The Encyclopedia britannica; a dictionary of arts, sciences, and general literatureWith new maps, and original American articles by eminent writersWith American revisions and additions, bringing each volume up to date . unterpoise 6, and thewhole is suspended by a fibre of unspun silk 11 from a torsionhead t, by means of which the arm db can be brought iutc ^ Cf. Lehmann, Nov. Comm. Petrop., 1766; Bnigraans, Magiieiismus sen de AiJUnitatibus Mttgnctieis Observations Acadeniic^Leyden, 1778; Coulomb, Mho. de IInst., 1812; Biot, Tfaiti dtPhysiqite, 1816, &c. ^Phil Trans., 1786 ; or Trcatisi on Ma
The Encyclopedia britannica; a dictionary of arts, sciences, and general literatureWith new maps, and original American articles by eminent writersWith American revisions and additions, bringing each volume up to date . unterpoise 6, and thewhole is suspended by a fibre of unspun silk 11 from a torsionhead t, by means of which the arm db can be brought iutc ^ Cf. Lehmann, Nov. Comm. Petrop., 1766; Bnigraans, Magiieiismus sen de AiJUnitatibus Mttgnctieis Observations Acadeniic^Leyden, 1778; Coulomb, Mho. de IInst., 1812; Biot, Tfaiti dtPhysiqite, 1816, &c. ^Phil Trans., 1786 ; or Trcatisi on Magnetism, 1787. *Phil. Trans., 1792. • Ritt«r, Gilb. Ann., 1800. « Loc. cit. Pogg. Ann., 1827. i BuU. Cmv. d. Si., 1828. See Von FdUtach, Karstens Ency., Bd. xvi. ; Wiederaaani Oaiivonismm, Bd. ii. p. 516. 262 MAGNETISM any repaired position, and if necessary kept there by theexertion of a known torsional couple. The arm andBuapension must be carefully guarded from draughts byenclosing it in a glass case, which fits over the poles of theelectromagnet, and is provided with a door and with meansfor bringing th^ torsion head t over any given part of themagnetic field. When a cylindrical piece is to be tested. In every experiment the magnetic behaviour of thesupport should be tested by itself beforehand, so thatif any residual effect be present it may be allowed greatest caution is also requisite in choosing thematerial to be experimented upon. There must be nochemical impurity, especially no trace of iron ; the spheresand cylinders must not be worked with iron tools or evenwith dirty hands. A source of error i to be speciallyguarded against in experiments with metals, or other goodconductors, is the action arising from induced currents inthe mass of the tested body caused by the increase anddecrease of the strength of the magnetic field when thecircuit of the electromagnet is made and broken. Thiserror is wholly avoided by waiting till the suspended bodyhas come to rest
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