SPACE CHARGE BETWEEN PARALLEL PLANE ELECTRODES 69 



separating the unstable region from the stable region is drawn on the 

 model and lies along the vertical surface of the cliff. The small 

 region lying between 7 values of and 0.5, for which direct transitions 

 between A solutions and D solutions occur, can be seen in both views 

 as a small vertical cliff. 



Solutions corresponding to varying conditions will be given by the 

 position of a movable particle confined to the surface of the model. 



Fig. 12 — -A three-dimensional model illustrating the current-voltage relationships. 

 Solutions of types C and D appear on the celluloid surface and solutions of the B 

 type appear on the concave surface. 



When this particle lies on the celluloid surface it may be moved around 

 at will and will not "fall off" unless an attempt is made to go to 

 larger values of the injected current 7 than is permitted by the bound- 

 curve a (the curved edge of the celluloid). The particle may, however, 

 "fall off" at this edge and land on the concave surface from which it 

 can escape only by climbing up the surface to the bounding curve b 

 which is common to both surfaces. The overhanging cliff is, however, 

 particularly treacherous, for unless the particle is supported by the 



