614 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



acteristic that this is only a rough approximation and can be considered 

 as valid only over a limited range in current. The value of the exponent 

 n, which on a logarithmic scale is the reciprocal of the slope of the volt- 

 age-current characteristic and which also is plotted on Fig. 1, is seen to 

 increase appreciably with increasing current. For example, n for the Vi 

 varistor covers a range from about 2 to 4. 



In a telephone set the alternating signal currents are small relative 

 to the direct currents. Therefore, in explaining how the silicon carbide 

 varistor controls the transmission equalization and sidetone, it is neces- 

 sary to examine how its ac resistance to small signals varies with direct 

 current. 



Equation (1) can be put in the form 



E 



ar- 



(2) 



Differentiating this with respect to / we get for the ac resistance to 

 small signals 



1/" 1 17^ 



(3) 



dE 

 dl 



= ( Jl I "" i T(l/n-l) _ ^ ^ 



\a) n n r 



where E/I is the dc resistance at the particular current at which n also 

 is evaluated. 



Thus, we see that the ac resistance of the varistor is the dc resistance 

 divided by the exponent n, and further, that the ac resistance is con- 



SWITCH 



O.lyUF 



rlDIAL 



O.AbflF 



JT-L 



L2 



^"OW 



Fig. 2 — Circuit schematic of 500D telephone set. 



I 



