OPERATION OF THERMIONIC VACUUM TUBE CIRCUITS 449 



lead one to believe. The method of arriving at the correct value 

 for Bgd, and hence for r^ and rg' is one of trial and error, for, after 

 several recalculations of Cgd have been made, it will be found that 

 check results are secured. Then rg and r/ may be determined from 

 this resulting value of Ego. 



In actually making these measurements, a dynamical method of 

 measuring rg will usually be found superior to the method of drawing 

 tangents to the static characteristic, for the grid-potential grid- 

 current characteristic of any tube is rather elusive because of the 



X 



co.ooo 



50,000 



40,000 



30.000 



?0,000 



10,000 



4 fc 8 



Fig. 4 — Grid resistance 



10 Eg- VOLTS 



very small values of current involved. In the dynamic method a 

 Wheatstone bridge circuit excited by a high frequency buzzer will 

 be found convenient. The value of rg' is, of course, obtained by 

 drawing tangents to the rg curve. Several examples of rg — Eg curves 

 are shown in P"ig. 4. 



It must be recognized that, for large values of buzzer excitation, 

 the dynamic value of rg differs somewhat from that found by drawing 

 tangents to the static characteristic. The dynamic value more nearly 



