A practical instrument whose operation depends on the torque developed on a current-carrying coil is the galvanometer . Its most direct application is for sensitive measurement of electric current.
Galvanometers are the basic component of many electrical meters.
A particularly reliable galvanometer was built in 1880 by the French physicist Jacques Ars ne d'Arsonval, who surrounded a cylindrical iron core with a movable coil placed between the poles of a magnet. When a current was passed through the coil, it produced a magnetic dipole whose interaction with the field of the permanent magnet created a torque on the coil. The coil turned through an angle that depended on the magnitude of the current causing the field. Precise measurements of the angle permitted precise measurement of the current I . In 1888 Edward Weston used d'Arsonval's idea to construct the first portable current-measuring instrument that could be read directly from its scale.