148 TRANSACTIONS OF SECTION A. 



The star was again observed in July and August 1919. Visually it showed 

 a planetary nebula disc surmounting a bright stellar point. Its usual spec- 

 trum too was concentrated in a tingle green line X 5007. The photographic 

 spectrum showed in addition A. 4363 very prominently, and other nebula lines 

 faintly. 



The sequence of progressive changes was therefore from a possible Procyon 

 type spectrum (FSG) through an a Cygni spectrum (A2F.p.), and a y Orionis 

 spectnim (B^), to that of a planetary nebula. This sequence is in agreement 

 with that generally adopted for giant stars rising in temperature. But in 

 the Nova it was accompanied by a gradual lowering in magnitude, and pre- 

 sumably of temperature. The nebula was therefore most probably present 

 at the very beginning of the changes. A solar eruption on a magnified scale 

 in a giant star situated in a dark nebula would square with the observed spectral 

 changes. The nebula would be put into sympathetic luminous ^abration by 

 the eruption. The a Cygni lines are chromospheric lines. The displaced hydro- 

 gen is characteristic of such eruptions. The 7 Orionis, the 0-type star and 

 nebula lines belong to the nebula in which the eruption took place. 



4. Report on Wave Motion. By Sir G. Gbeenhill, F.R.S. 



See p. 403. 



WEDNESDAY, SEPTEMBER 10. 

 Discussion on Thermionic Tubes. 



The following Papers were read : — 



Discussion on Thermionic Tubes, opened by Professor W. H. Eccles, 



D.Sc. 



Professor Eccles gave a general description of the history and development 

 of the three electrode valve, and explained its rectifying property and its uses 

 in amplifiers, heterodyne reception, and the arrangements necessary to produce 

 continuous waves. Experiments were shown illustrating theee uses of the valve; 

 and the way was thus prepared for the discussion of special points by subsequent 

 speakers. Professor Fortescue drew attention to the functions and properties of 

 the various parts of the valve in some detail. The hot filament is the source ot 

 the electrons upon which the action of the valve fundamentally depends, and 

 with tungsten filaments as at present used only 4^ per cent, of the energy heating 

 the filament is usefully employed as electron emission. This efficiency might be 

 improved by using oxide-coated filaments or higher temperatures, but at present 

 neither of these methods has been entirely successful in practice. The construc- 

 tion of the grid and the question of freeing the anode and containing vessel from 

 occluded gas were also discussed, and the importance of investigating the methods 

 of removing the last traces of gases and examining their nature was emphasised. 

 Dr. VVhiddington drew attention to tlie possibility of using valves and oscillating 

 circuits for making many standard physical measurements. Thus, for instance, 

 the coefficient of mutual induction between two coils can be measured by deter- 

 mining the degree of coupling at which oscillations are just started and main- 

 tained in the valve circuit. He also alluded to Professor Eccles' example of the 

 extreme sensitiveness of heterodyne reception as represented by the effect of 

 passing coal gas between the plates of a condenser in an oscillating circuit. The 

 temperature coefficient of resistance, the conductivity of flames, the permeability 

 of liquids and other quantities could also be measured by this delicate method. 



