Wattmeter
device that measures electric power
The wattmeter is an instrument for measuring the electric active power (or the average of the rate of flow of electrical energy) in watts of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.
A wattmeter reads the average value of the product v(t)i(t) = p(t), where v(t) is the voltage with positive reference polarity at the ± terminal with respect to the other terminal of the potential coil, and i(t) is the current with reference direction flowing into the ± terminal of the current coil. The wattmeter reads P = (1/T) ∫0T v(t)i(t) dt, which in sinusoidal steady-state reduces to Vrms Irms cos(φ), where T is the period of p(t) and φ is the angle by which the current lags the voltage.
History
The Hungarian Ottó Bláthy patented his AC wattmeter.
In 1974 Maghar S. Chana, Ramond L. Kraley, Eric A. Hauptmann Barry, and M. Pressman patented an early electronic wattmeter. This device is made up of power, current and voltage transformers, which measure the average power.
Electrodynamic
The traditional analog wattmeter is an electrodynamic instrument. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil.
The current coils are connected in series with the circuit, while the potential coil is connected in parallel.
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