262 MAGNETISM Be haviour fJfJFT any required position, and if necessary kept there by the exertion of a known torsional couple. The arm and suspension must be carefully guarded from draughts by enclosing it in a glass case, which fits over the poles of the electromagnet, and is provided with a door and with means for bringing the torsion head t over any given part of the magnetic field. Wheu a cylindrical piece is to be tested it is suspended | from the fibre piA.^ Msoas to hang horizontally. For this pur pose Faraday was in the habit of using a stirrup of carefully se lected writing paper attached to the lower end of the fibre. It is of the utmost importance to guard against magnetic ac tion on ths suspension ; the least trace of iron in the arm db for in stance, or in the paper stirrup, would in many cases be more than sufficient to mask the ac tion proper to a weakly mag netic body. Fig. 40. In every experiment the magnetic behaviour of the support should be tested by itself beforehand, so that if any residual effect be present it may be allowed for. The greatest caution is also requisite in choosing the material to be experimented upon. There must be no chemical impurity, especially no trace of iron ; the spheres and cylinders must not be worked with iron tools or even with dirty hands. A source of error 1 to be specially guarded against in experiments with metals, or other good conductors, is the action arising from induced currents in the mass of the tested body caused by the increase and decrease of the strength of the magnetic field when the circuit of the electromagnet is made and broken. This error is wholly avoided by waiting till the suspended body has come to rest, and attending only to deflexions which are permanent after the intensity of the field has become steady. The first substance with which Faraday experimented was a bar of the heavy glass with which he had discovered ^ e rotafc i n of; ^e plane of polarization of light. It took up the equatorial position between the poles of the electro magnet as soon as the current was established. There was no distinction between its ends, or according to the direction of the lines of force; the bar always took the shortest course to the equatorial position, and remained there in stable equilibrium. When placed in the axial position it was in unstable equilibrium, and on the slightest displacement either way it moved off in that direction to 1 See Faraday, Exp. Res., 2309 sq. the equatorial position. A further action was observed Setting : when the bar was placed with its centre of mass out of the e <>ngat<-. centre of the field ; it was then repelled as a whole away ^j from the nearest pole (no matter which). On testing a transla- small cube or sphere of the substance, no pointing tendency tion of was observed, but the mass as a whole when it was placed -r^crea unsymmetrically with respect to the poles tended to pass ai r away from the poles towards the centre of the field, and from the axial line outwards. Faraday sums up the matter by saying that every element Experi- of the heavy glass tends to move from places of stronger to Cental places of weaker resultant magnetic force. This is exactly law of the opposite of the law for bodies like iron (see mathematical ^"~] theory above, p. 247). All bodies that follow the same law netics as heavy glass he calls diamagnetics, all that follow the and dia- opposite law, like iron, paramagnetics. For the purposes of ma " experimental demonstration it is better to take some weaker llL paramagnetic than iron, e.g., a tube filled with a solution of ferric chloride ; for the order of magnitude of the effect obtained is then the same as with diamagnetics, and there is no danger of complications arising from the mutual action of the particles of the substance (see above, p. 245). Faraday found the following substances to be dia magnetic; i.e., pieces of them tended to set their longest dimension equatorial between pointed poles, and spheres and cubes of them tended to pass from places of stronger to places of weaker force : rock crystal, sulphate of lime, List sulphate of baryta, sulphate of soda, sulphate of potash, of clia ~ sulphate of magnesia, alum, muriate of ammonia, chloride !1 of lead, chloride of sodium, nitrate of potash, nitrate of lead, carbonate of soda, Iceland spar, acetate of lead, tar- trate of potash and antimony, tartrate of potash and soda, tartaric acid, citric acid, water, alcohol, ether, nitric acid, sulphuric acid, muriatic acid, solutions of various alkaline and earthy salts, glass, litharge, white arsenic, iodine, phos phorus, sulphur, resin, spermaceti, caffeine, cinchonia, mar- garic acid, wax from shellac, sealing wax, olive oil, oil of turpentine, jet, caoutchouc, sugar, starch, gum arabic, wood, ivory, mutton (dried), beef (dried), blood (dried or fresh), leather, apple, bread. In testing liquids Faraday used a very thin glass tube Liquids, of the form shown in figure 41 ; the opening being very fine, there was no need for a cork or other stopper which might have caused disturbance; the slight diamagnetic effect arising from the glass was allowed for. Another way of testing a liquid 2 is to place it in the bottom of a watch glass which rests on the edges of the pole of the electromagnet. "When the fluid is paramagnetic, it collects in the places of greater force, forming a depression in the centre of the field as in figure 42 ; when it is diamagnetic, it collects in the places of weaker force in the centre of the field, as in fig. 43. Yet another method 3 is to put a small quantity of the fluid in a narrow tube, and place the tube horizontally in the equatorial line so that the end of the liquid column is just on the axial line. When the electro magnet is excited the liquid will be driven away from the axial line or drawn in according as it is diamagnetic or paramagnetic. Faraday found that breaking a weakly magnetic body Powders into pieces, or even reducing it to powder, produced no effect upon its magnetic behaviour provided its general form was unaltered. In order to avoid disturbance from 2 Plticker, Poyg. Ann., 1848.
Quet, Conijotes Rendus, 1854.