Fig. 22.—Decomposing water into its chemical constituents by means of an electric current.
Experiment 9.—The previous experiment showed that an electric current heats a material through which it passes. If the thin iron wire be shortened to a length of about 1⁄2-inch, our battery will probably bring it to red-heat, thus demonstrating the principle of electric incandescent lamps. The difference between theory and practice, however, in this case consists in the use of carbon, or, very rarely, platinum, in place of the iron filament, and of inclosing this in a glass bulb free of air, so that combustion cannot proceed rapidly.
Experiment 10.—Immerse two wires from the battery terminals at some little distance apart in a glass of water, which has been slightly soured with sulphuric acid or spirits of salt. The weak acid readily conducts the electric current, which decomposes the water into its chemical constituents, hydrogen and oxygen, the former gas coming off in bubbles at the wire which leads from the battery tins, and the oxygen round the other conductor ([Fig. 22]). The hydrogen bubbles may perhaps be ignited as they are evolved by holding a lighted match just near the water’s surface; or another method is to seal the wires into separate glass tubes, so that both dip beneath the water, and light the hydrogen gas as it escapes from the tube’s upper end ([Fig. 23]). In this case great care must be taken to allow time for the expulsion of all air from the tube, because hydrogen and air in certain proportions form a very explosive mixture.
Fig. 23.—The hydrogen bubbles may be ignited by holding a match near the free end of the hydrogen tube.
Fig. 24.—Electro-plating in its infancy.
Experiment 11.—Repeat the foregoing experiment, using copper sulphate solution in place of the acidified water. After the current has passed for some time, one of the wires will be noticed to have become thicker whilst the diameter of the other has decreased. This behavior is owing to deposit of copper from the solution on the one conductor and abstraction of metal from the other, whose bulk diminishes in automatically maintaining the solution’s strength ([Fig. 24]). In this reaction is seen the basis of commercial electroplating—silver and nickel solutions being mostly employed instead of the copper bath, since these are the metals with which those of a baser nature are more frequently plated.
Experiment 12.—Electrotyping is a modification of electro-plating, where a mold of wax coated with some conducting substance like graphite is used to deposit the metal upon. Melt some quantity of sealing-wax on to a piece of cardboard, so that it spreads out slightly larger in diameter than a fifty-cent piece, and when just plastic press the “head” surface of the new coin into the wax, so that an exact replica is obtained. Fasten a copper wire by some extra wax to the cardboard disc (as in [Fig. 25]), and carefully cover the whole matrix with powdered blacklead, working it well into the crevices and up to the copper conductor, with a camel-hair brush. Hang this in a jar containing saturated copper sulphate solution—the copper wire being connected to the negative battery terminals (A, [Fig. 26]), whilst a sheet of copper or coil of wire is suspended in the solution some little distance from the sealing-wax mold, with a wire connecting to the other battery terminal (B, [Fig. 26]).