18 DISPLACEMENT INTERFEROMETRY APPLIED TO 



Capillary tubes were also tested, but with less satisfactory results. Energy 

 seemed to be uselessly frittered away in the channel. Thermometer capillaries 

 act like closed tubes. Capillaries o.i cm. in diameter and 0.7 cm. long gave 

 but moderate results, which were not improved by partially closing the tube 

 with thin wires. An ordinary small one-eighth inch gas stop-cock of brass, 

 with a loose plug, is preferable to a glass cock, when adjustable channels are 

 wanted. The sharp edge seems to be advantageous, though I later adapted 

 fine screw stop-cocks with good results. In case of the critically small leak, 

 pressure decrements frequently precede the pressure increments when the 

 telephone circuit is closed. 



If, in place of the quill-tubes c, the copper foil with an 0.04 cm. perforation 

 is cemented to the mouth of a thistle- tube, the deflections on closing the cir- 

 cuit are liable to be either absent or very small. The conditions for the valve - 

 like action of the pin-hole are thus quite involved; but with a given tube c 

 (fig. 14) the data are surprisingly constant. 



With the current on, if a drop of water is placed on the hole in c, figure 14, 

 the pressure is long retained, whether the current is thereafter broken or not. 

 Pressure is gradually dissipated, however, as the joint at the telephone plate is 

 rarely quite tight; and when the telephone sounds pressure tends toward 

 the negative, as above. If most of the water is removed by blotting paper, 

 a moderate, fairly constant pressure is usually observed for some time, until 

 (doubtless with the breaking of the film across the hole) the maximum is sud- 

 denly again attained. Thus from 60 fringes the deflection slowly fell to 10 

 fringes; thereafter rose to -f 10 fringes, from which it eventually jumped to 

 60 fringes again. The result may be interesting, inasmuch as some pressure 

 action through the film of water seems to be in evidence. 



1 9. Effect of resonance. While a relation of the maximum pressure parallel 

 to 1he frequency of the telephone note has been shown to exist, it is obvious 

 that the best conditions for high maxima will occur under conditions of 

 resonance between the natural R vibrations and the T vibrations (fig. 14) or 

 their harmonics. I therefore used the same small induction coil with two 

 storage cells, but with a commutator-like current-breaker, controlled by a 

 small electric motor with a variable resistance in circuit (electric siren). By 

 gradually increasing this resistance all chromatic intervals between about 

 f' and a" were obtainable. The speed of the motor is, however, liable to fluc- 

 tuate slightly in any given adjustment, while intervals within a semitone 

 often produce large pressure differences. Thus the determination of the 

 intervals of a somewhat flickering pitch in all chromatics is here quite difficult, 

 even for a musical ear. A series of organ-pipes within the given range seemed 

 to offer the best standards of comparison , as it was necessary to turn rapidly 

 from one series of observations to another. 



In this way the graphs given in figures 22, 23, 24 were worked out, the curves 

 showing the fringe displacement s to the logarithmic frequency n of the tele- 

 phone. In figure 22, to limit the deflections within the range of the ocular, 



