It is possible to show you an electrical experiment which has a close analogy with the above simple mechanical experiment. Here is a glass tube which has platinum wires sealed into the two ends, and the tube is partly exhausted of its air. Such a tube is called a _vacuum tube_, and when an electrical current is passed through this rarefied air, it causes it to become luminous, and, as you see when the room is darkened, the tube is filled with a reddish light. A tube, therefore, of this kind is very convenient in some experiments, because we can, in effect, see the electric current passing through it. If I connect one end of this tube with the earth, and the other with the terminal of an electrical machine, and if then the handle of the electrical machine is turned, the tube will continue to glow as long as the electrical machine is rotated. The electrical machine must be regarded as a pump which is forcing something called electricity through the vacuum tube, and as long as the pressure is continued the current flows. This corresponds with the case in which the tap at the bottom of the force pump was open and a continuous current of air could be forced out of it by pressing down the piston. Supposing, however, that I insert between the vacuum tube and the electrical machine a plate of glass, which is covered over with tinfoil on the two sides such an arrangement constitutes what is called a condenser, or Leyden pane. We now repeat the experiment, and begin to turn the handle of the electrical machine. You will notice that the vacuum tube glows as before, and is filled with a reddish light for a short time, but as we continue to turn the handle this dies away, and after a few moments there is no further evidence of an electric current passing through the vacuum tube. You will understand, therefore, that an electric current cannot be caused to flow for an indefinite time in one direction through a glass plate, although, by the application of electromotive force, it does evidently, as you see, pass through it for a short time. This is analogous to the operation of the force-pump when the tap at the bottom is closed. We then find that we can move the piston down a little way, compressing or straining the enclosed air, but that its motion is soon stopped by an opposing resistance. We therefore say that in the glass plate we have created an _electric strain_ by the action of the electromotive force, just as we describe the effect of the mechanical pressure on the air by saying that we have created a compression in it.