*THE POPULAR SCIENCE MONTHLY.*

It is only by the alternating system of distribution that we can realize this essential condition of economy. We have here no such limit to the electrical pressure in the generating apparatus as in the direct-current system, and through the medium of the converter it becomes possible to vary the two elements of electrical energy—current volume and pressure—to suit the most widely differing applications. It is this latter feature of the system which gives it its great range and flexibility, and its consequent economic value. It enables us, for instance, to generate a current of a certain voltage at the machine, then to raise this to ten, twenty, or fifty times the original pressure for transmission through the line, and then at the far end to step down to as low a pressure as we may want—a pressure suitable for entering dwellings, offices, and shops, and safe in the hands of the consumer. These successive transformations and retransformations, it should be noted, are effected in the simplest kind of a way. They involve no machinery with moving parts, but simply coils of wire placed in such relation to each other that the currents passing in one induce similar currents in the other. The practical value of this system arose with the discovery that the induction coil, like the dynamo, is reversible. This coil had long been used to transform a current of considerable volume and low pressure into one of very great pressure and small volume. The construction which enabled this to be done consisted in making the primary coil with a few turns of stout wire; and the secondary—that on which the induced current was produced—of a great many turns of fine wire. It was presently discovered, however, that this mode of operation might be reversed, and that, by passing a high-tension current of small volume through many turns of wire, a current of large volume and low pressure could be induced in a secondary circuit of few turns, and that the pressure and volume of the induced current in relation to that of the primary one depended only on the relative number of wire turns in the two circuits. If, for instance, the primary and secondary coils contained the same number of turns, the pressure and volume of the induced current would be precisely the same as the primary one. If, on the other hand, the induced circuit contained ten times the number of coils of the primary, the current in it would have a tenth of the volume and ten times the pressure of the primary one, while if the relation of the two circuits were reversed the induced current would have its volume increased to ten times and its pressure reduced to one tenth of that flowing in the primary.

In the field of lighting this method of electric distribution has taken a leading place, and it is no longer questioned that it is destined to displace entirely all methods of direct-current supply. It has heretofore found but little application to power transmis-