currents flowing in one circuit will cause similar, weaker currents to 

 flow in the other circuits. This is called induction or crosstalk and 

 experience with the New York-Philadelphia line showed that so much 

 induction between telephone circuits was obtained that intelligible 

 crosstalk resulted; in other words, one could overhear on one circuit 

 what was said on the others. To overcome this, systems were de- 

 veloped whereby, by suitably interchanging positions of the wires of a 

 circuit, the inductive effects in that circuit from an adjacent circuit 

 would tend to neutralize. This is illustrated in Fig. 2 for a simple 

 case of two circuits. In this figure the circuit sections "a" and "ft" 

 are equal in length, and voltages are being induced into circuit No. 2 

 from circuit No. 1. The arrows show the directions in which the 

 induced current would tend to flow in circuit No. 2, the wires of which 

 are interchanged in position between the two sections. It will be 

 noted that the induced voltage in section "6" is equal in magnitude 

 to that in section "a" and, by the interchanging of wires at the 



Fig. 2 — Side or non-phantomed circuit transposition. 



junction, is made to oppose that of section "a." The eff'ect of this 

 interchange or transposing of wires is such as to neutralize the in- 

 duction in sections "a" and "6" appearing at the circuit terminals. 

 With a large number of circuits, the induction between each two 

 circuits must be neutralized in each short section of line, and to ac- 

 complish this, more complicated arrangements, known as transposition 

 systems, were developed. The first system of this sort was worked 

 out by J. A. Barrett of the Bell System in 1886. The development of 

 transposition systems has continued constantly since that time, the 

 problem changing with the increase in the number of circuits on a line, 

 developments in the transmission of electrical power on lines which 

 sometimes are constructed near the telephone lines, the introduction 

 of phantom telephone circuits, and of repeaters and carrier telephone 

 circuits into the plant. Figure 3 shows a typical transposition system 

 in use in the Bell System today. 



7 



