wattmeter A wattmeter combines the function of voltmeter and ammeter to record power directly. A fixed coil, A in series with the load, carries the same current I as the load (see fig. w1). Another moving coil connected parallel across the load, carries current that is proportional to the voltage across the load. The high resistance, R in series with the moving coil limits the current through the moving coil. The deflection in the moving coil is proportional to the product of the current in the fixed coil (I) and the current in the moving coil.

Deflection µ (fixed coil current)´ (moving coil current)

µ IV/R

µ P

Therefore the deflection is proportional to instantaneous power dissipated. The joule meter (or watt hour meter) incorporates a revolution counter to record energy consumption.