1020] Thermionic Valve in Wireless Telegraphy & Telephony 187 



LiThe valve thus can be used to counteract the effect of resistance 

 on the line. In practice, however, the arrangements are a little 

 more complicated, because a telephone line has to be used in both 

 directions. This can be done, however, by means of an arrangement 

 used by Mr. Edison for a similar purpose. We have to contrive 

 matters so that telephonic currents arrive in either direction, and 



Trans former 



Transforms 



Grid Battery FtLammt 

 6 ClUt Ba-tUry 



3 oiUs 



Fig. 21. — Telephone Thermionic Repeater. 



yet to avoid letting the plate current of the valve re-act upon its own 

 grid, which would set up a loud howling in the receiving telephones. 

 The scheme of circuits for effecting this is shown in Fig. '1-1. 



The simple arrangement shown in Fig. 22 would, however, only 

 be suitable if the valve was placed exactly in the electrical centre of 

 the line, and this is not always possible. In the actual arrangement 



^ (ribMom a- /a'oocOoi ( 



Fig. 22. — Thermionic Repeater employed with an Edison 

 Circuit in Telephony with Wires. 



each section of the line on either side of the valve is balanced against 

 an equal artificial line, and a pair of valves are used, one of which 

 operates for currents in one direction and the other for the reverse 

 (Fig. 23). 



By the use of such a thermionic repeater, as it is called, speech 

 can be retransmitted with enhanced loudness, and a considerable 



