1884.] Stress and Strain on the Properties of Matter. 387 



traction on the thermo-electric qualities of cobalt was examined, and 

 it was found that temporary longitudinal traction renders cobalt tem- 

 porarily positive as regards its thermo-electric qualities, to cobalt not 

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

 time on the metal. The results obtained would, therefore, not only be 

 adverse to the above-mentioned hypothesis of the author, but also to 

 BidwelPs theory that the " Hall effect " is due to the joint action of 

 mechanical strain and certain " Peltier effects," were it not that in 

 Hall's experiments the metal is under very considerable magnetic 

 stress, which latter has been proved to alter very appreciably the 

 thermo-electric properties of iron, nickel, and cobalt,* and under these 

 circumstances both the effect of mechanical stress on the specific 

 resistancef and that on the thermo-electric qualities of cobalt may be 

 different to what they are, when there is no magnetic stress acting on 

 the metal. 



Longitudinal magnetic stress was found to render cobalt negative 

 thermo-electrical to cobalt not under stress. 



The cobalt used was in the form of a strip, and the section, which 

 was fairly uniform throughout the whole length, was 0'06168 square 

 cm. ; the length was 58'4 cm., and the mass 29'65 grams. 



The value of " Young's modulus " was determined by the method 

 of longitudinal vibrations, the strip being held in the centre, and 

 rubbed along. its length with a resined glove. 



The effect of longitudinal traction on the electrical resistance of 

 unannealed piano-steel was tried, the stress used being eventually 

 increased nearly to the breaking-point, with the view of ascertaining 

 whether the increase of specific resistance which had been previously 

 found to follow on moderate longitudinal traction might not be 

 changed to decrease. This was not the case ; but, on the contrary, 

 whatever the permanent load might be, a temporary addition to this 

 produced exactly the same temporary effect. The fresh experiments, 

 however, confirmed the results obtained with the older ones, and 

 agreed with these in making the alteration of resistance due to trac- 

 tion, both for unit stress per square unit of area and per unit increase 

 of length, less for steel than for iron. 



The electrical resistance of magnesium proved to be temporarily 

 increased by temporary longitudinal traction of moderate amount, but 

 the amount of increase was less than could be accounted for by mere 

 change of dimensions, so that the specific resistance of magnesium, 

 like that of aluminium, is decreased by the temporary stress. It was 

 found also that when the permanent load on the wire is very small, both 

 the temporary increase of length and the temporary increase of resist- 



* See Sir William Thomson's paper on " The Electrodynamic Qualities of 

 Metals," " Phil. Trans.," Part IV, 1856 ; also what follows. 

 t " Phil. Trans.," Part I, 1883, p. 152. 



