To measure the potential difference between two points in the circuit.
The only visible parts of this device 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 incorporated. The instrument also has got a group of graduated scales and several inputs to connect it to the circuit according to the magnitude we measure.
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. The same instrument allows us to measure intensities (connecting it in series), or differences in power (if we connect it in parallel).