Phenomena in X-Ray Bulbs. 201 



simple arrangement was found by means of which this diffi- 

 culty could be avoided. A circular piece of mica was fixed 

 on the rod supporting the cathode a few cm. behind it, so 

 as to screen the neck of the bulb from the sputtering 

 particles of the cathode. With this arrangement it was 

 found possible to carry on the experiments until polarization 

 of the bulb was reached. The degree of polarization, 

 however, was far from being as high as previously described, 

 and when the highest voltage was applied the maximum 

 pressure at which the discharge failed to pass was only *030 

 instead of *065 mm. in the former case. Further experi- 

 ments have shown that this cannot be ascribed to the change 

 in the metal of the cathode. The conditions of the discharge 

 were changed considerably in these experiments, owing to the 

 formation in the bulb of a metal film in contact with the 

 anode, thereby practically transferring the whole bulb into 

 one large anode, which in some parts approached the cathode. 

 When, again, an aluminum catnode was re-introduced into 

 the bulb in place of the copper one, the same lowering of the 

 degree of polarization, more or less, was still observed. 



An attempt was also made to repeat the same experiments 

 with a sodium cathode, in view of the peculiar electric 

 properties of the alkali metals. The cathode was cut from 

 a large piece of sodium under benzol, and then quickly 

 introduced into the bulb, so that a fairly fresh surface of 

 sodium was obtained. The experiments with this cathode 

 were, however, difficult and the results somewhat doubtful, 

 owing to the low melting-point of sodium. Only small 

 currents could be passed through the bulb on account of the 

 melting of the cathode and the accumulation of sodium 

 vapour. Finally, a stage was reached where a p.d. of about 

 20,000 v. was insufficient to produce a discharge at a pressure 

 of *015 mm. 



Experiments with source of Ionization inside the Bulb. 



6. Interesting results which have a bearing on the theory 

 of the phenomenon were obtained by introducing a source 

 of ionization into the bulb. A small disk coated with 

 polonium was placed in the middle of the bulb, and the gas 

 exposed to the action of a-particles. At such a low pressure 

 the ionization could not be intense, but still a certain number 

 of ions were constantly being produced in the apparatus. 

 This, however, did not affect the experiment, and after a 

 sufficiently long run a high degree of polarization was 

 still reached at a pressure of about ^ 5 mm. Under these 



