INACTIVE CONDUCTING VOLTAIC CIRCLES, — SULPHURET OF POTASSA. 69 
introduced at G. In this arrangement there were three metallic contacts of platinum 
and iron, a b and x : the two first being opposed to each other, may be considered as 
neutralizing each other’s forces ; but the third, being unopposed by any other me- 
tallic contact, can be compared with either the difference of a and b when one is 
warmer than the other, or with itself when in a heated or cooled state (1830.), or with 
the force of chemical action when any body capable of such action is introduced there 
(1831.). 
1825. When this arrangement is completed and in order, there is absolutely no 
current circulating through it, and the galvanometer-needle rests at 0°; yet is the 
whole circuit open to a very feeble current, for a difference of temperature at any one 
of the junctions a , b, or x, causes a corresponding thermo current, which is instantly 
detected by the galvanometer, the needle standing permanently at 30° or 40°, or 
even 50°. 
1826. But to obtain this proper and normal state, it is necessary that certain pre- 
cautions be attended to. In the first place, if the circuit be complete in every part 
except for the immersion of the iron and platinum plates into the cup D, then, upon 
their introduction, a current will be produced directed from the platinum (which ap- 
pears to be positive) through the solution to the iron ; this will continue perhaps five 
or ten minutes, or if the iron has been carelessly cleaned, for several hours ; it is due 
to an action of the sulphuretted solution on 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 (2049.). 
1827- 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 po- 
tassium, and the circuit be then completed, including a galvanometer, the arrange- 
ment, if perfect, will show no current ; but if one of the plates be lifted up into the 
air for a few seconds and then replaced, it will be negative to the other, and produce 
a current lasting for a short time *. If the two plates be iron and platinum, or of any 
other metal or substance not acted on by the sulphuret, the same effect will be pro- 
duced. 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'}- ; 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 reintroduction, almost certainly exhibit 
the negative state for a second or two. 
1828. These or other disturbing causes appear the greater in these experiments in 
* 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. 
t 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 re-immersion. Annales de Chimie, 1824, xxv. 405. 
