186 BELL SYSTEM TECHNICAL JOVRXAL 



between lines of a few hundred feet practically eliminates the noise- 

 frecjuency problem whereas the low-frequency problem may exist with 

 much greater separations. Since the same customers desire both 

 communication and power services, the two kinds of distribution lines 

 are necessarily often located on the same streets and highways. Power 

 transmission lines and toll telephone lines do not, in general, have to be 

 placed on particular routes and, therefore, separation can often be 

 employed where such lines are involved. Cooperative advance plan- 

 ning on the part of the utilities in laying out their plants makes it 

 possible to employ separation where it is readily feasible and economi- 

 cal. 



Frequency Separation. — Another method of fundamental impor- 

 tance is the use of frequency separation. By this method, circuits to 

 be coordinated are arranged so as to be responsive to different fre- 

 quencies or bands of frequencies, and comparatively unresponsive to 

 the frequency or band of frequencies employed for the other circuits. 

 It is thus possible to make many different uses of electricity involving 

 transmission in the same medium. This solution is familiar to us in 

 the coordination of radio services. 



Fig. 1 shows a diagram of the various uses of the frequency spectrum 

 for electrical transmission and the manner in which power and com- 

 munication services are coordinated by means of frequency selectivity. 



The first commercial electrical energy available was in the form of 

 direct current. Shortly thereafter, alternating current was used for 

 the transmission of power. The nominal frequencies of the current 

 used for this service in the earlier days range from I673 cycles to 133 

 cycles. In American practise the frequencies used for power purposes 

 have practically settled down to either 25 or 60 cycles. There is one 

 extensive 50-cycle system and a few odd frequency systems. These 

 latter of 30, ^i, and 40 cycles, and perhaps others, are being rapidly 

 eliminated, due to the importance of interconnecting them with 60- 

 cycle systems. At the present time, there is some tendency for the use 

 of higher frequencies in special machine shop applications. This use, 

 at present, is principally at 180 cycles and need not concern us here as 

 its extent is usually confined within a factory building. 



In message telephone transmission, the prime consideration is the 

 transmission of intelligible speech. While the range of response of 

 the human ear is from about 16 cycles to 15,000 cycles per second, 

 human speech occupies a narrower range and a still narrower band is 

 adequate for intelligibility. The present voice-frequency telephone 

 circuits, especially the longer ones, operate within a frequency band 

 of about 250 to 2750 cycles per second. The frequency selectivity at 



