492 Wellisch and Bronson — Distribution of the Active 



It was suggested that, if a large uniform electric field were 

 applied, possibly the number of uncharged carriers might 

 become insignificant. For this purpose the emanation was 

 introduced into a vessel consisting of' two parallel electrodes of 

 aluminium (each 58 ram in diameter) insulated by an ebonite 

 ring 20 ,1,m thick; the vessel contained air at a pressure of 1 

 atmosphere. Experiments made with applied potentials of 

 160 and 1000 volts gave values of 75 and 82'6 respectively for 

 the percentage of positively charged carriers. 



The second set of experiments had as object to determine 

 whether RaA was deposited on the case. To test this point 

 the cylindrical vessels were employed as usual and large 

 potentials applied, but exposures of only 1 minute duration 

 were made. It was found that the resulting curve of decay of 

 the case activity had the characteristic properties of the curves 

 of decay for the activity due to a short exposure to the radium 

 emanation. 



Mention might also be briefly made here of some experi- 

 ments which were performed to ascertain whether there was 

 any alteration in value of the percentage cathode activity 

 resulting from a long exposure with large positive potentials 

 applied to the case when throughout the exposure th? ioniza- 

 tion current was greatly increased by the passage of Rontgen 

 rays through the vessel. The alteration, if any, was very 

 small, and for the present, at any rate, we must assume that 

 the action of the Rontgen rays is without effect on the dis- 

 tribution of the activity. 



5. Experiments with Air at Reduced Pressure. 



The experimental results described in the preceding section 

 strongly suggest that over a wide range of potentials the per- 

 centage of the total activity which is deposited on the case 

 represents the percentage lack of saturation of the positive 

 ionization current. These experiments related to the activity 

 distribution in air at a pressure of 1 atmosphere. Now 

 Moulin and Wheelock have shown (loc. cit.) that the ioniza- 

 tion produced by a -particles is more readily saturated when 

 the pressure of the gas is reduced ; it was, therefore, of interest 

 to determine whether the percentage cathode activity would 

 follow the positive ionization current when the gas pressure 

 was reduced. For this purpose the emanation was introduced 

 into the testing vessel, which contained air at a pressure of 

 260 mm , and the percentage cathode activity was determined for 

 various applied jDositive potentials. The results are given 

 below (Table II), and are exhibited as a curve in fig. 2. 



