28 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



maxima are here dilatations, laid off positively for convenience in com- 

 parison with figure 36. The valve action (500 ohms in circuit) is much 

 weaker in figure 37 than in figure 36, but in every other respect figure 36 is 

 reproduced. This is a very disconcerting result, for it is not the impulsive 

 displacement of the telephone plate alone which produces the pressure 

 increments within, even if a reversal of the telephonic current (change of 

 poles) changes pressure increments into pressure decrements. The coni- 

 cal pin-hole vent in association with a given adjustment of telephone 

 current (poles not changed) will do the same thing if reversed. 



Suggestions to the same effect are contained in the above data for reversible 

 conical vents, but nothing quite so trenchant throughout all the harmonics. 



Further work with the resonator III consisted in modifying the motor, so 

 that with reduced frequency of current interruption the lower overtones could 

 be surveyed. The results are given in the graph, figure 38, for 1,000 ohms in 



50 







a' C" d" e" (f a 



the telephone circuit. Conformably with the large volume, the low tones are 

 very effective and the maxima occur at B, f, b, after which there is an hiatus in 

 which the motor refused to function, continuing with b', f", of figure 36 (lower 

 curve) appended to figure 38. The B maximum is represented by a displace- 

 ment of about 50 fringes, equivalent to a pressure increment of about 0.012 

 cm. of mercury when 100 ohms are in circuit. 



27. Resonator of very small capacity. Finally the resonator R was all but 

 closed (resonator IV), or at least reduced to a shell-like space by a cylindrical 

 inset, closed with a glass plate but a few millimeters above the mercury of the 

 U-tube. At n = ioo deflections of 50 fringes were obtained with the copper- 

 foil pin-hole, and at w = i2 but 25 fringes. The conical vent was less useful, 

 being too large. 



