On the transient magnetic state of which various substances are susceptible / [Sir W. Snow Harris].

  • Harris, W. Snow (William Snow), Sir, 1791-1867
Date:
1831
Catalogue details

Licence: Public Domain Mark

Credit: On the transient magnetic state of which various substances are susceptible / [Sir W. Snow Harris]. Source: Wellcome Collection.

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    nothing more than the presence of a temporary magnetic development induced in successive points of a body by the action of a permanent magnet, the body being supposed in motion. If the body therefore be at rest, this development will restrain the motion of a bar vibrating near it, and so diminish the ampli- tude of its oscillations; and thus by determining the number of vibrations per- formed in a given arc, we may from thence arrive at a comparative value of the force in action. To this effect the bar already mentioned (12) was suspended, and exposed to the influence of different substances, in the following manner. Two perpendicular rods of glass rr, r' r\ fig. 1, furnished with foot- and cap- pieces are fixed in a solid block of mahogany r r', fig. 1 ; these rods by the in- tervention of a short wood cylinder /, and two horizontal rods, also of glass, rl, rsl, sustain a glass tube tf which slides with friction through the cylinder. This tube is furnished with a cap-piece at t, through which passes a brass rod : the magnetic bar is suspended from this rod by a filament of silk, and is finally raised or depressed to the required altitude by a micrometer-screw at t: the altitude of the glass tube / being previously fixed. The centre of the maho- gany block is hollowed into a cylindrical cavity as represented in fig. 10, and is firmly fixed upon the pump-plate by means of the screw and shoulder pro- jecting through the brass collars (4). Its outer part is also depressed, leaving a cylindrical projection of about one fourth of an inch deep, 4.7 inches diameter, and 1.25 inch wide. The graduated circle of stout card-board D already mentioned (6) slides with friction between the glass rods, so as to be easily adjusted at any given point. The bar is retained at the given angle from its meridian, and again set free, when the exhaustion is sufficiently complete, by means of the lever and double stop before explained (4), and which moves in the interior circular cavity of the block, fig. 10. This method of arresting the bar seems to be of some consequence to the experiment; for if one pole only be checked, the force operating on the other, will for an obvious reason, give the bar a swinging motion, which is very undesirable, but which is here effectually prevented ; so that when set free it will appear to oscillate as if mounted on a fixed centre. The checks by which the bar is thus arrested, are so contrived as to be independent of each other.