This is a Kew pattern unifilar magnetometer that is marked "Dover, Charlton Kent No. 145" and that belonged to the Survey Branch of the U.S. Navy Hydrographic Office. While the "Dover, Charlton Kent" inscription was used between 1859 and 1920, the serial number suggests a date of around 1900. The U.S. Coast and Geodetic Survey repaired, cleaned, adjusted, and calibrated this instrument around 1950, and transferred it to the Smithsonian in 1959.
Ref: J. E. H. Gordon, A Physical Treatise on Electricity and Magnetism (London, 1880), vol. I, pp. 166-177.
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