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

But all this, of course, leaves quite untouched the question of the origin of the electrons themselves. That these go hurtling from one plate or pole of the battery to the other, along the wire, we can understand at least as a working theory; that, furthermore, the electrons have their origin either in the metal plates or in the liquid that connects them, seems equally obvious; but how shall we account for their development? It is here that the chemist with his atomic theory of matter comes to our aid. He assures us that all matter consists in the last analysis of excessively minute particles, and that these particles are perpetually in motion. They unite with one another to form so-called molecules, but they are perpetually breaking away from such unions, even though they re-establish them again. Such activities of the atoms take place even in solids, but they are greatly enhanced when any substance passes from the solid into the liquid state.

When, for example, a lump of salt is dissolved in water, the atoms of sodium and of chlorine which joined together make up the molecules of salt are held in much looser bondage than they were while the salt was in a dry or crystalline form. Could we magnify the infinitesimal particles sufficiently to make them visible we should probably see large numbers of the molecules being dissociated, the liberated atoms moving about freely for an instant and then reuniting with other atoms. Thus at any given instant our solution of salt would contain numerous free atoms of sodium and

of chlorine, although we are justified in thinking of this substance as a whole as composed of sodium-chlorine molecules. It is only by thus visualizing the activity of the atoms in a solution that we are able to provide even a thinkable hypothesis as to the development of electricity in the voltaic cell.

What puts us on the track of the explanation we are seeking is the fact that the diverse atoms are known to have different electrical properties. In our voltaic cell, for example, sodium atoms would collect at one pole and chlorine atoms at the other. Humphry Davy discovered this fact in the early days of electro-chemistry, just about a century ago. He spoke of the sodium atom as electro-positive, and of the chlorine atom as electro-negative, and he attempted to explain all chemical affinity as merely due to the mutual attraction between positively and negatively electrified atoms. The modern theorist goes one step farther, and explains the negative properties of the chlorine atom by assuming the presence of one negative electron or electricity in excess of the neutralizing charge. The assumption is, that the sodium atom has lost this negative electron and thus has become positively electrified. The chlorine atom, harboring the fugitive electron, becomes negatively electrified. Hence the two atoms are attracted toward opposite poles of the cell.

This disunion of atoms, be it understood, must be supposed to take place in the case of any solution of common salt, whether it rests in an ordinary cup or forms a part of the ocean. Here we have, then, material for the generation of the electrical current, if some

means could be found to induce the chlorine atom to give up the surplus electron which from time to time it carries. And this means is provided when two pieces of metal of different kinds, united with a metal conductor, are immersed in the liquid. Then it comes to pass that the electrons associated with the chlorine atoms that chance to lie in contact with one of these plates of metal, find in this metal an avenue of escape. They rush off eagerly along the metal and the connecting wire, and in so doing establish a current which acts-if we may venture a graphic analogy from an allied field of physics -as a sort of suction, attracting other chlorine atoms from the body of the liquid against the metal plate that they also may discharge their electrons. In other words, the electrical current passes through the liquid as well as through the outside wire, thus completing the circuit.

According to this theory, then, the electrical energy in evidence in the current from the voltaic cell, is drawn from a store of potential energy in the atoms of matter composing the liquid in the cell. In practice, as is well known, the liquid used is one that affects one of the metal poles more actively than the other, insuring vigorous chemical activity. But the principle of atomic and electrical dissociation just outlined is the one involved, according to theory, in every voltaic cell, whatever the particular combination of metals and liquids of which it is composed. It should be added, however, that while we are thus supplied with a thinkable explanation of the origin of this manifestation of electrical energy, no explanation is forthcoming, here

any more than in the case of the dynamo, as to why the electrons rush off in a particular direction and thus establish an electrical current. Perhaps we should recall that the very existence of this current has at times been doubted. Quite recently, indeed, it has been held that the seeming current consists merely of a condition of strain or displacement of the ether. But we are here chiefly concerned with the electron theory, according to which, as we have all along noted, the seeming current is an actual current; the ether strain, if such exists, being due to the passage of the electrons.

PRACTICAL USES OF ELECTRICITY

Various effects of the current of electrons have been hinted at above. Considered in detail, the possible ways in which these currents may be utilized are multifarious. Yet, they may be all roughly classified into three divisions as follows:

First, cases in which the current of electricity is used to transmit energy from one place to another, and reproduce it in the form of molar motion. The dynamo, in its endless applications, illustrates one phase of such transportation of energy; and the call-bell, the telegraph, and the telephone represent another phase. In one case a relatively large quantity of electricity is necessary, in the other case a small quantity; but the principle involved-that of electric and magnetic induction-is the same in each.

The second method is that in which the current, generated by either a dynamo or a battery of voltaic