Page:Text-book of Electrochemistry.djvu/303

 CHAPTER XVII. Development of Heat by the Electric Current.

Review. — ^When electricity passes through a circuit con- sisting of one or several conductors, a quantity of heat, W, is evolved which can be calculated from the formula (see pp. 11 and 203)—

IF = 0-24i€ cal.

where i is the current strength (in amperes), and c the poten- tial difference (in volts) between the two ends of the conductor. If the conduction takes place along a uniform metal wire, or through a column of liquid, the heat is developed uniformly throughout the conducting material. If the circuit is not homogeneous, i.e. if we have surfaces of contact of different substances, then the heat is not equally distributed over all parts.

In the former case the formula given can also be written —

W = 0-24 i^m = 0-24 '^ cal.

m

where m is the resistance between the ends of the circuit. This quantity of heat, developed in a uniform conductor by resistance analogous to friction, is called Joule heat. Besides this there occurs a change of heat at contact surfaces, e.</. between metals, which is known as the Peltier effect, and which is measured by the expression —

JFi = 0-24^?: cal.

where n is the electromotive force of the Peltier effect.

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