68 BELL SYSTEM TECHNICAL JOURNAL 



I. Different Types of Circuits 



The different types of circuits used in toll cables are illustrated in 

 diagrammatic form in Figure 1. Circuit "b" is a two-wire telephone 

 circuit employing a 21-type telephone repeater. This type of cir- 

 cuit is employed only for handling connections on which but one 

 telephone repeater is involved. Circuit "c" is a typical two-wire 

 circuit on which the familiar 22-type telephone repeaters are operated. 

 Circuit "d" is of the four-wire type which employs two transmission 

 paths, one for each direction. The function of the pilot wire cir- 

 cuits, "a," will be taken up later. 



With the exception of circuit "b", which possesses the limitation 

 that it cannot advantageously be connected to another circuit con- 

 taining telephone repeaters, the circuits shown in the figure may be 

 connected when required to circuits of the same or other types, such 

 as open-wire circuits, to build up various telephone connections. In 

 general, circuits such as "c", employing 22-type repeaters, are used 

 for handling connections of moderate lengths, while circuits such as 

 "d", of the four-wire type, are employed for the longer connections 

 where the transmission requirements are more severe. 



In addition to employing the cable conductors for furnishing tele- 

 phone service, these may also be arranged to furnish D.C. telegraph 

 service. Apparatus for compositing the circuits so as to permit this 

 superposition of the D.C. telegraph is indicated on the drawing. In 

 general, the method of compositing the small gauge cable circuits is 

 the same as that employed for compositing open-wire lines. The 

 telegraph circuits in cable, however, operate with a metallic instead 

 of a grounded return and employ much weaker currents than those 

 common on open wires. Telegraph currents employed in the cables 

 are comparable in magnitude with the voice currents. 



The two-wire circuits in toll cables employ conductors of No. 19 

 or No. 16 American wire gauge, while for the four-wire circuits, No. 

 19 gauge conductors are usually employed. (No. 19 gauge weighs 

 20^ pounds per wire mile or 5.8 kilograms per kilometer. No. 16 

 gauge weighs twice as much). 



II. Loading Characteristics 



Two weights of loading are usually employed. These are com- 

 monly known as "medium heavy loading" and "extra light loading" 

 and in this paper they will be referred to for brevity as "M.H.L." 

 and "X.L.L." respectively. The medium heavy loading employs 

 coils having an inductance of about 0.175 henry in the side circuits, 



