CARRIER IN CABLE 



259 



pilot wire circuit exposed to the same temperature variations can be 

 used to control gains and equalizer adjustments to overcome the 

 effect of this temperature variation. Fig. 7 shows a schematic 

 diagram of the pilot wire transmission regulation system used in the 

 Morristown experiments, while the photograph of Fig. 8 indicates the 

 appearance of the apparatus. This pilot wire regulation system takes 

 care of a 25-mile length of cable. The arrangement of the regulating 

 networks is such that variation of a single resistance causes the trans- 

 mission loss to be varied a different amount at different frequencies 



a. 1.25 



10 15 20 25 30 35 4 



FREQUENCY IN KILOCYCLES 



Fig. 6 — Transmission loss of 16-gauge cable pair. 



as required by the variation in the line loss shown in Fig. 6 above. 

 In Fig. 7 the relay system is omitted for the sake of simplicity. The 

 function of the relay system is, of course, to control the rotation of 

 the shaft carrying the variable resistances so that it follows the rota- 

 tion of the shaft associated with the master mechanism. The centering 

 cam is provided to avoid "hunting." 



The Morristown experiments have shown that this form of regulation 

 is adequate when underground cables are employed. Similar regula- 

 tion of aerial cables in which the transmission variation with time is 

 three times as large and several hundred times as rapid presents 

 greater but not insuperable difficulties. 



