current uses the same transmission path as the voice currents hut the 

 system is designed to discriminate between the voice and signaHng 

 current. The signaling current is transmitted from end to end over 

 the circuit without the use of intermediate ringers which had been 

 resorted to on the 135-cycle ringing system. This 1,000-cycle system 

 has extended rapidly in its field of use and is now used for the majority 

 of circuits over 100-150 miles in length. 



Telegraph System 



The early methods by which provision was made for the simultaneous 

 use of telephone circuits for telephone and telegraph service were 

 described in the section on " Early Developments." With the general 

 extension in the telephone plant of the improved types of telephone 

 circuits described above, and with the growth in extent and require- 

 ments of the private line telegraph service, it became important to 

 devise new types of telegraph circuits adaptable for use with the new 

 types of telephone circuit and suitable to meet the increased telegraph 

 requirements. 



One such new form of telegraph circuit was the metallic telegraph 

 system designed for use on telephone toll cables simultaneously with 

 the use of the same conductors for telephone service. In order to 

 avoid interference betw^een the telephone and telegraph circuits it 

 was necessary to use relatively low voltages and currents on the 

 telegraph circuit. With these low voltages and currents grounded 

 telegraph circuits were impracticable because of outside interference 

 and it was necessary to use metallic circuits in which no use was made 

 of the ground for the transmission path. With the new metallic 

 circuits voltages of 34 volts and currents of 4 milliamperes were used 

 compared with 130 volts and 60 milliamperes in the grounded direct- 

 current telegraph systems. 



Another form of telegraph circuit which was developed for use over 

 telephone toll cables is the so-called voice-frequency telegraph system. 

 With this system by carrier current methods the telephone channel is 

 split up into 12 telegraph channels, each of which is suitable for use as 

 an independent telegraph circuit, one telephone circuit thus providing 

 12 telegraph circuits. In this case, the telephone circuit cannot be 

 used simultaneously for telephone and telegraph circuits. This 

 system is designed to be applied either to a four-wire cable circuit, or 

 to a carrier telephone circuit which, like the four-wire cable circuit, 

 consists of two channels for transmission in the opposite directions. 

 It has large advantages for long circuits because of the fact that no 

 apparatus is required at points intermediate between the terminals, 



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