of Steel, Nickel , Cobalt, and Nick el- Steels. 7 i 



onlv feebly magnetic. The course of the magnetization curve 

 of nickel-steel resembles that of nickel. 



(3) (a) In cast cobalt, mechanical elongation in the 

 direction of magnetization produces diminution of magneti- 

 zation in low fields, which gradually lessens as the field 

 strength is increased. Ultimately there is increase of 

 magnetization by elongation. Thus, there is a critical point 

 in cobalt which is opposite in character to the Villari point 

 in iron. 



(b) In annealed cobalt, mechanical elongation in the 

 direction of magnetization produces diminution of magneti- 

 zation, which increases with the field. 



(-1) Mechanical elongation in the direction of magnetization 

 produces increase of magnetization in nickel-steel. 



Change of Length by Magnetization. 



1. In cobalt, the character of the change is different in the 

 cast and in the annealed state. 



(a) Cast cobalt contracts in low fields and assumes 

 minimum length in <5 = 130, whence it returns to its former 

 length in $ = 740, and goes on elongating at a slower rate 

 as the field is increased (result already obtained by Bidwell 

 with cobalt rod). This stands in reciprocal relation with 

 the effect of mechanically elongating cast cobalt on its 

 magnetization. 



(b) Annealed cobalt contracts without showing maximum 

 up to «§ = 1800. The character of the change is similar to that 

 in iron (after passing the maximum elongation). The effect of 

 mechanically elongating annealed cobalt is reciprocal to the 

 change of length by magnetization. 



2. Nickel-steel elongates by magnetization. The character 

 of the change is similar to that in nickel, but the sense is 



I4t)\. 



different. The rate of change I v * ' J in high fields is greater 



\ d$ I 

 in 29 per cent. Ni than in 36 per cent. Ni, in which it is again 

 greater than in 16 per cent. Ni. Nickel-steel elongates to a 

 greater degree in the annealed than in the hard-drawn state. 

 The elongation of nickel-steel in very low fields (comparable 

 with the terrestrial magnetic field) is generally less 

 than 10~ 7 . 



Change of Volume by Magnetization. 



1. Iron, steel, nickel, and nickel-steel increase in volume 

 by magnetization, but cobalt shows contraction. 



2. (a) Cast cobalt contracts at a rapid rate in low fields, 



