Elasticity of Ferromagnetic Substances by Magnetization. 461 



of the coils. It is supported at L and M by two fulcrums ; 

 one o£ them is an ordinary wedge fixed to the brass rod, 

 while the other consists o£ a cylinder which can rotate about 

 its own axis. Q is the weight suspended from the middle of 

 the bar. At the centre of the bar a fine copper wire, whose 

 diameter is about O08 mm., is soldered and stretched vertically 

 upwards by means of a weak spring P. This copper wire is 

 wound once round a rotating cylinder to which a reflecting 

 mirror is fixed, and stretched upwards, as used in Hertz's 

 dynamometer. The rotation of the cylinder is observed by 

 means of a vertical scale and a telescope. 



The dimensions of each part of our arrangement are as 

 follows : — 



Length of each coil = 39 90 cm. 



Its internal diameter = 5 "80 cm. 



4ttw =393-5 cm. 



Distance between the coils in j = 2'5 cm. for iron and steel. 



air-gap J = 20 cm. for nickel and cobalt. 



Distance between two fulcrums | = 59"91 cm. for iron and steel. 



Land M J = 21 - 95 cm. for nickel and cobalt. 



Diameter of rotating cylinder ... = 0'172 cm. 

 Scale-distance =261/3 cm. 



The sensibility of our apparatus was such that a displace- 

 ment of one division of the image of the vertical scale in the 

 field of the telescope corresponded to the change of depression 

 of 1*72 x 10~ 5 cm. at the middle of our ferromagnetic rod. 

 It was necessary to protect the mirror and the thin copper 

 wire from air-currents in order to get rid of minute vibrations 

 of the mirror. 



The measurement was conducted in the following way. 

 The bar to be tested was placed in the axial line of the coils 

 .and then loaded by a weight. The tension of the fine copper 

 wire was then suitably adjusted, and the mirror was directed 

 to the observing telescope. This adjustment was performed 

 as in the former experiment. To begin with, the bar was 

 demagnetized and then the initial reading taken. A current 

 was then passed through the coils and the corresponding 

 deflexion noted. These processes were repeated with suc- 

 -cessively increasing currents. 



Since the resistance of the coils did not exceed 3 ohms, no 

 trace of heating effect due to current was observed during 

 the time in which the deflexion was taken ; we therefore dis- 

 pensed with a water-jacketing arrangement. The lateral 

 contraction or elongation, which necessarily accompanies the 

 magnetization of a ferromagnetic rod, was at most of the 

 order 2 x 10~ 6 cm. for iron and 7 X 10 -6 cm. for nickel. 

 Hence with our arrangement, the lateral change of dimension 



