But there is an additional resemblance between the electrically strained glass and the mechanically compressed air. When any elastic object has been strained, and is suddenly released, it regains its position of equilibrium by a series of oscillations or vibrations. Thus, for instance, if we take a strip of steel and fix one end of it in a vice, and pull the other end on one side and then release it, the steel regains its position of equilibrium only after having executed a series of diminishing swings to and fro. In the same way, if we place some mercury or water in a glass tube bent in the shape of the letter ⋃, and displace the liquid by blowing into the tube, then, on releasing the pressure suddenly, the liquid will regain its position of equilibrium by a series of oscillations which die gradually away. You will not have any difficulty in seeing that this is really due to the inertia of the material, whether it be steel or mercury or water which is displaced. In an exactly similar manner, we find that when we have produced an electric strain in a sheet of glass by the application of electromotive force, and if we then remove the electromotive force and connect the two tinfoil or metal surfaces by means of a piece of wire, the electric strain in the glass disappears with a series of _electric oscillations_; that is to say, the electric strain in the glass does not disappear or die away gradually, but it is alternately reversed, at each reversal the strain becoming less and less in magnitude. The result of this oscillatory strain in the glass is to produce in the connecting wire an alternating electric current. [Illustration: FIG. 65.]