248 Prof. Thomson on the Forces experienced by inductively 



magnetic substances of feeble inductive capacity. — The pheno- 

 mena discovered by Faraday relative to the action of magnets 

 on substances not previously known to be susceptible of mag- 

 netic influence may be exhibited with great ease by means of 

 the apparatus described above. Small balls of the substances 

 to be experimented upon may be hung from one end of the 

 balance (the ball of soft iron being of course removed) by fine 

 threads of sufficient length to allow the arm, which may be of 

 any substance containing no iron, to be out of reach of any 

 sensible influence from the magnet employed. There is in these 

 cases no difficulty, regarding the length of the suspending- 

 thread, of the kind noticed above with reference to soft iron, 

 as the magnetic forces experienced are never strong enough 

 to produce lateral instability (that is, a want of stability in the 

 line of motion), even with the lightest of the substances ex- 

 perimented on, unless the suspending thread be far longer 

 than is necessary. In the experiments I have made, the 

 threads bearing the small balls have not been more than 

 four or five inches long. The diameters of the balls have 

 been from a quarter of an inch to an inch, or an inch and 

 a half. Instead of simple bar-magnets of steel, which are 

 not powerful enough to be convenient for these experiments, 

 I have used a bar electro-magnet of very moderate power, 

 consisting of a helix and soft iron core. This core is a cylin- 

 der of about an inch in diameter and a foot and a half long, 

 with round ends (nearly hemispherical), which, when the core 

 is in its central position, extend about an inch beyond the 

 helix on each side. By these means the repulsion of balls of 

 diamagnetic substance, and the attraction of very feebly ferro- 

 magnetic substances, may be shown with great facility. 



18. For example, I may mention that 1 have hung a small 

 apple, whole, by a thread three or four inches long, and 

 putting it at first at rest, pressing slightly (in virtue of toision 

 produced by the torsion head mentioned above) upon one end 

 of the soft iron core previously to the excitement of the elec- 

 tro-magnet, I have found that as soon as the galvanic current 

 is produced, the apple is repelled away ; and, by pushing for- 

 ward the soft iron core, I have chased it across the field 

 through a space of four or five inches. 



] 9. I have also used the same apparatus to show that a body 

 which is feebly attracted in air is repelled when immersed 

 below the surface of a sufficiently strong solution of sulphate 

 of iron in a small trough, so arranged that when, b} T the force 

 of torsion, the body immersed in the liquid is made to 

 press on a side of the trough, the electro-magnet may be 

 placed with one end of its core pressing on the outside of the 



