86 BELL SYSTEM TECHNICAL JOURNAL 



picks up a charge of electrons which in flowing off through the leak 

 creates an average negative grid potential. It is apparent that as the 

 amplitude of oscillation increases, the charge picked up at each posi- 

 tive swing of the grid potential increases, with the result that this 

 average negative potential tends to sink lower. The importance of 

 this control feature will be brought out presently. 



It is generally found that the oscillations build up to such a value 

 that the greatest positive potential of the grid, which will be repre- 

 sented by E gmax , becomes practically equal to the lowest plate po- 

 tential, E Pm - m . It is not difficult to understand why this condition 

 should represent a sort of limit. It is usually found that the dynamic 

 characteristic, as shown at B, Fig. 56, tends to bend rapidly downward 

 as E gmax becomes greater than E Pmin . When E gmax tends to become 

 greater than E Pm - m the electron current to the grid rises suddenly (see 

 cathode ray oscillogram, Fig. 57) with the result that the average 



current flowing through the grid leak increases very rapidly, which 

 in turn results in a marked depression of the average grid potential. 



The increased electron current to the grid as E g tends to become 

 greater than E Pmin also represents a greater dissipation of energy upon 

 the grid which is equivalent to an increase in the effective resistance R. 



It is quite possible that any one of these three factors, in the absence 

 of the others, would be sufficient to limit the oscillations; but as each 

 springs into importance when the amplitude of the oscillation has 

 reached about the same value, we shall not discuss the exact com- 

 bination of the three which actually determines the amplitude. 



Adopting the relation E gmax = E Pmin and making the additional 

 assumption that the dynamic characteristic is straight (which is very 

 nearly true) an expression for the amplitude of oscillation can readily 



