Analogue wattmeter

ANALOGUE WATTMETER This simple meter is primarily intended to establish how much power a mains load consumes and thus how much it adds to the electricity bill. 49 EE December 1986 by C Giorden We all know that the power con- sumption of electric appliances is generally stated in watts, and that the cost of using the appliance is roughly proportional to its power drain from the mains. It would ap- pear interesting to investigate the phenomenon "power" a little further, particularly since the simple notion that power is the product of current and voltage (P =I • U, whence P = /R and P = I"R) is not directly applicable to alternating voltage. A brief summary of terms may help to shed some light on the operational principle of the proposed wattmeter. Simply measuring the rms values of alternating current and voltage developed across a load R yields the apparent power : P = Irms • U. [W]. However this method can only be used where the load R is purely resistive; should it consist o...