Image: Astatic Galvanometer brass and ivory

Description: Closeup of an astatic galvanometer, showing the coils and the magnetic needle. the lower needle is inside the coils. An astatic galvanometer, invented by Leopoldo Nobili in 1825, is a historical instrument that measures electric current. It consists of two magnetized needles, suspended from their centers by a long thin fiber which can turn freely. One needle is inside a coil of wire. When electric current passes through the coil, its magnetic field causes the needle to turn from its equilibrium position perpendicular to the coil to be parallel to the coil's axis. The second needle (visible), located above the coil and magnetized in the opposite direction, serves to cancel out the effect of the Earth's magnetic field (that's what the "astatic" means), and also serve as a pointer to indicate tne magnitude of the current.
Title: Astatic Galvanometer brass and ivory
Credit: Own work Transferred from en.wikipedia
Author: Williamgelbart at en.wikipedia
Usage Terms: Creative Commons Zero, Public Domain Dedication
License: CC0
License Link: http://creativecommons.org/publicdomain/zero/1.0/deed.en
Attribution Required?: No