522 



Mr. H. Tomlinson. The Influence of 



porary stress is comparatively large ? The answer must be in the 

 affirmative, for, as we have seen, the alteration of resistance increases 

 with the load much more largely than the alteration of length : this 

 is shown in the next table. 



Limits of 

 temporary 

 load in kilos. 



Increase of resistance 

 per unit divided 

 by the increase of 

 length per unit. 



Alteration of specific 



resistance per unit 

 divided by the increase 

 of length per unit. 



+ signifies increase of 

 specific resistance. 



o-i 



0-7813 



-0-719 



f-1 



1-028 



-0-472 



1—1* 



1 • 307 



-0-193 



li-2 



1-605 



+ 0-105 



It would appear, then, at first sight, chat magnesium and nickel 

 behave in a somewhat similar manner as regards the effect of stress 

 on their specific electrical resistance, for with nickel the decrease of 

 resistance is changed to increase when the stress has passed a certain 

 limit. Closer examination, however, does not justify this view, for 

 with nickel the change from decrease to increase is conditioned by the 

 amount of the permanent load, so that if say 6 kilos, be the limiting 

 stress, and we maintain this stress permanently, an additional kilo, 

 will always produce increase of resistance, though this load of 1 kilo, 

 be taken off and put on any number of times, whilst with magnesium, 

 if we regard 2 kilos, as the limiting stress, and allow this load to 

 remain on, an additional J kilo, will only produce a temporary 

 increase or a temporary decrease of specific resistance when put on and 

 taken off respectively for the first time. 



The Alteration of the Electrical Resistance of Platinam-iridium pro- 

 duced by Longitudinal Traction. 



Platinum-iridium plays such an important part in the construc- 

 tion of standards of length and of electrical resistance, that having a 

 length of wire made of the alloy sufficient for my purpose, I resolved 

 to test its elasticity, and the change of electrical resistance due to 

 stress. 



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

 Rigidity, Density, fyc. 



The alloy was specially prepared for me by Messrs. Johnson and 



