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BELL SYSTEM TECHNICAL JOURNAL 



sometimes described as the fundamental theorem of the vacuum tube. 

 The theorem states that a potential change in the grid circuit appears 

 as a voltage generated in the plate circuit, the magnitude of which is 

 equal to the grid potential change multiplied by the amplification 

 factor ix. Solutions have been obtained for the output current 

 components with the aid of this relation through the work of Van der 

 Bijl and of Carson, which have been of great practical importance. 



These solutions are approximate because of the simplifying assump- 

 tion of the constancy of the amplification factor, which is certainly 

 not accurate as the curves of Fig. 2 demonstrate. In this diagram 



-25 -JO -15 



V0LT5 GRID POTENTIAL 



Fig. 2 — IX. as a function of the grid potential with plate potential as a parameter. 

 EL tube No. 109,150. 7/ = 1.1 amperes 



the amplification factor for small applied potentials is plotted as 



ordinate with the grid potential as abscissa and the plate potential 



as parameter. The variation of ^ is observed to be of the order of 



twenty per cent over the operating range. When we are interested 



in the distortion of the input wave, this variation cannot be ignored 



since to do so would in some cases yield results of another order of 



magnitude than those found experimentally. The treatment for our 



specific needs must therefore be modified to take account of the actual 



state of affairs.^ 



There are two ways open for a treatment involving the variation 



1 Early calculations to show the effect of a variable p. upon distortion were given 

 by H. Nyquist of the American Telephone and Telegraph Company in an unpublished 

 memoranfium of April, 1921. 



I 



