Some Uses of Electricity
: The Plating of Gold, Silver, and Other Metals.
If strips of lead or rods of carbon are connected to the terminals of an electric cell, as in Figure 208, and are then dipped into a solution of copper sulphate, the strip in connection with the negative terminal of the cell soon becomes thinly plated with a coating of copper. If a solution of silver nitrate is used in place of the copper sulphate, the coating formed will be of silver instead of copper. So long as the current flows and there is any metal present in the solution, the coating continues to form on the negative electrode, and becomes thicker and thicker.
The process by which metal is taken out of solution, as silver out of silver nitrate and copper out of copper sulphate, and is in turn deposited as a coating on another substance, is called electroplating. An electric current can separate a liquid into some of its various constituents and to deposit one of the metal constituents on the negative electrode.
Since copper is constantly taken out of the solution of copper sulphate for deposit upon the negative electrode, the amount of copper remaining in the solution steadily decreases, and finally there is none of it left for deposit. In order to overcome this, the positive electrode should be made of the same metal as that which is to be deposited. The positive metal electrode gradually dissolves and replaces the metal lost from the solution by deposit and electroplating can continue as long as any positive electrode remains.
Practically all silver, gold, and nickel plating is done in this way; machine, bicycle, and motor attachments are not solid, but are of cheaper material electrically plated with nickel. When spoons are to be plated, they are hung in a bath of silver nitrate side by side with a thick slab of pure silver, as in Figure. The spoons are connected with the negative terminal of the battery, while the slab of pure silver is connected with the positive terminal of the same battery. The length of time that the current flows determines the thickness of the plating.
FIG. - Carbon rods in a solution of copper sulphate.
FIG. - Plating spoons by electricity.