﻿364 Dr. G. Owen and Mr. J. H. T. Roberts on the 



than 1*38 was more dense when the radium was absent than 

 when the radium was in position. This point was tested 

 with the following results : — 



Expansion 1*436. 

 Current before expansion 230*0. 

 Current through cloud obtained with radium in position 



R x when expansion was made, 100*0. 

 Current through cloud, radium being absent when 



expansion was made, 73 5. 



The above numbers prove conclusively that far fewer 

 nuclei of cloudy condensation come into action when the gas 

 is subjected to strong ionizing rays. This effect may be due 

 to two causes : — 



(a) the ions, owing to their greater efficiency as conden- 

 sation nuclei, monopolize the water vapour, and, 

 therefore, the drops formed round the nuclei of 

 cloudy condensation do not attain the critical equi- 

 librium size. 



(It) the ionizing rays may, in some way, prevent the 

 accidental agglomeration of water molecules, to which 

 the nuclei of cloudy condensation are attributed, 

 e. g., by increasing the rapidity of their coalescence 

 to form fewer but larger drops. 



III. The manner in which the effect of a cloud upon the 

 current varies with the age of the cloud. 



AVe thought it would be interesting to see if any marked 

 change takes place in the cloud in the earlier stages after its 

 production. These changes might possibly be detected by 

 measuring the ionization current through the cloud at dif- 

 ferent times after its production. To detect any possible 

 interesting features, however, it is necessary that the ioniza- 

 tion current be measured over a very small interval of time, 

 say a fraction of a second, otherwise transitory effects would 

 be lost. To carry this out we arranged for the long pendulum 

 to swing through two mercury cups, some 17 cm. apart, the 

 total amplitude of the pendulum being about twice that 

 amount. The amplitude of the pendulum was kept constant 

 by releasing the iron bob from a fixed deflected position, in 

 which it had been held by an e^ctromagnet • thus the time 

 taken by the pendulum to pass from one mercury cup to the 

 other in its first swing was constant, and equal to 0*25 

 second. A metronome was arranged so that a wire attached 

 the oscillating lever passed through a mercury cup at the 



