Multiplying instantaneous values of volts and amperes will give the power curve, as before explained, whose average value is half-way between the zero line and the maximum of the curve; that part of the power curve above the line of average power WW, exactly filling the open space below the line WW. That is,
| average power | = | maximum power ÷ √2 |
| = | maximum voltage × maximum current | |
| √2 | ||
| = | virtual voltage × virtual current. |
This latter is simply the product of the voltmeter and ammeter readings which gives the watts just the same as in direct current.
Fig. 1,347.—Diagram of circuit containing only resistance; in such a circuit the power factor is unity.
Ques. What should be noticed about the power curve?
Ans. Its position with respect to the zero line; it lies wholly above the zero line which denotes that all the power delivered to the circuit except that dissipated by friction is useful, that is, the power factor is unity. Hence, to keep the power factor as near unity as possible is one of the chief problems in alternating current distribution.
Ques. Can the power factor be less than unity if the current and pressure be in phase?
Ans. Yes, if the waves of current and voltage be distorted as in fig. 1,348.
Effect of Lag and Lead.—In an alternating circuit the amount of power supplied depends on the phase relationship of the current and pressure. As just explained, when there is synchronism of current and pressure, that is, when they are in phase (as in fig. 1,346) the power factor is unity, assuming no distortion of current and pressure waves. In all other cases the power factor is less than unity that is, the effect of lag or lead is to make the power factor less than unity.