Ques. Why is the power curve positive in the second half of the period when there are negative values of current and pressure?
Ans. Because the product of two negative quantities is positive.
Ques. Does fig. 1,344 represent the usual way of drawing a power curve?
Ans. Since ordinates of the power curve are products of the current and pressure ordinates, they will be of inconvenient length if drawn to the same scale; it is therefore customary to use a different scale for the power ordinates, as in fig. 1,345.
The illustration is lettered identical with fig. 1,344, with which it should be compared.
Synchronism of Current and Pressure; Power Factor Unity.—The current and pressure would be in phase as represented in fig. 1,346 were it possible to have a circuit containing resistance only. In actual practice all circuits contain at least a small amount of reactance.
Fig. 1,346.—Synchronism of current and pressure. Power curve showing that the power factor is unity. This is indicated by the fact that the power curve does not project below the base or zero line.
A circuit supplying nothing but incandescent lamps comes very nearly being all resistance, and may be so considered in the discussion here. Fig. 1,347 illustrates a circuit containing only resistance. In such a circuit the pressure and current (as shown in fig. 1,346) pass through zero and through their maximum values together.