212 BELL SYSTEM TECHNICAL JOURNAL 



ment and theory aid us in identifying the shower-particles with the 

 electrons? As to experiment, there exist the records of a few studies 

 made by the Wilson chamber upon particles acknowledged to be 

 electrons, of energy-values ranging from about 2 Mev downward to 

 some 25000 electron-volts. In respect of the trend with energy, they 

 agree fairly well with the assertions of the quantal theory; but when 

 one inquires whether the absolute value for the number of clusters of 

 ions in unit length agrees with the absolute value of the quantal 

 expression for the primary ionization at any particular energy, one is 

 confronted with the fact that the quantal expression contains a 

 multiplying factor which depends on intimate details of the structure 

 of the molecule, and is not exactly known. The quantal theory, 

 however, predicts a minimum in the curve of primary ionization vs. 

 energy, at an energy of about 2 Mev. Such a minimum (Fig. 12) was 



I 70 - 



Fig. 12 — lonization-density (entire) along the tracks of cosmic-ray particles, 

 plotted as function of Hp. The continuous curve is that of a theoretical function 

 containing a multiplying factor which has been adjusted to get the best fit to the 

 data. (Corson and Erode) 



actually found by Corson and Erode in their study of some fifty 

 particles of the \:osmic rays, and probably is to be ranked as evidence 

 for the electronic nature of these particles quite as forcible, as would 

 be an absolute agreement between the observed ionization and the 

 predictions of a reliable theory. 



Street and Stevenson, with a row of counters and an interposed 

 cloud-chamber such as appeared in Fig. 3, adjusted their counters 

 in such a way that the chamber expanded only when the counters 

 above the chamber had simultaneous discharges and the counter 

 below did not. A thousand photographs yielded to them the track 



