1016] 



on The Genesis and Absorption of X-Rays 



in the film would escape defcecfcion. lu my experiments the Rontgeii 

 rays first became appreciable when the voltage on the cathode rays 

 was about 150 volts — the limit depend^ on the thickness of the film, 

 the magnitude of the stream of cathode rays, and the sensitiveness of 

 the electrometer — it then increased comparatively slowly at first, but 

 very rapidly when the voltage rose above a certain value. This value 

 was in the neighbourhood of 350 to 400 ; it seemed to depend some- 

 what on the nature of the target. The energy in the Rontgen rays- 

 at different voltages, the number of cathode rays being kept constant, 

 is shown below : the tarsret was made of celluloid : — 



Voltage 

 150 

 200 

 250 

 300 



The energy in the Rontgen rays does not continue to increase at 

 this rate with the potential ; it attains a maximum at a certain voltage, 

 and then, for a time at least, diminishes as the voltage increases. 

 The potential for maximum voltage is subject to considerable 

 variation ; the cause for this I have not yet had time to investigate ; 

 the thickness of the film, the pressure in the cathode-ray tube seem 

 both to affect this potential. 



I now proceed to considering the other method of investigating- 

 the quality of the rays, i.e. by the velocity of the electrons ejected 

 when the rays fall on a metal plate. To apply this method, the 

 ionizing chamber I used in the previous one was replaced by a sphere 

 silvered on the inside. At the centre of the sphere there was an 

 insulated metal plate connected with one pair of quadrants of a 

 Dolezalek electrometer. When the rays fall on this plate electrons 

 are given off, and the plate acquires a positive charge which is 

 registered by the electrometer. If the energy with which the 

 electrons are ejected is measured by V volts, then if the silvered 

 surface of the sphere is made negative to the plate by Y volts or 

 more, the electrons cannot escape from the plate and the electrometer- 

 will not be affected by the incidence of the rays. In this method it 

 is desirable to have the pressure of the gas in the bulb as low as- 

 possible, so that no diaphragm between the cathode ray tube i& 

 necessary, and thus the rays which may be absorbed by the diaphragm 

 and escape detection by the first method will be registered by this. 

 On the other hand, since the diaphragm prevents charges of electricity 

 which would vitiate the measurements from passing from the cathode 

 ray tube to the parts connected with the electrometer, it is necessary 

 to introduce some means for stopping these charges. This was- 

 effected by having in the tube, between the cathode ray tube and the 

 silvered sphere, two half-cylinders insulated from each other and 



