284 Scientific Proceedings^ Royal Dublin Society. 



The quartz tube passed through a box lined with tinfoil and earthed. 

 Inside the box, and opposite the quartz tube, were aluminium terminals 

 connected to the secondary of an induction coil. The shape of the terminals 

 enabled them to be placed, if necessary, right up against the quartz tube. 

 A motor-driven mercury interrupter was used with the induction coil. The 

 quartz tube was about half a millimetre thick. In most of the experiments 

 the light also traversed 2 or 3 mms. of air. 



The ultra-violet light of the spark was found, in a number of careful 

 experiments, not to ionize the air; but as a precaution the long terminal, Tj, 

 was used lo remove to earth any ions contained in the air before its passage 

 over the uranium oxide. The outer cylinder of the whole ionization tube 

 AB was kept at any desired voltage by means of cells. When the air was 

 passing through the apparatus, the ions produced by the uranium oxide took 

 on an average about twenty or thirty seconds to reach the terminal To. A 

 great number were consequently lost by recombination. The current due to 

 the uranium oxide itself we shall call C,. When Ci had been measured, the 

 spark was started. The air now passing over tlie uranium oxide contained 

 the nuclei produced by the ultra-violet light. Some of these nuclei 

 attached themselves to small ions. In this way large ions were produced 

 which recombined less rapidly than the small ones. More ions consequently 

 reached the terminal Tg than when the nuclei were absent, and consequently 

 the observed current Gj, when the spark was on, was greater than the 

 current C,. The current On will evidently increase with the number and size 



of the nuclei present in the air, and thus the ratio ^ gives an idea of the 



number of nuclei present. An idea as to the relative sizes of the nuclei and 

 small ions can be got from a knowledge of their mobilities under an electric 

 field. This mobility can be deduced from the saturation voltages in a 

 current-voltage curve when the dimensions of the terminal and ionization 

 tube and the C|uantity of air passing per second through the apparatus are 

 known. 



In our experiments, the quantity of air flowing per minute varied 



Q 



approximately from 1000 ccs. to 6000 ccs. The ratio -^ varied from 



about 8 for slow blasts of air to about 1'2 for quick blasts. The value of 



C-. 

 the ratio -- is increased for slow velocities of the air through the apparatus, 

 Gi 



not only on account of the larger time allowed for recombination, but 

 because the time of exposure of the air to the light is also increased, so that 

 more and larger condensation nuclei are produced. For the quickest air- 

 blasts used the nuclei had a mobility of about '3 cms. per sec. per volt/cm. 



