268. _Tension._--When a voltaic battery of 100 pairs of plates has its
extremities examined by the ordinary electrometer, it is well known that
they are found positive and negative, the gold leaves at the same extremity
repelling each other, the gold leaves at different extremities attracting
each other, even when half an inch or more of air intervenes.
269. That ordinary electricity is discharged by points with facility
through air; that it is readily transmitted through highly rarefied air;
and also through heated air, as for instance a flame; is due to its high
tension. I sought, therefore, for similar effects in the discharge of
voltaic electricity, using as a test of the passage of the electricity
either the galvanometer or chemical action produced by the arrangement
hereafter to be described (312. 316.).
270. The voltaic battery I had at my disposal consisted of 140 pairs of
plates four inches square, with double coppers. It was insulated
throughout, and diverged a gold leaf electrometer about one third of an
inch. On endeavouring to discharge this battery by delicate points very
nicely arranged and approximated, either in the air or in an exhausted
receiver, I could obtain no indications of a current, either by magnetic or
chemical action. In this, however, was found no point of discordance
between voltaic and common electricity; for when a Leyden battery (291.)
was charged so as to deflect the gold leaf electrometer to the same degree,
the points were found equally unable to discharge it with such effect as to
produce either magnetic or chemical action. This was not because common
electricity could not produce both these effects (307. 310.); but because
when of such low intensity the quantity required to make the effects
visible (being enormously great (371. 375.),) could not be transmitted in
any reasonable time. In conjunction with the other proofs of identity
hereafter to be given, these effects of points also prove identity instead
of difference between voltaic and common electricity.
271. As heated air discharges common electricity with far greater facility
than points, I hoped that voltaic electricity might in this way also be
discharged. An apparatus was therefore constructed (Plate III. fig. 46.),
in which AB is an insulated glass rod upon which two copper wires, C, D,
are fixed firmly; to these wires are soldered two pieces of fine platina
wire, the ends of which are brought very close to each other at _e_, but
without touching; the copper wire C was connected with the positive pole of
a voltaic battery, and the wire D with a decomposing apparatus (312. 316.),
from which the communication was completed to the negative pole of the
battery. In these experiments only two troughs, or twenty pairs of plates,
were used.
272. Whilst in the state described, no decomposition took place at the
point _a_, but when the side of a spirit-lamp flame was applied to the two
platina extremities at _e_, so as to make them bright red-hot,
decomposition occurred; iodine soon appeared at the point _a_, and the
transference of electricity through the heated air was established. On
raising the temperature of the points _e_ by a blowpipe, the discharge was
rendered still more free, and decomposition took place instantly. On
removing the source of heat, the current immediately ceased. On putting the
ends of the wires very close by the side of and parallel to each other, but
not touching, the effects were perhaps more readily obtained than before.
On using a larger voltaic battery (270.), they were also more freely
obtained.
273. On removing the decomposing apparatus and interposing a galvanometer
instead, heating the points _e_ as the needle would swing one way, and
removing the heat during the time of its return (302.), feeble deflections
were soon obtained: thus also proving the current through heated air; but
the instrument used was not so sensible under the circumstances as chemical
action.
274. These effects, not hitherto known or expected under this form, are
only cases of the discharge which takes place through air between the
charcoal terminations of the poles of a powerful battery, when they are
gradually separated after contact. Then the passage is through heated air
exactly as with common electricity, and Sir H. Davy has recorded that with
the original battery of the Royal Institution this discharge passed through
a space of at least four inches[A]. In the exhausted receiver the
electricity would _strike_ through nearly half an inch of space, and the
combined effects of rarefaction and heat were such upon the inclosed air us
to enable it to conduct the electricity through a space of six or seven
inches.
[A] Elements of Chemical Philosophy, p. 153
275. The instantaneous charge of a Leyden battery by the poles of a voltaic
apparatus is another proof of the tension, and also the quantity, of
electricity evolved by the latter. Sir H. Davy says[A], "When the two
conductors from the ends of the combination were connected with a Leyden
battery, one with the internal, the other with the external coating, the
battery instantly became charged; and on removing the wires and making the
proper connexions, either a shock or a _spark_ could be perceived: and the
least possible time of contact was sufficient to renew the charge to its
full intensity."
[A] Elements of Chemical Philosophy, p. 154.
276. _In motion:_ i. _Evolution of Heat._--The evolution of heat in wires
and fluids by the voltaic current is matter of general notoriety.
277. ii. _Magnetism._--No fact is better known to philosophers than the
power of the voltaic current to deflect the magnetic needle, and to make
magnets according to _certain laws_; and no effect can be more distinctive
of an electrical current.
278. iii. _Chemical decomposition._--The chemical powers of the voltaic
current, and their subjection to _certain laws_, are also perfectly well
known.
279. iv. _Physiological effects._--The power of the voltaic current, when
strong, to shock and convulse the whole animal system, and when weak to
affect the tongue and the eyes, is very characteristic.
280. v. _Spark_.--The brilliant star of light produced by the discharge of
a voltaic battery is known to all as the most beautiful light that man can
produce by art.
* * * * *
281. That these effects may be almost infinitely varied, some being exalted
whilst others are diminished, is universally acknowledged; and yet without
any doubt of the identity of character of the voltaic currents thus made to
differ in their effect. The beautiful explication of these variations
afforded by Cavendish's theory of quantity and intensity requires no
support at present, as it is not supposed to be doubted.
282. In consequence of the comparisons that will hereafter arise between
wires carrying voltaic and ordinary electricities, and also because of
certain views of the condition of a wire or any other conducting substance
connecting the poles of a voltaic apparatus, it will be necessary to give
some definite expression of what is called the voltaic current, in
contradistinction to any supposed peculiar state of arrangement, not
progressive, which the wire or the electricity within it may be supposed to
assume. If two voltaic troughs PN, P'N', fig. 42, be symmetrically arranged
and insulated, and the ends NP' connected by a wire, over which a magnetic
needle is suspended, the wire will exert no effect over the needle; but
immediately that the ends PN' are connected by another wire, the needle
will be deflected, and will remain so as long as the circuit is complete.
Now if the troughs merely act by causing a peculiar arrangement in the wire
either of its particles or its electricity, that arrangement constituting
its electrical and magnetic state, then the wire NP' should be in a similar
state of arrangement _before_ P and N' were connected, to what it is
afterwards, and should have deflected the needle, although less powerfully,
perhaps to one half the extent which would result when the communication is
complete throughout. But if the magnetic effects depend upon a current,
then it is evident why they could not be produced in _any_ degree before
the circuit was complete; because prior to that no current could exist.
283. By _current_, I mean anything progressive, whether it be a fluid of
electricity, or two fluids moving in opposite directions, or merely
vibrations, or, speaking still more generally, progressive forces. By
_arrangement_, I understand a local adjustment of particles, or fluids, or
forces, not progressive. Many other reasons might be urged in support of
the view of a _current_ rather than an _arrangement_, but I am anxious to
avoid stating unnecessarily what will occur to others at the moment.