Faraday's Researches
242
associated with iron and platinum in a circuit. The arrange-
ment is given in fig. 65, where D, E represent two test-glasses
containing the strong solution of sulphuret of potassium (800);
and ^also four metallic plates, about 0.5 of an inch wide and
two inches long in the immersed part, of which the three marked
x	P, P, P were platinum,

and that marked I, of
clean iron: these were
connected by iron and
platinum wires, as in
fig. 65, a galvanometer
being introduced at G.
In this arrangement there
were three metallic con-
tacts of platinum and
iron, a, b, and x: the first
two, being opposed to
each other, may be con-
sidered as neutralising
each other's forces; but
the third, being unop-
posed by any other
metallic 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 (818), or with the force of
chemical action when any body capable of such action is
introduced there (819).
 813.	When this arrangement is completed and in order, there
is'absolutely no current circulating through it, and the galvano-
meter-needle rests at o°; 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°.
 814.	But to obtain this proper and normal state, it is neces-
sary that certain precautions 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 appears 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