CONSTANCY OF MAGNECRYSTALLIC FORCE IN DIFFERING MEDIA. 161 
the force only varies ; for being included in the motion of the torsion index it has to 
be subtracted, as a constant quantity, from the observed result, and then leaves the 
true expression of the torsion force exerted between the upsetting points. 
3368. The magnet employed was that great one constructed by Logeman, and 
sent to the Exhibition of 1851. It could sustain a weight of 430 lbs., and is, I 
believe, very constant in power. It, with the torsion balance now used, is described in 
the Proceedings of the Royal Institution*. The sliding pole-pieces were of square iron, 
and presented either pointed terminations towards each other, or two flat faces, 1*7 inch 
square, which could be brought up to the opposite sides of the troughs or vessels 
containing the different fluids and media required for the experiments. These vessels 
were of various sizes and kinds ; but the outer ones were usually of copper, with flat 
sides, that the pole-pieces might bear against them, and be thus preserved in their 
position during the progress of a single experiment or a series of comparative results. 
3369. The torsion suspender was about 24-5 inches in length ; and was either 
a fine platinum wire, of which 28‘5 inches weighed 1 grain, or a finer wire of silver; 
or a bundle of cocoon silk fibres. The last was useful for certain delicate experi- 
ments, but could not be employed except in limited cases ; for its torsion force is 
liable to much variation under the influence of the vapour of water, camphine, &c. 
All these suspenders are liable to more or less of set, and that varies with the 
vibrations to which the apparatus is snbjeet ; but, by equalizing the time, by 
paying attention, and especially by alternating and combining right- and left-handed 
observations (3366.), the effect of this set may be obviated to a very great degree. 
The torsion wire terminated below by a hook, made out of a flat piece of copper 
foil, intended to receive on its edge a corresponding hook, attached to the object 
submitted to experiment. 
3370. The crystal, or other object, was held by one turn of a fine copper wire, 
which was continued upwards for 5’7 inches, and terminated by a flat hook like that 
just described. In this way different objects could be attached to the torsion wire, 
yet without any possibility of loose or uncertain motion about the point of attach- 
ment. Each loop had a horizontal bristle fixed to it, and this, by its position, not 
only showed the place of the crystal or other object beneath, but being retained 
between moveable stops associated with a horizontal scale, it indicated when the 
crystal was approaching the upsetting points, and being held within, and governed 
by, the stops, allowed them, through it, to govern the crystal. 
3371. The balance was enclosed by glass, to shut out currents of air as much as 
possible and prevent their production. 
3372. Experiments on the differential magnecrystallic force of bodies surrounded 
by different media required the bodies to be immersed in those media; and, as the 
latter varied from one another in specific gravity, so they exerted different degrees 
of buoyancy upon the same crystal, and thereby caused different degrees of tension 
* January 21, 1853, vol. i. p. 230. 
