Encyclopaedia Britannica > Volume 8, DIA-England

582 ELECTRICITY.
Phenome- 3. A number of bodies insulating electricity of low in-
na and tensity when solid, conduct it very freely when fluid, and
^laws- , are then decomposed by it.
4. There are many fluid bodies which do not sensibly
conduct electricity of this low intensity ; there are some
which conduct it and are not decomposed, nor is fluidity
essential to decomposition.
5. There is but one body yet discovered (periodide of
mercury) which, insulating a Voltaic current when solid,
and conducting it when fluid, is not decomposed in the
latter case.
6. There is no strict electrical distinction of conductors
which can as yet be drawn between bodies supposed to be
elementary and those known to be compounds.
The experiments of Dr Faraday with ice, in which
it appeared that electricity of exalted intensity passed
through it, while it completely stopped Voltaic electri¬
city, confirms the observations of M. Delarive on the
relation between the conducting power and the quantity
of electricity which traverses the conductor ; and the
phenomena seem to indicate that the electric fluid or
matter may consist, like solar light, of different parts pos¬
sessing different powers of conductibility and other pro¬
perties, which may facilitate or obstruct their passage
through solid, fluid, or gaseous bodies. An electric cur¬
rent, composed of different currents, may have some of
its component currents entirely stopped by some bodies,
while other currents are transmitted with the greatest
facility, in the same way as certain rays both of light and
heat are entirely absorbed by coloured bodies, while other
rays are copiously transmitted. Non-conductors, like
black bodies, stop every electrical current. Perfect con¬
ductors, like colourless transparent bodies, may transmit
every electrical current, or absorb a small portion of all
of them in an equal degree; while there may b6 imper¬
fect conductors, which, like coloured bodies, stop some
currents and transmit others. If this should prove cor¬
rect, two bodies which, when used separately, conduct
electricity, would be insulators when joined so as to trans¬
mit the electricity in succession, in the same manner as
two transparent coloured bodies which separately trans¬
mit light copiously, are opaque when combined, the light
which each transmits being absorbed by the other.
Distance have already seen that electricity was conveyed
to which through a distance of four miles. On the ground that
electricity these experiments were made imperfectly, and that an
has been electric charge will prefer a short passage through air to
tone ucte . a passage 0f twenty or thirty feet through thin wire, Mr
Singers has expressed his conviction that the results of
the experiments referred to are incorrect. We are un¬
able, we confess, to appreciate the reasons on which this
opinion is founded; but, even if they have any force,
the original fact has been more than confirmed by Mr F.
Ronalds, who erected at Flammersmith an electrical tele¬
graph, on which the inflections of the wire composed one
continuous length of more than eight miles. “ When a Can¬
ton’s pith ball electrometer was connected with each ex¬
tremity of this wire, and it was charged by a Leyden jar,
both electrometers appeared to diverge suddenly at the
same moment; and when the wire was discharged by being
touched with the hand, both electrometers appeared to
collapse as suddenly. When any person took a shock
through the whole length of wire, and the shock was com¬
pelled to pass also through two insulated inflammable air
pistols, one connected with each extremity of the wire,
the shock and the explosion seemed to occur quite simidtane -
ously. But when the shock was compelled to pass through
the gas pistols, and any one closed his eyes, it was im- Pheno
possible to distinguish more than one explosion, although Mai
both pistols were discharged. When people did not look lja"
at the pistols, and when I sometimes charged only one
highly, and sometimes both lowly, they could never guess,
except by mere chance, whether one or both were fired.
Thus, then, three of the senses, namely, sight, feeling, and
hearing, seemed to receive absolute conviction of the instan¬
taneous transmission of electrical signs through my pistols,
my eight miles of wire, and my own proper person”1
Sect. V.— On the Electric Spark.
Since the discovery of electric light by Otto Guericke Electr
and Dr Wall, the subject has attracted the particular at-spark,
tention of philosophers. In exciting a glass tube, or in
wmrking an electrical machine in the dark, sparks and
streams of light are distinctly visible ; but the phenomenon
is best seen when the knuckle or a brass ball is brought
near to an electrified conductor. A bright light, called
the electric spark, passes from the conductor to the knuc¬
kle or ball, and exhibits a great variety of phenomena,
varying with the nature and intensity of the electricity,
and with the form, magnitude, distance, and nature of
the bodies between which it passes.
Exp. 1. Having screwed into the prime conductor a Form
brass ball about two inches in diameter, and projecting the spj
about three inches, electrify the conductor positively, and
hold another ball near the first. Long ramified zigzag
sparks will pass between the two balls, as shown in fig. 6, PI. CC
where pos. is the positively electrified ball, and nat. the one Pig- 6,
held in the hand in a natural state of electricity. If the
ball on the conductor is very small, the spark will become
a faint divided brush of light. If the ball on the conduc¬
tor is electrified negatively, the spark will be as shown in
fig. 7, clear, straight, and more luminous. If one of the
balls is positively, and the other negatively electrified, the
forms shown in fig. 6 and 7 wall be combined, as in fig. 8.
When, in this last experiment, the distance of the balls
is not too great, the positive zigzag spark will strike the
negative straight spark about one third of the length of
the latter from its point, the-other two thirds becoming
very luminous. Sometimes the positive spark strikes the
negative ball at a distance from the negative spark.
Exp. 2. If two conductors PM, fig. 9, three fourths of Fig. S
an inch in diameter, and having spherical ends, are placed
parallel to each other, at the distance of two inches, so as
to have their ends pointing in different directions six or
eight inches asunder; then, if P is positively electrified,
its spark will strike the other conductor M in its natural
state, as in fig. 9. If M is electrified negatively, and P
connected with the earth, the conductor M will send the
negative spark to P, as. in fig. 10 ; and if the conductors Fig-1
have opposite electricities, the positive spark will appear at
one end, and the negative at the other, as shown in fig. 1L
Exp. 3. Upon the brass stem be, fig. 12, having a fine Fig. I
point at c, place a brass ball A, about three inches radius,
so that the point c can be protruded to any distance be¬
yond the ball, or be drawn within it, as shown in the
figure. In this last state the point produces no effect,
and the zigzag spark appears between the balls.
In proportion, however, as the point is protruded, its
transmitting power is increased, and it may be made to
have the same effect as any ball, from the smallest size to
one three inches radius. When the point projects to a
particular distance, it acts as if no ball were present.
1 Description of an Electrical Telegraph, Qc. p. 4. Lend. 1823.