Mirror Galvanometer

From the Philip Harris catalogue, 50th ed., 1950:

"6779  GALVANOMETER, REFLECTING, MOVING COIL, APERIODIC orBALLISTIC. This galvanometwer comprises a moving coil system with a 1 cm. diameter concave mirror, suspended between the poles of a powerful permanent magnet by means of a phosphor bronze strip attached to an adjustable torsion head. Arrestment is obtained by depression of the torsion head, so allowing the coild to rest upon the soft iron core. Mounted in the polished mahogany case fitted with terminals and levelling screws and a sliding glass front so that all parts are clearly visible"

The phosphor bronze strip is missing; the galvanometer is not in a working condition.

A mirror galvanometer is a mechanical meter that senses electric current, except that instead of moving a needle, it moves a mirror. The mirror reflects a beam of light, which projects onto a meter, and acts as a long, weightless, massless pointer. In 1826, Johann Christian Poggendorff developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a spot galvanometer after the spot of light produced in some models.

Mirror galvanometers were used extensively in scientific instruments before reliable, stable electronic amplifiers were available. The most common uses were as recording equipment for seismometers and submarine cables used for telegraphy.

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