The key feature of this circle is a second needle to measure the relative intensity of the field. Humphrey Lloyd, a professor of natural philosophy at Trinity College, Dublin, introduced the design in the 1840s. E. W. Creak, a Captain in the Royal Navy who served as Superintendent of the Compass Department of the Admiralty, improved the design around 1900, and the Admiralty paid for its development and tests. It soon replaced the Fox-style dip circles that had been in use since the 1830s.
This example is marked "Dover, Charlton Kent. Circle 168" and "C.&G.S. No. 35." The vertical circle is inside the box and viewed by opposite microscopes on the outside. This circle is silvered, graduated to 30 minutes, and read by opposite verniers to single minutes; and there is a curved thermometer along its top edge. The axle holding the needle rests in jeweled holes. The horizontal circle is silvered, graduated to 30 minutes, and read by vernier to single minutes. There are two level vials on the base.
The U.S. Coast and Geodetic Survey modified this instrument in 1906, making it more useful at areas within 30 or 40 degrees of the magnetic equator. The original deflection distance of 7.3 cm was replaced with two deflection distances of 7.9 and 9.4 cm. by placing an aluminum case on the frame between the reading microscopes. A small telescope was added in front of the instrument, and an auxiliary needle, used to determine magnetic declination, on top.
This instrument sailed on the Galilee, a ship owned by the Carnegie Institution of Washington, in 1905-1907 and was later used in Canada, Greenland, and Newfoundland. It came to the Smithsonian in 1959.
Ref: "The Lloyd-Creak Dip Circle for Observations at Sea," Terrestrial Magnetism 6 (1901): 119-21.
L. A. Bauer, "Results of Magnetic Observations," Report of the. . . Coast and Geodetic Survey (1903-04), App. 3.
D. Hazard, Directions for Magnetic Measurements (Washington, D.C., 1911), pp. 97-99.
ID Number: PH.316509
accession number: 225703
catalog number: 316509
Physical Description: metal (overall material)
Measurements: overall: 10 1/2 in; 26.67 cm
Measurements: overall: 10 7/8 in x 7 1/2 in x 8 in; 27.6225 cm x 19.05 cm x 20.32 cm
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Credit Line: U.S. Department of Commerce, Coast & Geodetic Survey
Place Made: United Kingdom: England, London, Charlton