CONQUEST OF DISTANCE BY WIRE TELEPHONY 383 



important commercially in the application of repeaters to long loaded, 

 small-gauge, toll cables. 



The maximum repeater gain which could be obtained in two-way circuits 

 without sustained singing that would block transmission, or near-singing 

 that would degrade intelligibility, was expressed by Campbell as a function 

 of the differences of the impedances of the circuits involved. 



In the 21-circuit, the single repeater element amplifies transmission in 

 both directions, and its usable gain is a function of the differences of the 

 impedances of the circuits between which it works. 



In the 22-circuit where the two repeater elements each function as one- 

 way repeaters, the usable gain is a function of the differences of the impe- 

 dances of the line East and the artificial line required to balance it, and of 

 the differences of the impedances of the line West and its own balancing 

 artificial line. In the general case, involving lines with irregular impedance- 

 frequency characteristics, twice the power amplification feasible with the 

 21-circuit would be allowable if the lines should be connected through a 

 22-circuit, assuming the use of balancing lines having "average" impe- 

 dances as described later. Statistically, the average balance obtainable 

 between any one line of a given t\"pe and the "average" balancing line here 

 assumed in the 22-circuit is 3 db better than the average balance obtainable 

 between any single line and others of its type as involved in the use of the 

 21-circuit. In the limiting theoretical case, which may be approached but 

 not attained in practice, if one of the lines using a 22-repeater should be 

 perfectly balanced by its associated artificial line, singing could not occur 

 at that repeater irrespective of the degree of the unbalance between the 

 impedance of the other line and its associated artificial line. 



The foregoing discussion is adequately summarized in the statement that 

 when high repeater gains are required the lines using the 22-repeater do not 

 need to be so uniform in their impedance frequency characteristics as would 

 be necessary with 21 -repeaters. This was very important in the trans- 

 continental telephone project because of the serious practical difificulties 

 involved in the reduction of line impedance irregularities to very small 

 values, as discussed later. 



Furthermore, the 22-circuit has an overwhelming superiorit}- in stability 

 over the 21-circuit, under conditions that require the use of repeaters in 

 tandem. This follows from its characteristic property of transmitting the 

 amplified energ}- in one direction only — the desired direction away from the 

 source — whereas each 21-repeater transmits in both directions and one half 

 of its amplified energ}-- starts backwards towards the source, thereby setting 

 up among the successive repeaters objectionable circulating currents which 

 impose severe restrictions on the repeater gains. 



