POWER, TRANSMISSION OF, HYDRAULIC. For Mechanical Transmission of Power, See BELTS. For Transmission by Compressed Air, see AIR COMPRESSORS. See, also, NIAGARA, UTILIZATION OF.) Dr. Pacinotti, in June, DM, mentioned the fact that his " electro-magnetic machine" could be used either to generate electricity on the application of motive power to the arma ture, or to produce motive power on connecting it with a suitable source of current. This, so far as can be determined, was the first mention of the now so well-known principle of the reversibility of the dynamo-electric machine, the practical utilization of which implies the electrical transmission of mechanical energy.
The principle of the reversibility of dynamo-electric machines appears to have been per ceived by Messrs. Siemens about 1867, but it was not heard of in practical application until the year 1873, when it was practically demonstrated by MM. Hippolyle Fontaine and Breguet at the Vienna Universal Exposition. In this case a Gramtne machine used as a motor to work a pump was run by the current produced by a similar machine connected by more than a mile of cable, and put in motion by a gas engine. This was the first instance of electrical transmission of mechanical energy to a distance.
Theoretical Considerations.—The work done by any electric motor is equal to the product of the current flowing through the circuit and the counter electromotive force the motor has set up. The efficiency of transmission is as the ratio of the electromotive force of the generator, E, to that of the motor (counter E.M.F.), e, that is, efficiency =E. As this expression does not contain the factor of resistance of the line or machines, Marcel Deprez deduced therefrom that the electrical transmission of power is independent of the distance of transmission. Theoretically this assumption is correct, but in practice various factors involved make its direct application impossible. According to M. Deprez, in order to obtain the same useful work, whatever be the length of the line, it suffices simply to vary the electromotive forces of the machine proportionally to the square root of the resistance of the circuit. In other words, if 11 represents the resistance of the circuit, and E and e.
respectively, the electromotive forces of the machines, and in such a circuit we obtain useful work at the motor me, then, in order to obtain the same amount of work with other values, ii', E', e', it is necessary to make the new values E' and such that they will satisfy the following equations: Er E .1?' /R' =1 / P R General Data.—The three elements of electrical transmission of power are: (1st) The generators which are placed at the power station and which are driven by the water-wheel or steam-engine or other prime mover ; (2d) the copper conductors which are placed on poles like telegraph wires, and which conduct the electric current from the generators to (3d) the motors, which deliver the electrical energy to all kinds of machinery. The motors are either belted or geared to these machines. Ordinarily electric manufacturers allow for motors up to 20 horse-power, 1.000 watts per mechanical horse-power, indicating 7:3 per cent. efficiency of the motor ; from 20 to 50 horse-power, 900 watts per mechanical horse-power. indicating 83 per cent. efficiency of the motor; over 50 horse-power, 830 watts per mechanical horse power, indicating 90 per cent. efficiency of the motor. A similar rule will hold good for generators. Up to 20 horse-power the output in electrical horse-power will be about 75 per cent. of the mechanical horse-power applied to the pulley. From 21 to 50 horse-power the output in electrical horse-power will be about 83 per cent. of the mechanical horse-power applied to the pulley. Over 50 horse-power the output in electrical horse-power will be about 90 per cent. of the mechanical horse-power applied to the pulley. 746 watts (one watt = ampere x volt) equal 1 electrical horse-power.