overall in case: 12 in x 9 3/8 in x 8 1/4 in; 30.48 cm x 23.8125 cm x 20.955 cm
Object Name:
dip circle
Place Made:
United States: District of Columbia, Washington
Description:
This Kew pattern dip circle is marked "U.S.C.&G.S. No. 23" and was probably made in the instrument shop of the U.S. Coast and Geodetic Survey. The vertical circle is silvered, graduated to 30 minutes, and read by opposite verniers and magnifiers to single minutes. The horizontal circle is graduated to 30 minutes and read by vernier to single minutes. On top of the box there is a compass needle in a box with peep sights that measures magnetic variation.
A brass housing on the front of the instrument holds an auxiliary needle (now missing) that serves as a deflector for the determination of total intensity; this technique was introduced by Humphrey Lloyd, professor of natural philosophy at Trinity College, Dublin, in the early 1840s.
Ref: Daniel Hazard, <i>Directions for Magnetic Measurements</i> (Washington, D.C., 1911), pp. 66-69.
Humphrey Lloyd, "On a New Magnetical Instrument, for the Measurement of the Inclination, and Its Changes," <i>Proceedings of the Royal Irish Academy</i> 2 (1840-1844): 210-217, 226-232.
