476 



Mr. G. Stead on the 



that the adjustment of the anode potential to obtain oscilla- 

 tions was ver} critical. The upper limit of pressure, however, 

 depends very much on the nature of the gas and also on the 

 design of the valve. Thus the amount of helium which can 

 be introduced into a valve is about ten times the amount of 

 nitrogen which is practicable, and this again several times ;.s 

 great as the amount of mercury vapour which is possible. 



In these experiments, in order to find the lower limit of 

 the oscillation range the anode potential was gradually raised 

 until the anode current showed a sudden drop. This in- 

 dicated that oscillations had started. The upper limit was 

 determined in a similar manner by starting with an anode 

 potential so large that no oscillations were generated and 

 gradually diminishing this until a sudden drop of current 

 occurred. As regards the determination of the potential of the 

 grid it is to be observed that the circuit shown in fig. 6 has an 

 insulated grid, and for this reason the method adopted was to 

 observe the anode current and then connect the grid to a 

 potentiometer and adjust this until the anode current was 

 the same as before. The range of: anode voltage over which 

 oscillations could be produced under the given conditions 

 was determined for each of the five valves for a series of 

 pressures lying between the limits 0*6 mm. and *0074 mm. 

 The oscillations were usually produced un<-er somewhat 

 difficult conditions, with an inductance of 1000 microhenries 

 and a capacity of - 005 microfarad, and an additional resist- 

 ance of 2 ohms in the oscillating circuit. Some observations 

 were also made under different conditions. 



In each case small portions of the valve characteristics 

 were drawn for the region containing the oscillation range. 

 These experiments show T ed clearly that considerably higher 

 gas pressures are practicable in cylindrical valves than 

 in those of the audion type. Observations for two different 

 valves are shown in the following table : — 



Pressure. 



Cylindrical type valve. 



Audion type valve. 



Minimum 

 voltage. 



Maximum 

 voltage. 



Range. 



Minimum 

 voltnge. 



Maximum 

 voltage. 



Range. 



06 mm. 

 0-2 „ 

 •05 „ 

 •015 „ 

 •0074 „ 



196 

 201 

 242 

 321 

 45-0 



19-8 

 212 

 261 

 375 

 656 



0-2 

 1-1 

 1-9 

 5-4 

 20-5 



200 

 21-9 



28-5 



20-2 

 239 

 33-2 



_ 

 0-2 

 20 

 4-7 



- ! 



! 



