Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From : Kimball, Arthur Lalane, A College Text-Book of Physics, 3rd ed., H. Holt and Co., New York (1923) pp.471-2.
© Copyright 1998 R. Paselk

692. Moving-coil Galvanometer. - In this type of instrument, known also as the D'Arsonval form of galvanometer, the suspended system is a coil of fine wire which hangs in a strong magnetic field due to a permanent steel horseshoe magnet. In figure 399 is shown a vertically placed horseshoe magnet, between the poles of which is hung a light rectangular coil of many turns of fine wire, the plane of the coil being parallel to the direction of the lines of force. The coil is suspended by a fine ribbon of phosphor-bronze which also serves to connect one end of the suspended coil to the outer circuit while the other connection is made through a spiral wound strip of the phosphor-bronze ribbon attached to the lower end of the coil.
A cylindrical mass of soft iron is fixed midway between the poles of the magnet so that as the suspended coil turns its vertical branches move in the gaps between the core and pole pieces. This arrangement secures a strong uniform field, across which the wires of the coil pass, and when a current is sent through it, it is deflected.
A small mirror mounted just above the coil and moving with it, enables the deflection to be determined by the telescope and scale, or reflected spot of light method.
The moving-coil galvanometer has the advantage that it is not affected by changes in the earth's magnetic field, and can be used near dynamo machines and where there is considerable magnetic disturbance. Also the coil damps strongly or comes almost immediately to rest when the wires leading to it are touched together, forming a short circuit, as it is called. This damping is due to electromagnetic induction.

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Last modified 22 July 2000