| reactance | 2πfL | ||||
| tan φ | = | = | (1) | ||
| resistance | R |
Fig. 1,293.—Steam engine analogy of current flow at zero pressure (see questions below). When the engine has reached the dead center point the full steam pressure is acting on the piston, the valve having opened an amount equal to its lead. The force applied at this instant, indicated by the arrow is perpendicular to the crank pin circle, that is, the tangential or turning component is equal to zero, hence there is no pressure tending to turn the crank. The latter continues in motion past the dead center because of the momentum previously acquired. Similarly, the electric current, which is here analogous to the moving crank, continues in motion, though the pressure at some instants be zero, because it acts as though it had weight, that is, it cannot be stopped or started instantly.
Ques. When an alternating current lags behind the pressure, is there not a considerable current at times when the pressure is zero?
Ans. Yes; such effect is illustrated by analogy in fig. 1,293.
Ques. What is the significance of this?
Ans. It does not mean that current could be obtained from a circuit that showed no pressure when tested with a suitable voltmeter, for no current would flow under such conditions. However, in the flow of an alternating current, the pressure varies from zero to maximum values many times each second, and the instants of no pressure may be compared to the "dead centers" of an engine at which points there is no pressure to cause rotation of the crank, the crank being carried past these points by the momentum of the fly wheel. Similarly the electric current does not stop at the instant of no pressure because of the "momentum" acquired at other parts of the cycle.
Ques. On long lines having considerable inductance, how may the lag be reduced?
Ans. By introducing capacity into the circuit. In fact, the current may be advanced so it will be in phase with the pressure or even lead the latter, depending on the amount of capacity introduced.
There has been some objection to the term lead as used in describing the effect of capacity in an alternating circuit, principally on the ground that such expressions as "lead of current," "lead in phase," etc., tend to convey the idea that the effect precedes the cause, that is, the current is in advance of the pressure producing it. There can, of course, be no current until pressure has been applied, but if the circuit has capacity, it will lead the pressure, and this peculiar behavior is best illustrated by a mechanical analogy as has already been given.