80 



Dr. R. A. Houstoun on Magnetostriction. 



magnetostriction and the effect o£ stress on magnetism has 

 been worked out several times, amongst others by^ J. J. 

 Thomson in his ' Applications of Mathematics to Physics and 

 Chemistry/ The result nearest mine is given by A. Heyd- 

 weiller * His equation is 



BE _ E 2 ^B 



^H~ 4ttBp 2 ' 



E being Young's modulus for the wire and p the stress per 

 unit area of cross-section. My equation can be pat in this 

 form. Heydweiller starts with another term in the expres- 

 sion for the external work done on the wire, namely, 

 HBdr/47r, but he makes approximations afterwards, which 

 are equivalent to neglecting this term. Heydweiller's 

 formula has been proved experimentally by H. Rensingt; and 

 has been attacked by R. Gans J. 



On the suggestion of Dr. C. G. Knott I have tested the 

 third relation with the data of Nagaoka and Honda§. 

 Before stating the results of the test I shall, however, prove 

 the relations in another way. 



Fk;-. L 







,K 



Suppose that the magnetic field intensity remains con- 

 stant and that the wire is carried through the cycle depicted 

 in the adjoining diagram, AB and CD being isothermals and 



* " Zur Theorie der magneto- elastisclien Wechselbeziehungen," Ann. 

 d. Phys. [4] xii. (1903) p. 602. 



t "Ueber magneto-elastisclieWeckselbeziehungenin para-magnetischen 

 Substanzen," Ann. d. Phys. [4] xiv. (1904) p. 363. 



X " Magneto-striktion ferromagnetischer Korper," Ann. d. Phys. [4] 

 xiii. (1904) p. 634. " Zur Heydweillerschen Kritik meiner Formeln 

 betreffend Maeneto-striktion ferro-magnetischer Korper," Ann. d. Phys. 

 [4] xiv. (1904) p. 638. 



§ " On Magneto -striction." H. Nagaoka and K. Honda. Phil. Mag. 

 [5j xlvi. (1898) p. 260. 



