The movable coil of the wattmeter is normally suspended so that its axis is at right angles to that of the fixed coil, and is constrained by the torsion of a spiral spring.
Hence this power can be measured by the torsion which must be applied to the movable coil of the wattmeter to hold it in the normal position against the action of the forces tending to displace it.
The wattmeter can therefore be calibrated so as to give direct readings of the power reckoned in watts, taken up in the circuit; hence its name, wattmeter.
In those cases in which the power - absorbing circuit is inductive, the coil of the wattmeter connected across the terminals of the power-absorbing circuit must have an exceedingly small inductance, else a considerable correction may become necessary.
If these conditions are not fulfilled, the wattmeter readings, assuming the wattmeter to have been calibrated with continuous currents, may be either too high or too low when alternating currents are being used.
In order that a wattmeter shall be suitable for the measurement of power taken up in an inductive circuit certain conditions of construction must be fulfilled.
The importance of this investigation resides in the fact that an electrometer of the above pattern can be used as a wattmeter, provided that the deflection of the needle is proportional to the potential difference of the quadrants.