280 



DR E. G. COKER ON 



that raising the elastic limit in one direction lowers it in the contrary direction does not 

 hold good here, since all critical points vanish. 



The further development of the idea that the distance apart of the limits is a con- 

 stant, appears to have no phy sical basis for the torsion of iron. 



IX. — The Influence of Tension on Torsion. 



1. Tension within the Mastic Limit, and Torsion within the Elastic Limit. 



Among the notable experiments made upon the influence of tension upon torsion are 

 those of M'Farlane upon steel pianoforte wire.* 



From the article it does not appear that any experiments were made to ascertain 

 whether tension within the elastic limit has any influence upon torsion within the elastic 

 limit, but in any case it was thought worth while to make the experiments, as speci- 

 mens of much larger diameter could be dealt with. 



The first specimen tried had the comparatively large diameter of f inch, and the 

 maximum tension load which could be applied was 3000 pounds. Repeated experiments 

 failed to show any difference in the torsional properties of the bar, whether loaded or 

 unloaded. 



A second bar was then prepared, having a diameter of \ an inch, and the experiment 

 was repeated with tension loads varying from 200 to 3000 pounds, the latter corre- 

 sponding to a stress of 15,300 pounds per square inch. The diameter of the pulley was 

 41 '62 inches, so that a weight of one pound in each pan corresponded to a torque of 

 41*62 inch pounds. 



The length of the specimen was 8 inches, and the calibration value gave one division 

 of the scale =16*85 seconds. 



The following Table gives a summary of the results obtained : — 



Table V. 





Mean Value of Reading corresponding to 





Tension Load on 



Specimen. 



Lbs. 



1 lb. in each pair = 41 "62 inch lbs. 



Mean of Columns 

 2 and 3. 









Torque increasing. 



Torque diminishing. 





200 



59-00 



58-98 



58-99 



600 



59-00 



58-74 



58-87 



1000 



58-56 



58-70 



58-63 



2000 



58-59 



58-70 



58-65 



3000 



58-75 



58-65 



58-70 



* Enc. Brit. Art. "Elasticity." 



