Page:Elementary Text-book of Physics (Anthony, 1897).djvu/396

382 of the other metals of the series. It is manifest, then, that in a circuit made up of any metals whatever, at one temperature, no electromotive force can be set up by changing the temperature of the circuit as a whole.

Thomson showed that it is not necessary for the production of thermal currents that the circuit should contain two metals; but that want of homogeneity arising from any strain of one part of an otherwise homogeneous circuit will also admit of the production of such currents. It has also been shown that when a portion of an iron wire is magnetized, and is heated near one of the poles produced, a thermal current will be set up.

Gumming discovered in 1823 that, if the temperature of one junction of a circuit of two metals be gradually raised, the current produced will increase to a maximum, then decrease until it becomes zero, after which it is reversed and flows in the opposite direction. The experiments of Avenarius, Tait, and Le Roux show that, for almost all metals, the temperature of the hot junction at which the maximum current occurs is the mean between the temperatures of the two junctions at which the current is reversed.

316. Thermoelectric Diagram.—The facts hitherto discovered in relation to thermoelectricity may be collected in a general formula or exhibited by means of a thermoelectric diagram.

Let us consider a circuit of two metals, copper and lead, in which both junctions are at first at the same temperature. We may assume that there is an equal electromotive force at both junctions acting from lead to copper. If one of the junctions be gradually heated, a current will be set up, passing from lead to copper across the hot junction. The heating has disturbed the equilibrium of electromotive forces, and has increased the electromotive force across the hot junction from lead to copper. The rate at which this electromotive force changes with change in the temperature is called the thermoelectric power of the two metals. That is, if $$E$$ represent the electromotive force, $$t$$ the temperature, and $$\theta$$ the thermoelectric power, we have $$\frac{E_{1} - E_{0}}{t_{1} - t_{0}} = \theta_{1},$$ in the limit