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34 RICHARDSON'S BLUEBOOK OF PROJECTION
if the circuit is working at capacity. If overloaded it becomes very heavy.
A water pipe of given diameter will convey water at any pressure sufficient to move the liquid without bursting the pipe. (3) However, a pipe of given diameter will convey only a certain volume of flow (number Ot gallons per minute) without excessive friction or resistance, regardless of the pressure. Beyond that point the pipe is overloaded. When the point is reached where resistance to flow becomes too great, the pipe has passed its normal capacity. .
(4) More water may be forced through despite excessive resistance but only at the expense of increased power consumption, which is largely wasted. It more water must be had it becomes necessary either to waste power by overloading or to increase the diameter of the pipe. By increasing the pipe diameter the friction or resistance to the water flow is decreased.
The length of the pipe must also be considered, bmce resistance comes from friction between the water and the walls of the pipe, it follows that the greater the pipe length the greater the total friction. Hence, with a given rate of flow, as the length of pipe is increased, resistance is increased; if the length of the pipe is decreased, resistance is decreased. .
We increase resistance bv (a) increasing the volume of water, (b) by reducing pipe diameter, (c) by increasing pipe length or (d) by increasing roughness of interior walls of pipe. .
We decrease resistance by (a) decreasing yolumeof flow (b) increasing diameter of pipe, (c) decreasing pipe length or (d) making interior walls of pipe more
smooth. . . a . '
What has been said of the action of water flowing through pipes under pressure is equally true of current flowing through metallic circuits.
(5) If we substitute circuits of wire for the water main and wire circuits for pipes E, A and G, in Fig. 8, and substitute voltmeters for the pressure gauges, with lamps or motors instead of the open ends of the pipes.