20 DISPLACEMENT INTERFEROMETRY APPLIED TO 



closing it suddenly. There has been an efflux of air corresponding to the influx 

 above; *. e., the acoustic pin-hole valve may be reversed. 



An auxiliary telephone placed in circuit with that of T, figure 14, affords 

 no suggestion of these occurrences. Its notes rather increase in strength 

 regularly with the pitch. Yet if the notes should happen to be near e", the 

 other telephone would show no fringe displacement. 



20. Inside and outside stimulation. I have as yet given but incidental at- 

 tention to this development. If, for instance, the branch tube and cock t"C t 

 in figure 14, is removed and the pin-hole tube c' is put within the rubber 

 connectors t, t', the apparatus is now completely closed. It functions, however, 

 though in a way totally different from the original adjustment, figure 14, as 

 I shall presently show. In this case, if the telephone connector t is even slightly 

 detached, no reaction is obtained, at least for the size of hole and pitch tested. 



When the tube c' is inserted in the absence of vents, there is much undesir- 

 able pressure disturbance at the outset, which is but very slowly dissipated. 

 Moreover, the closed space can not be opened again at pleasure without similar 

 commotion. I therefore used the apparatus, figure 25, in preference, in which 

 the pin-hole tube c' is provided with a branch tube t" and cock C. The 

 rubber tube t leads to the telephone (beyond T) and the tube t r to the mercury 

 U-tube (beyond /) K C is open, c' may be inserted or withdrawn with fa- 

 cility. If C is closed, the resonator R is closed, as in the above case. The 

 pin-hole O may point either toward U or toward T, without modifying the 

 results of this experiment. 



Using the mercury interrupter (frequency c#) with 2,000 ohms in circuit , 

 the deflection of the closed region was invariably negative. The deflection was 

 peculiar, moreover, inasmuch as it is a slow growth, within a minute or more, 

 to a maximum. On breaking the current the deflection died off in the same 

 slow, fluctuating manner, with a kind of loitering on the way, as if something 

 were removed in the first case and again supplied in the second. If the cock C 

 is opened, the zero is instantaneously recovered . In other words, the dilatation 

 is due to a loss of gas within the closed region, which loss is but slowly re- 

 covered after the telephone ceases to vibrate. 



True, the closed region is an air-thermometer, whose fringe displacements 

 also increase in this fluctuating manner; but these are pressures, resulting from 

 the very small increase of temperature in presence of the interferometer ray, 

 L'. They act against the dilatations just described. 



If the cock C, figure 25, is opened at the critical point, or if it is replaced by 

 the tube c, the deflection is again positive (about 14 fringes at 2,000 ohms 

 when c above would give about 40) . The action of c thus exceeds that of c' t 

 probably because the pin-hole in c happens to be nearer the critical size 

 than in c'. 



The question next at issue is the influence of pitch upon the dilatation of 

 the closed resonator R. The electric siren was here used with 2,000 ohms in the 



