Page:Elementary Text-book of Physics (Anthony, 1897).djvu/394

 314. Thermo-electric Currents.—The heating or cooling of a junction of two dissimilar metals by the passage of a current, referred to in § 270 as the Peltier effect, is the reverse of a phenomenon discovered in 1822–33 by Seebeck. He found that, when the junction of two dissimilar metals was heated, a current was sent through any circuit of which they formed a part. It has since been shown that the same phenomenon appears if the junction of two liquids, or of a liquid and a metal, be heated. This fact, as has been already shown in § 277, follows as a result of the Peltier phenomenon. If we designate by $$P$$ the heat developed at the junction by the passage of unit current for unit time, we may substitute it for the expression $$\frac{A}{t}$$ in the general equation of § 277, and obtain $$I = \frac{E - P}{R}\cdot$$ The counter-electromotive force set up at the heated junction is the coefficient $$P.$$

If the electromotive force $$E$$ and the current $$I$$ be reversed in the circuit, the junction is cooled and we obtain $$I = \frac{E + P}{R}\cdot$$ The electromotive force at the junction, therefore, tends to increase the electromotive force of the circuit. Since the current in this case is opposite to the current in the case in which the junction is heated, the direction of the electromotive force at the junction is the same in both cases. If there be no electromotive force $$E$$ in the circuit, we have $$I = - \frac{P}{R}$$ in case a unit of heat is communicated to the