Design of Soft Tliermionic Valves. 473 



The valve was exhausted in the usual way and was 

 rendered as hard as possible by means of charcoal and liquid 

 air, mercury vapour being also frozen out by liquid air. 

 Hydrogen was then admitted. The hydrogen was prepared 

 by electrolysis of a solution of barium hydroxide, and was 

 dried by phosphorus pentoxide, followed by the freezing 

 action of liquid air. Characteristics were then plotted for a 

 series of grid spacings. Specimens of these characteristics 

 are shown in tig. 4 (PI. VIII ) for grid spacings of 1, 5, 3, 

 and 4 mm. (By grid spacing is meant the width of the 

 openings between the grid wires, not the pitch of the helix.) 

 It will be noticed that for a given anode and grid voltage, 

 the anode current first increases and then diminishes as the 

 grid helix is pulled out, and the characteristics attain 

 maximum steepness for a particular value of the grid spacing. 

 The manner in which the anode and grid currents alter jis 

 the grid spacing is increased is shown in fig. 5 (PI. VIII. ). 

 The curves reproduced represent anode current (or grid 

 current) plotted against grid spacing for a fixed anode 

 potential, and for several different grid potentials. In this 

 particular valve the diameter of the £>rid helix was about 

 0*6 cm. and the diameter of the grid wire 0"5 mm. The 

 grid spacing which gives the maximum anode current 

 increases from 2'3 to 2*85 mm. as the grid potential increases 

 from 1-6 to 15 volts. 



II J. Preliminary experiments in Mercury Vapour, Residual 

 Air, and Argon. — The effect of the pressure and nature of the 

 contained gas on the characteristics of a valve of audion type 

 (with plane anodes and zig-zag grids) w r as examined in some 

 detail, the atmospheres employed being mercury vapour, 

 residual air, and argon. For 'the experiments on mercury 

 vapour a small tube containing a drop of mercury was 

 attached to the valve, and this tube was placed in a vacuum 

 flask containing water, or methylated spirit, at a known 

 temperature. In this way any desired vapour pressure 

 could be obtained. The range of temperature employed was 

 from — 1 3° C. to 4- 56° C, corresponding to a range of vapour 

 pressure from -00004 to '019 mm. " Residual air" was 

 obtained by simply pumping down the valve to the required 

 pressure, and doubtless con>ist"d chiefly of electrode gas, 

 i.e., hydrogen and oxides of carbon. Pressures between 

 *008 mm. and "00025 mm. were employed. The argon was 

 made from liquid air residues by prolonged sparking over 

 potash with excess of oxygen, the latter being finally 

 removed by phosphorus. The purity of the argon was 



