188 



Professor J. A. Fleming 



[May 21, 



saving effected in the copper necessary for good speech over a certain 

 distance. 



The Engineering Department of the General Post Office are now 

 thoroughly testing this thermionic repeater and using it in practice. 

 The Engineer-in-Chief has kindly lent an experimental set by which 

 you will be able to hear speech audibly transmitted over an artificial 

 line equal to forty miles of standard cable, which is about the 

 telephonic limit for that size of cable, and note that with the valve 

 repeater the speech over forty miles is as good as the speech over 

 twenty miles of the same cable without the valve. In other words, 

 the valve doubles the range. 



If our trunk telephone line system in Great Britain had to be 



^ $, 



Fig. 23. — Arrangement of Circuits using a Pair of 

 Thermionic Repeaters for Duplex Telephonic 

 Working in both Directions. 



laid over again afresh, it is perfectly certain a very great economy in 

 copper could be made by a widespread use of the thermionic valve as 

 a repeater and relay. 



It repeats so perfectly that we may certainly say it has completely 

 outclassed all previously invented forms of microphonic relay. The 

 longest telephone line in the world is the direct double line from 

 New York to San Francisco, a distance of 3400 miles. In this line 

 there are three places in which thermionic repeaters are inserted, and 

 by means of which telephonic speech is rendered possible over that 

 vast distance. Without the repeaters the copper invested in the line 

 would have had to be largely increased to obtain equal speech 

 efficiencv. 



