INDUCTIVE LOADING FOR TELEPHONE FACILITIES 451 



Their installed costs were about the same as those of standard heavy and 

 medium loading, respectively. The special importance and significance of 

 these new loading systems was that their use on the higher capacitance 

 cables avoided a degradation in the loading cut-off standards and the ob- 

 jectionable impairments in transmission inteUigibility that would otherwise 

 have resulted. The use of lower inductances at standard spacing, in order 

 to comply with the cut-off standards without raising costs, resulted in a 

 reduction of nominal impedance which was desirable and an increase in the 

 attenuation which was accepted as tolerable, under the circumstances. This 

 decision on the new loading systems was largely influenced by certain fun- 

 damental transmission-studies then under way which indicated that it would 

 eventually be desirable to adopt much higher cut-off frequency standards, 

 subsequently described. 



Table VII compares certain transmission properties of "medium-heavy" 

 and "light-medium" loading on the high-capacitance cables with those 

 which would have resulted from the use of standard medium and heavy 

 loading. 



(17) First Increase in Minimum Cut-Off Frequency for Loaded 

 Exchange Area Cables 



17.1 General 



During 1924 there occurred the first major improvement in loading 

 standards for exchange area cables, consisting of an increase in the minimum 

 cut-off frequency from about 2300 cycles to about 2800 cycles per second. 

 This decision implemented the conclusions reached in comprehensive fun- 

 damental theoretical and experimental studies of exchange area transmis- 

 sion that got well under way during the early 1920's. The improved loading 

 systems initially involved the use of available types of 135 and 175 mh 

 loading coils at spacings shorter than those previously used with these 

 coils, and the use of new 88 mh loading coils much smaller in dimension 

 and much lower in cost than the 135 and 175 mh loading coils. The new 88 

 mh loading inductance eventually became the most extensively used induc- 

 tance value in exchange area loading. 



17.2 New Technique for Computing Intelligibilily Indices for Complete Circuits 



In the theoretical aspects of the fundamental study, above' referred to, 

 use was made of a new technique developed by Dr. Harvey Fletcher for 

 computing the articulation index of complete telephone transmission sys- 

 tems, taking into account the effects of attenuation loss and circuit distor- 

 tion in the line, the subscriber loops and station sets, the effects of sidetone 

 in the station sets, and allowing for the masking effects of line noise and 



