The difference between watt-hour meter and var-hour meter

Hi.

You really give me a hard work here as I must move my head along the screen to read your big blue letters

What exactly are you planning to measure?

Do you want to know this because you're wondering how much wattages your householding consumes?
Then go for a watt-hour meter, as that is what you're paying for.

VA = measured volts X measured current. Watts = measured volts X measured current X cos Phi (angle between V and I) = real power. So if you put a VA meter on the feed to an electric motor you might read 1kW, if you replace it with a watt meter you would read 800W because the current and voltage are not in phase due to the inductive impedance of the motor.
Frank

I understand, that the question is asking for the design and operation principle of the Ferraris energy meter. The description can be hopefully found in profound electrical engineering textbooks, unfortunately I don't have a reference at hand. The below manufacturer applications has a short descriptions and principle drawing of a ferraris meter.
**broken link removed**

In addition, a short explanation of mine of the real/apparent power point. To produce a torque in the aluminium disc of the ferraris meter, he two magnetic fields representing voltage and current must be 90° phase shifted. This is simply achieved by the inductive impedance of the voltage coil. To make the meter measure apparent energy, the magnetic field of the voltage coil must be in-phase with the voltage. This can be achieved by a specially designed passive LR or LRC electriccal network.

I hesitated to answer the post due to the eye-offending layout.