
of paper on an insulated stand, and connecting it with the prime
conductor of the electrical machine; when the pieces of paper
rise rapidly after each other into the air, and descend on the
lecture-table like a fall of snow. The effect is greatly increased
when a metal disc is substituted for the basket.