188 BELL SYSTEM TECHNICAL JUCRNAL 



3>5 to 8000 cycles per second, thus overlapping the fundamental fre- 

 quencies used for power transmission. 



Control of Power Levels. — Coordination by frequency separation 

 becomes inadequate when the power levels of the various classes of 

 services differ greatly as with power and telephone services. Thus, 

 although incidental powers at harmonics of the power circuit funda- 

 mental frequency are negligible in comparison to the power at the 

 fundamental frequency, they are large compared to the power em- 

 ployed in the telephone circuits and fall directly within the frequency 

 range of the telephone circuits. 



While the powers involved in telephone transmission are small as 

 compared to those on power lines, they are in turn large as compared 

 to the acoustical power received from the talker or delivered to the 

 listener. The ordinary telephone transmitter is an amplifier, delivering 

 to the line several hundred times the voice power which actuates the 

 diaphram. On the other hand, the receiver requires an electrical 

 power a hundred or more times that which it delivers as sound to the 

 listener's ear. 



It is obvious that the relative levels of harmonic-frequency power 

 in the power circuits and voice-frequency power in the telephone cir- 

 cuits are of major importance in inductive coordination. These con- 

 siderations have had large influence in the power field in the control 

 of wave-shape of rotating machinery and transformers, and in the 

 telephone field in fixing limitations on such factors as wire sizes, spac- 

 ings of repeaters and instrument efificiencies. 



Balance. — Among the most important methods of coordinating 

 power and communication circuits is the control of their respective 

 balances to ground and to each other. A power circuit with absolutely 

 balanced voltages and currents impressed, and with the various con- 

 ductors arranged in such a way that they would not establish external 

 electric or magnetic fields, would not have any effect on any type of 

 neighboring communication line. 



Likewise, a telephone circuit in which there were no unbalances and 

 in which the conductors were arranged in such a way that in the pres- 

 ence of an electric or magnetic field they would not have any voltages 

 induced between them would not become noisy from any neighboring 

 power circuits. Such an ideal state is impossible, but much has been 

 accomplished by care in the design of the lines and equipment and by 

 the transpositions of the conductors. 



Shielding. — It is possible to materially reduce electric fields by inter- 

 posing between disturbing and disturbed conductors grounded con- 

 ductor surfaces known as shields. Magnetic fields can likewise be 



