Stress and Strain on the Properties of Matter. 



511 



1 grain per square centimetre was for the unannealed cobalt 

 242"3xl0 -1 ~. The ratio of the decrease of resistance per unit to 

 the increase of length per unit is 0'486, and the corresponding ratio 

 in the case of specific resistance is V986. 



ISTot only is cobalt remarkable for having, like nickel, its resistance 

 decreased by longitudinal traction, in spite of the increase of length 

 and diminution of section which is caused by the stress, but it also 

 presents a peculiarity not seen in nickel or indeed in any of the 

 other metals hitherto examined, namely, the extreme persistence with 

 which the same load when applied again and again continues to 

 produce permanent increase of resistance. With few metals is this 

 persistence anything like so noticeable with such stresses per square 

 centimetre as have been here employed, for we may observe from the 

 last two experiments that though a load of 22 kilos, had been twice 

 used on the lever, and afterwards a long rest given to the strip, 

 20 kilos., after having been put on and taken off fifteen times, still 

 continued to produce permanent increase of resistance.* Even 

 16 kilos, continued to produce a permanent effect, so that it would be 

 only after a very large number of loadings and unloadings that the 

 increase of resistance on taking off the load would be equal to the 

 decrease of resistance on putting on the load. The permanent 

 increase of resistance caused by the above-mentioned loads also was 

 greater the longer the time during which the stress was maintained. f 



The Effect of Permanent Longitudinal Extension on the Specific 

 Resistance of Cobalt. 



We have seen that cobalt behaves like nickel as far as the effect on 

 the specific resistance of temporary longitudinal stress is concerned, 

 and that both iron and nickel^ are decreased in specific resistance by 

 moderate longitudinal strain. This it appears is the case with cobalt 

 also. The distance between the points where the copper wires at the 

 upper and lower extremities of the cobalt strip were soldered, 

 was before stretching 49'8 cm. and after the stretching 50 cm. 

 The specific resistance before the stretching was 2289 X 10 -8 , and 

 after the stretching 2231 x 10 -8 , so that there was a decrease of 

 specific resistance of 2*6 per cent, for a permanent lengthening of 

 1'8 per cent. The permanent decrease of specific resistance per unit 

 divided by the permanent increase of length per unit, is for iron 

 0*02, for cobalt 1*44, and for nickel 237 ; so that the permanent 



* This was after-wards found to be the case when the same load had been put on 

 and taken off some fifteen times more. 



f This " running down" is also explained "with other metals, but not to the same 

 extent, as regards persistency "with such comparatively small stresses per square 

 centimetre. 



% Loc. cit., p. 100. 



