32 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



edly complicated, but consistent in their character throughout. In place of 

 the simple distribution of pressure in the usual closed organ-pipe, there is 

 a maximum pressure decrement near f' t another at d", a maximum nega- 

 tive pressure increment near a', and a very pronounced positive pressure 

 increment near f". A pipe of the same length, if wide and uniform, would have 

 responded to e". The low pitch is thus attributable to the plate space in the 

 telephone and possibly to the narrowness and roughness of the tube, which 

 was of hard rubber. 



However, the increase or decrease of pressures respectively, from the mouth 

 of t toward the telephone plate 12.5 cm. below, is well shown in figure 47, 

 in which the fringe displacements s for the large minimum d" and the large 

 maximum f" are recorded. Pressure rises (or falls) very rapidly from the 

 mouth inward and at 6 or 8 cm. of depth already reaches its maximum value. 

 Beyond this the curves of the figure often show a decrease, which is probably 

 not incidental (owing, for instance, to a heated telephone or to inevitable 

 changes in the motor-break inducing current, during so protracted a series 

 of observations) , as it occurs frequently below. 



A cylindrical enlargement was now attached to the pipe t, figure 43, con- 

 sisting of a glass tube (see fig. 48) 9 cm. long and 1.5 cm. in diameter, tapering 

 to a narrow tube fitting t, the total length of which was now 15 cm. Explor- 

 ations were made with the probe in the wider tube, with results also shown in 

 figure 48, at i, 4, 6, 8 cm. from the end of the tube. These results (pressures 

 positive upward) are again consistent. There are two definite maxima, one 

 near a' and the other (uncertain) near d". The latter may be a residue of the 

 case, figure 45, but the former is surely a repetition of the a'. Naturally, pres- 

 sure increments in figure 48 are much smaller; for the main pressure changes 

 will begin in the narrow tube / (see fig. 47). 



Finally, believing that the complicated distribution of maxima in figures 45 

 and 46 was due to roughness and narrowness or other similar incidental quality 

 of the pipe t, figure 43, 1 replaced it by a tube of brass, i cm. in diameter and 

 13 cm. in depth, as far as the telephone plate. Using the pipe-blower ( 45), 

 this tube was found to respond weakly to the note a" and strongly to a'". 

 The record obtained by aid of the 

 probe, as shown in figure 49, is, in 

 fact, an apparent simplification; 

 there were now but two maxima 

 determinable, both of them quite 

 sharp, a very large one near a' and 

 a small one near d". The one at 

 a' coincides with the response to 

 the blower and is accentuated 

 when evoked by the telephone. 



' e 1 qi a' c 1 d" e' a' 



o * 



The little maximum at d", however, is supernumerary, being the reversed d" 

 of figure 45, so that something else resonates, possibly the plate of the tele- 

 phone or the telephone space or the U-tube reservoir. 



