54 2 MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



consequently 



= 2s/2 L 



3 I 



This correction is not always negligable. It often happens, 

 especially for magnets of appreciable size, that the mean intensity 

 of magnetisation is not 2000 times greater than that of the earth ; 



we may then take = - 

 I a 2000 



The value of the coefficient / depends on the shape of the 

 magnet and the nature of the metal. 



If the magnet were a very long cylinder parallel to the plane 

 of the coil, which would be the best condition, we should have 

 (387) 



As the coefficient k is between 30 and 40, it follows that in effect 



27T/"= I. 



For a sphere (385) we should have 



= or 



If the magnet is a flat rectangular bar, of the usual form, it may 

 be compared to an ellipsoid. Let us suppose that the ratios of the 

 smaller axis c to the mean axis , and of this latter to the major axis a 

 are small, the coefficient M (387) for the mean axis has approxi- 

 mately the value 



b\ b ic* b 

 - i- -To/.4- 

 a |_ a 2 b 2 <: 



If a=iol> and t>=iot, we find 



M = n 

 we have then 



i 



f = - = about , 

 M 



or 



