OPEN-WIRE TELEPHONE LINES 



757 



Characteristics of Iron-Wire Circuits 



There now exists on the toll lines of the Bell System a small amount 

 of galvanized iron or steel wire. Nos. 12 and 14 B. W. G., with diam- 

 eters of 109 and 83 mils, respectively, are the gages most commonly 

 found. Steel wire and BB iron wire are both used, the latter being of 

 more frequent occurrence. These iron-wire pairs display such large 

 values of modulation and high-frequency attenuation that they are 

 generally quite unsuitable for carrier transmission. 



The skin effect resistance of iron wire may be computed by standard 

 theory, provided that the values of the resistivity p and permeability 

 fx are known. The values of p for BB iron and steel wire are about 12.8 



4.0 



UJ 



-I 3.0 



a. 



Hi 



a. 



CO 



a 

 I 



z 

 o 



D 

 Z 

 Hi 



^- 



2.0 



1.0 



10 20 30 



FREQUENCY- KILOCYCLES 



40 



50 



Fig. 20 — Attenuation of iron-wire circuits. 



and 14.8 microhm-centimeters respectively. It will be noted that these 

 values are about seven and eight times the resistivity of copper. For 

 currents of telephonic magnitude at frequencies from about 500 to 

 50,000 cycles, it has been found that the value of the permeability y. of 

 iron wire of the above sizes ranges from about 110 to 50. Study of the 

 available data indicates that the best average value of /x at 1000 cycles 

 is about 85 and that the effective value decreases with increasing fre- 

 quency. Because of the high d.-c. resistance and the large skin effect 

 ratio which results from the high permeability, the resistance of iron 

 wire for alternating currents is extremely great. 



