of Magnetic Stress in Magnetostriction. 461 



B 2 



This use of the expression ^- , incorrect according to 



existing theories, appears to have been first made by S. 



Bidwell*. 



As to the stresses inside the iron, Maxwell's theory 



H 2 

 (Art. 642) gives "a hydrostatic pressure — combined with 



BIT v7T 



a longitudinal tension ^— along the lines of force ;" while 

 Kirchhoff's theory gives a hydrostatic pressure 



2 f 47T J b7T 2 



combined with a tension 



along the lines of force. 



Other systems of stresses in an isotropic elastic medium, 

 which are equivalent to gravitational and electrostatic forces 

 in certain cases, are discussed by Chreef, and shown to bo 

 essentially different from that given by Maxwell (Art. 105). 



So far there does not appear to be sufficient experimental 

 evidence to enable one to decide between these theories. 

 Calculating the strain of an anchor-ring according to Kirch- 

 hoffs theory, we easily find that 



I 4,*(1 + 30) ^ } 



1 Sv 

 ~3 v' 

 n denoting the rigidity. 



As the principal factor K/ can be found from the measure- 

 ment of effect of pressure on magnetization, the easiest method 

 for making the crucial test would be to try experiments on an 

 anchor-ring. Moreover, it is worth while to notice that the 

 volume-change would in this case amount to the order 10 ~ 5 . v 

 in iron and to — 10~ 4 . v in nickel, calculating the value of K' 

 from the experiments of Bidwell on rings of these two metals. 

 In conclusion, we would express our thanks to Dr. H. 

 du Bois, at whose suggestion the preparation of this com- 

 munication was undertaken. 



g^' | March 25, 1896. 



* Phil. Trans, clxxix. p. 217 (1888). 



t Proc. Edin. Math. Sjc. xi. p. 107 (1892-93). 



