PHYSICS. 



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

 investigations in interferometry. (For previous reports see Year Books 

 Nos. 4, 5, 7-21.) 



During the course of the year Professor Barus continued his work with the 

 interferometer U-gauge. A new installation was made, in which the mercury 

 surfaces were 10 cm. in diameter, the object being to keep them more rigorously 

 in parallel throughout all displacements. The charge of mercury must be 

 introduced in vacuo. In this way the range of available pressures was 

 increased. 



The gage was first used in an endeavor to measure small pressure incre- 

 ments or decrements accruing slowly in the lapse of time and due to gas 

 evolution or absorption, vapor-pressure phenomena, etc. Though pursued 

 at some length, the results remained untrustworthy, because of the concomi- 

 tant temperature effects; for the closed region is also an exceedingly sensitive 

 air thermometer. These difficulties fall away, however, when the region is 

 closed by a liquid film; for the pressure is then determined solely by surface 

 tension of the film. Accordingly, a variety of experiments was made on the 

 pressures within bubbles, from which the surface tension could be computed 

 up to the point at which the film ceases to be contractile. In glue bubbles, for 

 instance, the surface tension rose from about 30 to over 80 dynes/cm. in the 

 first 5 minutes of exposure, after which, with incipient solidification, the 

 increase of pressure was suddenly arrested. The excess of air within the 

 bubble completely escaped through the solidifying iridescent film during the 

 ensuing 150 minutes or more. Similar work was done with solutions of sugar 

 in water, or resin in chloroform, or pitch in turpentine, and with collodion. 

 In case of a colloidal solution, by successively diluting the soap or glue content 

 the surface tension T of pure water was finally very closely approachable, 

 !r>60 being reached before the bubbles were too evanescent for use. 



The endeavor to apply interferometer methods to the capillary electrometer 

 was not successful; but with the apparatus at hand, a study of the march of 

 the phenomena on gradual dilution of the electrolyte to very pure water was 

 completed and certain marked advantages of a change of form of the ap- 

 paratus were indicated. Interferometer methods for the transpiration of gases 

 also fell short of interpretable results, chiefly because pin-holes fine enough 

 were not to be obtained. On the other hand, interferometer measurements of 

 the viscosity of gases passed through capillary tubes led to favorable results 

 at once. 



The chief application of the new interferometer U-tube throughout the year 

 embraced a continuation of the acoustic researches begun in the preceding 

 report. The acoustic pressures generated by paired pin-hole probes serve 

 admirably for ascertaining the mode of vibration encountered in closed 

 regions of any form. The results obtained with tubes closed at both ends were 

 particularly interesting, and the work was therefore developed systematically, 

 the tubes of all lengths ranging in diameter from 0.4 to 2 cm. It is somewhat 

 startling to find 4-inch quill tubes responding to the notes of the 4-foot 

 octave. In tests of this extreme severity, the results do not follow the theo- 

 retical equations, and an exhibit of the phenomena in terms of dimensions, 

 viscosity, and frequency was therefore attempted. A few supplementary 



experiments were made with horns and branched tubes. 



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