trial 



in grams -w 



1 



40 



2 



82 



3 



85 



4 



85 



5 



88 



6 



98 





sq. in. 



7'4X10* 



11000 



15-2 



22000 



18-3 



27000 



18-3 



27000 



18-9 



28000 



19-4 



28000 



18-0X10 8 



26200 



76 Benton — Strength and Elasticity of Spider Thread. 



Tensile Strength. — The following results were obtained for 



the tensile strength, or stress to produce rupture, the thread 



having been twisted as described : 



Stress at rupture 

 Number of Load at rupture Cross- section at Dynes per cm' : Pounds per 

 t point of rupture 



5-28Xl0- 5 cm 2 

 5-28 

 4-55 

 4-55 

 4-55 

 4-95 

 Final value : (Mean of trials 2, 3, 4, 5, and 6) 1 8*0 X 1 8 



It would seem justifiable to assume that the first trial, giv- 

 ing a value less than half the mean of the other, represents 

 some anomalous condition (such as a flaw in the material). It 

 is accordingly rejected. 



It appears then that the material of this thread possesses 

 quite a high tensile strength, about double that of most kinds 

 of wood (the value for pine being about 10,000 and for other 

 woods ranging between 6,000 and 23,000 pounds per square 

 inch). 



Variations in Length. — The length of the thread was found 

 to vary irregularly from day to day, the stretching force being 

 eonstant. This was probably due to absorption of moisture, 

 but was not definitely investigated. 



Elastic After-effect. — If the stretching force was changed, 

 the thread stretched (or contracted) ; but the strain so pro- 

 duced did not at once assume a constant value, but gradually 

 varied. Thus in one set of experiments, the stress was 

 increased by 5 X 10 7 dynes per cm 2 (730 pounds per square inch) 

 or 1/36 of the mean breaking strength, and readings taken at 

 intervals of 30 seconds after applying the load gave the fol- 

 lowing values for the strain (mean of three trials) : 



•00200 

 •00203 

 •00207 

 •00210 

 •00218 



Upon removing the load as soon as these readings were taken, 

 the following values of the strain were observed at intervals 

 of 30 seconds : 



•00048 



•00040 



•00037 



•00034 



•00029 



