**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. - 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. LDG TW-1 The TW-1 talking**Wattmeters**LDG TW-1 The TW-1 talking**wattmeters****Wattmeter**provides an aural spoken indication of power and SWR using a digitally recorded voice.