CARRIER SYSTEMS ON TELEPHONE LINES 587 



conditions, however, much larger harmonics are present in the voice- 

 frequency range, so that the induction in the normal telephone circuit 

 is much more severe than the carrier circuit. 



A much more severe source of carrier interference has been found to 

 result from the abnormal actions of power line circuits in which arcing 

 phenomena occur. Interference of this sort has been noted and 

 traced to such sources as arcing insulators, tree leaks, pantograph and 

 trolley collector sparking, charging lightning arresters, unusual com- 

 mutator or slip ring sparking, switching, etc. In the early days of 

 operation of carrier systems, interference of this type formed a not 

 uncommon source of disturbance. The situation was remedied in 

 some cases by cooperation with the power companies concerned. 

 On the whole, this source of interference has been greatly reduced in 

 the past few years. 



On occasions the carrier telephone systems have been interfered 

 with by power line carrier systems operating on near-by power lines. 

 Considering the widespread use of power line carrier telephone 

 systems and the fact that they normally involve a transmitting power 

 many times that of the systems described in this paper, this would, 

 no doubt, be a more common source of difficulty if it were not a fact 

 that such power systems adjacent to the telephone systems are 

 operated well above the frequency range of the telephone line carrier 

 systems. 



Energy picked up from the high-power transoceanic radio telegraph 

 stations, transmitting at frequencies in the carrier range, is an 

 occasional source of interference, particularly in the east where carrier 

 systems are located relatively close to the radio stations. The open- 

 wire telephone lines act as long-wave antennae and intercept the radio 

 energy. This, of course, enters initially on the longitudinal wire 

 circuit to ground. Due to residual line unbalances, some energy is, 

 however, unavoidably passed on to the metallic circuits on which 

 the carrier systems are operated, and enters the speech channel in 

 the form of a tone or note similar to a heterodyne signal at a radio 

 telegraph receiver. 



Lightning and general static disturbances form a substantial part 

 of the background noise which is found on all carrier lines. Its 

 general magnitude is ordinarily small, except under certain conditions 

 such as the case of near-by storms. 



Transmission Levels. In the design and laying out of type "C" 

 installations, the transmission level of a system is ordinarily not 

 permitted to fall below a certain figure, which under particular 

 circumstances might be about — 25 TU, with respect to the trans- 



