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BELL SYSTEM TECHNICAL JOURNAL 



This consists essentially of first measuring the input and output a-c. currents 

 at the two ends of the test pair by means of thermocouple-milliammeter 

 arrangements and then immediately sending d-c. over another pair (called 

 reference pair in Fig. 1) built out to a convenient fixed d-c. resistance, the 

 same for all measurements, and adjusting resistance networks at both 

 ends of the line until the meter readings are the same as for the a-c. Suit- 

 able charts then enable readings of attenuation (insertion loss) corresponding 

 to the d-c. (and therefore also to the a-c.) readings to be made very 

 rapidly. 



Fig. 1 — Simplified attenuation measuring circuit 



Toll Cable Characteristics Below 100 Kc. 

 Primary Line Parameters 



The four primary line parameters, R, L, G and C — series resistance, series 

 inductance, shunt leakage and shunt capacitance — are of the same sort for 

 all kinds of transmission lines, but the relative importance of the various 

 elements changes considerably with frequency and the type of structure 

 considered. The old name primary "constants" is obviously a misnomer, 

 and it is simpler to speak of them as line "parameters," since this does not 

 necessarily imply anything regarding their constancy or inconstancy under 

 various conditions. 



The "true" or distributed values of these parameters are usually obtained 

 from measurements of the open circuit admittance, G' -\-jojC', and the short- 

 circuit impedance, R' -+- jcoL' of the actual pair in a short length of cable, 

 by means of the following formulas: 



