40 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



As would be nearly zero throughout in consequence of the symmetrical in- 

 duction utilized. The magneto employed above, however, was too weak to 

 test the case adequately Like the reversible As n , even s n would nevertheless 

 depend on the mode of vibration of the telephone plates. 



On completion of the interferometer work of the next chapter, I took up 

 this pipe again for a more detailed survey of the pressure distribution in rela- 

 tion to frequency, at different depths below, on the mouths of the pipe. The 

 graphs (5,000 ohms in circuit) are given in figure 61, where the numbers on 

 the curve show the position of the pin-hole in centimeters from the nearer end, 

 16 cm. being at the middle, and 8 cm. half-way to it. The a! maximum, so 

 loud to the ear in the i6-cm. curve, is followed at once by a near a' minimum, 

 which is easily overlooked, as it runs at once into the next c" maximum. 

 The e" i? actually a minimum followed by the maximum near f" . 



At a quarter tube-length from the end (8 cm.) these features have been 

 thoroughly modified. The a' maximum is now isolated, the former e" mini- 

 mum has been reversed. The 8-cm. curve is somewhat low throughout, 

 probably because the telephone gets heated in so long a series of experiments. 



Figure 62 shows the effect of reversing the f 

 poles of the telephone with the pin-hole at / 

 the middle of the tube. The position // of 

 the switch corresponds to the preceding fig- 

 ure; / is the alternate position. The effect 

 produced is obvious near c" , whereas e" 

 retains its negative character, as do also 



the other positive parts of the graph. - - 



/T n 1 n rl* 0" n" n 1 f>" H P" n" n' 

 In an adequate discussion of these results 



the telephone current would have to be measured as one of the parameters. 



35. Open pipe on the interferometer. It seemed to me of sufficient interest 

 to make a direct measurement of the compression in the open pipe, by passing 

 the component ray of the interferometer longitudinally through it, as explained 

 in Chapter V and shown in figure 78, 47. The reed R of that figure is here 

 to be replaced by the telephone as in figure 57 (inset). Using the motor- 

 break, a survey of compressions was made between a' and a" . On removing 

 the resistance from the telephone, very sonorous and regular sinusoidal wave- 

 forms were obtained at the harmonics a' and g" (nearly), each about one fringe 

 in double amplitude. The occurrence of the latter note is peculiar. Referring 

 to the a', the mean compression would be (notation as in 42), 



C\ 



16cm. 



when the height of crest above trough is one fringe. Since 



the mean value in question is thus Ap/p = 6.2 X io~ 3 per fringe and the maxi- 

 mum compression would be IT/ 2 times larger or io~ 3 X9-7, practically io~ 2 . 



