Stress and Strain on the Properties of Matter. 



527 



The Effect of Longitudinal Traction on the Thermo-electric Properties 

 of Platinum-iridium. 



Experiment XIIL 



The above-mentioned large increase of resistance resulting from 

 the temporary traction of platinum-iridium, together with the ap- 

 parent relationship between the effect of mechanical stress on the 

 thermo-electric properties of metals, and that on the specific resist- 

 ance, rendered it probable that stress would act on the thermo-electric 

 properties of iron and platinum-iridium similarly as regards nature, 

 but with greater intensity with the latter metal than with the former, 

 but here, again, a surprise was met with, for when the wire was 

 tested in the same manner as the cobalt had been, the unstr etched 

 platinum-iridium was found to be positive to the temporarily stretched 

 metal, and therefore the alloy is affected by stress, thermo- electrically, 

 in a manner similar to nickel. Thus we see that, though these new 

 experiments would in the case of cobalt and magnesium largely con- 

 firm us in our opinion respecting the above-mentioned relationship, it 

 is quite the contrary in the case of platinum-iridium. In the next 

 table will be found drawn up the results obtained by Hall, Bidwell, 

 and myself. 



Hall says of the numbers in the second column : "I cannot vouch 

 for the quantities within 50 per cent., but I think I can vouch for the 

 direction of the effect." Bearing this statement in mind, and the diffi- 

 culties which lie in the way of obtaining accurate values for the ratio 

 of lateral contraction to longitudinal extension, which difficulties will 

 affect the numbers in the fourth and fifth columns, one cannot help 

 being struck with the fact that with most of the metals the order of 

 the " rotational coefficients " is the same as that of the alteration of 

 specific resistance caused by traction. Cobalt and platinum are, 

 however, conspicuous exceptions, but with regard to the former 

 metal it has been already observed, and of course the same observa- 

 tion would apply equally to nickel and iron, that longitudinal traction 

 might produce different effects in the magnetised and unmagnetised 

 metals. The exception furnished by platinum cannot be thus ac- 

 counted for, nor does it seem fair to attribute the discrepancy either 

 to errors of observation or to difference in the purity of the specimens 

 examined by Hall and myself respectively.* In the fifth column is 

 given the difference between the alteration of the specific resistance 

 of lead by traction, and that of the other metals, and, with the excep- 

 tions just mentioned, f it may be fairly said that, within the errors of 



* The specimen of platinum used by myself was obtained from Messrs. Johnson 

 and Matthey as chemically pure. 



f I ought to remark here that according to Bidwell the " Hall effect" in 

 aluminum is + . In a trial made by myself on the pure specimens of aluminium in 



