474 Mr. G. Stead on the 



tested spectroscopically. Characteristics were obtained for 

 a range of pressures from '032 mm. to '00005 mm. 



These preliminary experiments showed that the general 

 effect on the characteristics of variation of gas pressure was 

 the same for all the gases studied, but that the pressure at 

 which an audion works most satisfactorily is at least five 

 times as great in residual air and argon as in mercury 

 vapour. 



IV. General remarks on Soft Valves. — The chief difficulty 

 to be overcome in the design of a satisfactory soft valve is 

 that caused by variations of gas pressure whilst the valve 

 is in operation. It seemed likely, at first sight, that changes 

 of pressure would be less marked if larger amounts of gas 

 could be employed, but in valves of the audion type, with 

 plane anodes, a general glow readily occurs throughout the 

 bulb if the anode voltage is too high, and the valve is at all 

 soft This glow, which is accompanied by rapid disinte- 

 gration of the tungsten filament, is probably associated with 

 ionization along the tracks of electrons which travel from 

 the filament to the anode by long paths outside the electrodes. 

 The probability of such paths is greatly diminished by the 

 adoption of a cylindrical construction for the electrodes. 

 Hence a higher gas pressure should be possible in cylindrical 

 valves than in those of the audion type. Experiment showed 

 this to be the case. The cylindrical construction has the 

 further advantage that all four leads can be carried by the 

 same stem ; the electrodes can therefore be completely 

 assembled outside the bulb, and then sealed info the bulb 

 at one operation. 



In order to test the theory of the general glow r given 

 above, a special cylindrical valve was constructed in which 

 one end of the anode w^as permanently closed, and the. other 

 end could be closed when required by means of a small brass 

 cup which was carried on a stem attached to a glass bulb 

 floating on mercury. It was found that when the end of 

 the cylinder w T as closed by the cup the general glow was 

 entirely eliminated, and " kinks" in the characteristics could 

 lie avoided at pressures high enough to produce them if the 

 anode were open. 



V. Soft Valves as Oscillators. — A detailed investigation was 

 made in order to determine the pressures at which various 

 types of valve would generate oscillations in two different 

 circuits. A diagram of one of these circuits is shown in 

 fig. 6. Nitrogen was the gas first selected for these experi- 

 ments, and five different valves were employed. Two of 



