To understand the operation of this common household appliance we must bear in mind two or three familiar experimental facts in reference to the action of the voltaic cell. Briefly, such a cell consists of two plates of metal--for example, one of copper and the other of zinc--with a connecting medium, which is usually a liquid, but which may be a piece of moistened cloth or blotting-paper. So long as the two plates of metal are not otherwise connected there is no electricity in evidence, but when the two are joined by any metal conductor, as, for example, a piece of wire--thus, in common parlance, "completing the circuit"--a current of electricity flows about this circuit, passing from the first metal plate to the second, through the liquid and back from the second plate to the first through the piece of wire. The wire may be of any length. In the case of your call-bell, for example, the wire circuit extends to your door, and is there broken, shutting off the current. When you press the button you connect the broken ends of the wire, thus closing the circuit, as the saying is, and the re-established current, acting through a little electromagnet, rings the bell. In another case, the wire may be hundreds of miles in length, to serve the purposes of the telegrapher, who transmits his message by opening and closing the circuit, precisely as you operate your door-bell. For long-distance telegraphy, of course, large cells are required, and numbers of them are linked together to give a cumulative effect, making a strong current; but there is no new principle involved. The simplest study of this interesting mechanism makes it clear that the cell is the apparatus primarily involved in generating the electric current; yet it is equally obvious that the connecting wire plays an important part, since, as we have seen, when the wire is broken there is no current in evidence. Now, according to the electron theory, as previously outlined, the electric current consists of an actual flow along the wire of carriers of electricity which are unable to make their way except where a course is provided for them by what is called a conductor. Dry air, for example, is, under ordinary circumstances, quite impervious to them. This means, then, that the electrons flow freely along the wire when it is continuous, but that they are powerless to proceed when the wire is cut. When you push the button of your call-bell, therefore, you are virtually closing the switch which enables the electrons to proceed on their interrupted journey. THEORIES OF ELECTRICAL ACTION