626 BELL SYSTEM TECHNICAL JOURNAL 



respect to the filament, that on the other grid is negative. Since the 

 even order products are proportional to an even power of the input 

 voltage, these currents will flow through the high side winding of 

 the output transformer non-inductively producing no flux in the trans- 

 former, and hence no current in the low side windings. To realize 

 this ideal condition, the two currents flowing in the output trans- 

 former windings must be equal in amplitude, and 180 degrees out of 

 phase. Like amplitudes can be obtained in several ways since the 

 plate current is a function of a number of tube constants. Tubes 

 may, therefore, be selected which will give the same harmonic current, 

 that is, tubes in which the net effect of the several factors is the same. 



Conclusion 



Use in Telephone Plant. The carrier systems are meeting success- 

 fully and economically the requirements of long distance telephone 

 service. From what has already been written, it is evident, however, 

 that the apparatus is by its nature complex and to a fair degree 

 expensive, so that for the relatively short distances it is cheaper to 

 string additional wire. The exact distance beyond which it is more 

 economical to employ carrier methods is obviously dependent on the 

 circumstances surrounding each particular case. Systems are oper- 

 ating for distances of 150 miles and upwards. 



Traffic growth often requires additional circuits for the shorter 

 distances, where there are longer haul continuous physical circuits on 

 the same line. In this case it is not uncommon to break up the long 

 haul physical circuits into sections to satisfy the short haul circuit 

 growth and to install a carrier system to meet the long haul needs. 



The growth of the use of carrier systems has already been pictured. 

 How the systems are distributed over the lines of the Bell System is 

 shown on Figure 44. The heaviest density of use occurs in the middle 

 and western sections and in general where the circuit demand and 

 growth have not reached the large figures required to justify the 

 installation of toll cables. In particular, the section west of the 

 Mississippi is a promising field for the application of carrier systems. 



Future. While the type "C" system satisfies those circuit growth 

 demands for moderate and long haul, there has remained undeveloped 

 a considerable field for carrier methods over the shorter distances 

 where only wire stringing has hitherto been economical. Very recent 

 developments have resulted in the trial and early field applications of 

 a simple single-channel carrier telephone system designed particularly 

 to meet these shorter haul demands and thereby to secure the greatest 

 practicable economy in providing facilities by carrier methods in the 



