This variometer measures slight changes in the vertical component of the earth's magnetic field. The surrounding Helmholtz coil enables its scale value to be determined electrically. This example is marked "RUSKA INSTRUMENT CORPORATION HOUSTON TEXAS No. 5728 MADE IN U.S.A." The U.S. Coast and Geodetic Survey acquired it in 1957, probably for use during the International Geophysical Year. The U.S. Geolgical Survey acquired it in 1973 when it assumed responsibility for the geomagnetic program of the federal government, and transferred it to the Smithsonian in 1982.
Ref: Ruska Instrument Corporation, Magnetic Instruments (Houston, 1955).
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