520 Mr. Phillips on the Slow Stretch in Indiarubber, 



To sum up the results for indiarubber : — 



1. When indiarubber is stretched by a given load it 

 stretches according to the law 



stretch = a + b log (time) , 



and b is directly proportional to the load. 



2. When the load is removed the stretch still remaining is 

 equal to 



b(\ogt-\ogt ), 



where t is the time since putting on the load, and f is the 

 time since removing the load. 



3. When two loads are put on successively their effects are 

 not exactly superposable, but just after the second load is put 

 on, the effect is as if the last load only were acting, and then it 

 gradually alters till finally the effect is the same as if both 

 loads had been put on simultaneously. 



4. When indiarubber is stretched to a fixed length and 

 retained there, the load required to maintain that stretch 

 alters according to the law 



S = a — blogt, 



where S is the stress and t is the time since the rubber was 

 stretched ; b is proportional to the initial stretch. 



5. The temperature coefficients of expansion and contrac- 

 tion of indiarubber in tension are quite different. 



6. The decrement of the amplitude of vibrations in india- 

 rubber obeys the ordinary law. 



The Sloiv Stretch in Glass. 



Having established these results for indiarubber, it was 

 thought desirable to investigate whether the properties were 

 possessed by other substances. 



Glass being a substance which is known to exhibit a slow 

 creep when under stress, it was next investigated. As the 

 motion is so much smaller in glass than in indiarubber, it 

 was necessary to use a much more exact measuring device. 



A diagram of the apparatus is given in fig. 1. /is a glass 

 fibre made by drawing out a glass rod to a fairly thin neck 

 and then heating the neck and drawing it out to a 

 length of about 1 foot. By this means a fairly uniform 

 fibre was obtained, but it was still a little thinner in the 

 centre than it was elsewhere. One of the thick ends of the 

 fibre was attached as shown to a firm support S. 



To the other end was attached a little table T, and a weight 



