INDUCTIVE LOADING FOR TELEPHONE FACILITIES 161 



ments were in controlling or correcting the small accidental unbalances 

 that were unavoidable in manufacture. 



The transmission performance in loaded side circuits was about the same 

 as that of loaded non-phantom circuits on similar-size conductors. A slight 

 attenuation impairment resulted from the non-inductive resistance of the 

 phantom coils. 



The phantom coils were located at side circuit loading points and the 

 phantom inductance was chosen to give a cut-off frequency of about 2300 

 cycles, the same as in the side circuits, and in non-phantomed circuits. 

 In consequence, the nominal impedance of the loaded phantoms was ap- 

 proximately 60% of that of the associated side circuits. The attenuation 



Fig. 3— An early installation of open-wire phantom group loading. Individually potted 

 coils; phantom coil on pole; side-circuit coils on crossarms. 



was about 13% better than that of the associated side circuits of open-wire 

 lines, and from 15 to 20% better in loaded cables, depending upon con- 

 ductor size. 



The great commercial unportance of the phantom-group loading de- 

 velopment is indicated by the fact that nearly two-thirds of all the voice- 

 frequency loading coils installed on quadded toll and toll entrance cables 

 are coils of side circuit type, and nearly one-third are phantom loading 

 coils. Over the years during which phantom loading and quadded cable 

 have been available, only a relatively small amount of non-phantom type 

 loading has been used in voice-frequency toll cable facilities. Important 

 facilities in this special category are the loaded cable program-transmission 

 circuits subsequently described and the "order wire" maintenance circuits 

 in coaxial cables. Also, during the 1940's, there was some occasional use of 



