630 



Miss J. M. W. Slater on the 



thorium for the same time and at the same potential were 

 used in nearly all cases, so that the activities before and after 

 the exposure of one of them to the cathode rays could be 

 compared. The sensitiveness of the electrometer varied some- 

 what in the course of an experiment, as the charge on the 

 needle leaked away ; the effect of this was to make the ap- 

 parent value of the rate of decay rather too large. When 

 comparative values were taken it was not generally necessary 

 to correct for this ; when the absolute constant of decay was 

 required, the electrometer was standardized at intervals by 

 connecting a uranium cell to the insulated quadrants. The 

 saturation current across such a cell is constant, and therefore 

 the rate of deflexion is directly proportional to the sensitiveness 

 of the electrometer. 



The substance made active was in nearly all cases alumi- 

 nium. This is particularly suitable as it neither oxidizes nor 

 sputters to any appreciable extent in a discharge-tube. In 

 the earlier experiments long strips of aluminium-foil were 

 used, and were coiled round in the tube so as to expose as 

 large a surface as possible to the cathode rays; later on it 

 was found better to concentrate the activity on a small area 

 in the centre of a disk of the metal. The discharge-tube 

 used with these disks is shown in fig. 1. 



Kir. 1. 



To Pump. 



The cathode C was about 10 cm. from the active disk D, 

 the anode in the form of a ring being placed between them, 

 so as to minimize the risk of sputtering from the cathode to 

 the active surface. The disk could be earthed, or connected 

 to either electrode, by means of the terminal B. The tube 

 was closed by a ground-glass stopper, so that the active disks 

 could be put in and taken out without the necessity for sealing 



