504 



Mr. H. Tomlinson. The Influence of 



mechanical stress on the specific electrical resistance of metals. A 

 comparison of the two tables shows, that with the exception of plati- 

 num the metals stand in nearly the same order in both, and that iron 

 and nickel are very conspicuous, the former at the top and the latter 

 at the bottom of both lists. 



Mr. Shelford Bidwell has also brought forward* very strong evi- 

 dence in favour of his assertion, that the " Hall effect " can be ex- 

 plained by the joint action of mechanical strain and certain " Peltier 

 effects," and has shown that those metals in which the " Hall effect " 

 is positive are rendered by temporary traction thermo-electrically 

 negative to pieces of the same metal unstretched. It seemed, then, 

 a matter of considerable interest to ascertain whether cobalt would 

 act like iron or nickel as far as the effect of stress on the electrical 

 resistance is concerned, and I endeavoured — for some time in vain — 

 to obtain either wires or strips of cobalt suitable for the purpose in 

 view. At length, through the courtesy of Mr. Wiggin, jun., Birming- 

 ham, I found myself in possession of two stripsf of cobalt, upon which 

 I was able to make the necessary experiments. 



Preliminary Determination of the Value of " Young's Modulus" 



Density, 8fc. 



The length of each strip was 58'4 cm., and the thickness and width 

 of one of them, when gauged at ten places at equal distances apart, 

 were as follows : — 



Number of 



Thickness in 



Width in 



Section in square 



observation. 



centimetres. 



centimetres. 



centimetres. 



1 



0-0864 



0-7091 



0-06127 



2 



0-0870 



0-7076 



-06156 



3 



-0851 



0-7131 



-06069 



4 



0-u858 



0-7104 



-06095 



5 



0-0857 



-7282 



-06241 



6 



-0848 



0-7250 



0-06148 



7 



0-0850 



0-7326 



-06227 



8 



-0849 



0-7557 



0-06416 



9 



-0846 



0-7556 



06392 



10 



-0854 



-7474 



-06383 





0-08547 



0-72847 



-06226 



The strip was therefore fairly uniform in section throughout, the 

 mean value of the section as determined by the gauge being - 06226 

 square centimetre. 



* " Phil. Mag.," April 1884, p. 249. 



f Mr. Wiggin informed me that he found it impossible to draw wires of cobalt, 

 as the metal was so hard that it destroyed the tools with which it was brought into 

 contact. Messrs. Johnson and Matthey were also good enough to attempt to draw 

 for me wires of cobalt, but they too failed through a like cause. 



