16 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



opened and the ordinates show the fringe deflections s obtained. The max- 

 imum or pressure is yery much larger than the minimum or dilation. The 

 experiment may be repeated at pleasure in either direction with the same 

 results. A loose plug moving easily with a minimum of oil is essential; but 

 many trials to and fro are necessary, nevertheless, to set the plug at the 

 optimum. The maximum pressure obtained in these initial experiments was 

 the equivalent of about 50 fringes; i. e., A= 1.5 X io~ 8 cm. or about 2 X io~ B 

 atmosphere, for a frequency of about 12 per second. At higher frequencies 

 this datum is much increased. 



These pressures are real ; for on suddenly closing the cock at the maximum 

 and breaking the current, they are retained until discharged on opening the 

 cock. There has thus been an actual influx of air into the resonator R in B 

 through the crevice in the cock C, acting like a valve while the telephone is 

 sounding. Screw-cocks are not at once adapted for these experiments, since 

 (on closing) a pressure is introduced owing to the viscosity of air. On open- 

 ing, the minimum is accentuated. 



16. Pressure depending on the frequency and on the intensity of vibra- 

 tion. The maxima are observable for very considerable reductions of the 

 intensity of vibration. In figure 16 I have given the observed fringe location 

 (here incidentally decreasing when pressure increases) when different resist- 

 ances r, given by the abscissas in io 3 ohms, are put in the telephone circuit. 

 The frequency is n = 1 2 per second. In curves i and 2 the rift in the cock was 

 either slightly too large or too small; in curve 3 it was nearer the optimum. 

 In case of i and 2 the curious result of an initial increase of pressure with the 

 first few hundred ohms inserted shows itself. This disappears in curve 3. 



Figure 17 contains similar results 5' when the frequency is n = ioo per 

 second. The sensitiveness has obviously greatly increased and in a general 

 way this is the case for higher frequencies. Anticipating results obtained in a 

 better adjustment below, the following data may be cited. There were 2,000 

 ohms in circuit. The zero-point is at s. 



100 



S Q -S'=II 



300 per sec. 

 29 fringes 



250 

 IS 23 



It will be shown, however, that the 

 magnitude of the phenomenon depends 

 essentially on resonance, so that these 

 data have but a general bearing. 



17. Fringe deflection varies as current intensity. If the readings in figures 

 16 and 17 be changed to actual deflections 5 s' s, the curves (3) and (5) 

 take the form given in figure 18. They are roughly hyperbolic, so that thfe 

 equation rs C(r being the high resistance inserted into the telephone circuit) 

 may be taken to apply within the errors of observation for resistances exceed- 

 ing 1,000 ohms. So computed for convenience, rs is 24X10* in series 3 



