The type of circuit arrangement most commonly used to associate 

 two repeater elements with two sections of telephone line in such a 

 way that the operation will be satisfactory is shown schematically in 

 Fig. 10. This is known as a 22-type repeater. Attenuated telephone 

 current transmitted from a distant point over the west section of line 

 passing through the transformer ^ of a special design (sometimes called 

 a hybrid coil) enters the input of amplifier element B and is amplified. 

 From the output of amplifier element B it passes through a second 

 transformer C associated with the east section of line and a balancing 

 network E. An essential function of the repeater circuit lies in the 

 design of the transformer in such a way that the two halves (between 

 the midpoints of which the input to the amplifier element D is con- 

 nected) are equal and in the design of the east line and of the balancing 

 network E in such a way that they offer the same impedance to the 



LINE 

 WEST 



^()QQ J (J^Q^ NETWORK 



-viKKLr 



^sm^r- 



REPEATER 

 ELEMENTS 



LINE 

 EAST 





Fig. 10 — Schematic 22-type telephone repeater circuit. 



flow of current. Coil C is so designed that if this condition is exactly 

 met, none of the ouput current from amplifier element B is transmitted 

 across to the input element of amplifier D. However, currents trans- 

 mitted in from the east section of line pass through Coil C to amplifier 

 D, are amplified and retransmitted to Coil A where the condition of 

 balance also must be applied between the west line and the balancing 

 network W. 



The complete repeater circuit includes many other things beside 

 the bare essential elements shown in Fig. 10. Important among these 

 are electrical filters to control the band of frequencies, potentiometers 

 to control the amplification, transformers to efficiently interconnect 

 different parts of the circuit, equalizers (see chapter on Associated 

 Technical Developments) and arrangements for the supply of electric 

 power to the vacuum tubes. 



Obtaining a condition of balance requires that each section of line 

 offer the same impedance to the flow of current as the balancing net- 



18 



