What happens inside an electric discharge tube?
We shall now return to the electric discharge tubes described earlier in this chapter. The processes which go on inside these tubes are complex and difficult to investigate, so that physicists hold different views regarding explanations of the various dark
spaces, striations and so on.
However, all agree that ions play the most important part in the conduction of electricity through gases. The meaning of the term gaseous ion was explained on page 375. In a discharge tube at low pressure the ions which are always present are accelerated by the electric field between the electrodes; positive ions towards the cathode and free electrons and negative ions towards the anode. Collisions between electrons and atoms occur resulting in the production of further ions.
It is believed that the impact of positive ions on the cathode causes electrons to be knocked out of the metal of the cathode itself, and these constitute the cathode rays which stream towards the anode at high velocity. The various discharge tubes discussed earlier in this chapter played a very important part .in our knowledge and understanding of the nature and properties of electrons. But, in order to obtain a copious stream of electrons in tubes such as the low gas pressure Maltese cross tube of Fig. 44.3, very high voltages are required and this can cause undesirable X-radiation when he electrons strike the walls of the
tube. The danger can be avoided by the use of evacuated tubes which have a thermionic electron gun. It is therefore preferable to use this type of tube for investigating the properties of electrons since they produce a good beam of cathode rays at lower accelerating voltages, with the added advantage of reduced danger from radiation