184 Prof. Thomson on the Theory of Magnetic Induction 



quantities be positive, the force on the ball in each position will 

 be in the direction in which the force of the field increases ; if 

 any one of these quantities be negative, the force on the ball 

 when the corresponding principal axis is in the direction of the 

 lines of force, ■n'ill be in the contrary direction, or that in which 

 the force of the field decreases most rapidly. 



13. If A, B, and C be all positive, the body is called ferro- 

 magnetic ; if they be all negative, it is called diamagnetic. No 

 substance has as yet been found to have some of the quantities 

 A, B, C positive, and others negative. 



14. If the inductive capacities be veiy small, all the preceding 

 conclusions will be applicable to the actions experienced by bodies 

 in air (ferro-magnetic), or in any magnetizable fluid of either 

 ferromagnetic or diamagnetic inductive capacity, provided, instead 

 of the absolute inductive capacities of the substance in each case, 

 we use for A, B and C, or for the " principal inductive capacities " 

 in the verbal enunciations, the excesses of the absolute principal 

 inductive capacities of the substance, above the inductive capacity 

 of the fluid. 



Curious experiments might be made by means of a variable 

 field of force occupied by a magnetizable fluid, and a ball of cry- 

 stalline substance allowed to move freely in the line of most 

 rapid variation of the force. If the inductive capacity (whether 

 positive or negative) of the fluid be intermediate between the 

 greatest and the least of the absolute principal inductive capaci- 

 ties of the substance, the ball will be urged from places of weaker 

 towards places of stronger force when its axis of greatest induc- 

 tive capacity is placed along the lines of force, and in the con- 

 trary direction when the axis of least inductive capacity is placed 

 in the same direction. 



It would be easy to adjust the strength of a solution of sul- 



crystallic axis along the lines of force, that axis must, according to § 8, be 

 the principal axis of greatest inductive capacity, or, bismuth being diamag- 

 netic, the axis of least diamagnetic capacity. 



It is right to add, that what, according to the theory explained in the 

 text, must be the correct explanation of the peculiar phaenomena of mag- 

 netic induction depentUng on magnecrystallic properties, was clearly stated 

 in the form of a conjectm-e by Faraday in his 22nd Series (2588) in the 

 foUo^iing terms : — " Or we might supjiose that the crystal is a little more 

 apt for magnetic induction, or a little less apt for diamagnetic induction, in 

 the direction of the magnecrystallic axis than in other directions. But, if 

 so, it should surely show * * in the case of diamagnetic bodies, as bismuth, 

 a difference in the degree of rei)ulsion Avhen presented with the magne- 

 crystallic axis parallel and perj>endipular to the lines of magnetic force (2552) ; 

 which it does not do." [Read before the Royal Society, December 7, 1848.] 

 The faihu-e of the first experiment (2552) to detect this differential action 

 need not be wondered at, when we consider how minute it must probably 

 be ; and the conjecture, apparently abandoned at the time by the author 

 for want of experimental support, may be considered as fully established by 

 his own subsequent experimental researches. 



