240 Mr Appleton, A method of testing Triode Vacuum Tubes 



which the conductance is required. To determine the value of dljdv 

 the resistance R is varied until the potential difference between the 

 points C and D is constant. The value of dljdv is then given simply 

 as IjR. An amplifying-telephone is connected across the resistance 

 r, the constancy of the potential difference between C and D being 

 indicated by a note of minimum intensity in the telephone. 



To determine the actual voltage amplification obtainable in 

 any particular case (e.g. in a resistance-coupled amplifier such as 

 has been suggested for the measurement of small ionization cur- 

 rents) a suitable high resistance Rq may be included in the anode 

 circuit while the test is made. The voltage amplification then avail- 

 able is given hj Rq-^ . 



To 

 Amplifier 



Fig. 1. 



Fig. 2 shows some typical results obtained with a hard valve 

 (Type K). For comparison the actual v-I characteristic is shown 

 in the same figure. 



Proof of Formula. Before the alternator current is switched on 

 the potential difference between the ends of the resistance r is seen 



Rr 

 *° ^® ^0 .RT7 ^^®^® ^^ ^^ *^® stationary value of the thermionic 

 anode current. If the alternating voltage appUed between the grid 

 and filament is v^mip the change in the potential difference 

 between C and D may be regarded as the algebraic sum of two 

 oppositely directed changes. The first is due to the direct effect of 



the appHed alternating voltage and is equal to ^"^o ^^P ^^^ ^^^^^^ 



T -\- R 

 may be regarded as due to the alteration of anode current from Z^ 



