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BELL SYSTEM TECHNICAL JOURNAL 



between the quality of the h"ght falHng upon the cell and the electric 

 current produced. We must in addition consider certain other physical 

 variables which must be met with in practice, notably temperature. 



Characteristics of Cells of Various Structures 



For purposes of discussion we may classify photoelectric cells in 

 regard to structure as central cathode cells and central anode cells. As 

 the terms imply, these two extreme types of cells differ in the position 

 of the photoelectric material. The central anode type of cell is shown 

 in Figure 1. The sensitive material entirely covers the walls, so that 

 the cathode is of relatively large area. The central cathode cell is 

 illustrated in Figure 2, in which the symbols are the same as in Figure 1. 



w, 



Fig. 2 — Schematic central cathode photoelectric cell. 



In this the walls of the cell are covered with a non-light sensitive 

 material (e.g. silver), and the sensitive material is coated upon a 

 relatively small centrally placed electrode. 



Central Cathode Cells 



Central cathode cells possess certain decided advantages for the 

 theoretical study of photoelectric phenomena and have consequently 

 been used in many of the more important photoelectric investigations. 

 The simplest case to consider first is that of the high vacuum cell, that 

 is one containing no appreciable amount of gaseous atmosphere. When 

 a constant light is incident on the light sensitive cathode, and a series 

 of voltages are applied to the terminals of the cell, voltage-current 

 relationships are obtained of the character shown by any one of the 

 curves of Figure 3. Several significant points are to be noted about 

 these characteristic curves. We find that the photoelectric current 

 starts at a definite positive value of the voltage. This voltage is 

 called the "stopping potential." It varies with the wave length of the 



