A great deal of attention has been given, in recent years, to the economical transmission of power by electricity, and the various problems that are involved have been solved so satisfac torily that enormous quantities of power are now transmitted electrically, in instances to a distance of upwards of 200 miles. The mechanical energy that is to be transmitted is first converted into electricity by means of a dynamo; and the electricity thus generated is led along a conductor (usually of copper) to the point where the power is to be used, it being there reconverted into mechanical energy by means of electric motors. In certain shops and mills, the power is transmitted in this way from the engine room to the various machines that are to be operated, each machine being pro vided with its own separate motor. Installa tions of this kind are especially successful when the work is of such a nature that the machines are idle for a considerable part of the time, be cause there is no loss, in electrical transmission systems, when the circuit is interrupted and no current is flowing; whereas in a mill that is fitted up with shafting and belts the shafting runs all the time, and the losses due to its friction go on all the time, whether the ma chines are running or not. The use of indi vidual motors, as described, is also advisable in plants where the machines are run at high speeds, since the desired speeds can be attained electrically, without the losses incident to the use of pulleys and belts, of trains of gears, or any other method of direct mechanical multipli cation.
In the transmission of power by electricity to considerable distances, the chief losses are those due to direct leakage of electricity along the line, and to the dissipation of energy in the form of heat in the conductor. The losses due to the former cause can be kept down to a reasonable limit by paying proper attention to insulation, though the problems that are here involved are very serious, when the potential of the conductor is maintained at 60,000 volts or over. The loss due to the development of heat on account of the resistance of the conductor can theoretically be diminished as much as we please, by merely increasing the size of the conductor; but conductors sufficient in size to render the heat loss negligible are too expen sive to be commercially practicable. Lord Kel vin, in an attempt to determine the most econo mical size of conductor for electrical trans mission, came to the conclusion that the maxi mum economy is attained when the conductor is of such a size that °the annual interest on the capital outlay is equal to the annual cost of energy wasted.* Under ordinary conditions, this is found to lead to the conclusion that the most economical current-density in the conduc tor is about 380 amperes per square inch of sectional area of the conductor. For the case in which a given horse power is to be delivered at a given distance, the condition is somewhat different, and not so simple. Professors Ayrton and Perry, who have investigated this case, have given somewhat complicated formula: for determining the most economical cross-section of the conductor. (Consult Kent, Pocket Book,' and the references there given). In long distance transmission it is customary to adopt, for the transmission line, a far higher potential than is desired for the operation of the machinery at the delivery end, since this artifice makes it possible to transmit a given quantity of electrical energy over a given conductor with a smaller heat loss than would be involved if the transmission were effected at a lower potential. When the transmitting current is alternating, the reduction of potential at the delivery end may be effected by means of a transformer; but when the transmitting current is direct, it is customary to employ a °motor-generator,* which consists of a motor, actuated by the transmission current, coupled on the same shaft with a dyna mo, which gives out a current adapted to the work to be done. The motor-generator is also used to transform an alternating transmission current into a continuous current, when the con tinuous current is desired for delivery purposes. Some of the problems of electrical transmission have been solved on a very large scale at Niagara Falls, where the water power is par tially utilized for mechanical purposes. A por
tion of the power that is developed in the tur bine houses is transmitted electrically to Buffalo, for the operation of electric railways and for other purposes, and the remainder is utilized, also in the electrical form, in factories in the more immediate neighborhood of the falls. The modern electric transmission line for high voltages is of aluminum cable on a steel core— to give it strength and lessen the sag between towers — or of copper-clad steel, known to the trade as metal.* The latter is an in geniously devised steel core which is first al loyed with copper on its surface, and then covered with a heavy coating of pure copper. The aluminum cable has to be 61.6 per cent larger in volume than a copper cable to obtain an equally low resistance. As the weight of aluminum is 302 per cent that of an equal volume of copper, the weight of aluminum re quired for a stated conductivity is 48.8 per cent of the weight of copper having the same con ductivity. An aluminum line is, therefore, much cheaper than a copper line. For voltages up to 66,000 the type of insulators, set upon iron pins. are commonly used. The or porcelain (bells' are nested one within another until the insulation is sufficient to prevent the current from the cable from leap ing to the supporting pin. The protective power ranges about 15,000 volts to each bell; that is, for a current of 44,000 volts, three bells nested are required; for 66,000 volts, at least four bells in the nest. The great weight of these porcelain masses limits their use to the smaller voltages, so that above 66,000 volts the suspension type of insulation is used. This consists of a series of porcelain discs hanging in horizontal posi tions one below another and suspended through their centres. As many discs may be added to the °string') as are needed to insulate the par ticular current. On 110,000-volt lines five or six discs are used. On 140,000 volt lines the num ber of dics is increased to 10 and the space be tween the horizontal cables is not less than 12 feet. On lines of low voltage wooden poles of white cedar (arbor vita) or chestnut are generally used. They have a of from 10 to 12 years. Concrete poles are practically in destructible, but very expensive to set, on ac count of their great weight. For high voltage transmission lines the steel tower of lattice con struction is favored. These are set from 400 to 1,000 feet apart. The longest known span is one reaching across the Saint Lawrence River at Three Rivers (Canada) which is a clear span of 4,800 feet from towers 315 feet high; the cable carrying 140,000 volts. Underground con duits are common for electric transmission in cities. They are variously constructed: of arti ficial stone— made by mixing limestone and Portland cement; or of fibre pipe — made of woodpulp saturated with a bituminous com pound. The cables used in underground trans mission are usually three-phase, three-conductor cables of the so-called (cloverleaf" pattern, the conductors being insulated by manila-rope paper of the finest quality saturated with resin and resin oil. This type of insulation is as durable as the lead sheath in which the cable is en cased; which is all that can be desired. The voltage which such an underground cable can carry without loss, or perforation of the insula tion, ranges up to a maximum of 300,000 volts.
Bibliography.— Collins, H. E.,
Pulleys, Belting and Rope
(New York 1908) : Fairbairn, Sir Wm., (Prin ciples of Mechanism and the Machinery of
(Philadelphia 1903) ; Hiscox, G. D.,
Appliances, Movements and Novelties of Construction) (New York 1914) ; Kapper, F.,