512 



Mr. EL Tomlinson. The Influence of 



decrease of specific resistance as well as the temporary decrease is 

 greater with nickel than with cobalt. 



TJie -Effect of Longitudinal Traction and of Longitudinal Magnetisation 

 on the Thermo-electric Properties of Cobalt. 



It will be seen that contrary to the expectation of myself, who had 

 regard to the relationship which apparently exists between the 

 " rotational coefficient " of Hall and the alteration of specific resis- 

 tance caused by traction, cobalt behaves like nickel and not like iron. 

 I now, therefore, turned to examine the effect of traction on the 

 thermo-electric properties of the metal, for the purpose of ascer- 

 taining whether the strip would act in respect to these properties 

 like iron or nickel. According to Bidwell* cobalt acts like iron, but 

 it appears that this experimenter did not subject his bar of cobalt to 

 traction but to torsion, and finding that copper under torsion behaved 

 similarly to copper under longitudinal traction, assumed that cobalt 

 would do so likewise. The above-mentioned assumption is hardly, I 

 think, justifiable, and the following experiments show that cobalt is 

 altered by traction in a manner similar to nickel, provided the metals 

 are not at the same time under the influence of any magnetising stress. 



Experiment IV. 



The two strips of cobalt in the unannealed condition were clamped 

 together at their centres, the clamp projecting to a distance of about 

 3 inches from and at right angles to the strips. One of the strips 

 could be stretched as before by means of the lever, and insulated 

 copper wire soldered near the lower extremities of the two strips 

 served to connect them with the galvanometer. The clamp was then 

 heated at the extremity furthest away from the strips, and the heat 

 conducted along the clamp to the strips was such that in a short time 

 a temperature, as judged roughly by the touch of about 60° C, was 

 attained, stress of moderate amount was then put upon the lever, and 

 this caused a current from stretched to unstretched through the 

 heated junction : consequently stretched cobalt is thermo-electrically 

 positive to unstretched cobalt. As soon as the needle of the galvano- 

 meter was fairly steady, the stress was removed and a deflection in 

 the opposite direction ensued. The temporary stretching and un- 

 stretching were repeated several times, but always with the same 

 result as regards the direction of the deflection. The unannealed 

 cobalt therefore under mechanical stress behaves thermo-electrically 

 like nickel. f 



* " Phil. Mag.," April 1884, p. 261. 



f In this experiment the strips were under the influence of the earth's vertical 

 magnetic stress, but this last is so much smaller than that due to the helix, that I 

 have not deemed it necessary to take it into account. 



