Signedon dial: E. Ducretet & Cie / à Paris
on glass cover: Thomas Hall / Boston
FunctionThe chief purpose for the development of the astatic galvanometer was to completely eliminate the use of the earth's magnetic field in precision measurements. It was first introduced by Leopoldo Nobili (1784-1835) in 1825. The principle is simple. A pair of needles is mounted parallel to each other, but with the poles reversed. This combination has a net magnetic dipole moment of zero and thus has no preferred direction in the earth's magnetic field. The lower of the magnetic needles is inside the coil which carries the current under test, and alone experiences a torque due to the resulting magnetic field. The above needle, usually suspended by a silk thread, as this example, was there to counteract the effect of the earth's magnetic field.
For the above description and more on astatic galvanometers, see the following website.
On Nobili and this type of galvanometers, see also the following website.
Primary SourcesE. Ducretet et Cie, Petit catalogue des instruments de précision de E. Ducretet et Cie (élève de M. Froment) (Paris, 1880), part 3, 41, no. 2335.
ProvenanceEssex Institute, Salem (acc. 10.5.88); loaned permanently to the Peabody Museum, Salem (no. T.62); received by David P. Wheatland, November 1957, on permanent loan to CHSI.