Galvanometer, an instrument used to indicate the presence, direction, or strength of a small electric current. The typical galvanometer is a sensitive laboratory instrument used mainly to detect and compare currents.
The galvanometer makes use of the fact that an electric current flowing through a wire sets up a magnetic field around the wire. In the galvanometer, the wire is wound into a coil. When current flows through the coil, one end of the coil becomes a north magnetic pole, the other a south magnetic pole. When a permanent magnet is placed near the coil, the two fields—the one from the coil and the one from the magnet—interact. The like poles will repulse each other and the unlike poles will attract. The amount of attraction and repulsion increases as the strength of the current increases.
In the moving-magnet galvanometer, the permanent magnet is a needle (much like a compass needle) mounted on a pivot and surrounded by the coil. In the moving-coil galvanometer—the most common type—the coil is mounted on pivots or suspended by thin metal strips. The coil lies between the poles of a permanent magnet in such a way that it rotates when current flows through it. The direction of the rotation depends on the direction of the current through the coil, and the amount of rotation depends on the strength of the current. A galvanometer is often used to indicate when the current in a circuit has been reduced to zero, as in the operation of the Wheatstone bridge, a device for measuring electrical resistances precisely.
A moving-coil mechanism similar to that used in a galvanometer is used in some ammeters. Like the galvanometer, these instruments measure the strength of a current but they can handle a stronger current; unlike the galvanometer, they cannot indicate the current's direction. A moving-coil mechanism is also used in some voltmeters (which measure the voltage in a circuit) and ohmmeters (which measure the resistance in a circuit). In some instruments, a selector switch connects the moving-coil mechanism to different internal circuits so that a single mechanism can be used in making all three types of measurements.
The principle upon which the operation of the galvanometer is based was discovered in 1820 by Hans Christian Oersted when he observed that a magnetic needle could be deflected by an electric current. The first galvanometer was made by Johann Schweigger in 1820. In 1882, Jacques Arsene D'Arsonval introduced the moving-coil galvanometer. Edward Weston made important improvements to the device a few years later.