Page:Elementary Text-book of Physics (Anthony, 1897).djvu/400

386 continually decreases in numerical value, until at $$t_{1} = t_{n}$$ it becomes zero. At that temperature, then, the metals are thermoelectrically neutral to one another, and a small change in the temperature does not change the electromotive force at the junction.

317. The Thomson Effect.—Thomson has shown that, in certain metals, there must be a reversible thermal effect when the current passes between two unequally heated parts of the same metal. Let us suppose a circuit of copper and iron, of which one junction is at the neutral temperature and the other below the neutral temperature. The current then sets from copper to iron across the hot junction. In the hot junction there is no thermal effect produced, because the metals are at the neutral temperature. Across the cold junction the current is flowing from iron to copper, and hence is evolving heat. The current in the circuit can be made to do work, and since no other energy is imparted to the circuit this work must be done at the expense of the heat in the circuit. Since heat is not absorbed at either junction, it must be absorbed in the unequally heated parts of the circuit between the junctions.

To show this, Thomson used a conductor the ends of which were kept at constant temperatures in two coolers, while the central portion was heated. When a current was passed through this conductor, thermometers, placed in contact with exposed portions of the conductor between the heater and the coolers, indicated a rise of temperature different according as the current was passing from hot to cold or from cold to hot. The heat seems therefore to be carried along by the current, and the process has accordingly been called the electrical convection of heat. In copper the heat moves with the current, in iron against it. In another form of statement it may be said that, in unequally heated copper, a current from hot to cold heats the metal, and from cold to hot cools it, while in iron the reverse thermal effects occur. The experiments of Le Roux show that the process of electrical convection of heat cannot be detected in lead. For this reason lead is used as the standard metal in constructing the thermoelectric diagram.