Elasticity of Ferromagnetic Substances by Magnetization. 59 



<p = : - and &>= — — r-, 



where d=M'S and a=MM'. The actual deflexion observed 



is 



Now U-M'^r{4>—#) 





("'-sK 1 -?)] 



In our experiments 

 ^==624*5 cm.. 



65*83 cm., p = 0'795 mm., 



and 



= 0-1054. 



Since n 1 was very small compared with s, and — appears 



as a small correction, it was sufficient for our purposes to 

 determine n' roughly only once for a specimen throughout 

 the sets of observations. This was easily done by directing 

 the telescope to M' and reading off the deflexion. Usually 

 n' did not exceed 7 divisions of the scale, while s was greater 

 than 90 divisions. 



To prevent shocks by loading, the face of the initial weight 

 was covered with a mat of cotton wool. The wire to be 

 tested was surrounded bj a tube of paper to prevent 

 disturbance due to air-currents. 



Denoting the weight, by which the elongation SI — SI' is 

 produced, by P, we have for the coefficient of elasticity 



P 



A 



E = 



61-81' 



where A is the section of the wire. P in our experiments 

 was 100 grams, which gave a deflexion of more than 90 

 divisions of the scale. To test the working of the present 

 apparatus, weights of 10, 50 and 100 grams were used with 

 the following results : — 





Table 



XXII. 



t 



= 12° C. 



Weight. 



Tungsten-Steel. 



Nickel. 



28" 



74 per cent. A T i. 



10 



50 



100 



div. 



9-7 



48-1 



962 



div. 



91 



47-3 



960 





div. 



92 



4(>-3 



92*9 



