458 CARNEGIE INSTITUTION OF WASHINGTON. 



PHYSICS. 



Earns, Carl, Brown University, Providence, Rhode Island. Continuation of 

 investigations in interferometry. (For previous reports see Year Books 

 Nos. 4, 5, 7-19.) 



Dr. Barus has presented to the Carnegie Institution a report em- 

 bodj'ing applications of displacement interferometry, together with 

 the correlative work which has grown out of such applications. 



In the introductory paper the open mercury manometer is made 

 directly available for pressure measurement. The sensitiveness per 

 fringe displacement being easily a few hundred thousandths of a centi- 

 meter of mercury, applications to air thermometry on a micronometric 

 scale and an attempt to revive the old absolute electrometer are made 

 incidentally. 



A more suitable field for testing the immediate capabilities of the 

 interferometer U-tube is detailed in the following chapters, which ex- 

 hibit the pressure and dilation observable in a region vibrating acous- 

 tically. If this is quite closed, or quite open to the atmosphere, the 

 record of the U-tube within is of little interest; but if the region is all 

 but closed — 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 

 semitone are registered and the ear becomes a poor apparatus for dis- 

 crimination. 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. 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 dilations in different orders of 

 frequency. 



Developments of the direct interferometry of the compression of a 

 sound-wave follow; but only as much is given as is necessary for the 

 coordination of the other chapters. The method used is superior to 

 the earlier solutions of the problem, in its greater flexibility and the 

 ease with which fringes in any orientation may be produced and short- 

 ened to a string of silvery beads. The simple organ-pipe blower, or 

 adjustible embouchure described, will be found serviceable for many 

 other purposes, both of research and instruction. 



