﻿Ionization by Positive Rays, 791 



given by 



h 

 h — h 



i 2 and i$ being measured while the values of R were taken 

 from the extrapolated part of "fig. 4. This procedure doubt- 

 less appears rash ; but it must be remembered that, if the 

 real values of R are greater than the assumed, the real 

 decrease in P will be more and not less marked than that 

 shown. If, on the other hand (and it will be noted presently 

 that this alternative is more probable), the real values of R 

 are less, then, since R cannot be <0, the values of P given 

 will be at most 10 per cent, too small. It is impossible by 

 any assumption concerning R to explain away the apparent 

 decrease of P after the maximum. It may be added that 

 any serious error on this account is highly improbable, for 

 the measured values of i z were constant, so far as could be 

 detected, for all values of V greater than 20,000 volts so 

 long as the conditions of S (temperature and so on) were 

 maintained constant. The decrease in P shown in the curve 

 was due to a decrease in i 2 , not to a change in i 3 , and must 

 therefore be attributed to a change in the electronic current 

 from C. 



10. Assuming that the form of the curve relating P and 

 V is in the main correct, I think that we can obtain some 

 idea of its significance by the light of a very interesting 

 suggestion due to Ramsauer *. Noting that the velocity at 

 which «-rays have their maximum ionizing power is nearly 

 the same as the velocity at which electrons have their 

 maximum ionizing power (about 10 9 cm. /sec), Ramsauer 

 suggests that the ionizing power of a charged particle is a 

 function of its velocity rather than of its energy, so long 

 as this velocity is above a certain value. He shows that the 

 form of the Bragg ionization curve for a-rays can be deduced 

 with considerable accuracy by assuming that the ionization 

 of a-rays of a given velocity is always proportional to (about 

 10 times greater than) the ionizing power of electrons of the 

 same velocity. The limiting velocity, at which this relation 

 ceases to hold, is the velocity of an electron (about 2 x 10 s 

 cm. /sec.) corresponding to its ionization potential (11 volts 

 in hydrogen) : particles which have less than this velocity, 

 whether they be electrons or atoms, cannot penetrate within 

 the atom, and the ionization (if any) which they produce is 

 * K. Ramsauer, Jalir. d. Bad. u. Elek. ix. p. 515 (1012). 



