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BELL SYSTEM TECHNICAL JOURNAL 



12007ra/ is equal to 0.3 volt. It will be convenient to treat o- as if it 

 were expressed in volts, i.e., as if a stood for 12007ra/. If the patch 

 were very large a in volts would be the decrease in the work function 

 due to the patch. It is to be noted that a typical curve starts along a 



60 80 100 120 140 



Yf" (V volts per cm ) 



160 



180 200 



Fig. 9 — Variation of emission current with applied field for circular patches of 

 various sizes; T and <j constant. Comparison with experimental curve for thoriated 

 tungsten. 



line having the Schottky slope; but soon it rises at a much more rapid 

 rate and continues until it almost reaches an upper line having the 

 Schottky slope; then it bends rather sharply and approaches this line 

 asymptotically. For the larger patches, the curve starts to rise at 

 very small values of V ^ and it is very steep. For smaller values of R, 

 the curve follows the lower Schottky line for an appreciable distance 

 and its slope never attains very large values; also the place at which 

 it bends toward the upper Schottky line moves to large values of V F. 

 Note also that as long as a is constant all curves are bounded by the 

 same two Schottky lines. 



Figure 10 shows the effect of varying a while R is kept constant. 

 The distance between an upper Schottky line and the lower Schottky 

 line is directly proportional to a. In fact this shift is given by 



A log ?//() = aejlMT 



(69) 



