Apparatus for the Calibration of Extensometer s. 133 



diameter of a ring at the cross-wires of the microscope. A 

 displacement exactlv equivalent to ten rings was then pro- 

 duced and the movement of the extensometer scale noted. 

 The process being continued, the readings in Table I. were 

 obtained. 



Table I. 



Calibrator 

 Xo. of Ring?. 



Extensometer. 



Reading. 



Differences 

 in ^V inch. 





 10 

 20 

 30 

 40 

 50 

 60 

 70 



50-0 



64-5 



794 



93-4 



107-8 



122*2 



1366 



1510 



1 

 14-5 



146 



14-3 



14-4 



14-4 



144 



14-4 



Mean difference for 10 Rings 1443 



The mean difference of 14*43 corresponds to a displacement 

 of 2 9 '46 x 10~ 5 cms. The magnification of the extensometer 

 is therefore 3012. 



On another occasion, the instrument was arranged to give a 

 difference of 13*75 units for a displacement of ten rings, and 

 was then tested for accuracy in reversing. Readings were 

 taken for a displacement of five rings followed by a return 

 to the original position. Three such series are given in 

 Table II. 



Table II. 



Calibrator 







Extensometer. 







No. of 



Rings. 















Readings 

 1633 



Diff. 



Readings 



Diff. 



Readings 



Diff. 







119-9 





120-0 









6-8 





70 





6-8 



5 



1701 



6-9 



112-9 



6-9 



1132 



6-9 



1 " 



163*2 





119*8 





1201 





