work associated with that section of Hne in the repeater circuit. A 

 difficulty to be met arises from the fact that telephone currents are 

 very complicated in wave form as previously indicated and involve 

 components varying all the way from 200 or 300 to 2700 cycles per 

 second or more. In order to provide for suitable operation, the 

 condition of balance must be met within a close approximation for 

 currents of all of the frequencies within this range. The reason why 

 this requirement reacts on the construction of telephone lines is very 

 simply illustrated by the curves of Figs. 11 and 12. Figure 11 shows 

 the impedance of a long telephone circuit for all frequencies within 

 that range when the circuit is of very uniform construction throughout. 

 Figure 12 shows the corresponding impedance curve obtained if there 

 are some irregularities in construction in the line. It is possible to 

 design balancing networks which have the same characteristics as 

 those indicated in Fig. 11 for the uniform line but it is not practicable 

 without too great expense to design such networks having the same 

 characteristics as the irregular line shown in Fig. 12. This is true 

 since the characteristics of no two irregular lines are the same, the 

 characteristics varying widely depending upon the nature and the 

 location of the irregularities. 



Means for the general use of repeaters on telephone circuits therefore 

 involved the development by the Bell System of line balancing net- 

 works and the development of long telephone lines with uniform 

 impedance characteristics over the range of frequencies used in 

 telephony. In some cases this could be done by making the lines 

 uniform in construction. For example, loading coils had to be de- 

 signed so that they had, very accurately, equal amounts of inductance 

 and had to be spaced at exact equal intervals along the line. Further- 

 more, it was found that the types of loading coil in previous use were 

 affected by lightning and other causes so that the amounts of induct- 

 ance changed enough to interfere with repeater operation. It was, 

 therefore, necessary to develop new types of coils of very stable 

 materials which would avoid this change in inductance. 



In some cases, it is not possible to construct the line in a uniform 

 way throughout. For example, it is often necessary for open-wire 

 lines to be brought into towns and cities through sections of cable. 

 For such cases, for each type of open-wire construction, a type of 

 cable construction was worked out having such characteristics that it 

 could be connected to the open wire without spoiling the impedance 

 characteristics of the circuit. This involved the development of new 

 loading systems for use on cables of this sort. In the case of circuits 

 entirely in cable, improved uniformity in the manufacture of the cable 



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