38 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



the U-tube (when i ,000 ohms were in circuit) gave a deflection of but 5 fringes. 

 This maximum (curiously enough) is in the middle, figure 58, in spite of the 

 open hole there. The result appears more clearly when the resistance is re- 

 duced about one-half (500 ohms) . It is the d" ', probably, which is impressed on 

 the graph of the adjutage, figure 57. 



The a' pipe (hole opposite adjutage closed) showed two maxima, a large one 

 of 25 fringes at a' and a smaller one of 12 fringes at e" when 1,000 ohms were 

 in circuit. The latter is possibly impressed on pp by the e" note in the adju- 

 tage. The distribution of compression in depth is also given in figure 58 for 

 this pipe. It increases more gradually toward the center than in the pre- 

 ceding case, a result which suggests a tendency to a re-entrance in the middle 

 for the d" curve. The a' pipe is also much stronger than the d" pipe under 

 the same conditions ; but both are usually less than half as strong in their 

 compressions as the closed organ-pipe of the preceding section. 



The quill-tube glass probe functioned about equally well at d" and a', but 

 the somewhat narrower aluminum-tube probe often refused to give results at 

 d". As I could not detect any defect in the probe, it is probable that the 

 phenomena of 25, relating to nodes in the pin-hole tube, are here in question. 

 If so, pressure would be transferred to the U-tube by means of a vibrating 

 column of air. 



4 8 



34. Reversal of poles of telephone. The more or less complete exchange of 

 the maxima and minima of 5, of a closed region, when the telephone current 

 (motor break) is reversed, has already been instanced. 



Open pipes, however, do not quite conform to this rule. In wide tubes (2.6 

 cm.) of organ-pipe shape, pressure increments only are found, and these are 

 indifferent to the direction of the telephone current. 



