An early D'Arsonval galvanometer, which became the design used in modern moving coil ammeters. It consists of a small coil of fine wire suspended between the poles of a horseshoe magnet by a torsion fiber. When an electric current passes through the coil it turns to be parallel with the field, twisting the fiber, moving the pointer up the scale. When the torque of the coil balances the torque of the fiber, the pointer stops. From Hooke's law, the torque of the fiber is proportional to the angle of twist, so the angle of the pointer is proportional to the current.