526 



Mr. H. Tomlinson. The Influence of 



Kilos, on 

 the lever. 



Temporary alteration of 

 resistance in terms of the 

 divisions of the graduated 

 platinum-iridium wire. 

 First series. 



Temporary 

 alteration of 

 resistance, &c. 

 Second series. 



Mean 

 alteration 

 from the 

 two series. 



1st kilo. 



230 



23 5 



23 25 



2nd „ 



23 5 



24-5 



24-00 



3rd „ 



29-5 



29-5 



29-50 



4th „ 



32 



335 



32-75 



5th „ 



350 



29-0 



32-00 



6th „ 



27-0 



28-0 



27-50 



7th „ 



23-0 



26-5 



24-75 



8th „ 





235 



23-50 



described in the previous portions of this memoir,* and, moreover,, 

 there is a like change in the elasticity, f but with platinum-iridium 

 the effect, as far as alteration of the resistance is concerned, is more 

 marked than is the case with other metals. Equally noteworthy is it 

 that at the third kilogram there is a sudden increase in the tem- 

 porary alteration of resistance produced by the load, and a like 

 phenomenon is plainly discernible in the above-mentioned previous 

 experiments. Moreover, the stress which produces this sudden 

 increase is certainly not far from the stress which produces the first 

 sudden leap in the value of the ratio of the permanent alteration of 

 resistance to the load, when the annealed wire is stretched for the 

 first time ;J nay, more, a careful examination of Experiment VII § 

 shows undoubted evidence of the existence of the same number of 

 critical points where temporary alteration is concerned, as there are 

 in the case of permanent extension. 



Finally, it must be added, that if we take the mean effect produced 

 by all the loads on the resistance of the wire, we obtain an increase of 

 resistance which is exactly equal to that already recorded in Experi- 

 ment X, and therefore we must regard the comparatively large 

 increase of resistance of platinum-iridium caused by traction as a 

 well-established fact.|| 



* Loc. cit., p. 50, where by taking the alteration of resistance caused by con- 

 secutive loads of 2 kilos, each, this can be plainly discerned, 

 f Loc. cit., p. 16. 



t Compare Experiments VII and XXIII, loc. ext., pp. 50, 82. As regards 

 Experiment XXIII, it should be remarked that more recent observations have shown 

 that there is a critical point at the third kilogram (fifth kilogram if we include the 

 weight of the scale-pan itself), and that here therefore we must look for the first 

 critical point and not at the eighth kilogram (tenth kilogram including the weight 

 of the scale-pan), where is the second critical point. 



§ Loc. ext., p. 50. 



|| This comparatively large increase of resistance produced by traction is rather 

 against the use of platinum-iridium in the construction of standard resistance coils. 



