Physics at the British Association

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ONE day of Section A was devoted almost entirely to matters relating to wireless telegraphy. Prof. Eccles opened a discussion on thermionic valves, giving a general description of the history and development of the three-electrode valve, explaining its rectifying property, the method of heterodyne reception, and the arrangements necessary j to produce continuous waves. Experiments were shown illustrating these uses of the valve, and the j way was thus prepared for the discussion of special points by subsequent speakers. Prof. Fortescue directed attention to the functions and properties of the various parts of the valve in some detail. The hot filament is the source of the electrons upon which ! the action of the valve fundamentally depends; 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 construction of the grid and the question of freeing the anode and containing vessel from occluded gas during pumping were also discussed, and the importance of investigating the methods of removing the last traces of gas and examining their nature was emphasised. Dr. Whiddington directed attention to the possibility of using valves and oscillating circuits for making many standard physical measurements. Thus, for example, the coefficient of mutual induction can be determined by observing the degree of coupling at which oscillations are just started and maintained. He also alluded to Prof. Eccles's example of the extreme sensitiveness of heterodyne reception as illustrated 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.