Stress and Strain on the Properties of Matter. 



519 



enable the pair of wires to attain a constant resistance-ratio, various 

 amounts of temporary stress from J to 2 kilos, were employed. The 

 results are given below : — 



Permanent 

 load on the 

 wire in kilos.* 



Temporary 

 load in kilos. 



Temporary alteration 

 of resistance in 

 divisions of the 

 platinum-iridium 

 wire. 



JL CIJJIJUI ell V 



alt eration of 

 resistance per 

 kilo. 



i 



i 



4-4 



8-8 





l 2 



10-8 



10-8 



1 



2 



H 



19-5 



13-0 



2 



2 



31-4 



15-7 



2 





4-4 



6-8 



The wire completely recovered itself when each of the temporary 

 loads was removed, and yet it will be noticed that the alteration of 

 resistance per kilo.f varies largely with the load, so much so, indeed, 

 that with a temporary load of 2 kilos, the temporary alteration per 

 kilo., which is of the nature of increase on loading, was nearly double 

 that when only J kilo, was employed for the temporary load. This 

 marked increase of alteration of resistance per kilo, with the increase 

 of the temporary load does not depend upon the amount of permanent 

 stress, as we obtain the same alteration for ^ kilo, when the per- 

 manent load is \ kilo, as we do when the permanent load is 2 kilos., 

 but at the same time it should be observed that with the permanent 

 load of 2 kilos, the first effect both of loading and unloading with 

 \ kilo, was considerably greater % than that which took place after three 

 loadings and unloadings with the ^ kilo.§ The results given above 

 undoubtedly point to imperfect elasticity due to the rotation of the 

 molecules about their axes, for though the wire recovered its original 

 resistance on the removal of the load, this is evidently due to the 

 slight shock caused by unloading. || 



Experiment IX. 



In order to examine more fully the relationship which might exist 

 between the alteration of resistance and the alteration of length 

 caused by temporary traction, the wire used in determining the value 



* "Weight of clamp, hook, &c, included. 



t The means of ten closely according trials with each load. 



X More than twice as great. 



§ After this the effect remained constant. 



|| "With some metals a marvellously small agitation suffices to make the molecules 

 spring back to their original position after they have been permanently deflected 

 from their positions by mechanical or other stress. 



