in contact will produce a current by chemical action, and Seebeck
showed that heat might take the place of chemical action. Thus, if
a bar of antimony A (fig. 22) and a bar of bismuth S are in
contact at one end, and the junction is heated by a spirit lamp to
a higher temperature than the rest of the bars, a difference in
their electric state or potential will be set up, and if the other
ends are joined by a wire W, a current will flow through the wire.
The direction of the current, indicated by the arrow, is from the
bismuth to the antimony across the joint, and from the antimony to
the bismuth through the external wire. This combination, which is
called a "thermo-electric couple," is clearly analogous to the
voltaic couple, with heat in place of chemical affinity. The
direction of the current within and without the couple shows that
the bismuth is positive to the antimony. This property of
generating a current of electricity by contact under the influence
of heat is not confined to bismuth and antimony, or even to the
metals, but is common to all dissimilar substances in their
degree. In the following list of bodies each is positive to those
beneath it, negative to those above it, and the further apart any
two are in the scale the greater the effect. Thus bismuth and
antimony give a much stronger current with the same heating than
copper and iron. Bismuth and selenium produce the best result, but
selenium is expensive and not easy to manipulate. Copper and
German silver will make a cheap experimental couple:--

POSITIVE
Bismuth
Cobalt
Potassium
Nickel