[ 470 j 



XLI. On tlie Design of Soft Thermionic Valves. 

 By G. Stead, M.A* 



[Plate VIII.] 



Preliminary Note. — The experiments described in this 

 paper were undertaken on behalf of ELM. Signal School, 

 Portsmouth, and were carried out in the Cavendish Labor- 

 atory, Cambridge, at various times during 1916-17. They 

 are now published with the consent of the Admiralty. The 

 writer wishes to acknowledge his great indebtedness to the 

 members of the W/T staff of ELM. Signal School, and in 

 particular to Professor C. L. Fortescue and Mr. B. S. Gossling, 

 with whom he was in close touch throughout the work. 



I. Introduction. — A general account of the development 

 of thermionic valves for Naval purposes has been given in a 

 recent paper by B. S. Gossling f, and in section (7) of the 

 paper the evolution of the " soft " or gas-filled valve is dis- 

 cussed. It is thought that a more detailed account of some- 

 of those portions of the work with which the present writer 

 has been directly associated may be of general interest. The- 

 experiments described deal in the main with the effect on the 

 valve characteristics of the pressure and nature of the con- 

 tained gas, and of the position and form of the electrodes. 

 Chronological order has been followed as far as possible, but 

 has not been adhered to where rearrangement seemed likely 

 to render the account more logical and connected. 



II. Position and Form of Electrodes. — In 1916, when the 

 work described in this section was undertaken, little was 

 known about the effect on valve characteristics of changing 

 the position and form of the three electrodes, and experi- 

 mental work in this direction seemed urgently required. 

 Two kinds of experiment were carried out, one to determine 

 the effect of altering the distance between the grid and the 

 filament, and the other to ascertain the effect of altering the 

 spacing of the grid wires. 



(i.) Experiments with a movable grid. — A special valve 

 was designed in which it was possible to move the grid 

 whilst the valve was exhausted. The arrangement is shown 

 diagram matically in fig. 1. The anode was an aluminium 

 disk A, and the filament was in the form of a V with its. 



* Communicated b} r Professor Sir J. J. Thomson, F.E.S. 

 t Journai 1. E. E. Iviii. p. 670 (1920).- 



