Wattmeter n. : An instrument for measuring power in watts, -- much used in measuring the energy of an electric current.

Example:
How do I make a digital Wattmeter?

Taxonomy
coil, wattmeter, movable, circuit, torsion, instrument, electrodynamometer, needle, reading, torque, free encyclopedia, true power, power factor, electrodynamic, scale, proportional, pointer, currents.

Defination from the web:
wattmeters, the potential coil carries a needle that moves over a scale to ... The two circuits of a wattmeter can be damaged by excessive current. ... A modern digital electronic wattmeter/energy meter samples the voltage and current thousands of times a second. ...