§ 7. _Identity of Electricities derived from different sources._ § 8.
_Relation by measure of common and voltaic Electricity._
[Read January 10th and 17th, 1833.]
§ 7. _Identity of Electricities derived from different sources._
265. The progress of the electrical researches which I have had the honour
to present to the Royal Society, brought me to a point at which it was
essential for the further prosecution of my inquiries that no doubt should
remain of the identity or distinction of electricities excited by different
means. It is perfectly true that Cavendish[A], Wollaston[B], Colladon[C],
and others, have in succession removed some of the greatest objections to
the acknowledgement of the identity of common, animal and voltaic
electricity, and I believe that most philosophers consider these
electricities as really the same. But on the other hand it is also true,
that the accuracy of Wollaston's experiments has been denied[D]; and also
that one of them, which really is no proper proof of chemical decomposition
by common electricity (309. 327.), has been that selected by several
experimenters as the test of chemical action (336. 346.). It is a fact,
too, that many philosophers are still drawing distinctions between the
electricities from different sources; or at least doubting whether their
identity is proved. Sir Humphry Davy, for instance, in his paper on the
Torpedo[E], thought it probable that animal electricity would be found of a
peculiar kind; and referring to it, to common electricity, voltaic
electricity and magnetism, has said, "Distinctions might be established in
pursuing the various modifications or properties of electricity in those
different forms, &c." Indeed I need only refer to the last volume of the
Philosophical Transactions to show that the question is by no means
considered as settled[F].
[A] Phil. Trans. 1779, p. 196.
[B] Ibid. 1801, p. 434.
[C] Annnles de Chimie, 1826, p. 62, &c.
[D] Phil. Trans. 1832, p. 282, note.
[E] Phil. Trans. 1892, p. 17.
"Common electricity is excited upon non-conductors, and is readily
carried off by conductors and imperfect conductors. Voltaic
electricity is excited upon combinations of perfect and imperfect
conductors, and is only transmitted by perfect conductors or imperfect
conductors of the best kind. Magnetism, if it be a form of
electricity, belongs only to perfect conductors; and, in its
modifications, to a peculiar class of them[1]. Animal electricity
resides only in the imperfect conductors forming the organs of living
animals, &c."
[1] Dr. Ritchie has shown this is not the case. Phil. Trans. 1832, p.
294.
[F] Phil. Trans. 1832, p. 259. Dr. Davy, in making experiments on the
torpedo, obtains effects the same as those produced by common and
voltaic electricity, and says that in its magnetic and chemical power
it does not seem to be essentially peculiar,--p. 274; but he then
says, p. 275, there are other points of difference; and after
referring to them, adds, "How are these differences to be explained?
Do they admit of explanation similar to that advanced by Mr. Cavendish
in his theory of the torpedo; or may we suppose, according to the
analogy of the solar ray, that the electrical power, whether excited
by the common machine, or by the voltaic battery, or by the torpedo,
is not a simple power, but a combination of powers, which may occur
variously associated, and produce all the varieties of electricity
with which we are acquainted?"
At p. 279 of the same volume of Transactions is Dr. Ritchie's paper,
from which the following are extracts: "Common electricity is diffused
over the surface of the metal;--voltaic electricity exists within the
metal. Free electricity is conducted over the surface of the thinnest
gold leaf as effectually as over a mass of metal having the same
surface;--voltaic electricity requires thickness of metal for its
conduction," p. 280: and again, "The supposed analogy between common and
voltaic electricity, which was so eagerly traced after the invention of
the pile, completely fails in this case, which was thought to afford the
most striking resemblance." p. 291.
266. Notwithstanding, therefore, the general impression of the identity of
electricities, it is evident that the proofs have not been sufficiently
clear and distinct to obtain the assent of all those who were competent to
consider the subject; and the question seemed to me very much in the
condition of that which Sir H. Davy solved so beautifully,--namely, whether
voltaic electricity in all cases merely eliminated, or did not in some
actually produce, the acid and alkali found after its action upon water.
The same necessity that urged him to decide the doubtful point, which
interfered with the extension of his views, and destroyed the strictness of
his reasoning, has obliged me to ascertain the identity or difference of
common and voltaic electricity. I have satisfied myself that they are
identical, and I hope the experiments which I have to offer and the proofs
flowing from them, will be found worthy the attention of the Royal Society.
267. The various phenomena exhibited by electricity may, for the purposes
of comparison, be arranged under two heads; namely, those connected with
electricity of tension, and those belonging to electricity in motion. This
distinction is taken at present not as philosophical, but merely as
convenient. The effect of electricity of tension, at rest, is either
attraction or repulsion at sensible distances. The effects of electricity
in motion or electrical currents may be considered as 1st, Evolution of
heat; 2nd, Magnetism; 3rd, Chemical decomposition; 4th, Physiological
phenomena; 5th, Spark. It will be my object to compare electricities from
different sources, and especially common and voltaic electricities, by
their power of producing these effects.