ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 5 



work which they do on electric circuits is, with the exception of a few 

 men, relatively small, and their work is also very largely based on the 

 use of economic principles, knowledge of the telephone system, general 

 business principles and common sense. 



General Principles 



Communication circuits in general are very complicated networks 

 and the application of electric principles to these networks involves 

 the solution of numerous new problems and the development of a 

 great many practical approximations. To be effective in this work it 

 is of prime importance that the young engineer have a good grounding 

 in the general fundamental principles of direct and alternating current 

 theory. By a good grounding we mean an appreciation of the gener- 

 ality of these principles so that they can be applied by the student to 

 the problems of his particular work, no matter in what branch of 

 electrical engineering that work may be. He needs also to have a 

 facility in their application to new problems. The relations of re- 

 sistance, reactance, conductance and susceptance, the use of Kirch- 

 hoft's laws, the relations of resonance and conditions for maximum 

 transfer of energy between two branches of a network — -all these we 

 would list as a matter of course. We should include also in the list of 

 basic fundamentals some simple but extremely useful practical 

 theorems, including the reciprocity theorem and Pollard's or Thev- 

 enin's theorem. 



Along with these fundamental principles we believe it helpful to a 

 man in any branch of electrical work to have absorbed the idea of the 

 equivalence of networks of different types; for example, the expression, 

 in terms of equivalent T or Pi circuits or other convenient form of the 

 characteristics of any 4 pole network (that is, a network with two 

 input and two output terminals) from the measurements which can be 

 made at its terminals. Fundamental training of this sort gives the 

 man a mastery in the solution of electrical problems, not only through 

 ability to place a given problem in its most convenient form, but by 

 assisting the engineer in the formation of correct and simple physical 

 ideas regarding the processes which are taking place. 



A very good illustration of this point is given by the transformer. 

 Most young men graduating from engineering schools, I believe, think 

 of the operation of the transformer in terms of its vector diagram. 

 This is a valuable way of getting a physical picture of the effect of a 

 transformer which is useful in certain types of problems but less con- 

 venient in others. If in addition to the vector diagram the student 

 is taught the equivalent network of a transformer, as is done in some 



