196 



BELL SYSTEM TECHNICAL JOURNAL 



The reader has now been introduced to charged particles which 

 bore through quantities of lead, apparently without doing or suffering 

 anything. Next he is to be introduced to particles which begin to do 

 something startling, when they have scarcely more than entered into 

 a thin metal plate. This is vividly shown to him in Fig. 4, in which — 

 after he can detach his eyes from the pretty sight beneath the trans- 

 verse leaden plate — he will see that two of the "showers" beneath 

 spring from the places where the metal was entered by two charged 

 particles coming from above. These are accordingly called "shower- 

 producing particles." 



Fig- 5 — Shower begun by a charged particle impinging on a 6.3-mm lead plate, 

 and multiplied as it passes through a second such plate; in the third plate, 0.7 mm 

 thick, only deflections occur. (Fussell, Harvard University) 



Figures 5 and 6 and 7 show examples of showers even more gorgeous 

 — regular cloudbursts, to continue with the metaphor (and indeed 

 the term "burst" is often used as a synonym for "very large shower"). 

 Of these, the special value of Fig. 6 is that the tracks that start in the 

 gas itself bear witness to corpuscles of light — photons — included in 

 the shower; for these are the tracks of electrons ejected by photons 



