Thomson Effect.
In an unequally heated conductor the differential heating is either
increased as in iron, or diminished as in copper by a current. In lead
the phenomenon does not occur. It is termed the Thomson effect. It is
intimately related to the Peltier effect.
In a thermo-electric couple a heated junction is the source of
electro-motive force, if heated more than other parts of the circuit.
The current in a copper-iron junction flows from the copper to the iron
across the heated junction. A hot section of an iron conductor next to a
cold section of the same is a source of thermoelectricity, in the sense
that the hot section is negative to the colder. A current passing from
the hot to the cold iron travels against rising potentials, and cools
the iron in the cooler parts. As it passes to the hotter parts it
travels against falling potentials and hence heats the iron in these
parts. In this way a current intensifies differential heating in an iron
conductor.
In copper the reverse obtains. In it the thermo-electric relations of
hot and cold copper are the reverse of those of iron, and a current
tends to bring all parts of a differentially heated copper conductor to
an identical temperature.
As a current travels in iron from hot to cold it absorbs heat; in copper
traveling from cold to hot it absorbs heat.
The convection of heat by a current of electricity in unequally heated
iron is negative, for it is opposed to that convection of heat which
would be brought about by the flow of water through an unequally heated
tube. In copper, on the other hand, the electric convection of heat is
positive. (Daniell.)
The above effects of the electric current upon an unequally heated
conductor are termed the Thomson effects. In iron, at low red heat, they
are reversed and are probably again reversed at higher temperatures.