326 Mr. G. A. Campbell on Loaded 



actual relation between gross costs and effective time con- 

 stants at telephonic frequencies, and a variety of practical 

 requirements are involved which will materially modify the 

 above results. In special cases it may be necessary to connect a 

 loaded line of high impedance directly to an unloaded line 

 or terminal apparatus designed for present lines. Diagram I. 

 (PL V.) will give the reflexion loss and reduction in range. 

 From the formulae already deduced, the proportions may be 

 determined for maximum efficiency with a given total weight 

 of copper and given terminal conditions. 



Experimental Work. 



In January 1899, I was assigned the problem of investi- 

 gating the possibilities of improving the efficiency of cables 

 for telephonic service. After considering some other methods 

 I concluded that the loaded line presented the greatest pro- 

 mise, and, as I felt that more progress would be made by 

 experimental tests than by mathematical work, I immediately 

 planned to have made an artificial line with 100 loading coils 

 on a twenty-mile cable circuit. Before this line was com- 

 pleted, becoming more confident of the success of loading, 

 an experiment on an actual cable was planned, and for these 

 tests three reels of 100-pair telephone cable, commonly known 

 as ''Conference Standard"" cable, were brought to the labo- 

 ratory. Each reel contained about 600 feet of cable, so that 

 the entire circuit, when connected back and forth, formed a 

 metallic cable pair thirty- five miles in length, with a resistance 

 of about 87 ohms per mile and a mutual capacity of about 

 •057 microfarad per mile. For a laboratory test a circuit 

 thirty-five miles in length could not be stretched out to its 

 full length, and we actually used the cable on the reels with 

 the circuit looped back and forth, fifty times through the 

 first cable, then into the second and third cables. The equi- 

 valence of a looped circuit of this kind to a straight-away 

 circuit had been shown, provided the circuit was balanced as 

 all telephone circuits must be balanced in order to eliminate 

 cross talk and noise. This point was also carefully tested 

 during the investigation. 



For the loading of this cable 300 coils were manufactured. 

 A cross-section of this loading coil, known as the T-14 

 coil, is shown in fig. 2. On a wooden spool a primary of 

 578 turns of Xo. 20 single cotton-covered wire was wound, 

 and a secondary of ±6d turns of No. 20 single cotton-covered 

 wire. The turns were so chosen as to give the primary and 

 secondary the same inductance, and they also had approxi- 

 mately the same resistance. The cable circuit, as has been 



