. The Bell System technical journal . ofside circuits of phantoms. In this case the transposition pattern maychange when the side circuit changes pin positions at a phantomtransposition. The fundamental types may be extended to involve 64, 128, 256,etc., transposition poles. Types involving 128 transposition poles areoften used. A long line, say 100 miles, is divided into short lengths calledtransposition sections. With the latest transposition designs, sectionshaving 128, 64, 32, 16 and 8 transposition poles are provided. Thenominal lengths of these sections vary from to .25 mile. Thepurp
. The Bell System technical journal . ofside circuits of phantoms. In this case the transposition pattern maychange when the side circuit changes pin positions at a phantomtransposition. The fundamental types may be extended to involve 64, 128, 256,etc., transposition poles. Types involving 128 transposition poles areoften used. A long line, say 100 miles, is divided into short lengths calledtransposition sections. With the latest transposition designs, sectionshaving 128, 64, 32, 16 and 8 transposition poles are provided. Thenominal lengths of these sections vary from to .25 mile. Thepurpose of these sections is to provide an approximate balance againstcrosstalk (and induction from power circuits) in short lengths andthus to allow for unavoidable discontinuities in the exposure between 216 BELL SYSTEM TECHNICAL JOURNAL circuits such, for example, as points where circuits branch off the arrangements must be chosen for each circuit in eachtype of section to ensure this approximate crosstalk Fig. 28—Fundamental types for 32 transposition poles. Certain lines have few, if any, discontinuities and a succession of thelongest type of section is used. To improve the effectiveness of thetranspositions, junction transpositions are used at the junctions ofsuccessive similar sections. For such lines it would be more effectiveto use longer transposition sections and not require that all circuits beapproximately balanced in a short length. Such a special designwould be impracticable, however, since it would be too inflexible inregard to circuit changes, etc. In choosing the transposition arrangements for a section it mustbe kept in mind that the object is to meet certain crosstalk limits fora succession of sections considering both type unbalance and irregu-larity crosstalk. The method of procedure is discussed below. OPEN-WIRE CROSSTALK 217 Evolution of Transposition Designs In designing transposition systems it must be kept in mind thatmuch o
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