Design of Soft Thermionic Valves, 475 



them were o£ auction type, and the other three were cylin- 

 jdricalValyes closely resembling the French type. The grid 



Fig-. 6. 

 T 



U 



B e 



'I 



VJ 



i ! 



A 



h 



-r a 



A = anode. 

 G =grid. 



F = filament. 

 Bi = filament battery 

 C), c, 



B 2 = anode battery. 

 T = telephones. 

 L = inductance. 



2 , c 3 , Ci, and c 5 = condensers. 



cylindrical valves were of different dia- 



m ires of the three 



meters, and so possessed different projected areas, bnt the 



three grids were of the same effective degree of closeness, 



i.e., possessed the same value of the quantity m in the 



formula 



m 



_ 7rN(/'log (did') 



l0g[J/(7TN^)J 



where d and d' are the anode and grid diameters, respec- 

 tively, d is the diameter of the grid wires, and N is the 

 number of turns of grid helix per centimetre. This formula 

 Mas communicated to H.M. Signal School by Professor 

 Sir J. J. Thomson and is an adaptation to the cylindrical case 

 of a formula given by Maxwell * for a plane anode, grid and 

 cathode. (See also the paper by Grossling, already cited.) 

 Of the two auction valves employed in this investigation, one 

 had a normal type of grid and the other a very open grid. 



Experiment showed that under given conditions a soft 

 valve would not generate oscillations unless the anode potential 

 lay between certain limits. Both limits rose as the pressure 

 of the gas was diminished, but the upper limit rose much 

 more rapidly than the lower limit, so that the oscillating 

 as the gas was pumped out. For high gas 

 of anode voltage over which oscillations 

 would take place was very limited, e. g., less than one voir, so 



pressure the range 



1 Electricity and Magnetism,' i. p. 312 (1892). 



