Galvanometer
The galvanometer, a device used to measure extremely small electrical currents, traces its origin back to 1820. In that year Hans Christian Oersted (1777-1851) discovered that an electric current flowing in a wire created a magnetic field around it, deflecting a magnetized needle. This effect became the basic principle behind the galvanometer. In the same year André Ampère (1775-1836) used the effect to invent a device to measure electric current. He suggested it be called the galvanometer, in honor of Luigi Galvani (1737-1798), a pioneer in the investigation of electricity.
The first practical use of the galvanometer was made by Karl Friedrich Gauss in 1832. Gauss built a telegraph that sent signals by deflecting a magnetic needle. This style is known as a moving-magnet galvanometer. More commonly used today is the moving-coil or moving-mirror galvanometer, sometimes called a D'Arsonval galvanometer.
The invention of the moving-coil galvanometer is credited to Johann Schweigger in 1825, three years later Italian physicist C. L. Nobilli designed an astatic type. It consists of a coil that has been wound with very fine wire mounted between the poles of a permanent magnet. Attached to the coil is a pointer. When electric current is turned on, the coil turns and the deflection angle is measured as the pointer moves along a graduated scale.
In the case of a moving-mirror galvanometer, a mirror is attached to the coil, and illuminated with light. When the coil moves the deflection of the light is measured along a scale. The mirror galvanometer was of major use in laying the transatlantic telegraph cable between the United States and Europe in 1866. William Thomson, later known as Lord Kelvin, used it to keep track of how much electric current was coursing through the cable. Thomson also invented a "siphon recorder," which was a more sensitive galvanometer. Ink was siphoned through a thin glass tube that was attached to the coil of wire which was mounted between the poles of a horseshoe magnet. The moving tube carried the ink onto a paper tape where it traced a line.
Galvanometers come in a variety of types. Ultraviolet recorders use light-sensitive paper and ultraviolet light in place of ink. A photoelectric galvanometer amplifies the signal using a photocell. The ballistic galvanometer is used to measure an electric pulse or burst. A cousin of the galvanometer is the direct current ammeter, which is a calibrated galvanometer that measures larger currents. Another cousin still is the direct-current voltmeter, which uses Ohm's Law to measure voltage. Digital display galvanometers, the best of which can measure a current as small as one hundredth billionth (10-11) of an amp, have almost entirely replaced the early analog galvanometers of yore.
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