386 Mr. H. Tomlinson. The Influence of [Nov. 20, 



III. *' The Influence of Stress and Strain on the Physical 

 Properties of Matter. Part II. Electrical Conductivity 

 continued. The Alteration of the Electrical Conductivity of 

 Cobalt, Magnesium, Steel, and Platinum-Iridium produced 

 by Longitudinal Traction ; Discovery of simple Relations 

 between the ' Critical Points '* of Metals." By HERBERT 

 TOMLINSON, B.A. Communicated by Professor W. GRYLLS 

 ADAMS, M.A., F.R.S. Received October 7, 1884. 



(Abstract.) 



The effect of temporary longitudinal traction on the electrical 

 resistance of cobalt was determined by a method similar to that 

 already described in a former portion of this memoir,t and it was found 

 that, like uickel, this metal has its resistance decreased by moderate 

 temporary stress, in spite of the changes of dimensions which ensue. 

 Whether the decrease of resistance would be changed to increase, as 

 it is with nickel, by a greater amount of stress, has not yet been 

 ascertained, but should this be the case, the magnitude of the stress 

 per unit area which would suffice for the purpose must be much 

 greater with cobalt than with nickel. As with nickel, permanent 

 extension and rolling diminish the effect of temporary longitudinal 

 traction, so that there is a larger decrease of resistance caused by a 

 given stress with annealed than with unaunealed cobalt. 



Cobalt is remarkable for the extreme persistence with which the 

 same load, when applied again and again, continues to produce per- 

 manent increase of resistance, and probably increase of length, but for 

 a moderate amount of permanent extension the increase of resistance 

 is more than accounted for by the permanent increase of length and 

 diminution of section which take place ; so that, as with iron and 

 nickel, the specific resistance is decreased by moderate permanent 

 extension. The permanent decrease of specific resistance per unit 

 for unit permanent increase of length is, for iron, cobalt, and nickel, 

 O02, 1'44, and 2*37 respectively ; thus the permanent decrease of 

 specific resistance, as well as the temporary decrease of resistance, is 

 greater with nickel than with cobalt. 



Since contrary to the expectation arising from the apparent relation- 

 ship which exists between the " rotational coefficient " of Hall, and 

 the alteration of specific resistance caused by mechanical strees,J 

 cobalt behaved like nickel, and not like iron ; the effect of longitudinal 



* Points at which there is a sudden increase in the ratio of the permanent 

 extension produced by any load and the load itself, 

 t " Phil. Trans.," Part I, 1883, p. 58. 

 J Loc. cit., pp. 167, 168. 



