Inactive Conducting Circles 243

oxide of iron, and not to any effect on the metallic iron; and
when it has ceased, the disturbing cause may be considered as
exhausted. The experimental proofs of the truth of this
explanation I will quote hereafter (1037).

815. Another precaution relates to the effect of accidental
movements of the plates in the solution. If two platinum
plates be put into a solution of this sulphuret of potassium, and
the circuit be then completed, including a galvanometer, the
arrangement, if perfect, will show no current; but if one of the
plates be lifted up into the air for a few seconds and then re-
placed, it will be negative to the other, and produce a current
lasting for a short time.1 If the two plates be iron and plati-
num, or of any other metal or substance not acted on by the
sulphuret, the same effect will be produced. In these cases.,
the current is due to the change wrought by the air on the film
of sulphuretted solution adhering to the removed plate;2 but
a far less cause than this will produce a current, for if one of
the platinum plates be removed, washed well, dried, and even
heated, it will, on its re-introduction, almost certainly exhibit
the negative state for a second or two.

816. These or other disturbing causes appear the greater in
these experiments in consequence of the excellent conducting
power of the solution used; but they do not occur if care be
taken to avoid any disturbance of the plates or the solution, and
then, as before said, the whole acquires a normal and perfectly
inactive state.

817. Here then is an arrangement in which the contact of
platinum and iron at x is at liberty to produce any effect which
such a contact may have the power of producing; and yet what
is the consequence? absolutely nothing. This is not because
the electrolyte is so bad a conductor that a current of contact
cannot pass, for currents far feebler than this is assumed to
be pass readily (801); and the electrolyte employed is vastly
superior in conducting power to those which are commonly
used in voltaic batteries or circles, in which the current is still
assumed to be dependent upon contact. The simple conclusion
to which the experiment should lead is, in my opinion, that
the contact of iron and platinum is absolutely without any
electromotive force (823, 847, 877).

1 Marianini observed effects of this kind produced by exposure to the air, of
one of two plates dipped in nitric acid.—Annales de Chimie, 1830, xlv. p. 42.

2 Becquerel long since referred to the effect of such exposure of a plate,
dipped in certain solutions, to the air. Generally the plate so exposed
became positive on reimmersion.—Annales de Chimie, 1824, xxv. 405.ndorf s Annalcn, xlii.