Such a circuit, when used for lighting, obviously has the disadvantage that if a lamp filament breaks, the resistance of the group is increased, thus reducing the current and decreasing the brilliancy of the lamps. This arrangement accordingly does not admit of turning off any of the lights.
Series Parallel Circuits.—The arrangement of circuits of this kind is shown in fig. 2,134; they are used to economize in copper since by joining groups of low pressure lamps in series they may be supplied by current at correspondingly higher pressure.
Thus, if in fig. 2,134, 110 volt, ½ ampere lamps be used, the pressure on the mains, that is, between any two points as A and B would be 110 × 3 = 330 volts. Each group would require ½ ampere and the five groups ½ × 5 = 2½ amperes.
Fig. 2,138.—44,000 volt lines entering the Gastonia sub-station of the Southern Power Co. The poles used are of the twin circuit two arm type, built of structural steel, their height varying from 45 to 80 feet, the latter weighing 9,000 pounds each. These poles have their bases weighted with concrete.
Transformer Systems.—Nearly all alternating current systems are transformer systems, since the chief feature of alternating current is the ease with which it may be transformed from one pressure to another. Accordingly, considerable economy in copper may be effected by transmitting the current at high pressure, especially if the distance be great, and, by means of step down transformers, reducing the voltage at points where the current is used or distributed.
Ordinarily and for lines of moderate length, current is sent out direct from the alternator to the line and transformed by step down transformers at the points of application.
With respect to the step down transformers, there are two arrangements: