304 ELEMENTARY LESSONS ON [CHAP. v. 



the facts. There can be no doubt that the phenomenon 

 is due to magnetic induction : and it has been pointed 

 out (Art. 89) that the amount of induction which goes 

 on in a medium depends upon the magnetic inductive 

 capacity (or " permeability ") of that medium. This 

 magnetic permeability may be specified in terms of a 

 "coefficient of magnetic induction," 1 which represents 

 the ratio between the actual induction and the magnetis- 

 ing-force producing it. This coefficient will always be 

 positive ; it has values greater than i for magnetic 

 media, less than i for diamagnetic media : for empty 

 space it is i. The student may think of it in the 

 following way : Suppose a certain magnetising-force to 

 act in a certain direction, there would naturally result 

 from its action induction along a certain number of 

 lines of induction (or so-called. " lines of force"), and 

 in a vacuum the number of " lines of induction " would 

 numerically represent the force. But if the space con- 

 sidered were occupied by iron, the same magnetising- 

 force would induce many more " lines of induction " 

 through it, since iron has a large coefficient of magnetic 

 induction. If, however, the space considered were 

 occupied by bismuth, the same magnetising-force would 

 induce in the bismuth fewer " lines of induction " than 

 in vacuum. But those lines which were induced would 

 still run in the same general direction as in the vacuum ; 

 not in the opposite direction, as Weber and Tyndall 

 maintain. The result of there being a less induction 

 through diamagnetic substances can be shown to be 

 that such substances will be urged from places where 

 the magnetic force is strong, to places where it is 

 weaker. This is why a ball of bismuth moves away 

 from a magnet, and why a little bar of bismuth between 



l The student must not confound this " coefficient of magnetic induction," 

 for which we may use the symbol , with the " coefficient of magnetisation " 

 &, in Arts. 313 and 340. The two coefficients are, however, related in a 

 manner expressed by the equation u = i + 47T/. 



