W. R. Barss — Measurements of Radio-activity. 549 



ligible compared to the recombination between ions of the 

 same column. 



As the intensity of the source is increased, the number of col- 

 umns of ions existing together is also increased. The proba- 

 bility that different columns will cross each other is greater and 

 therefore the amount of recombination between ions of dif- 

 ferent columns will be greater. So that, as in the case of the 

 perpendicular field, the ratio of the larger current to the smaller 

 will probably increase as the potential gradient is increased. 



In the present experiments, a cylindrical tin chamber w^as 

 used 13*5 cm high and 10*5 cm in diameter. A central brass 



Fig. 1. 



8 16 



40 



200 

 Field in volts 



electrode, provided with an earthed guard ring, was connected 

 to a tilted electroscope of the Wilson type, the leaf of which 

 was observed by means of a microscope having a graduated 

 scale in the eyepiece. This central electrode and the leaf of 

 the electroscope were grounded through a potentiometer by 

 means of which each deflection due to the ionization current 

 was calibrated in terms of potential. The capacity of the sys- 

 tem was kept constant, so that these calibrated readings varied 

 directly as the actual ionization currents. Different potentials 

 were applied to the case of the chamber. Radium emanation 

 was used as an ionizing agent ; it has a half value period of 

 about four days, so that it provided a suitable source of varying 

 intensity. 



One series of observed data is given in the following table 

 V is the potential in volts applied to the case. C x represents 

 the corresponding ionization current for a given intensity ; C 2 

 the ionization current for a weaker intensity, etc. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 198. 

 36 



-June,' 1912. 



