fifSloic Stretch in Indiarubber, Glass, and Metal Wires. 513 



data ; and that, therefore, what it deals with is the succession 

 of simpler events which would take place within a model, 

 which the procedure he must of necessity adopt obliges him 

 to substitute for the more complex machinery of nature — viz. : 

 that model which is defined by the data which he has to 

 make the foundation of his reasoning. 



G. Johnstone Stone r. 



30 Ledbury Road, W. 

 February 9, 1905. 



XLIX. The Slow Stretch in Indiarubber, Glass, and Metal 

 Wires when subjected to a Constant Pull, By Percy 

 Phillips, M.Sc* 



The SI oio Stretch in Indiarubber. 



IN an investigation of the adiabatic and isothermal elas- 

 ticities of indiarubber, it was observed that whenever 

 indiarubber was subjected to a sustained pull it continued to 

 yield slowly after the completion of the first large stretch. 

 This slow yielding, being quite considerable in size, was very 

 easily investigated, and the following general results were 

 obtained. 



Firstly, with a particular pull the stretch is a linear function 

 of the logarithm of the time elapsed after the establishment 

 of the pull. Thus, if x is the stretch and t the time since the 

 pull was established, 



x = a-\-b \ogt, 



where a and 6 are constants for the particular pull. This 

 obviously does not hold for very small values of t. No doubt 

 t \< to be reckoned from an instant a little before the establish- 

 ment of the pull, but if t has any value above a few seconds 

 it is sufficiently exact to measure it from the instant of 

 establishment of the pull. 



Secondly, the constant b is proportional to the pull, and 

 when t is measured in minutes the constant a is also roughly 

 proportional to the pull. 



Thirdly, if a pull is established for a short time and then 

 removed, the indiarubber returns to its original length, ac- 

 cording to the law x=b log - where x and b denote the same 



° V 



tilings ns before, t represents the time which has elapsed 

 since the pull was established, and t the time since the pull 

 was removed. 



* Communicated by the Physical Society : read January 27, 1905. 



