184 BELL SYSTEM TECHNICAL JOURNAL 



Millikan & Cameron, ■ibid., 32, 533-557 (1928) (interpretation). 



Millikan, ibid., 36, 1595-1603 (1930) (approach to north magnetic pole). 



Millikan & Cameron, ibid., 37, 235-252 (1931) (most recent and complete ionization- 



t'5. -depth curve). 

 To this list should be added Millikan's address before the B. A. Symposium on the 

 Evolution of the Universe (Nature, 128, 709-715; 24 Oct. 1931), and the account of 

 C. D. Anderson's work probably soon to be published. 



Regener's observations at various depths of water are described in Naturwissen- 

 schaften, 17, 183-185 (1929), with a later and regrettably brief note in Phys. ZS., 31, 

 1018-1019 (1930). Piccard's observation at the altitude of 16 km. is mentioned in 

 the Bulletin of the French Physical Society for 4 Dec. 1931. Swann's observations 

 at three altitudes appear in Jour. Franklin Inst., 209, 151-200 (1930), and an obser- 

 vation by A. H. Compton in Phys. Rev. (2) 39, 55 (1932). 



Skobelzyn's experiments by the expansion-chamber method are published in 

 ZS.f. Phys., 54, 686-702 (1929); see also P. Auger & D. Skobelzyn, C. R., 189, 55-57 

 (1929). L. M. Mott-Smith and G. L. Locher made simultaneous observations by 

 this and the Geiger-counter method {Phys. Rev. (2), 38, 1399-1408; 1931). 



The absorption-experiments with Geiger counters cited in this article are due to 

 W. Bothe & W. Kolhorster, ZS.f. Phys., 56, 751-778 (1929), and B. Rossi, ibid., 68, 

 64-84 (1931); see also Rossi, Lincei Rendiconti (6), 13, 600-606 (1931). 



Experiments with Geiger counters to seek for deflection of ionizing particles by 

 magnetic fields: Alott-Smith, Phvs. Rev. {?.), 37, 1001-1003 (1931); Rossi, Lincei 

 Rendiconti (6), 11, 478-482 (1930), and Nature, 128, 300-301 (22 Aug. 1931); L. F. 

 Curtiss, Phys. Rev. (2), 35, 1433; (1930). 



Search for influence of earth's magnetism: Millikan, //. cc; K. Grant, Nature, 127, 

 924 (20 June 1931) (approach to south magnetic pole); Bothe & Kolhorster, Berl. Ber. 

 (1930) (cruise in Arctic waters); J. Clay, Proc. Amsterdam Acad., 30, 1115-1127 

 (1927), and 31, 1091-1097 (1928) (cruise to tropics); Rossi, Nuovo Cirnento, 8, 3-15 

 (1931); W. M. H. Schulze, Nature, 128, 837-838 (14 Nov. 1931) (magnetic storms). 



On the dependence of cosmic-ray ionization on time there have been many 

 researches, mostly by people desirous of finding (or of not finding) influences of 

 barometric pressure, weather, variations of the earth's magnetic field, the position of 

 the sun, the position of the Milky Way or other remote bodies. Most if not all of 

 the attempts to determine the absolute value of the ionization to be ascribed to 

 ■ cosmic rays, on or above the ground, have involved by necessity studies of fluctu- 

 ations. Fig. 9 of this article comes from G. Hoffmann, ZS. f. Phys., 69, 703-718 

 (1931); his paper contains references to earlier German work, some of it in a geo- 

 physical journal. Fig. 10 above comes from G. von Salis, ZS.f. Phys., 50, 793-807 

 (1928). Millikan discusses the fluctuations in his 1930 paper. The references to 

 Hess' estimate of the influence of the sun are: Nature, 12'7, 10-11 (3 Jan. 1931) and 

 ZS.f. Phys., 71, 171-178 (1931); earlier work is mentioned there. 



The estimate made by Kolhorster and Tuwim of the average number of ion-pairs 

 produced (in air under standard conditions) per centimetre path of one of the high- 

 speed ionizing particles detected by the Geiger counters, is stated in ZS.f. Phys., 73, 

 130-136 (1931) ; for the notation it seems to be necessary to study an earlier paper in 

 Berliner Berichte, 1931. 



For effects associated with the "transition-zone" mentioned above on page 180, 

 see E. Steinke, Phys. ZS., 31, 1019-1022 (1930) and the earlier work there cited; A. 

 Corlin, ibid., 1065-1071. 



For the variation of the ionization inside a high-pressure air-chamber with the 

 pressure of the air, see J. W. Broxon, Phys. Rev. (2), 37, 1320-1337 (1931) and the 

 literature there cited; E. G. Steinke & H. Schindler, Naturwiss. 20, 15-16 (1932) 

 and literature there cited. For the theory mentioned on page 170 supra, see Milli- 

 kan & Bowen, Nature, 128, 582-583 (1931); A. H. Compton, R. D. Bennett & J. C. 

 Stearns, Phys. Rev. (2) 38, 1565-1566 (1931); Steinke & Schindler, loc. cit. 



