Wattmeter
Use
To measure the potential difference between two points in the circuit.
Description
The only visible parts are a moving needle and a fixed graduated scale (as it corresponds to an analogical measuring instrument). However, we can find a fixed horseshoe-shaped magnet, among whose poles a coil is set. The magnet is in a fixed position, but the coil is attached to a moving axis which, in turn, has got a needle and a spiral spring.
Functioning
The device is connected in series for the fixed coil and in parallel for the moving one. The permanent magnetic field of the magnet begins to exert a force over the coil. As a consequence, this moves around its axis and the needle deviates a bit from its initial position. The steering angle is equivalent to the intensity of the current that goes through the veil. This allows to graduate the scale of the device if we employ currents with a known value.
