of Steel, Nickel, Cobalt, and Nickel- Steels. 51 



that the further diminution of length will be but slight, even 

 if the field be increased to several thousand units. 



(c) Cobalt (fig. 2). 



One of us has already examined the change of length in a 

 cobalt ovoid, which unfortunately was broken in two pieces. 

 The result was notwithstanding in close conformitv with that 

 already discovered by Bidwell. 



Rods of cobalt, obtained from Johnson and Matthey, were 

 turned into ovoids of the same dimensions as for the two 

 former metals. One of the ovoids was examined in the state 

 just as it issued from the lathe, while the other was annealed in 

 a charcoal fire for about four hours, after carefully wrapping 

 it with asbestos paper. As the change of length by magneti- 

 zation and the intensity of magnetization were characterized 

 by a remarkable difference in character, it would be well to 

 describe the phenomena separately for cobalt ovoids which 

 underwent different treatments. 



Cast Cobalt. — The behaviour of cast cobalt, as regards the 

 length- change, is similar to that of nickel in weak fields. 

 Instead of reaching an asymptotic value, as in nickel, the 

 contraction of cobalt reaches a maximum at about § = 160, 

 from which the metal gradually recovers with increase of 

 field-strength, till it attains its initial length in § = 740. 

 The metal, however, goes on elongating but at a less rapid 

 rate up to § = 2000, which is the strongest field employed in 

 the present experiment. Representing the change of length 

 by means of a curve (see fig. 2) we notice a singular trend, 

 somewhat resembling the inverted form of the curve showing 

 the same change for iron and steel. If the existence of the 

 maximum elongation in iron warrants the existence of the 

 Villari point, a point of opposite character will exist in cobalt 

 if the metal be subjected to loading. 



Annealed Cobalt. — The cast cobalt has a silvery hue, similar 

 to nickel only lacking the yellowish lustre of the latter. By 

 annealing cobalt the surface-colour turns ashy grey, and the 

 permeability of the metal diminishes (see § 1) in a remarkable 

 degree, as will be seen from the curves of magnetization 

 (Hg. 1). The change of length by magnetization takes place 

 at first slowly, but goes on steadily increasing till it amounts 

 to nearly 25 x 10" 6 for § = 2000. The curve (see fig. 2) re- 

 presenting the change is therefore very simple, approximating 

 to a straight line. As will be found later on, we found the 

 reciprocity between the strain caused by magnetization and 

 th^ effect of stress on the magnetization again established, 

 since the longitudinal pull only produces diminution of 

 magnetization. 



E2 



