astatic galvanometer
Date: 1865-1875
Inventory Number: 0415
Classification: Galvanometer
Subject:
Maker: E. Ducretet & Cie (1864 - 1931)
Maker: Thomas Hall (fl. 1840 - 1875)
Donor: Essex Institute (1821-1992)
Donor: Peabody Museum of Salem (1799-1992)
Cultural Region:
Place of Origin:
Dimensions:
30.3 × 20.8 × 20.8 cm (11 15/16 × 8 3/16 × 8 3/16 in.)
Bibliography:
Catalogue des instruments de précision de E. Ducretet & Cie
Description:
This galvanometer is made of a double coil, beneath the dial, with wires wound on brass form. The dial is made of a silvered scale (0°-90°-0°). A pair of vertical brass rods surmounted by angular brackets support a knob and suspension thread. The coil and frame are set on a circular base, itself centered on a larger wooden base with three leveling screws. A bell jar encloses the apparatus.
The sticker on the bell jar, mentioning a Boston's instrument maker, suggests the original one was broken and later replaced by this one.
The sticker on the bell jar, mentioning a Boston's instrument maker, suggests the original one was broken and later replaced by this one.
Signedon dial: E. Ducretet & Cie / à Paris
on glass cover: Thomas Hall / Boston
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.
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.