Page:Elementary Text-book of Physics (Anthony, 1897).djvu/341

§ 283], or, if solid, can be weighed. This arrangement is called a voltameter. If the current strength be desired, the current must be kept constant in the voltameter by suitable variation of the resistance in the circuit during the time in which electrolysis is going on.

Two forms of voltameter are in frequent use.

In the first form there is, on the whole, no chemical work done in the electrolytic process. The system consisting of two copper electrodes and cupric sulphate as the electrolyte is an example of such a voltameter. The weight of the copper deposited on the cathode measures the current.

The second form depends for its indications on the evolution of gas, the volume of which is measured. The water voltameter is a type, and is the form especially used. The gases evolved are either collected together, or the hydrogen alone is collected. The latter is preferable, because oxygen is more easily absorbed by water than hydrogen, and an error is thus introduced when the oxygen is measured.

282. Measure of the Counter Electromotive Force of Decomposition.— In the general formula developed in § 377 the quantity $$IA$$ represents the energy expended in the circuit which does not appear as heat developed in accordance with Joule's law. In the present case it is the energy expended during electrolysis in decomposing chemical compounds and in doing mechanical work. In many cases the mechanical work done is not appreciable; but when a liquid like water is decomposed into its constituent gases, work is done by the expansion of the gases from their volume as water to their volume as gases. In many cases some of this energy is also used in keeping the temperature of the electrolyte constant. These cases occur when the electromotive force developed varies with the temperature.

In case no such variation with the temperature occurs, we may calculate the electromotive force developed in terms of heat. Let $$e$$ represent the electro-chemical equivalent of one of the ions, and $$\theta$$ the heat evolved by the combination of a unit mass of this ion with