Motor, ELECTRIC

The action of the electric motor depends on the complete reversibility of the dynamo electric machine. It was not till 1873, after the substitution of electro-magnets for permanent ones in electric generators, that this principle was fully recognized at the Vienna exhibition. An electric motor, therefore, is a dynamo employed to convert electric energy into mechanical energy. The motor made by the Edison General Electric Company has a high efficiency and is widely used. A coil through which a current is pa ing, and which is capable of rotating about a diameter as an axis, experiences, when placed in a magnetic field, a force tending to rotate it so as to make the flow of magnetic force through it a maximum. The work done per second, in thus rotating it, is the product of the moment of the rotating couple by the speed. This mechanical activity is derived from the electric energy supplied to the motor, and so it is the more nearly equal to the electrical work expended in the same time, as the motor is more efficient. Two laws of efficiency may be here mentioned: the first is Jacobi's law of maximum rate of work; for the maximum rate of work the motor must run at such a speed as to reduce the current to one-half that pa ing when the motor is at rest, or the efficiency is only 50 per cent when the motor does work at the maximum rate. Siemens, however, pointed out theoretically that a higher efficiency may be obtained from a motor if it be made to work at le than its maximum rate. True, it does le work, but the amount of work done upon it is also le ened, and to a greater extent, so that the efficiency is higher. The ratio of the work done by the motor to the work done upon it is simply the ratio of the counter electromotive force developed by the motor itself to that supplied to it by the generator. So that by increasing this counter electromotive force, either by increasing the strength of the field or the speed of rotation, the efficiency may be made very nearly unity, exactly as is the case with the generating dynamo; this is the second law. Electric motors are made in many forms and are used for a great variety of purposes. In cities they are usually placed or supplied by power supplied by powerful dyn dynamos at central power houses. The convenience and economy of the electric motor have led to its common use in operating many forms of light machinery such as fans, sewing machines, lathes, printing pre es, etc. On a larger scale, it is used for running elevators and street cars, and may soon displace the steam locomotive on railways.