Components of electrodynamical wattmeter and varmeter.

[0fibonacci1]

Electrodynamical wattmeter has 4 connections, 2 for current and 2 for voltage. Why do we have both, how that works inside? Those 2 drive in current and those 2 voltage so it calculates how much power is it in there in some sort of mechanism?
What about in varmeter, why we connect one inductor next to it, so we find the reactive power somehow through it? I have seen regulation resistor connected between entry of voltage connection and exit of current connection, it's used to regulate current, for protection or other reasons?

 

[The Electrician]

You might have a look at this:

http://en.wikipedia.org/wiki/Wattmeter