Deposit of Radium in an Electric Field. 493 



Table II. 

 Emanation in air at pressure of 260 n 



Potential in 



V.oltS. 



Percentage cathode 

 activity 



80 



82-5 



150 



82-5 



775 



81-8 



790 



82-2 



1130 



81-8 



2250 



83*8 



2250 



84-2 



It is noticeable that although the percentage cathode activity 

 for 150 volts is greater than the corresponding value for a 

 pressure of 1 atmosphere, nevertheless when the higher 

 potentials are reached the values are smaller at the lower 

 pressure. 



When the ionization due to the emanation in equilibrium 

 with its activity and in air at a pressure of 260 mm was measured 

 for various applied positive potentials, it was observed that 

 the alteration of ionization with potential was so extremely 

 slow as to suggest saturation. Over the range for which the 

 ionization could be measured this alteration was too small to 

 justify comparison with the figures given for the percentage 

 cathode activity ; the striking feature is that both curves 

 approach more nearly to the horizontal than the corresponding 

 curves for a pressure of 1 atmosphere. The figures for the 

 lower pressure show that for large potentials the activity is 

 farther from saturation than at 1 atmosphere ; it is therefore 

 only reasonable to conclude that in the case of ionization the 

 percentage lack of saturation is greater at the lower than at 

 the higher pressure. 



The figures given in Table II, although sufficiently con- 

 sistent to justify the conclusion just given, nevertheless exhibit 

 slight irregularities which are being made the subject of 

 further investigation. In the first instance the values for the 

 percentage cathode activity for potentials in the neighborhood 

 of 1000 volts show a slight falling off as compared with those 

 corresponding to the smaller potentials. The explanation of 

 this effect appears to lie in a distortion of the field in the 

 neighborhood of the ebonite plug, arising from some action of 

 the a-radiation on the ebonite ; this distortion would result in 

 some of the cathode activity being deposited on the plug 

 instead of on the central electrode. Corresponding difficulties 

 arose for the ionization measurements at the reduced pressures. 



