To measure the difference in potential or intensity in the current between two sections of the circuit.
A transparent protective case contains a horseshoe-shaped permanent magnet. We can find a coil among its poles. The magnet is fixed, but the coil is attached to a mobile axis that, in addition, 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).
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.