459 
1909-10.] On Two Relations in Magnetism. 
cross-section. My equation can be put in this form. Heydweiller starts 
with another term in the expression for the external work done on the 
wire, namely, HB^/47r, but he makes approximations afterwards, which 
are equivalent to neglecting this term. Heydweiller’s formula has been 
proved experimentally by H. Rensing,* and has been attacked by R. Gans.f 
This paper will apply to the case of a twisted wire if F denote the 
twisting couple and x the angle of twist of the lower end. 
*“ Uber magneto-elastische Wechselbeziehnngen in para-magnetischen Substanzen,” 
Ann. d. Phys. (4) 14 (1904), p. 363. 
t “ Magneto-striktion ferromagnetischer Korper,” Ann. d. Phys. (4) 13 (1904), p. 634. 
“ Zur Heydweillerschen Kritik meiner Formeln betreffend Magneto-striktion ferro- 
magnetischer Korper,” Ann. d. Phys. (4) 14 (1904), p. 638. 
{Issued separately July 28, 1910.) 
