20 



DISPLACEMENT INTERFEROMETRY BY 



The first series is irregular. The present data are a considerable improve- 

 ment on the preceding, but they are far from indicating the rigidity of brass. 

 A concluding series, made with but slight tapping, showed the following two 

 rates : 



Turns 

 Ae 



Xio- n 

 A0 per turn 



Finally, in a series of measurements in triplets, between three and four turn 

 of the forcing-screw, values of EXio~ 11 between 1.3 and 2.4 were obtained, 

 showing therefore no improvement on the preceding work. 



The interferometer was now modified to guard against displacements on 

 tapping, large fringes were installed, and readings were made several times 

 before and after tapping. There was but little difference. Examples of the 

 results are given in figure 17, (i) and (2), where Ae/A/VX io 3 = 43, and therefore 



In the first series A per turn was 8.0, and hence -EXio- n = 2.4; in the 

 second A^ = 7.8 and -EXio- u = 2.5. Seeing that a scale-part in figure 17 is 

 but 23Xio- 6 cm., these results are experimentally very good. But their 



X 



absolute value, as given by E, is nevertheless very low. The rates for the 

 outgoing and return series are identical. The back-lash, as it were, on passing 

 from one to the other is probably in the apparatus. 



In the triplets naturally higher values of E appeared; for instance, between 



