D'Arsonval Meter Movement

The most sensitive form of early current measurement was the Thomson or mirror galvanometer, patented in 1858 by William Thomson (Lord Kelvin). It could detect very rapid current changes by using small magnets attached to a lightweight mirror, suspended by a thread. A light beam deflected from the mirror greatly magnified the deflection induced by small currents.

However, moving-magnet galvanometers were disadvantaged through being affected by any nearby magnets or iron masses, so their deflection did not remain linearly proportional to the deflecting current. Accordingly, in 1882, Jacques-Arsène d’Arsonval and Marcel Deprez developed a version with a stationary permanent magnet and a moving wire coil, suspended by fine wires which provided both an electrical connection to the coil and the restoring torque to return to the zero position.

An iron tube between the magnet's pole pieces defined a circular gap through which the coil rotated. This gap produced a consistent, radial magnetic field across the coil, giving a linear response throughout the instrument's range. A mirror attached to the coil deflected a beam of light to indicate the coil position. The concentrated magnetic field and delicate suspension made these instruments sensitive; d'Arsonval's initial instrument could detect ten microamperes.

A common variation on the d’Arsonval movement is the more rugged Weston movement, which employs jewelled supports for the core and a heavier winding in the electromagnet.