Page:Elementary Text-book of Physics (Anthony, 1897).djvu/395

§ 315] junction and absorbed by it in unit time, and $$I = \frac{P}{R}$$ in a case a similar quantity of heat is removed from the junction by cooling.

If two strips of dissimilar metals, for example antimony and bismuth, be placed side by side, and united at one end of the pair being everywhere else insulated from one another, the combination is called a thermoelectric element. If several such elements be joined in series, so that their alternate junctions lie near together and in one plane, as indicated in Fig. 89, such an arrangement is called a thermopile. When one face of the pile is heated, the electromotive force of the pile is the sum of the electromotive forces of the several elements. Such an instrument was used by Melloni, in connection with a delicate galvanometer, in his researches on radiant heat.

When a thermoelectric element is constructed of any two metals, that metal is said to be thermoelectrically positive to the other from which the current flows across the heated junction.

315. Thermoelectric Series.—It was found by the experiments of Seebeck himself, and those of others, that the metals may be arranged in a series such that any metal in it is thermoelectrically positive to those which follow it, and thermoelectrically negative to those which precede it.

If a circuit be formed of any two metals in this series, and one of the junctions be kept at the temperature zero, while the other is heated to a fixed temperature, there will be set up an electromotive force which can be measured. If now the circuit be broken at either junction, and the gap filled by the introduction of any other metals of the series, then, provided that the junction which has not been disturbed be kept at the temperature which it previously had, and that the other junctions in the circuit be all raised to the temperature of the junction which was broken, there will be the same electromotive force in the circuit as existed before the introduction