true power = volts × active amperes (1)
The active component of the current is easily obtained graphically as in fig. 1,358.
With any convenient scale draw AB equal to the current as given or read on the ammeter, and AC, equal to the pressure, making the angle φ between AB and AC equal to the phase difference between the current and pressure.
From B, draw the line BD perpendicular to AC, then BD will be the wattless component, and AD (measured with the same scale as was used for AB) the active component of the current, or that component in phase with the pressure.
Hence from equation (1)
true power = AC × AD (2)
Now in the right triangle ABD
| AD | ||
| = | cos φ | |
| AB |
from which
AD = AB cos φ (3)