DISTRIBUTI NB SYSTEMS are almost invariably of copper wire. but aluminium is beginning, to be employed extensively, though its use depends largely on the relative priees of the two metals. I'nder certain conditions it may be economical. and it is usually preferable where lightness is essential. l'p to the year 1901 aluminium has been employed chiefly for transmission lines,. rather than distribution proper. Copper is used on account of its high conductivity. The con ductivity of aluminium is only about one-half that of copper. but owing to its lesser density an aluminium conductor weighs only 61 per cent. as much as copper wire of equal length and re sistance: on the other hand, the greater cross-sec tion of aluminium wire calls for more insulating material and also for more space. an important consideration in work. Wires of either material are most commonly supported on poles. erected along the highways. or less fre quently on private land. There is au increasing tendency. however, to place street wires for all classes of electrical :ervice underground. ( For it discussion of underground wires. see CABLE, ELECTRIC, and SUBWAYS.) Distributing systems are divided broadly into series, constant-eurrent systems, and parallel constant-potential systems. These ğmy be complicated by various combing. dons, but only the simplest forms will be de scribed here.
In the series SySifill the current passes through each lamp on the circuit in succession and then returns (as is always the ease) to the dynamo. The current is generally constant, and of high initial potential, the latter dropping at each lamp as many volts as is lost there by the re. sistance. Parallel systems employ constant po tential. or voltage, but the current varies accord ing to the number of lamps in circuit. The lamps are placed between the two wires of the circuit, the current to each lamp returning di reetly to the station. Nearly all incandescent and many arc lamps and most electric motor: are operated on this system. Tlwe- and fire wire systems are designed to save copper. Taking up the three-wire system as the simplest. it may be considered as the combination of two parallel systems into one, by uniting the positive main of one circuit with the negative main of the other. If one of the lamps in a series is extinguished the current for the other returns by the common conductor. The five-wire system is merely an extension of the same principle as the three wire. fleeent improvements in incandescent lamps permit the use of current at 220 volts or more. instead of 110 volts, thus partly doing away with the necessity for the multiple-wire system.