168 BFXL SYSTEM TECHNICAL JOURNAL 



The above figures are based on favorable conditions for obtaining 

 low resistance ground connections on the cable sheaths. Such ground 

 connections are necessary to provide the full shielding benefits, since the 

 shielding is brought about by induced currents on the cable sheath 

 flowing along the sheath and through ground. These sheath currents, 

 because of the close coupling between the sheath and pairs, induce 

 voltages into the pairs tending to neutralize the voltages induced into 

 the pairs directly from the power system. The use of the tape armor, 

 which is a magnetic material, increases the coupling between the sheath 

 and pairs. The grounding conditions necessary for satisfactory shield- 

 ing effects can usually be obtained, but situations sometimes arise in 

 the case of aerial construction where it is difficult or impossible to 

 obtain them. 



While as noted above, the cable circuits are effectively protected 

 from noise induction, the efficiency obtainable over the long circuits is 

 limited in part by the noise currents occurring in the open-wire lines 

 which may be switched to the long cable circuits. This is because the 

 efficiency of the long cable circuits depends upon voice-operated 

 switching devices which must not be operated by the noise currents. 

 This is also true of the intercontinental circuits mentioned above. The 

 extension of the circuits controlled by voice-operated devices tends 

 therefore to increase the importance of good coordination of the entire 

 plant. 



Telephone Repeaters. — Another important trend of practise is the 

 extended use of telephone repeaters. The purpose of these devices is 

 to amplify the voice currents and thus make possible higher efficiency 

 and greater extension of long distance telephone circuits. Their use 

 is essential to the great development of toll cable. Moreover, they are 

 used widely on open-wire circuits. Without repeaters it was necessary 

 on the long open wire circuits to permit the power level of voice currents 

 to sink to relatively low values. An extreme example of this is given 

 by the New York-Denver circuit which, before repeaters were available 

 for use on this circuit, had an overall equivalent, using the highest 

 grade of telephone construction which had been developed up to that 

 time, of about 31 dhJ With the application of repeaters to this circuit 

 the level of voice currents could be kept relatively high throughout the 

 circuit. This is illustrated in Fig. 6 giving level diagrams for the cir- 

 cuit as originally set up and later when provided with repeaters. 



The use of repeaters contributes to reducing the susceptiveness of 

 the telephone plant and thus aids coordination. On such a circuit as 

 the original New York-Denver circuit just mentioned, a relatively 



' This means that the ratio of output power to input power of this circuit is O.OOOS. 



