592 



SCIENCE 



[N. S. Vol. XL VI. No. 1198 



was necessary to measure their relative ioniz- 

 ing effects at a given point in air. This was 

 accomplished by sucking the ionized air from 

 the vicinity of the given point through a metal 

 tube into the chamber of an electroscope 

 placed at some distance, as shown in Fig. 1. 

 By noting the rate of deflection of the gold 

 leaf for different air velocities curves corre- 

 sponding to decay curves were plotted, using 

 ionization in divisions per minute as ordi- 

 nates and the times of passage of the ions 

 through the tube as abscissse. By contin- 

 uing these curves back to zero time an ap- 

 proximation was obtained of the relative 

 ionization originally present. A more exact 

 estimate was made by obtaining the recom- 

 bination constants for the two ionizing agents 

 and, assuming the square law, calculating the 

 original ionization when the ionization after 

 a given time was known. This work sug- 

 gested a further study of the recombination 

 constants by this method, using various ion- 

 izers, and an examination of the recombina- 

 tion constants for ions produced by "hard" 

 X-rays or the more penetrating 7-rays as com- 

 pared with these values for the softer and less 

 penetrating radiations. 



Before using the electroscope as an indi- 

 cator of the number of ions present at any 

 instant, it was necessary to determine the de- 

 flection to which the gold leaf must be charged 

 in order to obtain satiiration conditions for 

 the maximum velocity utilized. This was 

 found by passing the ionized gas through the 

 chamber of the electroscope to be used, then 

 through the chamber of a second electroscope 

 of high sensibility in close proximity to the 

 one to be tested. The gold leaf of the latter 

 was then charged to a potential sufficient to 

 give no leak in the auxiliary electroscope. 

 For lower potentials ions escaped into the 

 second electroscope and the rate of leak of 

 the first did not give a true indication of 

 the number of ions passing into it. When the 

 potential to which the leaf is charged is con- 

 siderably lower than that necessary for satu- 

 ration the decay curves obtained may show a 

 maximum point, since there may be a critical 

 velocity at which a maximum number of ions 



will give up their charges to the electroscope. 

 At such a velocity the gain in the number 

 entering the chamber will be counterbalanced 

 by the number escaping without giving up 

 their charges. 



The order of experiment was then as fol- 

 lows : Determine the saturation potential 

 necessary for a given position of an ionizing 

 agent at the maximum velocity to be used. 

 Obtain the natural leak of the electroscope 

 when the ionizing agent was present, but with 

 no current passing through the tube. Obtain 

 rates of deflection of the leaf in divisions per 

 minute for each of as large a number of dif- 

 ferent velocities as time and the capacity of 

 the suction pump would permit. The leaf 

 was charged to a given deflection and allowed 

 to leak over the same number of divisions 

 for each reading. The mean of several ob- 

 servations was taken at each velocity. Succes- 

 sive times for the flow of .5 cubic foot of gas 

 through the meter at a given velocity were 

 also recorded. These values were then plot- 

 ted using ionization in divisions per minute 

 as ordinates and cubic feet per minute as 

 abscissae. From the smooth curve thus ob- 

 tained a number of points were chosen and 

 the time of decay of the ionization to these 

 given amounts calculated from the rates of 

 flow. Two of the ionization values were then 

 selected as representing N and n in the for- 

 mula 



H N 



where t was the difference between the cal- 

 culated times of decay for the values chosen. 

 Thus assuming the recombination law, a the 

 recombination constant was calculated in ar- 

 bitrary units. Using this value for a, a num- 

 ber of values for n were computed and com- 

 pared with the experimental values. The ion- 

 izing agent was then placed at different dis- 

 tances from the electroscope and similar de- 

 cay curves plotted as a series of checks and 

 with the purpose of obtaining a better idea 

 of the part played by diffusion. This was 

 repeated for brass tubes of different diameters, 

 using X-rays, 7-rays, electric sparks and black 



