LOADING rOR TELEPHONE CIRCUITS 



TABLE III 



First Standard Open Wire Loading 



229 



Note. Transmission efficiency figures assume dry weather insulation conditions, 

 5 megohm-miles, or better. 



lated fine wires on a stiitably shaped spool. The core wire was 38 

 A. w. g. (0.004 in. diameter). 



The wire used in the cable loading coil cores was a commercial 

 grade of mild steel, hard drawn under conditions which gave it an 

 initial permeability of 95. The term "initial permeability" signifies 

 the permeability at very weak magnetizing forces; i.e., below 0.1 gilbert 

 per cm. The core wire used in the open wire loading coils was drawn 

 from the same stock, but difTerences in the drawing and annealing 

 treatments gave it an initial permeability of about 65. This core 

 wire had lower eddy current and hysteresis losses than the 95-perme- 

 ability wire. A black enamel insulation was used on the 95-permeabil- 

 ity wire. A celluloid-shellac compound which could be applied at a 

 lower temperature was used on the 65-permeability wire. 



As illustrating the magnitudes involved, it may be noted that in 

 order to meet the service requirements, the coils were designed so 

 that for telephone currents of the order of 0.001 ampere, the magne- 

 tizing force H has a value of about 0.04 gilbert per cm., corresponding 

 to a flux density of approximately B = 2 gauss. 



The winding space on the cores was divided in half by means of 

 fiber washers, and the winding was applied in two equal sections, one 

 being located on each half of the core. In installing the coils, one of 

 these windings was inserted in one line wire and the other winding 

 in the other line wire, so connected that the mutual inductance be- 

 tween windings aided the self-inductance for current flowing around 

 the circuit through both windings. 



The high costs of the open wire lines warranted considerable re- 

 finement in the design of the open wire coils. They were, therefore, 

 made much more efficient and correspondingly larger than the cable 

 coils. They were wound with insulated stranded wire and had much 



