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BELL SYSTEM TECHNICAL JOURNAL 



mile as compared with about .062 microfarad for conventional cable 

 pairs. This permits more practical loading coil spacings. These 

 disc-insulated units are made up as spiral-fours, that is, two pairs 

 (.051" diameter wire) which form the diagonals of a square. When 

 these cables are loaded with small coils at intervals of approximately 



Fig. 6 — Disc-insulated cable. Sheath diameter 2.3 inches. 



600 feet, they present impedance characteristics substantially equiva- 

 lent to that of an open-wire pair over the desired frequency range. 

 Accordingly, they form a desirable, although somewhat expensive, 

 solution of the problem of intermediate or entrance cables. As shown 

 in Fig. 6, the spiral-four units are bound together in complements of 

 seven or less under a lead cable sheath similar to standard toll cables. 

 It should be noted that the low-capacity disc-insulated loaded cables 

 not only provide a satisfactory solution of the impedance matching 



