538 



Dr. Norman Campbell : Further 



distance between the electrodes, or a pressure of 1*5 with the 

 larger. At the highest of these pressures collisions between 

 the rays and the molecules must have been frequent, for 

 (1) calculation shows that the mean free path of the electrons 

 is not large compared with the distance between the elec- 

 trodes, and (2) when P.D.'s of some 50 volts were employed, 

 unmistakable signs of ionization by collisions appeared. It 

 is clear, therefore, that the measurements cannot be expected 

 to conform to the theory given, but, though no explanation 

 of them can be given as yet, they seem of sufficient interest 

 to be mentioned. 



In fig. 4 the relation is plotted between the P.D. and the 

 current expressed as a fraction of that which would be carried 



Fig. 4. 



LO 



02 

































































r 





















i 























































































































> i 





«■ » 



J t 



i I 



< 



} ■ 



r < 



3 1 



/o 



by all the ions liberated by the alpha rays, i. e. eN. The 

 value of N was extrapolated from that found at higher 

 pressures, when a true saturation current could be obtained 

 and Was found to be accurately proportional to the pressure. 

 The effect of the delta rays from the electrodes was eliminated 

 in all cases by substracting from the current at any given 

 pressure that obtained with the same P.D. when the pressure 

 Was zero. 



It will be seen that the curve is curiously like that obtained 

 with much higher pressures and higher P.D.'s, where satu- 

 ration is obtained over a certain range of P.D/s and the 

 current subsequently increases as ionization by collision 

 enters. But the " saturation " which is obtained in this 

 case occurs when only about 0'7 of the ions produced reach 

 the electrodes. 



