THE i.o.miii) siHM.iKixii Ti-.i.i.c.i<.\t'ii c.im.r. m 



for a triinsoi-iMiiif tfli'Krapli c.ihk' and it is pritnariK' with ri'^;ar(l t<i 

 roiitinunus loading that the following discussion is ronrurni-d. 



KiKIXTS OF I.OADINt, 



Most of the proposals to load telegraph cables have had the ol)j«^c^ 

 of reducing; or eliminating distortion, and accordingly most of the 

 mathematical treatments of loading have heen from that point of 

 view. The reduction of distortion is, however, not the only benefit 

 to Ik- obtained from loading and, in fact, may not always be secured 

 in the high speed operation of a loaded cable. The principal benefit 

 of loading from the practical standpoint is to decrease the attenua- 

 tion of the signals so that for a given freciuency more current will be 

 received or so that the minimum permissible current may be received 

 with a greater speed of signalling. From the mathematical stand- 

 point there are two ways of treating the problem of the loaded cable, 

 first with regard to the transmission of a transient impulse, and 

 secontl with regard to setting up steady alternating currents of definite 

 frequency. In the ultimate analysts the solution of either problem 

 can be got from the other. However, for practical purposes they 

 are two distinct means of attack. Which should be used depends 

 on the object to be secured. If one is concerned primarily with the 

 effect of the cable on the wave shape of the signal transmitted over 

 it, it is fairly obvious that the transient treatment has advantages. 

 If, however, one is concerned only with the strength of the received 

 signal, as is the case if there is assurance that the signal shape can in 

 any event be corrected by terminal networks, then the steady state 

 treatment is sufficient and much more convenient to apply. In 

 the case of the real loaded cable the complete transient solution is 

 extremely comple.x and the steady state treatment relatively simple. 

 The solution of the transient problem of an ideal loaded cable is, 

 however, very valuable to give a physical picture of how inductive 

 loading aids the high speed transmission of signals. 



The transient solution of the problem of an i^deal heavily loaded 

 cable has been worked out by Malcolm * and more rigorously by 

 Carson ', who have determined the curve showing the change of 

 current with time at one end of the cable if a steady c.m.f. is applied 

 at zero time between the cable and earth at the distant end. Such a 

 curve is called an "arrival curve" and for an ideal loaded cable com- 

 prising only constant distributed resistance, capacity and inductance 

 may have a form like that shown in Curve b of Fig. ',i, which is to be 



•Theory of the Submarine Telegraph and Telephone Calile, I-onclon, 1917. 



•Trans. A. I. E. E., Vol. 38, p. 345, 1919. 



