ELECTRONIC MAGNITUDES 315 



individual systems which it passes to receive by inter- 

 action a component of velocity. On the other hand, 

 in Fig. 42 are given the paths of some electrons with 

 smaller velocities which ionize more frequently but 

 are also deflected. The slower speed electron ionizes 

 a larger number of molecules per centimeter than does 

 the higher speed particle. 1 Both are gradually retarded, 

 and below a certain speed ionization does not occur. 



In the cases of Fig. 40 and Fig. 41 the electrons 

 which are knocked off from the molecules leave at 

 such small speeds that they may not themselves serve 

 for ionizing agents (unless accelerated by an impressed 

 electrical field as was discussed in Chapter XIV). 

 In Fig. 42, however, is shown the trace of an X-ray. 

 This radiation does not merely jar loose electrons as 

 did the a and ft particles, but seems to shake them 

 loose with such violence that each electron leaves 

 behind itself a trail of ionization. 



The ability of electromagnetic radiations to shake 

 out electrons is not limited to X-rays and 7 rays but, 

 as we saw in Chapter XIV, is characteristic also of 

 ultra-violet radiations. Metals, especially zinc and 

 the alkali metals, emit electrons when exposed to 

 ultra-violet light. The effect, known as photoelectric, 

 was recognized, before the theory of electrons was so 

 fully developed, by the fact that under these condi- 

 tions the metal acquires a positive charge. 



In this phenomenon are some curious facts which 

 have supported Planck's suggestion that energy (in this 



1 Millikan estimates that this particle (Fig. 41) on the average 

 passed through 10,000 gas molecules for every time it reacted suf- 

 ficiently to jar loose an electron. Cf. Millikan, "The Electron, "p. 186. 



