WATTMETER, an instrument for the measurement of electric power, or the rate of supply of electric energy to any circuit.
If an electro-dynamometer, made as above described, has its fixed circuit connected in series with the power-absorbing circuit and its movable coil (wound with fine wire) connected across the terminals of the power-absorbing circuit, then a current will flow through the fixed coil which is the same or nearly the same as that through the power-absorbing circuit, and a current will flow through the high resistance coil of the wattmeter proportional to the potential difference at the terminals of the power-absorbing circuit.
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.
This correcting factor has the follow - ing value: If Ts stands for the time-constant of the movable circuit of the wattmeter, commonly called the potential coil, the time constant being defined as the ratio of the inductance to the resistance of that circuit, and if T R is the time-constant similarly defined of the power-absorbing circuit, and if F is the correcting factor, and p = 2r times the frequency n, then,' + p2Ts2 1 For the proof of this formula see J.
1 d-p2T,T„' Hence an electrodynamic wattmeter, applied to measure the electrical power taken up in a circuit when employing alternating currents, gives absolutely correct readings only in two cases - (i.) when the potential circuit of the wattmeter and the power-absorbing circuit have negligible inductances, and (ii.) when the same two circuits have equal time-constants.
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.
Sumpner, however, has devised forms of wattmeter of the dyna - mometer type in which iron cores are employed, and has defined the conditions under which these instruments are available for accurate measurements.
For methods of employing the heating power of a current to construct a wattmeter see a paper by J.
Wilson, "The Kelvin Quadrant Electrometer as a Wattmeter," Proc. Roy.
Swinburne, "The Electrometer as a Wattmeter," Phil.
Addenbrooke, "The Electrostatic Wattmeter and its Calibration and Adaptation for Polyphase Measurements," Electrician (1903), 51, 811; W.