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.
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).
In 1882, Jacques-Arsène d´Arsonval conceived the first device that measured the intensity of the current. It was made up of a permanent magnet and a moving coil that hangs from spiral springs.
In 1888, Edward Weston developed a commercially available form of this instrument. It became a standard component in measurement equipment for electric and electronical circuits.