Subclass 1/24: Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps (details of electron-optical arrangements or of ion traps H01J 3/00) Main electrodes Solid thermionic cathodes Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment Insulating layer or body located between heater and emissive material