510 



Mr. H. Tomlinson. The Influence of 



The sixth, column shows that the temporary alteration of resistance 

 is of the nature of a decrease, but the decrease becomes less and less 

 as the permanent strain increases, at first rapidly, and afterwards 

 more slowly, until the load of 18 kilos, has produced its permanent 

 effect. Moreover, we learn from this column that when a load of 

 12 kilos, had been employed the temporary effect per kilogram is 

 always greater for the highest load than for the smaller ones, so that 

 for the loads 16, 18, and 20 kilos., when employed for the first time, we 

 get about twice the temporary decrease per kilogram which is obtained 

 with the loads 2 and 4 kilos. Experiment III, however, teaches us 

 that ultimately, when all the loads have been applied a great number 

 of times, the temporary decrease is exactly proportional to the load in the 

 case of the loads 4, 8, 12, 16, and 20 kilos., nor is there any sign, as 

 was the case with nickel, that the decrease of resistance would 

 ultimately be changed to increase when the stress was increased 

 beyond a certain limit. The greatest total stress on the strip, 

 including that caused by the weight of the lever itself, was 1860 kilos, 

 per square centimetre, whereas with nickel the point of loading where 

 the above-mentioned limit was reached, was about 1500 kilos, per 

 square centimetre.* It would seem probable that there is a limiting 

 stress beyond which, as in nickel, the resistance begins to increase 

 with the load, but it is evident from what has been said that this 

 limiting stress is much higher for cobalt than nickel. 



From the last values recorded in the sixth column of Experiment 

 III it was calculated that the decrease of resistance per unit produced 

 by a stress of 1 gram per square centimetre at the temperature 

 of 16° C. is 386*8 X 10~ 12 . The ratio of the decrease of resistance 

 per unit to the increase of length per unitf is 0*703, and, if we 

 assume the ratio of lateral contraction to linear elongation to be 

 0*250, the ratio of the decrease of specific resistance per unit to the 

 increase of length per unit of the stress is 2*203. All the values 

 just given are much less than the corresponding ones in the case of 

 nickel. 



With the cobalt in the unannealed state temporary traction also 

 produced decrease of resistance, but, as might be expected from what 

 has been said above with reference to the effect of permanent strain, 

 the amount of decrease per unit was less than with the annealed 

 metal. J The decrease of resistance per unit produced by a stress of 



* Loc. cit., p. 60, where the stresses given should be increased by that caused 

 by the weight of the scale-pan if the total stress on the wire is, as in this case, 

 required. 



+ : — , where E is the resistance and AR is the decrease of resistance which 



R I 



results when the length, I, is increased to I + Al. 



X Notice the similarity in this respect between cobalt and nickel. Loc. cif., 

 p. 61. 



