304	Faraday's Researches
sharp and sudden. So if silver and copper be associated in a
dilute solution of sulphuret of potassium, the copper will be
chemically active and positive, and the silver will remain clean;
until of a sudden the copper will cease to act,,, the silver will
become instantly covered with sulphuret, showing by that the
commencement of chemical action there; and the needle of
the galvanometer will jump through 180°. Two pieces of silver
or of copper in solution of sulphuret of potassium produce the
same effect.
 1025.	If metals be used which are inactive in the fluids
employed, and the latter undergo no change during the time,,
from other circumstances, as heat, etc. (826, 925), then no
currents, and of course no such alterations in direction, are
produced.
 1026.	Where no chemical action occurs no current is produced.
—This in regard to ordinary solid conductors is well known
to be the case, as with metals and other bodies (855).    It has
also been shown to be true when fluid conductors (electrolytes)
are used, in every case where they exert no chemical action,
though such different substances as acid, alkalies and sulphurets

have been employed (831, 841,,
813, 817). These are very
striking facts.
Fig. 77-
1027. But a current will occur
the moment chemical action com-
mences.—This proposition may
be well illustrated by the fol
lowing experiment. Make an
arrangement like that in fig.
77: the two tubes being charged with the same pure, pale,
strong nitric acid, the two platinum wires p p being connected
by a galvanometer, and the wire i, of 'iron. The apparatus is
only another form of the simple arrangement, fig. 78, where, in
imitation of a former experiment (624), two plates of iron and
platinum are placed parallel, but
-11	i	f    .	•    -	Tron*
separated by a drop of strong nitric     >	~T~7]l
acid at each extremity.   Whilst in     d—    ' ptaftwm,      —
this state no current is produced in	Fig. 78.
either apparatus; but if a drop of
water be added at b, fig. 78, chemical action commences, and a
powerful current is produced, though without metallic or any
additional contact. To observe this with the apparatus, fig. 77,
a drop of water was put in at b. At first there was no chemical