[Illustration: FIG. 204.—An electric stove.]
290. Electric Lights. The incandescent bulbs which illuminate our buildings consist of a fine, hairlike thread inclosed in a glass bulb from which the air has been removed. When an electric current is sent through the delicate filament, it meets a strong resistance. The heat developed in overcoming the resistance is so great that it makes the filament a glowing mass. The absence of air prevents the filament from burning, and it merely glows and radiates the light.
[Illustration: FIG. 205.—The heating element in the electric stove.]
291. Blasting. Until recently, dynamiting was attended with serious danger, owing to the fact that the person who applied the torch to the fuse could not make a safe retreat before the explosion. Now a fine wire is inserted in the fuse, and when everything is in readiness, the ends of the wire are attached to the poles of a distant battery and the heat developed in the wire ignites the fuse.
[Illustration: FIG. 206.—An electric pad serves the same purpose as a hot water bag.]
292. Welding of Metals. Metals are fused and welded by the use of the electric current. The metal pieces which are to be welded are pressed together and a powerful current is passed through their junction. So great is the heat developed that the metals melt and fuse, and on cooling show perfect union.
293. Chemical Effects. 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.
[Illustration: FIG. 207.—An incandescent electric bulb.]
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.
[Illustration: FIG. 208.—Carbon rods in a solution of copper sulphate.]
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.