788 Mr. S. Ratner on the Nature of Discharge of 



15 mm.), the discharge suddenly sets in, and the current 

 through the tube abruptly rises to the order of a milli- 

 ampere *. The external current heating the strip is then 

 broken, and the experiments repeated with a cold anode, 

 other conditions remaining the same. The discharge is 

 found to take place at exactly the same potential as when 

 the strip was heated and the cathode intensely bombarded by 

 a stream of positive ions. 



The same procedure was followed at a lower vacuum, 

 when a potential difference of the order of 1000 v. applied 

 to the electrodes was enough to produce the discharge. In 

 this case the voltage was supplied by a battery of small cells, 

 and the sparking potential could be determined with great 

 accuracy. The results obtained were as before, and the 

 positive emission from the strip did not lower the sparking 

 potential by as much as one per cent. 



4. These results are hardly compatible with the usual 

 theory of discharge of electricity through vacuum-tubes, for 

 it is equivalent to admitting that positive ions falling through 

 a potential of more than 20,000 v. in a high vacuum do not 

 carry with them sufficient energy to liberate electrons from 

 the cathode on striking against its surface ; while in other 

 cises, on the other hand, a fall through less than 1000 v. in 

 a lower vacuum enables them to produce this effect. If we 

 suppose that the electrons necessary to start the discharge 

 may be bombarded not necessarily only out of the cathode, 

 but also from the gaseous molecules by collision with the 

 positive ions, the same difficulties will still remain in the 

 interpretation of these experiments. One might perhaps 

 suppose that in a high vacuum, where the number of gaseous 

 molecules present in the tube is comparatively small, the 

 potential necessary for the discharge is necessarily high in 

 order to ensure ionization at every collision, but such a 

 theory would require a gradual increase in the discharge 

 current as the potential rises from about 1000 v. and goes 

 on increasing. It is well known, however, that the discharge 

 in a well-exhausted tube starts 'abruptly when the applied 

 potential attains a value considerably higher than 1000 volts. 



Whatever the nature of the discharge may be, the expe- 

 riments described above show distinctly that it is not caused 

 by the impact of positive ions on the surface of the cathode. 

 It seemed important, therefore, to carry the experiments 



* It may be mentioned here that the walls of the apparatus were 

 previously carefully freed from occluded gases by electronic bombard- 

 ment and heating of the strip, so that the pressure in the bulb remained 

 sufficiently constant throughout the experiments. 



