752 



BELL SYSTEM TECHNICAL JOURNAL 



which has become almost a tradition in so many phases of telephone 

 work, will be obvious from Fig. 15. The impedance curves for pairs with 

 8-inch and 18.25-inch spacing are similar to those of Fig. 15, the resist- 

 ance values for these spacings being about 45 ohms lower, and 40 ohms 

 higher, respectively, than the values shown for 12-inch spacing. 



Changes of resistance and leakage conductance due to changing 

 weather conditions have very little effect on the characteristic imped- 

 ance at frequencies above 1000 cycles. Changes of insulator capaci- 

 tance due to changing weather conditions or the use of different num- 

 bers or types of insulators have an appreciable effect on the impedance. 

 Deviations from the normal spacings between wires which result from 

 the use of drop bracket transpositions also have an important effect 

 upon the impedance. 



750 



500 



to 



^250 



O 



-250 



10 20 30 



FREQUENCY— KILOCYCLES 



40 



50 



Fig. 16 — Impedance measured on a well-transposed 128-mil open-wire pair with 8-in. 



spacing. 



Like the measured attenuation, the impedance which is measured for 

 an open-wire pair is affected by the presence of line irregularities. 

 Hence, the measured impedance is never a smooth function of fre- 

 quency, but displays slight irregularities throughout the entire range. 

 This is apparent from Fig. 16, which gives a curve of the impedance 

 measured on a well transposed pair. This curve is in remarkably close 

 accord with the generalized values of impedance, the maximum devia- 

 tion from the theoretical curve being about 2 per cent. 



