ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 



11 



they appear to have in clarifying the students' ideas about electric 

 transmission, and because few students appear to be familiar with this 

 method of treatment. 



It is no doubt true that in many cases the student can best start 

 transmission line theory with a simple approximation. It would seem, 

 however, that before he gets through his study of the principles of 

 electric circuits he should have a clear picture of the physical processes 

 involved in the propagation of electric power over transmission lines, 

 such as is given by equation 2, and of the assumptions involved in the 

 various approximations which may be presented to him. If the scope 

 of the course is not such as to permit the derivation of the general 

 equations from the differential equations, it is possible to get equation 



100 Mile, 8BWG Copper Telephone 



Circuit, 1000 ~ 



Terminating Impedance Equals 



Characteristic Impedance 



Ratio of Terms: 



Zl 



Zr 



= YlZr 



= 3.5/83°[l U i JUI •••] 



Fig. 4 



4 by a method of successive approximations as is shown in at least one 

 textbook, and from 4 to derive the general equation 2. 



In contrast with the power line cases, Fig. 4 indicates the results 

 obtained by the series computation of a relatively short telephone toll 

 line, an open wire circuit 100 miles long, and using 1,000 cycles as one 

 typical telephone frequency. Although this line is only a little more 

 than half a wave-length, the solution by the series for this case is quite 

 laborious and indeed impracticable, and of course, would be even more 

 so with the longer lengths or the higher attenuations ordinarily in- 

 volved in telephone circuits. 



The form of the general equation under discussion which is found 

 most convenient for telephone use is given in equation 6. 



