6 BELL SYSTEM TECHNICAL JOURNAL 



texts, he can more easily apply it to other types of problems, and the 

 equivalent T network of the transformer is so simple that it is very 

 helpful in showing the variation in the performance of the circuit with 

 changes in the constants of any part of the circuit. 



Familiarity with the fundamental principles of networks and equiv- 

 alent circuits is particularly helpful in case of a man who comes in 

 contact with problems associated with networks made up of a number 

 of similar sections, as for example, electric filters, which already play 

 such an important part in certain branches of the communication art. 



In the study of fundamental principles it would appear to be very 

 helpful in enabling the student to get an appreciation of their generality 

 if the specific problems and illustrations used in his work are drawn 

 from the various fields of electric work rather than from a single field. 

 The communication field is replete with specific problems illustrating 

 these principles which are very suitable for the use of the student. 



Transmission Line Theory 



In discussing the application of the principles of electric circuits to 

 communication it is natural to give particular attention to transmission 

 line theory because of its importance in connection with the trans- 

 mission of electrical energy for any purpose whatever, including both 

 power and communication services, and because of the interest of the 

 problems it involves. Transmission line theory in one sense dates 

 back to Lord Kelvin who in 1855 applied laws of diffusion of heat to 

 the determination of the flow of electricity through long submarine 

 cables. This solution ignored the effect of line inductance which was 

 unimportant in the particular problem to which Lord Kelvin applied 

 this solution, but which is very important in any general transmission 

 line theory. Through the work of Heaviside and others the general 

 transmission line theory was at an early date applied to telephony. 

 It is, of course, in relatively recent years that the great development of 

 long distance power transmission lines has made the general theory of 

 value in power transmission work, the performance of early alternating 

 current systems being adequately represented by approximate for- 

 mulas, entirely neglecting the effect of the capacity of the line. It is 

 indeed not long ago that the effect of line capacity, assumed lumped at 

 one or two points, became important and still more recently that it 

 became necessary in power problems to take more accurate account of 

 the distribution of the capacity. 



It is no doubt partly as a result of the historical development of the 

 application of transmission line theory to telephony and to power 

 transmission problems, and partly the result of differences in conditions 



