34 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



pendent measurements both at a' (now with 500 ohms in the telephone circuit) 

 and at d" with 200 ohms in circuit). At a' the pressure increments increase 

 rapidly with the depth until within i cm. from the telephone plate, when they 

 drop off a little. At the minimum d" the pressure decrements are continually 



g! a' c" 



larger until a point a little below the middle of the tube is reached by the 

 pin-hole, after which they continually wane as far as the telephone plate. 



This tube (diameter i cm.) was now cut down to a length of about 10 cm. 

 When placed in the telephone and blown with the pipe-blower it responded 

 weakly to c' and strongly to d'". The survey made with the pin-hole valve 

 is given in figure 52, the curves showing the pressure increments at depths 2, 

 4, 6, 8, 10 cm. below the mouth of the tube. The arrangement was remarkably 

 sensitive, so that 500 ohms had to be put in the telephone circuit to keep the 

 fringes in the field. Moreover, in sharp contrast to figure 51, the curves are 

 throughout of the same kind, there being now no pressure decrements. 

 Whereas the blown tube responded to a clear c", the chief maximum evoked by 

 the telephone is a decidedly flatted c". There remain the two supernumerary 

 resonances as in the preceding figure, the one at a' considerably dwarfed and 

 soon vanishing and the one near d", now not only sharpened in pitch, but re- 

 versed, so that it becomes a pressure increment. The accentuation of the 

 notes near c" and d" was obvious to the ear. 



In the inset I have given an independent survey of the distribution of 

 pressures in depth below the mouth of the tube. To keep the fringes well in 

 the field, 1,000 ohms were put in the telephone circuit. Pressures increase 

 continually, almost as far as the telephone plate, both at the c" (flat) maximum 

 and at the d" (sharp) maximum. One may note that at the flat c" the maxi- 

 mum fringe displacement at 1,000 ohms is 35, so that these pressure increments 

 (about o.o i cm. of mercury) are of the same order as occur in most of the com- 

 pletely closed regions above. 



The brass tube was then further cut down to a length of 7 cm. from the 



