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BELL SYSTEM TECHNICAL JOURNAL 



Group II shows the e.m.f.'s impressed upon the circuits when re- 

 ceiving and when transmitting, and the assumed positive directions 

 of the resulting mesh currents.^ The schematics in Fig. 4 will be used 

 in the later explanations of the receiving condition ; but for the trans- 

 mitting condition the equivalent component schematics evolved in 

 Group III will be employed instead of Fig. 5. 



Receiving 

 Kig. 4 



Transmitting 

 Fig. 5 



Group II — The applied E.M.F.'s and assumed 

 and transmitting 



mesh currents, receiving 



By the well-known principle called Thevenin's Theorem, under the 

 receiving condition the line and the set at its distant end may be 

 replaced by an impedanceless generator in series with the line im- 

 pedance, the e.m.f. of this generator being equal to the open-circuit 

 voltage across the terminals of the line. Also, under receiving condi- 

 tions, the transmitter will be treated as a passive impedance; and 



* The curved arrows in the above and in all subsequent circuit schematics, it 

 should be noted, do not purport to indicate the relative directions of the actual 

 currents; but merely to denote the directions assumed as the positive sense of mesh 

 currents and voltages. The selection of these directions is purely optional. As a 

 matter of convenience, they have been so chosen that the currents through the 

 transmitter and receiver are in every case the algebraic sum of the two contributing 

 mesh currents. 



'The following conventions and nomenclature are employed, see Group II: — 

 The directions around the meshes, chosen as positive for all voltages and currents, 

 are indicated by curved arrows. Subscripts differentiate impressed e.m.f.'s with 

 regard to the meshes in which they are applied, and identify currents with respect to 

 the meshes (or circuit elements — see Group IV) in which they flow. Currents are 

 further identified with respect to the e.m.f. or e.m.f.'s to which they are due, by 

 superscripts corresponding with the activating voltage subscripts. 



