D'Arsonval Movement

The most generally used sensing mechanism used in DC voltmeters, ammeters, and ohm meters is a current-sensing device known as a D'Arsonval meter movement. A D'Arsonval movement is a DC moving coil-type movement in that an electromagnetic core is suspended among the poles of a permanent magnet.

The current measured is directed through the coils of the electromagnet so in which the magnetic field generated through the current opposes the field of the permanent magnet and causes rotation of the core. The core is restrained through springs so in which the needle will deflect or move in proportion to the current intensity. A more current applied to the core, the opposing field the stronger   and the larger the deflection that is up to the limit of the current capacity of the coil. Whenever the current is interrupted, the opposing field collapses, and the needle is returned to zero through the restraining springs.  The limit of the current which could be applied to this type movement is commonly less than one milliampere.

Figure: D'Arsonval Meter Movement

A general variation of the D'Arsonval movement is the Weston movement, that uses essentially the same principle built to a more rugged construction by employing jeweled supports for the core and employing a heavier winding in the electromagnet. Notice that the D'Arsonval movement is a DC device and could only measure DC current or AC current rectified to DC.