CONQUEST OF DISTANCE BY WIRE TELEPHONY 385 



uses with loaded open-wire lines. Later on, when the 22-repeater circuit 

 was used on non-loaded Imes, new types of simple basic networks were 

 devised to simulate the impedance of the non-loaded lines. It is also appro- 

 priate at this point to mention the fact that when commercial service started 

 with 22-repeater circuits, the balancing networks of the repeaters included 

 not only the basic networks, and buildmg-out devices when required, but 

 also apparatus for balancing line terminal apparatus which otherwise would 

 have contributed objectionable impedance irregularities to the line. Such 

 auxiliary apparatus usually included a repeating coil to balance the line 

 repeating coil, and a simple 4-terminal network to balance the composite 

 telegraph sets, when involved. 



An additional very important new feature provided in 1912 for the 22- 

 repeater circuits was the use of a low-pass electrical wave filter in the 

 branches of the circuit where each repeater element functions as a one-way 

 amplifier. As their cut-off frequency was about 300 cycles below that of 

 the loading cut-off, these filters suppressed the unwanted and unneeded 

 frequencies near and above the loading cut-off, thereby making these fre- 

 quencies negligible factors in repeater singing phenomena. This was of 

 special importance because the simple basic balancing networks above 

 described did not simulate loaded line impedance at these frequencies, and 

 had added importance where vacuum tube repeaters were involved in 

 consequence of their tendency to amplify the same amount at all frequencies. 



Subsequently, low-frequency filters were included in the 22-circuit to 

 suppress unwanted frequencies below approximately 250 cycles, in which 

 band are occasionally present currents resulting from the operation of super- 

 posed telegraph circuits, and noise current produced by induction from 

 power circuits. Line irregularities are not important factors in the repeater 

 balance problem at these frequencies when proper t^-pes of basic* networks 

 are used. 



The line experiments made with the improved mechanical repeater and 

 the mercury arc and vacuum tube repeaters and the commercial installa- 

 tions previously mentioned used the 22-t}'pe repeater circuit. Different 

 t}'pes of auxiliary apparatus (input and output transformers, etc.) were of 

 course required to obtain optimum results with the different types of re- 

 peater elements. \Mien the experiments involving tandem 22-repeaters 

 showed objectionable impedance irregularities to be caused by the impe- 

 dances presented by the repeaters to the line, improvements in the repeaters 

 were made to reduce these effects. 



In due course, the 22-repeater became the standard two-way repeater. 

 The use of the 21-repeater was restricted to special situations not requiring 

 more than one repeater of relatively low gam and located near the middle 

 of the circuit. 



