Stress and Strain on the Properties of Matter. 529 



e X cc 1 is, with most metals, a constant, where e denotes " Young's 

 modulus." Taking the mean of the values of e and of A for cobalt in 

 the annealed and unannealed conditions, we have a=l"927, and 

 e X «7=1886 X 10 8 . For magnesium *=2'399, and e X a7=1969 x 10 8 . 



In the next table will be found collected together most of those 

 results of the present inquiry which can be expressed by numbers. 



It has been suggested with regard to the method of finding the 

 effect of strain on the resistance, that unless the strain was fairly 

 uniform, thermo-electric effect might have to some extent vitiated the 

 results. It will be seen, however, that the strain must have been 

 fairly uniform, and even if there had been considerably more lack 

 of uniformity than actually existed, the small change which can be 

 wrought by stress and strain in the thermo-electric power of any of 

 the metals hitherto used, would preclude the possibility of any appre- 

 ciable vitiation. Further, in many cases, very different lengths of the 

 same wire were tested and found to yield the same results, which 

 would not have been the case had the strain on the connexions, a 

 point also suggested for consideration, introduced error. 



Summary. 



1. The electrical resistance of cobalt, like that of nickel, is tem- 

 porarily decreased by temporary longitudinal traction. 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. 



2. Permanent extension and rolling diminishes with cobalt as with 

 nickel, the effect of longitudinal traction alluded to in 1. 



3. 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. 



4. Moderate permanent extension decreases permanently with cobalt 

 as with nickel and iron, the specific resistance. 



5. Temporary longitudinal traction renders cobalt temporarily 

 positive as regards its thermo- electrical qualities to cobalt not under 

 traction, provided there is no magnetic stress acting at the same 

 time. 



6. Temporary longitudinal magnetic stress renders cobalt tem- 

 porarily negative as regards its thermo-electrical qualities to cobalt 

 not under magnetic stress, provided there is no mechanical stress 

 acting at the same time. 



7. The effect of temporary longitudinal traction, even when carried 

 to very great excess, is to increase the resistance of unannealed piano- 

 steel ; and this increase, though less than with iron, both for uni t 



