In order to successfully apply this idea it was necessary to develop 

 repeating coils that were carefully balanced, that is, which had the 

 two halves of their windings very exactly equal in electrical charac- 

 teristics, so that the current from the phantom circuit would divide 

 equally between these two halves of the windings and not influence 

 the other circuits (called the "side" circuits). Also, an improved 

 technique of line construction was necessary in order to avoid high 

 resistance joints and other irregularities in construction which would 

 result in overhearing betvveen the phantom and the side circuits. 

 Furthermore, in order to avoid overhearing between different phantom 

 circuits on the same pole line, it was necessary to interchange not 

 merely the two wires of each pair but also all four wires of the phantom 



H) 



■9 



W 



H) 



Fig. 5 — Diagram of phantom circuit using balanced transformers (called repeating 



coils). 



group as is indicated in the transposition system shown in Fig. 4. 

 This greatly complicated the design of transposition systems. An 

 important feature of the problem is the high degree of balance required 

 since the transfer from the phantom circuit to the side circuit of more 

 than one-millionth of the electrical energ>^ carrying the telephone 

 currents in the phantom circuit or vice versa might be sufficient to 

 make overhearing possible. This high degree of balance was achieved 

 by years of painstaking work and resulted in the first successful 

 phantom circuits in the year 1903. 



Today 12,400,000 miles of wire in the Bell System are installed in 

 such a way as to be suitable for phantom operation. Without phan- 

 toming, 6,200,000 additional miles of wire would be required for the 

 same circuit mileage. 



Superposed Telegraph on Telephone Circuits 

 From the ver>^ beginnings of long distance telephony, the telephone 

 wires were used also for private line telegraph service. At first, 



10 



