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



covered with sheet-lead 3 mm. thick. The general arrange- 

 ment of the apparatus is shown in fig. 1. Special precautions 

 were taken to prevent losses from the external electrodes of 

 the bulb and to prevent discharges over its surface. For 

 this purpose, the dust collecting on its surface was regularly 

 removed and the surface washed with alcohol. 



Fiff. 1. 



As the machine after stoppage occasionally reversed its 

 voltage, suitable cross connexions (see fig. 1) were arranged 

 to rectify rapidly the direction of the current. The acces- 

 sories for the Ooolidge tube, viz., the battery, adjustable 

 resistance, etc., w T ere placed on an insulated stand and com- 

 pletely covered with a rounded metal case. The current 

 could be controlled by an insulating handle coming through 

 a small opening. The conductors for the heating current 

 passed inside the hollow metal tubes. 



Measurement of Voltage and Current. 



In order to determine the absorption curve of the radiation 

 with accuracy, it was necessary to keep the voltage very 

 constant, and to have some method of knowing the voltage 

 at any moment. For this purpose, the conductors to the 

 bulb were shunted through a high resistance in series with a 

 galvanometer. The resistance consisted of two capillary 

 tubes, Rx and R 2 , filled with xylol and thoroughly insulated 

 by a thick layer of paraffin. The moving-coil galvanometer 

 O with suitable shunts was placed between the two resist- 

 ances, so that its potential was never far from zero. In 



