Radiation produced by slowly moving Cathode Rays. 3 Go 



to get the radiation outside the tube ; and as the results were 

 much more regular than with the coil, and the current through 

 the tube and the potential-difference between the electrodes 

 much more easily measured, the coil was abandoned and the 

 subsequent experiments made with the battery of cells. 



The radiation obtained in this way is very easily absorbed, 

 for if a screen of the very thin aluminium-foil used for the 

 windows was placed between the windows and the disk, the 

 rate of leak from the disk was reduced to about 1/6 of its 

 previous value. The radiation is also absorbed by a thin 

 layer of air ; for when the tube containing the disk was ex- 

 hausted and the rate of leak taken through gas at various 

 pressures, it was found that the rate was not affected by the 

 pressure until this fell below a certain value. This shows that 

 it was not until this reduction of pressure took place that the 

 radiation was able to reach the disk, which was about 1 cm. 

 from the window. The following numbers show the variation 

 in the rate of leak with the pressure in the outside tube, the 

 conditions in the discharge-tube being kept constant. 



Pressure in 



Outer Tube. 



Leak 



fr 



om. Disk in one minute. 



770 



millims. 







87 



270 



jj 







90 



100 



5> 







64 



45 



>y 







37 



10 



?? 







11 



5 



5? 







3 



Origin of the Radiation. — It was found that the radiation 

 started at the place where the negative glow hit against a 

 solid surface. Thus, when the window was cathode and the 

 negative glow extended to the anode, the anode was the 

 source of the rays. This was proved by deflecting the negative 

 glow by a magnet. In this way the negative glow could 

 gradually be displaced from the anode. As soon as the glow 

 began to leave the anode the leak began to diminish, and when 

 the glow was clear of the anode the leak ceased entirely. If 

 the exhaustion of the tube was carried so far that the negative 

 dark space reached to the anode, the rate of leak was very 

 small : this point will be considered at more length later on. 



After the source of the radiation had thus been determined, 

 it seemed desirable to make experiments with a tube in which 

 the window was not itself an electrode, and. in which the 

 position of the window in the negative glow could be varied. 

 The tube (fig. 2) was constructed for this purpose. The 



2B 2 



