244 Faraday's Researches

818. If the contact be made really active and effective,
according to the beautiful discovery of Seebeck, by making its
temperature different to that of the other parts of the circuit,
then its power of generating a current is shown (812). This
enables us to compare the supposed power of the mere contact
with that of a thermo contact; and we find that the latter
comes out as infinitely greater than the former, for the former
is nothing. The same comparison of mere contact and thermo
contact may be made by contrasting the effect of the contact c
at common temperatures, with either the contact at a or at b,
either heated or cooled. Very moderate changes of tempera-
ture at these places produce instantly the corresponding current,
but the mere contact at x does nothing.

819. So also I believe that a true and philosophic and even
rigid comparison may be made at x, between the assumed effect
of mere contact and that of chemical action. For if the metals
at x be separated, and a piece of paper moistened in dilute acid,
or a solution of salt, or if only the tongue or a wet finger be
applied there, then a current is caused, stronger by far than
the thermo currents before produced (818), passing from the
iron through the introduced acid or other active fluid to the
platinum. This is a case of current from chemical action with-
out any metallic contact in the circuit on which the effect can
for a moment be supposed to depend (614); it is even a case
where metallic contact is changed for chemical action, with
the result that where contact is found to be quite ineffectual,
chemical action is very energetic in producing a current.

820. It is of course quite unnecessary to say that the same
experimental comparisons may be made at either of the other
contacts, a or b.

821. Admitting for the moment that the arrangement proves
that the contact of platinum and iron at x has no electromotive
force (823, 847), then it follows also that the contact of either
platinum or iron with any other metal has no such force. For
if another metal, as zinc, be interposed between the iron and
platinum at x, fig. 65, no current is produced; and yet the test
application of a little heat at a or b will show by the corre-
sponding current that the circuit being complete will conduct
any current that may tend to pass. Now that the contacts
of zinc with iron and with platinum are of equal electromotive
force is not for a moment admitted by those who support the
theory of contact activity; we ought therefore to have a result-
ing action equal to the differences of the two forces, producingplate so exposed