22 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



sure within R is raised as a whole by the amount which the pin-hole air-valve 

 will withstand. Again, if the closed region TU, figure 27, contains a pin-hole 

 valve within only, it does not differ essentially from the corresponding case 

 in figure 26. But if an additional pin-hole leak 0' is supplied on the T side, 

 figure 27, the U side gradually develops a large pressure decrement; whereas 

 if the pin-hole is supplied on the U side, this develops the usual pressure in- 

 crement. In the foimer case air leaks out of 0' diffusively; in the latter it 

 leaks into 0". 



In figures 22, 23, 24, marked pressure decrements occur near the minima at 

 c" and o" in case of these prolonged tests. One may therefore suspect that 

 (as above) the decrements result from an insufficiently tight joint at the tele- 

 phone plate. The telephone with sealed plate was therefore carried through 

 the chromatic series of notes from /' to a", as recorded in figure 28. The 

 curve resembles figures 22 to 24 in character, except (as was anticipated) that 

 there are no pressure decrements at the minima. In fact, the maxima (below 

 /' at c", g", and above a") came out more sharply than in figuies 22 to 24, and 

 the now positive minima (near g', d", a") equally so. It seems as if the ordi- 

 nary overtones in the key of C were in question, but further work will be needed 

 to establish these and other relations. 



To obtain corroborative evidence, I finally replaced this telephone with a 

 common bipolar, containing the usual clamped plate, and worked out the 

 results of figure 29. These obviously again reproduce figures 22 to 24 with 

 marked dilatations at the minima, except that the maxima at a', e", a," are now 

 in the key of A, conformably with the changed internal volume. Thus far 

 the argument seemed to be conclusive ; but when I re-examined the original 

 unipolar telephone with sealed plate at a low r er frequency c' (n = 26o), an 



