safe current is conveyed from the transformer to the wires connected to the lamps. In this way a current of two thousand volts, an unsafe and unsuitable pressure for incandescent lighting, is exchanged for one of about one hundred volts, which is quite safe. In this way, also, the supply station is enabled to reach a customer too far away to be supplied directly with current at one hundred volts, without enormous expense for copper conductors. The alternating current transformer not only greatly extended the radius of supply from a single station, but also enabled the station to be conveniently located where water and coal could be had without difficulty. It also permitted the distant water-powers to become sources of electric energy for lighting, power, or for other service. For example, a water-power located at a distance of fifty to one hundred miles or more from a city, or from a large manufacturing centre where cost of fuel is high, may be utilized as follows: A power-station will be located upon the site of the water-power, and the dynamos therein will generate electricity at, say, two thousand volts pressure. By means of step-up transformers this will be exchanged for a current of thirty thousand volts for transmission over a line of copper or aluminum wire to the distant consumption area. Here there will be a set of step-down transformers which will exchange the thirty-thousand-volt line current for one of so low a pressure as to be safe for local distribution to lamps, to motors, etc., either stationary or upon a railway. The same transmission plant may simultaneously supply energy for lighting, for power, for heat, and for charging storage batteries. It may, therefore, be employed both day and night. These long-distance power transmission plants are generally spoken of as “two-phase,” “three-phase,” or “polyphase” systems. Before 1890 no such plants existed. A large number of such installations are now working over distances of a few miles up to one hundred miles. They differ from what are known as single-phase alternating systems in employing, instead of a single alternating current, two, three, or more, which are sent over separate lines, and in which the electric impulses are not simultaneous, but follow each other in regular succession, overlapping each other’s dead points, so to speak. Early suggestions of such a plan, about 1880, and thereafter, by Bailey, Deprez, and others, bore no fruit, and not until Tesla’s announcement of his polyphase system, in 1888, was much attention given to the subject. A widespread interest in Tesla’s work was invoked, but several years elapsed before engineering difficulties were overcome. This work was done mainly by the technical staffs of the large manufacturing companies, and it was necessary to be done before any notable power transmissions on the polyphase system could be established. After 1892 the growth became very rapid.