24 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



inductor) there should be less tendency to marked fiinge deflection. Tests 

 made between g" and c'" seemed to bear out this inference ; but the current 

 of the small magneto was too weak to decide the case. 



23. Change of volume of reservoir. The marked effect of resonance met 

 with in the above experiments induced me to provide resonators of different 

 volume. The open shank K of the U-tube, figure 14, was therefore sealed with 

 a salient cylindrical cap, closed on top by a glass plate. The volume thus added 

 was about 135 cm. 3 , which is 2.8 times the original volume R' of 48 cm. 3 . 

 Testing this with the mercury break of frequency 12 per second, the high 

 cylinder (R f ) was less responsive than the low cylinder, in the ratio of about 

 20 to 30 fringes; but at the higher frequency, n= 100 per second, this disparity 

 was reversed, being in the ratio of 65 to 50 fringes, both data being larger. The 

 two cases were thus unequally near a resonance maximum. 



With the motor break I obtained the resonance curves, figures 31 and 32, 

 having 1,000 and 2,000 ohms in the telephone circuit, respectively, and using 

 the sealed telephone. The harmonics found are apparently in the key of A. 

 Strong negative and positive fringe deflections are here successively encoun- 

 tered; i. e., pressure increments and decrements occur together, though in 

 different frequencies. The fringes at the maxima, or the minima, or both, left 

 the field of the telescope ; but it does not seem that the response for the three- 

 fold additional volume is less strong than for the original volume. If anything 

 it is larger, which is rather an unexpected result, unless it refers to the greater 

 mass vibrating in the former case. 



24. U-tube used differentially. I now placed the two different reservoirs R 

 and R' of the U-tube in communication by the branch-pipe bb' (fig. 33) leading 

 to the telephone T by way of the quill-tubes t and t'. The pin-hole is in the sec- 

 ond branch t" at and a cock C (here open) is interposed. I had anticipated a 



