JACOBI'S LAW concerning the maximum power of an electric motor supplied with currents from a source of given electromotive force, is the following : The mechanical work given out by a motor is a maximum when the motor is geared to run at such a speed that the current is reduced to half the strength that it would have if the motor was stopped. This, of course, implies that the counter electromotive force of the motor is equal to half the electromotive force furnished by the battery or generator. Now, under these circumstances, only half the energy furnished by the external source is utilized, the other half being wasted in heating the circuit. Dr. Siemens proved, in fact, that if the motor be arranged so as to do its work at less titan the maximum rate, by being geared so as to do much less work per revolution, but yet so as to run at a higher speed, it will be more efficient : that is to say, though it does less work, there will also be still less electric energy expended, and the ratio of the useful work done to the energy expended will be nearer unity than before. Hence, to get maximum work
per second out of an electric motor, the motor must run at such a speed as to bring down the current to half the value which it would have if the motor were at rest. That is to say, the efficiency is but rid per cent. when the motor does its work at the maximum rate.
When a dynamo is used as a motor, the power is supplied to it electrically in the form of electric currents delivered at a certain potential or pressure. if Cu is the number of amperes of current which flow through the motor, and EN be the number of volts of potential as measured at the terminals of the motor, then the electric power, in watts, Pa-, given by the mains to the motor, will be found by multiplying together the amperes and the volts, or = (watts). This may be expressed as electric horse power by dividing by 746, since 746 watts equal one electric horse-power.
(Eva v)