RELATIVE MAGNETISATION. 365 



We may, in fact, consider k^ as the relative coefficient of magneti- 

 sation of a body, in reference to the medium which surrounds it, 

 k and /', being coefficients of the two media in respect of air. If the 

 coefficient k of the body is greater than the coefficient K of the 

 medium, the value of k^ is positive, and the apparent magnetisation 

 of the body is positive. If, on the contrary, k<k\ the value of k^ 

 is negative and the body will appear diamagnetic. When the co- 

 efficients k and K are equal, the magnetisation of the body A will 

 appear to be null, which ought to be the case, for it is in a medium 

 identical with itself, and the induced magnetism is superficial. 



We are thus led to assume that there is no real opposition of 

 properties between magnetic and diamagnetic bodies, and that the 

 difference of the effects is due to the greater or less magnetic 

 character of the external medium. As diamagnetic bodies retain 

 their characteristic properties in the most perfect vacuum which has 

 been produced, we must assume, on this view, that a vacuum is a 

 magnetic medium, and that its coefficient of magnetisation is greater 

 in absolute value than that of all known diamagnetic substances. 



If, on the contrary, we assume the value zero for the coefficient 

 of magnetisation of vacuum, a negative value must be assigned to 

 those of all diamagnetic bodies. In this case the coefficient of 

 induction ft is greater than unity for magnetic bodies, and is less 

 than unity for diamagnetic bodies. No body is known in which ft is 

 negative since the coefficient of diamagnetic bodies is never greater 



than in absolute value ; we have already said that for bismuth, the 



47T r 



most diamagnetic of all known substances, k is about = . 



400,000 



The coefficient of induction of diamagnetic bodies only differs from 

 unity by an infinitely small quantity. For soft iron and nickel the 

 coefficient k being comprised between 30 and 40, the value of ft is 

 nearly 500. The ratio of the two absolute values of k for iron and 

 bismuth is then almost 



40 x 400,000 = 1,6 . io 7 . 



We may however remark that the influence of a magnetic 

 medium could only be exactly compared with that of a fluid, and 

 the principle of Archimedes be applied, provided that k^ = k - k 1 . 

 From the preceding remark it appears that this ratio is very nearly 

 verified for all diamagnetic bodies, and for those also which are 

 very slightly magnetic; but it would be far from the truth if the 

 surrounding medium had a coefficient of magnetisation near unity, 



