in order to provide circuits of high efficiency, as was done in the first 

 cable between Washington, New York, and Boston, the losses were 

 still high because of the close crowding together of conductors in the 

 same circuit and of the fact that even the best insulation which could 

 be provided, namely dry paper, resulted in considerably more energy 

 losses to the telephone conversations than take place on open-wire 

 circuits in which the conductors are separated at very considerable 

 distances by air. 



The use of repeaters in cable circuits made it possible to get high 

 net efficiency over long distances using small conductors, since it was 

 possible to compensate for the relatively large loss by the repeated 

 gains introduced into the circuit by repeaters suitably located about 

 40 to 50 miles apart. That this might be done, however, required a 

 reduction of manufacturing tolerance limits for cables, loading coils, 

 and apparatus and care in the design, construction, and maintenance 

 of the cable circuits. Also, new loading systems were designed which 

 transmitted a broader band of frequencies than those transmitted by 

 the earlier systems. This was desirable both because of the improved 

 clearness of speech resulting from the broader band of frequencies 

 itself and also because the use of the new loading systems made it 

 possible to provide for better repeater balance within the band trans- 

 mitted. 



While these improvements made possible a very large extension in 

 the distances over which good transmission could be given on small 

 gauge cable circuits, it was found that, with many repeaters, the bal- 

 ance difficulties were still sufficient to justify the development and use 

 for the longer circuits of a different arrangement. This arrangement, 

 which is shown schematically in Fig. 14, consists of using for each 

 telephone circuit two pairs or two transmission channels, each equipped 

 simply with one-way amplifiers and thus arranged to transmit the 

 telephone currents in one direction only. Two such one-way channels, 

 oppositely directed, are connected together at the terminals of the 

 circuit by apparatus similar to that used for associating amplifiers 

 with two sections of line in the ordinary telephone repeater, including 

 apparatus for balancing the line or the equipment to which the circuit 

 is connected when in use. The complete circuit is thus reduced at its 

 terminals to two wires like any other telephone circuit. From the 

 fact that it uses two channels for transmission in opposite directions, 

 it is called a four-wire circuit. These channels may, however, be 

 either side or phantom circuits as in the case of an ordinary repeatered 

 telephone circuit. 



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