238 BELL SYSTEM TECHNICAL JOURNAL 



It is interesting to note in this connection that, though, in most 

 previous proposals to load long telegraph cables, loading had been 

 advocated primarily as a means of reducing distortion, practical 

 consideration of the problem uncovered new types of distortion which 

 were absent in the non-loaded cable. The nature of distortion of 

 signals by a non-loaded cable was well understood, the problem having 

 been solved long ago by Lord Kelvin. The distortion of a loaded 

 cable is a much more complex affair since there are involved in it 

 not only the effects of distributed inductance, capacity, resistance 

 and leakance of the ideal cable for which the distortion is readily 

 calculable, but also the factors of change of inductance and resistance 

 with frequency and current, and the effects of magnetic hysteresis 

 which are unavoidable in a practical loaded cable. Though the 

 effect of these factors on distortion could be approximated by theo- 

 retical analysis it was considered necessary to have experimental 

 proof that a signal could be restored in shape after passing over a 

 loaded cable and it was primarily on this account that tests were 

 made with an artificial loaded line. These tests showed that even 

 the distortion of a loaded cable could be corrected by using suitable 

 terminal networks in connection with the vacuum tube amplifier. 



With the factor of distortion thus eliminated there remained duplex 

 balance, sending voltage and received interference as possible limits 

 to the speed of the loaded cable. 



Duplex balance would, of course, set the limit of speed of operation 

 if the cable were to be operated simultaneously in two directions as 

 is commonly done with non-loaded cables, since it would obviously 

 be more difficult to build an artificial line electrically equivalent to a 

 loaded cable with its variable inductance and resistance than one 

 equivalent to a non-loaded cable in which only resistance and capacity 

 have to be considered. Even with non-loaded cables the difficulty of 

 balancing is so great that the double-duplex speed is usually much 

 less than twice the possible simplex speed and with the loaded cable, 

 which is more difficult to balance, the relative gain in traffic capacity 

 to be obtained by duplexing is certain to be less than with non-loaded 

 cables. On the other hand, simplex, or one-way, operation offers 

 very great advantages especially when used in connection with 

 automatic operation, since it disposes of the necessity for an intricate 

 and costly artificial line and permits dividing the full traffic capacity 

 of the cable most efficiently to accommodate the traffic it must carry, 

 which with most transoceanic cables is usually unequal in the two 

 directions. For these reasons it was decided to design the first loaded 

 cable primarily to secure efficient simplex operation. Subsequent 



