ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 19 



out that the classic circuit theory applied to transmission Hnes involves 

 the following assumptions: 



1. That the solution is concerned only with conditions at some distance 



from the terminals of the circuit and can therefore ignore the 

 changes in distribution of electric and magnetic fields near the 

 terminals known as end effects. That means that the electric 

 and magnetic fields are propagated along the line in the same 

 way as the currents and voltages. 



2. That the propagation constant (per centimeter) is very small com- 



pared with unity and that the real part of the propagation con- 

 stant is not large compared with the imaginary part, that is, 

 the attenuation is not large compared with the phase change. 



3. That in the conductors the loss due to the transmission current 



(that is, the axially flowing current) is large compared with the 

 loss due to the charging currents. 



4. That in the dielectric the propagation of energy is nearly parallel 



to the axis of the conductors and the dissipation in the dielectric 

 is negligible. 



5. That the fields of the currents and charges are propagated at an 



infinite velocity, that is, that radiation is neglected. 



These assumptions, it can be shown, are very good for the ordinary 

 case of an efficient transmission system. The effect of modification 

 of the field at the terminals influences only a few feet of the line and is 

 negligible in amount. Assumptions 2, 3 and 4 can very readily be 

 shown to be true from the characteristics of the conductors and di- 

 electrics involved in transmission, and as regards neglecting of radia- 

 tion Mr. Carson has shown that for the ordinary transmission system 

 the losses by radiation are in the order of one ten-thousandth of those 

 in the conductors. 



It is to be noted, however, that these assumptions place certain 

 limitations on the application of the classic theory which are important 

 in certain cases. The limitations are as follows: 



1. The electric and magnetic fields are accurately expressed only for 



points relatively near the conductors. At great distances from 

 the conductors the radiation field becomes important in com- 

 parison with the inductive field because of the much more 

 rapid rate of decrease in intensity of the inductive field with 

 distance. 



2. The electric and magnetic fields are not accurately expressed near 



the terminals of the circuits. 



