Assuming current to be supplied at about 460 volts by different and separate steam dynamos, Dy1, Dy2 (fig. 2), the machines are connected through proper amperemeters and voltmeters with _omnibus bars_, O1, O2, O3, on a main switchboard, so that any dynamo can be put in connexion or removed. The switchboard is generally divided into three parts--one panel for the connexions of the positive feeders, F1, 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, D1, D2, D3, respectively called the positive, middle and negative distributing mains. The lamps in the houses, H1, 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_, B1, B2, connected respectively between 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, S1, S2, which are placed in a separate battery room and connected between the middle omnibus bar and the two outer omnibus bars. The extra electromotive force required to charge these batteries is supplied by two small dynamos b1, b2, called _boosters_. It is not unusual to join together the two balancing dynamos and the two boosters on one common bedplate, the shafts being coupled and in line, and to employ the balancing machines as electromotors to drive the boosters as required. By the use of _reversible boosters_, such as those made by the Lancashire Dynamo & Motor Company under the patents of Turnbull & M^cLeod, having four field windings on the booster magnets (see _The Electrician_, 1904, p. 303), it is possible to adjust the relative duty of the dynamos and battery so that the load on the supply dynamos is always constant. Under these conditions the main engines can be worked all the time at their maximum steam economy and a smaller engine plant employed. If the load in the station rises above the fixed amount, the batteries discharge in parallel with the station dynamos; if it falls below, the batteries are charged and the station dynamos take the external load.