250 Prof. Thomson on the Forces experienced by inductively 



for instance, we take the arrangement (described for Exp. 2 

 above) of two bar-magnets, fixed with similar poles near one 

 another, we have obviously between these poles a point where 

 the resultant force vanishes, and towards which consequently 

 a small diamagnetic ball placed anywhere sufficiently near it 

 would be repelled. It is easily shown that, actually under 

 the action of gravity, a ball of diamagnetic substance would 

 be in stable equilibrium a little below this position, without 

 any external support or constraint whatever, if only the 

 magnets were strong enough. It is, however, extremely im- 

 probable that any attempt to realize this by experiment will 

 succeed, since, even in the most favourable cases, no diamag- 

 netic repulsion upon a solid has yet been obtained which at 

 all approaches in magnitude to the weight of the body. Still 

 we must consider that a true theoretical solution of the cele- 

 brated physical problem * suggested by " Mahomet's coffin " 

 has been obtained, which is not the least curious among the 

 remarkable consequences of Faraday's magnetic discoveries. 



On the relations of Ferromagnetic and Diamagnetic Magneti- 

 zation to the Magnetizing Force. 



21. In the mathematical investigation by which the result 

 stated above was obtained, it is assumed that the magnetization 

 of the substance of the ball in each case is proportional to the 

 magnetizing force (although this assumption may of course be 

 avoided by merely supposing /-t to have a value varying with 

 the force, which will not affect either the investigation or the 

 form of the result). It appears to me very probable that this 

 assumption is correct for all known diamagnetic substances^ and 

 for homogeneous feebly ferromagnetic substances ; since it is 

 equivalent to an assumption that inductive magnetization of 

 a substance does not impair or in any way alter its suscepti- 

 bility for fresh magnetization by means of another magnet 

 brought into its neighbourhood. This opinion cannot how- 

 ever at present be regarded but as a mere conjecture, being 

 as yet unsupported by experiment. It is indeed directly op- 



* It is I believe often thought that this problem is solved in the expe- 

 riment in which a needle is attracted into a galvanic helix held with its 

 axis vert'cal ; but I have convinced myself that the needle always touches 

 somewhere on the sides of the tube ("if there be one round it) or on the 

 wire of the helix : and I have also ascertained that, when a powerful helix 

 is used with, in place of the needle, a tin-plate cylinder, even if it be very 

 little less in diameter than the inner cylindrical surface of the helix, there 

 is never stable equilibrium without contact between them. Thephaenomenon 

 of a solid body, hovering freely in the air, in stable equilibrium, without 

 any external support or constraint, has never, I am convinced, been wit- 

 nessed as the result of any electrical or magnetical experiment. 



