This magnetometer measures the vertical component of the earth's magnetic force. The form was designed by Adolph Schmidt in Germany around 1915 and widely used for prospecting purposes. The "MADE BY G. R. & D.C. PITTSBURGH, PA M-9" inscription refers to the Gulf Research & Development Corp. That firm donated it to the Smithsonian in 1964.
Ref: John A. Fleming, ed., Terrestrial Magnetism and Electricity (New York, 1939), pp. 112, 120-127.
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