Determining the Elastic Constants of Glass. f)53 

 Stra nbel's Values for Poisson's Ratio for Glass. 



Width ... 



3-0 cm. 



2-5 cm. 



2-0 cm. 



2 cm. 



175 cm. 



10 cm. 



Width 



.0 



8-3 



6-7 



10 



7 



5 



Thickness. 





Distance 















apart of 

 knife-edges. 















7 cm. ... 



•216 



•221 



•228 



•225 



•224 



•228 



5 cm. ... 





•220 



•223 



•223 



•226 



•228 



3-4 cm. ... 





•215 



•225 



•218 



•221 



•230 



Straubel remarked that the nearness of the knife-edges in 

 some of the tests produced a marked diminution in the 

 cross-bending, and stated that he was unable to obtain 

 consistent results without the introduction of thin rubber or 

 cardboard pads between the knife-edges and the glass. He 

 considered, however, that for beams of widths 2 cm. and 

 under, the values obtained with the knife-edges 7 or 5 cm. 

 apart were reliable. 



S trail bel found also that it was practically impossible to 

 obtain thin glass beams with accurately plane surfaces, and 

 that as the load was increased, a progressive change was 

 observed in the value of Poisson's Ratio for most specimens, 

 due to initial curvature of the surface of the beam. To correct 

 errors from this source he took a series of observations with 

 different loads, and by applying the method of least squares 

 to his results deduced the value which would have been 

 obtained had the beam initially been free from curvature. 



Method of the Present Investigation. 



The method used in this investigation was substantially 

 the same as that employed by Cornu. The glass beam was 

 supported on two knife-edges, and the load was applied by 

 means of weights suspended from two wooden arms clamped 

 to the ends of the beam. By this means a large moment 

 could be applied without introducing great pressure on the 

 knife-edges, thus reducing the action of disturbing end- 

 effects. The cover-glass either rested on the beam or was 

 held against it by means of small screws, and it was found 

 to be a simple matter in either case to adjust so that the 



Phil. Mag. S. 6. Vol. 42. No. 250. Oct. 1921. 2 P 



