Vol.. 7, 1921 
PHYSICS: C. BARUS 
207 
the destruction of the active centers by the ultra violet light are two in- 
dependent phenomena the existing theory must be altered. Further 
experiments are needed to decide this point. The writer's thanks are 
due to Prof. R. A. Millikan for his kind interest and criticism. 
1 B. Marsden, London, Proc. Roy. Soc, 83A, 1910 (549-561). 
2 Nichols & Merritt, Phys, Rev. Ithaca, 28, 1909 (349). Pospielow, Ann. Physik, 
Leipzig, 45, 7 pp., 1039-1062. No. 17, 1914. 
3 H. Baerwald, Ann. Physik, 39, 4 pp., 849-880, Nov., 1912. G. Berndt, Zs. Physik, 
Leipzig, pp. 42-44, 1920. 
4 J. Perrin, Ann. Physique, Paris, IX, 10, pp. 133-159, Sept., 1919. 
6 R. W. Wood, "High Power Phosphorescence and Fluorescence." Lecture be- 
fore the A. A. A. S., Chicago, Dec, 1920. 
6 E. Rutherford, Proc. Roy. Soc, 83A, 1910 (561). 
7 Wiedemann & Schmidt, Ann. Physik, 56, 1895 (177). P. Lenard, Ibid., 15, 1904 
(665). P. Lenard, Ibid., 31, 1910 (667). Nichols & Merritt, Physic. Rev., 27, 1908 
(367). 
ACOUSTIC PRESSURE DISTRIBUTIONS, CHIEFLY IN RESER- 
VOIRS AND IN PIPES* 
By Carl Barus 
Brown University 
Communicated May 12, 1921 
Introductory. — In Science (May, 1921) I described an available method 
of pressure measurement in connection with acoustic phenomena, the 
method consisting essentially of a shallow mercury U-tube, the head of 
which is read off by displacement interferometry. Thus the fringe dis- 
placement 5 (occurring throughout the following paragraphs) will vary 
directly as the head in question and therefore with the pressure in the 
region under investigation. When the shank of the U-tube communi- 
cating with the sounding region is quite closed or quite open, significant 
results are either absent or without interest; but when the sound leak is 
reduced to the dimensions of a pinhole in foil, or the like small aperture, 
relatively large pressure increments or decrements are usually found to 
reside in the vibrating medium, varying with frequency and particularly 
marked when the region is in resonance with the sound generator. As an 
instrument for this purpose, a telephone, actuated by a small induction 
coil and a motor current interruptor is preferable; since non-symmetric 
inductions conduce to greater sharpness and strength in the phenomena to 
be observed. The current so obtained may be reversed, changing all the 
pressure increments into corresponding dilatations. The effect of the 
symmetrical induction of the magneto does not admit of commutation; 
*From a forthcoming Report to the Carnegie Inst., Washington, D. C. 
