708 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



The radiation from the wire ionized the gas inside the 

 testing cylinder and the current between the electrodes was 

 observed with the sensitive electrometer in the usual way. 

 On account of the weak ionization of the air by the radiation 

 a P.D. of 100 volts was sufficient to remove all the ions to 

 the electrodes before appreciable recombination, and to give 

 the maximum current through the gas. 



2, 1 shows the decay-curve for a copper wire exposed 



Fig. 



Fia-. 2. 



)4o l6o ISO ZOO 



210 minutes inside the laboratory at a P.D. of —26000 volts: 

 fig. 2, II the curve for the same wire exposed 270 minutes in 

 the open air at —24000 volts. The ordinates represent 

 divisions per sec. of the electrometer and the abscissa? time in 

 minutes. 



In most of the experiments (especially when the wire was 

 exposed for several hours) it was found necessary to use a 

 condenser in parallel with the electrometer to decrease the 

 rate of movement of the needle. 



There was always a current (about 2*5 divisions per sec.) 

 in the testing vessel when the wire was inactive, due to the 

 spontaneous ionization of the air in the cylinder. Allowing 

 for this it will be seen that the current (which is proportional 

 to the intensity of the radiation) falls off in a geometrical 

 progression with the time, falling to half value for both cases 

 in about 45 minutes. 



