346 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson: 



aluminium sheets. The plates 1, 3 were connected to the 

 high potential battery and the plate 4 earthed. The plate 2 

 was connected with a mica condenser M of suitable capacity, 

 which could be connected at will through the insulating 

 key K to the string electrometer. The method of conducting 

 an experiment was as follows. By means of the key K, 

 the standardizing vessel was disconnected from the electro- 

 meter and the connexion of the latter with earth broken. 

 A thick lead slide L, which completely stopped the radia- 

 tion, was rapidly drawn aside and the radiation allowed to 

 pass into the ionization vessel for a period varying from 

 10 to 60 seconds in different experiments. The slide was 

 then closed, and the steady deflexion of the electrometer 

 read. After discharge of the latter, the standardizing vessel 

 was connected, and the deflexion again read. The corrected 

 deflexions for the two vessels should be proportional to each 

 other provided the fluctuations are in the current and not 

 in the voltage ; for variations of the latter alter the pene- 

 trating power of the radiation as well as the current through 

 the bulb. In this way, provided the voltage was kept steady, 

 it was found possible to correct for any changes in the 

 intensity of the radiation over the long interval of an hour 

 or more required to obtain a complete absorption curve. 



It was necessary to measure the absorption curve, espe- 

 cially for high voltages, over a very wide range of thickness 

 of aluminium where the current in the ionization vessel was 

 reduced in some cases to 1/10000 of its initial value. This 

 was done as follows. An air-condenser F of capacity about 

 400 e. s. units was placed parallel with the electrometer, and 

 the width of the opening of the ionization vessel adjusted 

 till there was a convenient deflexion of the electrometer in 

 10 seconds. Successive screens of aluminium were intro- 

 duced, and the currents in the testing and standardizing 

 vessels compared as the current diminished. The air con- 

 denser was removed, and before the current became too small 

 to measure with accuracy, the opening of the ionization 

 vessel was widened until again a convenient deflexion was 

 obtained when the condenser was in the circuit. This process 

 was continued two or three times, depending on the variation 

 in range of the ionization current to be measured. For the 

 end part of the curve, the testing vessel was always com- 

 pletely open to the radiation. In this way it was possible to 

 determine the complete absorption curve over a very wide 

 range without introducing any uncertainty in regard to 

 saturation. The capacities of the circuits were carefully 



