Image: D'Arsonval ammeter movement

Description: Drawing of D'Arsonval Galvanometer (ammeter) movement, of Weston type, from around 1900. This is the mechanism used in most moving-pointer (galvanometer) meters today. Only the pole pieces of the horseshoe-shaped magnet are shown, and part of the left-hand pole piece is broken out by the artist to show the coil. It consists of a light coil of wire with an attached pointer, suspended on jewel bearings between the poles of a magnet, with a spiral hairspring to return it to zero. When a current is passed through the coil, it creates a magnetic field that opposes the magnet's field, creating a torque that rotates the coil against the restoring force of the spring. The coil turns through an angle until the restoring force of the spring is equal to the force of the coil. Since the angle of deflection is proportional to the force, which is proportional to the current, the deflection is proportional to the current. The screwdriver slot at the top of the axis is for adjusting the meter to zero. Alterations: removed caption, and faint background discoloration.
Title: D'Arsonval ammeter movement
Credit: Downloaded from Flather, John J. (1900) Dynamometers and the Measurement of Power, 2nd Ed., John Wiley & Sons, New York, p.236, fig.81 on Google Books. The drawing is signed 'Bartlett & Co. N.Y.'
Author: Bartlett & Co. N.Y.
Permission: Public domain - published in US before 1923
Usage Terms: Public domain
License: Public domain
Attribution Required?: No