physics. 383 



PHYSICS. 



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

 investigations in interferometry . (For previous reports see Year Books 

 Nos. 4, 5, 7-20.) 



Dr. Carl Barns has submitted to the Institution a report on the develop- 

 ment of the acoustic investigations with the pin-hole probe, begun in the 

 preceding communication (Carnegie Inst., Wash. Pub. No. 310, 1921). The 

 introduction shows that the pin-hole probe responds effectively to nodes in 

 organ pipes and that it ignores the antinodes. In the case of the usual air- 

 blown diapason pipes, the presence of the ait-current naturally interferes 

 with the acoustic experiment. If, however, the pin-hole is surrounded by a 

 minute bag of porous material, a consistent registry of nodes again appears, 

 but constructed on a base of increasing pressures, positive or negative. 



With a device so sensitive to nodal regions in vibrating pipes, the con- 

 struction of a pin-hole resonator suggests itself. The latter, if designed with 

 but a single mouth and responding correlatively to a closed organ-pipe of the 

 same pitch, is preferred in the interest of simplicity. However, unless the 

 pin-hole is adapted to the resonator, no acoustic pressures whatever are 

 evoked. It was eventually found that not only the size, but the slope of 

 the walls of the pin-hole are critical; that a salient pin-hole generates acoustic 

 pressure, a reentrant pin-hole acoustic dilation, and there is neutral behavior 

 between the two. With a properly designed pin-hole, pressures, whether posi- 

 tive or negative, will be about equal; it follows, therefore, that by coupling 

 them with the corresponding shanks of the mercury U-tube, an advantage 

 in sensitiveness is secured; but it does not seem feasible to push the sensitive- 

 ness of the pin-hole indefinitely farther, for it appears to be the nature of these 

 occurrences to drop off exponentially. 



Though the project of relaying pin holes would seem to be quite at hand, 

 all experiments with this end in view remained persistently negative. With 

 so many pin holes in series that displacement of the mercury in the U-tube 

 was absolutely dead-beat, the acoustic response was no better than for a single 

 pin-hole. 



The work has a direct bearing on the behavior of sensitive flames. If an 

 adjusted pin-hole burner becomes turbulent by slight increase of pressure 

 within, a sound-wave passing the burner will supply this pressure acoustic- 

 ally. Experiments with branched tubes and one or two (salient and re- 

 versed) pin-holes present many points of interest, particularly on account 

 of the eventual bearing of the data obtained on the sensitiveness of tele- 

 phones. 



Within its restricted field, the pin-hole resonator serves admirably for the 

 acoustic survey of the interior of a room in which an organ-pipe is sounding. 

 For a given position of the pipe, nodal regions alternate with antinodal 

 regions, quite irregular in distribution, but none the less fixed in position. 

 The author has cut through this acoustic topography in all directions from 

 the position of the pipe, with the expectation of arriving at some general 

 facts as to distribution; but it is difficult to state the results, otherwise than 

 by the graphs in the text. These show a difference in the character of a 

 survey between walls contrasted with one toward an open door; but (a priori) 

 one would not be able to predict the occurrence of either type. 



