WATTMETER, an instrument for the measurement of elec tric power or the rate of supply of electric energy to any circuit.
For direct current (D.C.) circuits the power supplied is given in watts by multiplying together the current (in amperes) and the voltage; but, when alternating currents (A.C.) are used with inductive circuits, this product has itself to be multiplied by the power factor—a quantity which depends on the phase rela tionship of the current and the electromotive force. Wattmeters
measuring this combined product are of three types: (a) electro static—used only in standardising laboratories, (b) dynamometer instruments based on the principle of the Siemens electrodyna mometer, and (c) induction instruments. Types (a) and (b) may be used for both A.C. and D.C. supplies, but type (c) can be used only for A.C. circuits.