§ 10. _On Conducting Power generally._[A]
[A] In reference to this § refer to 983 in series viii., and the
results connected with it.--_Dec. 1838._
418. It is not my intention here to enter into an examination of all the
circumstances connected with conducting power, but to record certain facts
and observations which have arisen during recent inquiries, as additions to
the general stock of knowledge relating to this point of electrical
science.
419. I was anxious, in the first place, to obtain some idea of the
conducting power of ice and solid salts for electricity of high tension
(392.), that a comparison might be made between it and the large accession
of the same power gained upon liquefaction. For this purpose the large
electrical machine (290.) was brought into excellent action, its conductor
connected with a delicate gold-leaf electrometer, and also with the platina
inclosed in the ice (383.), whilst the tin case was connected with the
discharging train (292.). On working the machine moderately, the gold
leaves barely separated; on working it rapidly, they could be opened nearly
two inches. In this instance the tin case was five-eighths of an inch in
width; and as, after the experiment, the platina plate was found very
nearly in the middle of the ice, the average thickness of the latter had
been five-sixteenths of an inch, and the extent of surface of contact with
tin and platina fourteen square inches (384.). Yet, under these
circumstances, it was but just able to conduct the small quantity of
electricity which this machine could evolve (371.), even when of a tension
competent to open the leaves two inches; no wonder, therefore, that it
could not conduct any sensible portion of the electricity of the troughs
(384.), which, though almost infinitely surpassing that of the machine in
quantity, had a tension so low as not to be sensible to an electrometer.
420. In another experiment, the tin case was only four-eighths of an inch
in width, and it was found afterwards that the platina had been not quite
one-eighth of an inch distant in the ice from one side of the tin vessel. When this was introduced into the course of the electricity from the machine (419.), the gold leaves could be opened, but not more than half an inch; the thinness of the ice favouring the conduction of the electricity, and permitting the same quantity to pass in the same time, though of a much lower tension.
421. Iodide of potassium which had been fused and cooled was introduced
into the course of the electricity from the machine. There were two pieces, each about a quarter of an inch in thickness, and exposing a surface on each side equal to about half a square inch; these were placed upon platina plates, one connected with the machine and electrometer (419.), and the other with the discharging train, whilst a fine platina wire connected the two pieces, resting upon them by its two points. On working the electrical machine, it was possible to open the electrometer leaves about two-thirds of an inch.
422. As the platina wire touched only by points, the facts show that this
salt is a far better conductor than ice; but as the leaves of the
electrometer opened, it is also evident with what difficulty conduction, even of the small portion of electricity produced by the machine, is effected by this body in the solid state, when compared to the facility with which enormous quantities at very low tensions are transmitted by it when in the fluid state.
423. In order to confirm these results by others, obtained from the voltaic
apparatus, a battery of one hundred and fifty plates, four inches square, was well-charged: its action was good; the shock from it strong; the discharge would _continue_ from copper to copper through four-tenths of an inch of air, and the gold-leaf electrometer before used could be opened nearly a quarter of an inch.
424. The ice vessel employed (420.) was half an inch in width; as the
extent of contact of the ice with the tin and platina was nearly fourteen
square inches, the whole was equivalent to a plate of ice having a surface
of seven square inches, of perfect contact at each side, and only one
fourth of an inch thick. It was retained in a freezing mixture during the
experiment.