t ' •) II machines, for supplying the demand during the day and later part cf the evening, the remainder of the machines being called into requisition only for a short time. Provision must be made for sufficient reserve of plant, so that the breakdown of one or more sets will not cripple the output of the station. Assuming current to be supplied at about 460 volts by different and separate steam dynamos, Dyi, Dy2 (fig. 2), the machines are connected through proper amperemeters and voltmeters with omnibus bars, Oi, 02, Os, on a main switchboard, so that any dynamo can be put in connexion system. or removed. The switchboard is generally divided into three parts — one panel for the connexions of the positive feeders, FI, with the positive terminals of the generators; one for the negative feeders, F3, and negative generator terminals; while from the third (or middle-wire panel) proceed an equal number of middle-wire feeders, F2. These sets of conductors are led out into the district to be supplied with current, and are there connected into a distributing system, consisting of three separate insulated conductors, DI, D2, D3, respectively called the positive, middle and negative distributing mains.
\nThe lamps in the houses, HI, H2, &c., are connected between the middle and negative, and the middle and positive, mains by smaller supply and service wires. As far as possible the numbers of lamps installed on the two sides of the system are kept equal; but since it is not possible to control the consumption of current, it becomes necessary to provide at the station two small dynamos called the balancing machines, BI, B2, connected respectively between FIG. 2. the middle and positive and the middle and negative omnibus bars. These machines may have their shafts connected together, or they may be driven by separate steam dynamos; their function is to supply the difference in the total current circulating through the whole of the lamps respectively on the two opposite sides of the middle wire. If storage batteries are employed in the station, it is usual to install two complete batteries, Si, S2,
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