Page:Electricity (1912) Kapp.djvu/254

250 expenditure of copper for feeders as to make the system commercially impossible on any but fairly short lines. It thus becomes necessary to transmit electricity from the power-house at high pressure to sub-stations placed at intervals along the line, and to convert the three-phase high-pressure current at these sub-stations into continuous current, which is supplied by means of feeders to the trolley line. The apparatus used is of the type shown in Fig. 26, with this difference, that the middle wire o on the D.C. side is omitted and that the negative brush is connected to the rails or by way of a switch-board to the negative return feeders bringing the current back from the rails, whilst the positive brush is connected with an omnibus bar on the main switch-board from which all the feeders start.

The current taken by electric tramways and railways is very fluctuating. In the case of tramways the fluctuations are the less felt the larger the system, that is to say, the greater the number of cars drawing current from a particular power-house or sub-station. If only a small number of cars is on the system the fluctuations may become very great. This is especially the case if a block in the