Galvanometer

Galvanometer, device used to measure small values of electric current. Galvanometers make use of the fact that an electric current flowing in a coil of wire creates a magnetic field that can deflect a magnetized needle placed near the coil. The deflection angle of the magnetized needle is related to the quantity of electricity flowing in the wire and can therefore be used to measure the current passing through the wire.

The most common galvanometers in use are the moving-coil and the moving-mirror galvanometers. The moving-coil galvanometer consists of a coil of wire with a pointer attached to it placed in the magnetic field of a permanent magnet. When a current passes through the wire coil, the coil experiences a torque, or twisting force, caused by the interaction of the coil’s magnetic field with the magnetic field of the permanent magnet. This torque forces the coil, which rotates about its axis, to turn. The pointer attached to the coil measures the angle that the coil turns through by moving over a graduated scale. This scale can be made proportional to the current passing through the wire by attaching a counterspring to the coil. The counterspring is necessary because the force on the coil due to the magnet’s torque depends on the angle between the coil and the magnet. The counterspring makes this force constant so that it can be measured on a linear scale.

The moving mirror galvanometer is similar to the moving coil galvanometer, except that the pointer is replaced with a mirror. A narrow beam of light is reflected off the mirror and onto a fixed scale. As the coil and the attached mirror turn, the beam of light moves across the scale, giving a measure of the current in the wire coil.

Various forms of the galvanometer are used to measure different electric properties of a circuit. When large currents need to be measured, a low resistance shunt, or current bypass, is placed across the terminals of the galvanometer, forming an ammeter. To measure voltage, a large resistance is placed in series with the galvanometer’s coil, resulting in a voltmeter. Resistance can be measured by attaching a battery to a voltmeter, turning it into an ohmmeter. These measuring devices are the primary tools used to analyze electric circuits.

The principle upon which the galvanometer works—that a current in a coil exerts a force on a magnetized needle—was discovered in 1820 by Danish scientist Hans Christian Oersted. In that same year French physicist André Ampère took Oersted’s discovery and applied it to the measurement of current, thereby inventing the galvanometer. Ampère suggested that the device be named after Luigi Galvani, an Italian pioneer in the study of electricity. The moving-coil galvanometer was invented in 1825 by the German physicist Johann Schweigger. The moving-mirror galvanometer was invented in 1882 by French scientist Jacques-Arsène d’Arsonval and is commonly called the d’Arsonval galvanometer. The galvanometer was first put to practical use in 1832 by German physicist Carl Friedrich Gauss, in the form of a telegraph that sent signals by deflecting a magnetic needle at each end of the telegraph line.