THERMIONIC VACUUM TUBES 



67 



wise the output impedance of the amplifier should equal 2R±. The 

 input impedance of the vacuum tube voltmeter is so high as not to 

 shunt the resistances across which it is connected appreciably. The 

 grounds at the mid points eliminate the disturbing effects of capacities 

 to ground. Under these conditions the voltage amplification a is 

 given by the equation : 



= ar 4R 3 (2R 1 + R 2 )+2R 1 R 2 ^L 

 a pL2R 3 {2R i +R2)+2R 1 R 2 J' 



in which a, /3 are the fractions of Ri and Ri respectively, across which 

 the voltmeter is connected to obtain equal readings when the switch 

 W is thrown from one position to the other. In case R 2 is made quite 

 small with respect to R it R 3 , the expression for a reduces approximately 



2a 



to a= — . An expression for current amplification can readily be 



derived. 



Another simple measuring circuit is shown in Fig. 34a in which 

 is an oscillator of the desired frequency, F is a filter to remove 

 harmonics from the oscillator current, R\R 2 and R 3 R± are attenuating 

 networks consisting of resistances, WWW are switches by which the 

 telephone receiver T can be joined either to the output of the amplifier 



Fig. 34a 



or directly to the oscillator, and also provide means for removing the 

 known attenuation in the shunt Sh (receiver shunt) at the same time 

 the amplifier is removed. By the proper design of the receiver shunt, 

 which will be discussed presently, the attenuation required to give the 

 same volume of sound in the receiver whether the amplifier is in or 

 out may be read directly. 



In setting up the circuit of Fig. 34a, special attention must be given 

 to the networks R]R 2 and R 3 Ri. In addition to reducing the input 

 to the amplifier to a value safely below the overload point, R\R 2 should 

 be designed to present an impedance (when seen from the amplifier) 

 equal to that out of which the amplifier is to operate in service. Other- 

 wise the measurements of amplification may be without significance. 



