31 



L 



i: 



bv recombination in crossing the chamber will be o I I n^ dt 



where N is the density of the ions in the space of length, I, 

 at a time, t , irom the start : 



Hence a curve can be plotted as in fig. 9, showing the rela- 

 tion between field strength and the number of ions which 

 reach the electrode starting with a given initial density. 



In fig. 9 the full line curve is drawn according to the 

 formula which has just been deduced, the value of X being 

 determined in the usual way from a knowledge of the value 

 of the saturation current, the capacity of the system, and 

 the dimensions of the ionization chamber. The value of the 

 X was S'lO*^ ions per cub. cm. The circles show the results 

 of experiments upon air, and it will be seen that they fall 

 very nearly upon the theoiretically determined curve, show- 

 ing that ordinary recombination is sufficient to account for 

 the observed effects. However, it is necessary to show that 

 the effect due to initial recombination was large enough to 

 be detected had it been present. 



Using for comparison currents at field strengths of 390 

 and 80 volts per cm., it will be seen from the curves given 

 by Bragg and Kleeman, in their paper upon "Initial Re- 

 combination," that between these values of field strength 

 a very considerable percentage of initial recombination is 

 observable when the ions are collected in the presence of 

 the ionizing agent; but the exact amount depends upon the 

 speed of the a particles producing the ionization. By re- 

 moving the pendulum and obtaining saturation curves ni 

 the ordinary manner with the apparatus used in the fore- 

 going, it was found that after allowing for the effect of 

 ordinary recombination there remained between these values 

 of field strength a rise of about 5 per cent., due to the effects 

 of initial recombination. This amount was such as could have 

 been readily detected in the pendulum experiments. 



As the time which elapsed between the stopping of the 

 action of the ionizing agent and applying the field in these 

 experiments was l-20th sec, it appears then that no evi- 

 dence can be obtained from the experiments of the non-com- 

 pletion of the act of initial recombination in air within a 

 period of l-20th sec. after the act of ionization. 



