514 Mr. Phillips on the Slow Stretch in Indiarubber, 



Fourthly, if indiarubber is stretched to a certain length 

 and retained at that length, then the pull required to keep it 

 so stretched decreases for about the first day, according to 

 the 



law 



P = a — Mog t, 



where P is the pull, t is the time which has elapsed since the 

 initial stretch was established, and a and b are constants for 

 a particular extension, b is proportional to the amount of 

 the initial extension. 



The apparatus used to investigate the slow stretching in 

 indiarubber was very simple. One end of a strip of india- 

 rubber bandage 30 cms. long was attached by a clip to a 

 wall bracket, and a light scale-pan and a photographic scale 

 with divisions of '210 mm. were attached by another clip to 

 the other end. A telescope' with cross wires in the eyepiece 

 was used to view the scale. 



The mode of procedure was as follows : — 



1st, the reading in the telescope was noted. 



2nd, the required load was placed very gently in the scale- 

 pan at a given time. 



3rd, as each division of the scale passed the cross wire of 

 the telescope the time was taken. 



The following is a typical set of readings : — 



Pull. 



Beading on 

 Scale. 



Stretch in Scale- 

 divisions. 



Time. 



log*. 







3012 









500 grams 







12 noon 







76*8 



46-68 



2 m. 59 s. 



•47 





76-9 



46-78 



3 50 



•58 





77-0 



46-88 



4 42 



•67 





77-1 



46-98 



6 



•78 





77-2 



47-08 



7 40 



•88 





77-3 



47-18 



9 24 



•97 





77-4 



47-28 



11 40 



107 





77-52 



47-4 



15 36 



119 





77-61 



47-49 



20 18 



1-31 





77-73 



4761 



27 45 



1-44 





78-43 



48-31 



2 30 



2-18 



Plotting the scale-readings against log t, as in Curve No. I., 

 we get a straight line, i. e. we have the relation 



OC — Oi 



x=a + b\ogt or J^T t = h > 



the symbols having the same meaning as before. 



