ELECTROSTATIC FIELD IN VACUUM TUBES 



309 



reduces to the familiar approximate formula derived, for example, in an 

 early paper by R. W. King^^ 



M = — , (23a) 



a In 



27rc 



■0.2 



0.05 OJO 015 0.20 0.25 0.30 0.35 0.40 0.45 50 



DISTANCE ALONG CATHODE, X/^ 



Fig. 3 — Variation of cathode field strength in a triode. 



Some calculated curves for \x and Eg are shown in Figs. 2 and 3, Figure 2 

 shows the amplification factor as a function of the distance along the cathode 

 with the ratio of grid-cathode separation to grid wire spacing as a parameter. 

 The ratio of grid-anode separation to grid wire spacing is held constant at 

 five. Only half the grid spacing interval is included since the curves are 

 symmetrical. The increase in ^i-variation as the grid-cathode separation 

 becomes small is clearly demonstrated. For negligible ju-variation we must 

 select d\la of the order of 2 or greater. 



