



PREFACE. 



The experiments of the present volume are either direct applications of 

 displacement interferometry or they embody the correlative work which has 

 grown out of such applications, often at widely different times. In the ar- 

 rangement of the chapters it has therefore been expedient to depart from 

 chronological order in favor of an arrangement of subjects which belong 

 together. 



In a former report* I had already used a U-tube in connection with the in- 

 terferometer, but the design of the apparatus was limited in scope. In the 

 present paper (Chapter I) the open mercury manometer is made directly 

 available for pressure measurement, and as the attainable sensitiveness is 

 easily a few hundred thousandths of a centimeter of mercury per fringe dis- 

 placement, it is well worth while to see what can be gained by using it. The 

 applications to air thermometry on a micrometric scale and an attempt to 

 revive the old absolute electrometer in Chapter II are merely incidental, 

 though in each case much more may be done than I have here attempted, as I 

 hope to show at some other opportunity. The manometer also admits of 

 further improvement in ways which can not be included in the present report. 



A more suitable field for testing the immediate capabilities of the mercury 

 U-tube is detailed in Chapters III and IV, where the endeavor is made to give 

 an account of the pressures and dilatations observable in a region vibrating 

 acoustically. If this is quite closed or quite open to the atmosphere, the rec- 

 ord of the U-tube within is without interest; but if the region is all but closed 

 up open, for instance, through a pin-hole less than 0.5 mm. in diameter the 

 gage shows pronounced fringe displacements as a rule and particularly at 

 the frequency of the harmonics. If the sound generator is a telephone, the 

 displacements are proportional to the effective currents actuating it and at the 

 harmonics much more than 10,000 ohms may be put in circuit before the 

 fringes cease to move appreciably. Similarly under low resistance, very 

 small fractions of a semi-tone are registered and the ear becomes a poor ap- 

 paratus for discrimination. The investigations are made along two lines, in 

 the first of which the sound generator and the U-tube lie within the boundary 

 carrying the pin-hole; in the second the sound generator lies without and is 

 independent, so that the pin-hole valve carried on a long tube becomes an 

 appropriate probe, or sonde, for the pressures within sounding pipes and cavi- 

 ties. As all the harmonics are thus saliently registered, there should be no 

 serious difficulty in exploring the acoustics of the mouth cavity uttering word 

 sounds, for instance. A curious result of the survey of the distribution of 

 pressure increments in relation to pitch is the replacement of pressures by 

 dilatations in different orders of frequency. 



* Carnegie Inst. Wash. Pub., No. 249, II, Chap. V, 1917; ibid, Chap. VI, ? 72. 



iii 



