which will record the total quantity of electricity that has passed through the solution in a given time, such as a recording electricity meter used in the sale of electricity to householders. We shall find that the copper plate at which the current enters gets gradually thinner, and that at which the current issues gradually stouter exactly in the same measure, showing that the current has the effect of transporting copper from the one plate to the other.
An arrangement of this kind is called an electrolytic cell, and the process going on in the cell is called electrolysis. The plates are called the electrodes, and the liquid between them is the electrolyte. The electrode by which the current enters is also called the anode, and that by which it leaves is called the kathode. The current flows from the anode into the electrolyte and from the electrolyte to the kathode, taking the copper with it. If instead of copper sulphate the electrolyte contained some other metallic salt, the current would split this up chemically and take the metal, whatever it may be, with it and deposit it on the kathode. This is the principle on which objects are silver- or nickel-plated. It is also the principle on which copper is refined on a large scale. The
