194 BELL SYSTEM TECHNICAL JOURNAL 



charged particles through thicknesses as great or even much greater 

 is amply attested by the scheme of apparatus sketched in Fig. 3, even 

 without the cloud-chamber there indicated by "Ch." 



OC2 



Pb 



Ch 



OC3 



Fig. 3 — Scheme of apparatus for observing very penetrative particles with counters 



and cloud-chamber. 



In this sketch of Fig. 3, the objects Ci and C2 and Cz are Geiger- 

 Miiller counters: that is to say, gas-filled discharge-tubes of a very 

 special design, the two electrodes of each being an axial wire and a 

 coaxial cylinder, and the electrode-size, voltage, and gas-content 

 being very carefully adjusted. These long large cylinders, usually 

 called simply "counters" without the prefixed names, are familiar 

 sights in almost every laboratory where cosmic rays are studied. 

 If a charged flying corpuscle penetrates such a tube, a momentary 

 discharge takes place in the gas. If such discharges spring up simul- 

 taneously in all the three tubes of such a system as Fig. 3 exhibits, 

 the event is recorded by a mechanism. ("Simultaneously" is of 

 course a word which requires detailed exegesis; it meant at first that 

 in all tubes discharges began within 0.01 second of each other, but 

 this interval has been pushed down to .0001 second and lower.) 



These events, the "threefold coincidences," do actually occur. 

 Of course, since in each of the tubes a discharge occurs now and then 

 by itself, some of the coincidences must be the result of chance; but 

 the probable number of these meaningless ones can easily be estimated 

 from the frequency and the duration of the individual discharges, and 

 in the best experiments they are a small minority. For the great 

 majority, the simplest of explanations is to attribute each of them to 

 a single vertically-flying particle cutting through all of the counters 

 in succession. Yet there are other thinkable causes, and confirmation 



