Benton — Strength and Elasticity of Spider Thread. 77 



Similar, but much less distinct, effects were observed, when the 

 change in stress was only half as great. 



On account of the hygroscopic properties of the substance 

 it was not feasible to follow these changes over any great 

 length of time. 



Young's Modulus. — In view of the variations of strain 

 just described, it is clear that any value obtained for Young's 

 modulus, or the ratio of stress to the corresponding strain, 

 must involve some arbitrariness depending upon the time 

 when the strain is observed. A similar situation exists for 

 most other substances, however, though it is not usually 

 emphasized in publications as much as, in the writer's opinion, 

 it ought to be. Most (if not all) substances exhibit some 

 elastic after-effect after any change of stress, even when the 

 change of stress is very small. It is customary, in computing 

 the modulus of elasticity, to use the value of the strain observed 

 immediately after changing the stress. The result obtained 

 in this way will be approximately the same as if any later 

 value of the strain were used, the approximation being more 

 or less good according as the elastic after-effect is more or less 

 small. 



In the present experiments a load was applied long enough 

 (about three minutes) to obtain five readings for the increase 

 in length ; then the load was removed, and five readings were 

 immediately taken. The change in length, as determined from 

 the mean of the five readings with the load, and the mean of 

 the five readings after removing the load, was made the basis 

 of computation for Young's modulus. 



The first experiments were made with a thread 6'2 cm long 

 and 6 - 19xl0~ 5 cm 2 in mean cross-section. The mean of ten 

 experiments gave for Young's modulus 3*27 XlO 10 dynes per 

 cm 2 (480,000 pounds per square inch), the different determina- 

 tions varying from 2 - 98 to 3-62 XlO 10 dynes per cm 2 . The 

 change in stress employed was 2*5 X 10 7 dynes per cm 2 . With 

 larger change in stress the after-effects were so great as to pre- 

 vent proper determination of the modulus. 



Ten experiments were also made with another piece of the 

 thread, 16-5 cm long, and 5-30 XlO -5 cm 2 in mean cross-section, 

 twisted with a specific twist of three revolutions per centi- 

 meter of length, the same range of stress being used. Thej^ 

 gave a mean value for Young's modulus of 2-TOxlO 10 dynes 

 per cm 2 (390,000 pounds per square inch), the extremes being 

 2-10 and 3-29X10 10 dynes per cm 2 . 



Adopting as the final value for Young's modulus 3-0 XlO 10 

 dynes per cm 2 , it appears that the material of the spider thread 

 is, like most organic materials, much more stretchy than the 



