JOINT DEVELOPMENT AND NOISE FREQUENCY INDUCTION 197 



situations of proximity. Transpositions operate by neutralizing, in 

 one section, inductive effects which arise in a closely adjacent section. 

 It is evident that, in order for transpositions to l)e fully effective, condi- 

 tions must be substantially alike among the various sections to be 

 neutralized as regards relations of the power and telephone circuits 

 to each other, to ground, and among the various circuits on each line. 

 This latter condition more often applies to the telephone lines, as they 

 usually comprise many circuits. 



These conditions require that balanced and coordinated systems of 

 transpositions be provided between each point of discontinuity in the 

 exposure. By "discontinuity" is meant any point at which an impor- 

 tant change takes place in the physical or electrical conditions of the 

 circuits, such as loads, branch circuits, series impedances, etc.; any 

 change in configuration, in the separation of the two classes of circuit 

 or in their position relative to ground or to some other circuits which 

 may be associated with either power or telephone circuits closely 

 enough to appreciably modify the induction. 



In addition to meeting these conditions, the telephone transpositions 

 must also satisfy the requirements for minimizing cross talk among the 

 various telephone circuits. This, in general, requires telephone trans- 

 position arrangements of considerable complexity. For this purpose 

 standard transposition arrangements are available,* adapted for differ- 

 ent lengths depending upon the distances between the successive dis- 

 continuities. 



In most cases unavoidable irregularities occur in the spacing of poles, 

 in distances between power and telephone circuits, in presence of 

 shielding objects, such as trees, and in height of poles, which it is not 

 possible to treat as discontinuities and take into account in the trans- 

 position design. In cases where these irregularities are large, the effec- 

 tiveness of the transposition arrangements is greatly impaired. The 

 extent to which the effectiveness of such arrangements is imparied due 

 to these non-uniform conditions is a problem not easily susceptible to 

 mathematical analysis and reliable information is not now available. 

 The subcommittee is planning to investigate this problem experi- 

 mentally by tests on a number of situations involving operating circuits. 

 Susceptiveness Factors.— The degree to which telephone transmis- 

 sion is adversely affected by noise-frequency induction depends not only 

 upon the magnitudes of the induced voltages as determined by influence 

 and coupling factors, but also upon the susceptiveness factors of the 

 telephone system. These include the manner in which the induced 

 voltages and currents are propagated to the circuit terminals together 

 with the reactions of the circuit unbalances, thus relating the current 



