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Electricity meter - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Electricity_meter)

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http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Basic%20Electrical%20Technology/pdf/L-44(GDR)(ET)%20((EE)NPTEL).pdf

I think I already answered this sometime back. The meter has two coils(current and voltage. The induction disc moves freely in the field provided by the voltage coil at a speed proportional to the current or the load,thus rotating the counter which registers the reading. Nowadays they have digital meters too.

How electrical energy is measured in dc and ac circuits?

how electrical energy is measured in dc and ac circuits?
In dc power is Vx I but in ac it is Vx I x pf.To get energy multiply by hrs.The kvh or energy meter is made to measure kwh. In DC there will be no kVA but in ac you get KVA,KW & KVAr.