of MM. P. Curie and C. Ckeneveau. 361 



Hence Erom equations (3), (4), and (5) 



from which 



K =^V (7) 



* CM 



This formula (7) thus permits the absolute value of the 

 coefficient K to be determined for a body if the other 

 quantities are accurately determined. By an analogous 

 method P. Curie * has determined the absolute value of the 

 coefficient of specific magnetization of water. 



But the apparatus is especially adapted for relative 

 measurements, and in this case it becomes extremely simple 

 and practical. 



If we have a body of unknown coefficient K and of mass 

 m, we have from equation (6) 



K»*H,^=g (8) 



For a body of known coefficient K/ and mass m occupying 

 the same volume 



dH, 7 dD 

 Hence by division 



*™^=K «> 



*- - = -.. . . . . . (10) 



Equation (10) thus enables K to be determined. 



It is evident that it is much more convenient to measure 

 the displacements D and D' of the spot corresponding to the 

 angular deviations 2« and 2a! between the two positions 

 0' and 0" of the body (fig. 2), for, without altering equation 

 (10), the displacements of zero are eliminated and the 

 accuracy of measurement doubled. 



If the body is placed in a tube of glass or other substance, 

 this tube being subjected like the body under test to the 

 magnetic forces, an experiment must be made with the tube 

 alone, and the effect due to the tube represented by a 

 displacement of the spot D" added or subtracted. 



* P. Curie, Annates de Chimie et de Physique, 7 e Serie, t. v. p. 344 

 (1895). 



Phil. Mag. S. 6. Vol. 20. No. 116. Aug. 1910. 2 B 



