along wires, and part of the voltage is lost in overcoming the ohmic resistance of these wires. This loss of pressure varies directly with the current. At the time that the greatest number of lamps in any particular district are switched on, the loss of pressure in the cables supplying that district is greatest, and in order that the lamps may still burn with normal brightness, the pressure at the home end of the distributing system must be raised to a value just sufficient to make up for the loss of pressure due to ohmic resistance. But an exact adjustment is impossible; some lamps are nearer and some farther away from the home end of the cable. The current when it reaches the nearer lamps has not lost quite as much of its voltage as when it reaches the farther lamps. To make the delivery voltage absolutely right for every lamp is obviously impossible, but we can approach this ideal condition by the adoption of the following principles: First use cables stout enough so as to limit the total loss of pressure to a moderate amount, say 10 to 15 per cent. of the lamp voltage; secondly, divide the distributing plant into two distinct portions, namely "feeders" and "mains."
To explain what is meant by these terms,
