McCi-iELLAND AND M'HiiNRY — Uncharged Nuclei. 

 Ionization Ciorve ( Ur allium and Spark). 



289 



It will be evident from the tables above that positive and negative 

 numbers are materially the same, indicating the equal behaviour of positive 

 and negative ions. 



Eesnlts similar to the preceding were obtained by Lenard and Eamsaiier 

 (loc. cit). In their experiments the large ions were produced directly by 

 Schumann — violet rays, which at once ionized the air and produced the 

 nuclei. They found that the ions increased in size with the intensity of the 

 light, the mobilities being measured, as in our experiments, by plotting a 

 current-voltage curve. Increasing the time of exposure of the air to the 

 light by diminishing the velocity of the air past the source of light also 

 increased the size of the ions, as in our experiments. Lenard and Eamsauer 

 give curves showing the decrease of the currents by recombination (cf. 

 Curves 1) ; the rate of decrease being slower, the more vaporous impurities 

 are contained in the air. Finally, the mobilities of the positive and negative 

 ions were shown by them to be the same. The mobilities were deduced from 

 the saturation points in a current-voltage curve. 



Effect of Drying the Air. 



The number and size of the nuclei depend upon the amount of moisture 

 in the air. Thus the effect almost disappears in air dried by calcium 

 chloride, but even on drying with phosphorus pentoxide a small number of 

 nuclei seem always to be produced, as shown by a slight increase in the 

 uranium current on switching on the spark. Curves 3 show the relative 

 effects in moist and dry air. As will be seen, the current C^, consisting of 

 condensation nuclei charged by the uranium ions, is in the case of dry air 

 only slightly greater tha.n Ci, the current due to uranium alone, while in the 

 case of moist air Ca is 3-^- times as great as d. The saturation point in moist 

 air is 40 volts, indicating an ion of mobility •014. Furthermore, these ions 



