INDUCTIVE LOADING FOR TELEPHONE FACILITIES 



177 



investigations of experimental installations also became necessary. For cost 

 reasons, it seemed desirable that the improved loading should be used at 

 standard ''heavy loading" spacing, i.e., 6000 ft. In consequence, the increase 

 in cut-off was proportional to the increase in velocity. Also, there was a 

 proportional reduction in the nominal impedance, accompanied by an in- 

 crease in the unit-length attenuation. 



Loading Systems- 



Table IV 

 -Small Gauge Repeatered Toll Cables 



Notes: (a) Nominal coil spacing is 6000 feet in cable having a capacitance of 0.062 

 mf/mile in the side circuits and 0.100 mf/mile in the phantom circuits. 



(b) The code numbers of the first standard compressed, unannealed, powdered- 

 iron core coils used in the loading systems. 



(c) These attenuation values apply at 55°F. Under extreme high or low tem- 

 perature conditions, the actual attenuation may be approximately 12 per 

 cent larger or smaller, due principally to changes in conductor resistance 

 with temperature. In long repeatered cable circuits these variations of 

 attenuation with temperature require special corrective treatment by 

 means of automatic transmission regulators. 



(d) These particular length-limitations were set by velocity-distortion effects. 

 By using velocity-distortion corrective networks in the Hnes or at repeater 

 stations, it would have been possible to extend the circuit lengths beyond 

 the listed limits, provided also that adequate steps could be taken to con- 

 trol echo currents. Under actual service conditions, however, echo currents 

 might Hmit the circuit lengths to considerably lower values than those 

 listed above, depending upon the grade of balance of the lines and the 

 permissible overall loss. 



The transmission development work resulted in the standardization of 

 two new phantom-group loading systems, designated HI 74-106 and 

 H44-25('"> in Table IV. Several years later (1923), a new H63 phantom 



(«") The letter-number loading designations used in Table IV, and in the remainder of 

 the text, were simplifications adopted for general use in 1923. The letter prefix symbolizes 

 the geographical spacing in feet; the numbers correspond to the nommal mductances (in 

 milUhenrys) of the associated side circuit and phantom loading coils, m the sequence noted. 

 The letter "H" designates "Heavy" loading spacing. In the early days this was about 

 1.25 miles; later it became 6(XX) ft. 



