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126 on the objects to be treated. Conversely, it is also possible to draw up a balance sheet showing how much zinc will be used up in a voltaic cell relatively to the amount of electricity obtained from the cell. Such a balance sheet shows that the production of electric currents in large quantities by voltaic cells is far too expensive for commercial use. It is only when feeble currents are required that it pays to produce current by electrolytic process; when current is required on a large scale, such as for lighting and power, and also for metallurgical work, copper refining and electro-plating on a large scale, it must be produced by dynamo machines. Electrolysis, then, is not of much importance for the production of electricity, but it is of enormous importance in the utilisation of electricity, forming the basis of copper refining and electro-plating, on which large industries carried on in Swansea, Birmingham, Sheffield and other places are built up.

Thermal action of an electric current.—An electric current flowing through any conductor heats it. The amount of heat developed depends on the material of the conductor, its length, cross section and the strength of the current. As the current is increased