242 Mr Appleton, A metliod of testing Triode Vacuum Tubes 



The definiteness of the point of null telephone resjjonse depends 

 on the fidelity with which the anode current changes follow the 

 grid voltage variations. The circuit is therefore suggested as a 

 possible method of testing whether any temporal 'lag' exists be- 

 tween these two quantities for any particular tube. So far as tubes 

 of extreme exhaustion have been tested no such inertia has been 

 found to exist. For cases of ultra-acoustic frequencies the ordinary 

 wireless methods of testing the constancy of the potential across 

 CD are applicable. 



In addition to its use as an amplifier the vacuum tube has a 

 distinct field of utility as a detector of high-frequency oscillations. 

 When operation takes place with conditions represented by a 

 curved portion of the characteristic a symmetrical grid voltage 

 change produces a variation in the mean value of the anode cur- 

 rent. As a first approximation in such a case one may consider 

 that the magnitude of the anode current alteration is proportional 

 to the value of d^jdv^ and also to the square of the amplitude of 

 the grid voltage variation. 



It is possible to determine the value of dHjdv^ directly. To do 

 this the alternator of Fig. 1 is replaced by a small voltage battery 

 (positive to grid) and tapping key, and the resistance CD by a 

 milli-ammeter. The resistance R is adjusted so that the milli- 

 ammeter deflection is unaltered when the key is closed. If the 

 value of the resistance so obtained is R^ , we have approximately 

 1 dl IdH ^ 



i; = ^ + 2^-^"' (1) 



where Av is the e.m.f. of the small battery. 



If the same adjustment is now made with the battery reversed 

 (negative to grid) we have 



1 dl Id^I ^ 



Rrdv~2w^-^''- (2) 



Thus from (1) and (2) we have 



dllfl^ l\ 

 dv~ 2\R^^ RJ ' 



and ^=fi__ M.i_ 



dv^ Ui RJ Av' 



With such an arrangement the results are accurate to within 

 3 or 4 per cent. If extreme accuracy is required the milli-ammeter 

 may be replaced by a resistance r (see Fig. 1) and a potentiometric 

 scheme and sensitive galvanometer used to indicate when the 

 voltage drop along CD is constant. 



