762 THE BELL SYSTEM TECHNICAL JOURNAL, MAY H).17 



250 



7 8 9 



SPACING, d, IN CM X PRESSURE, po, IN MM OF Hg 



Fig. 4 — Breakdown voltage as a function of spacing and pressure for small 

 wire anodes parallel to cathode surface. 



eration at the minimum of the p^d curve can be retained and the 

 breakdown voltage made higher by resorting to non-uniform geometry. 

 Typical curves are shown in Fig. 4. The cathode was a small rectangular 

 plate and the anode was a wire placed parallel to the cathode surface. 

 It is seen that as the anode diameter is decreased, the minimum of the 

 breakdown curve is increased. The practical limit is set by mechanical 

 stability of the anode and by transmission requirements, as will be dis- 

 cussed later. 



The rise in minimum breakdown voltage as the anode size is reduced 

 can be explained on the basis of the distortion of the electric field. Near 

 the cathode, E is lowered, and near the anode, E is increased, as com- 

 pared to the parallel plane case. If the spacing is adjusted for optimum 

 E/pi) with parallel planes, then rj is necessarily less than optimum for the 

 distorted fields. 



Returning now to Fig. 2, we note that, as the current is increased 

 beyond breakdown, the tube voltage falls to a lower sustaining value and 

 again is relatively constant with current. This lowei' voltage corresponds 

 to the development of a space-charge layer of positive ions near the 

 cathode and an increased voltage gradient at the cathode. This highei" 



