ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 13 



more complicated in form than the simple circuit indicated above. 

 It is, however, necessary to have for practical telephone purposes, 

 means for readily determining to a good degree of approximation the 

 overall efficiency of these complicated circuits as a part of the every- 

 day work of certain departments of the telephone companies. The 

 process of expressing the efficiency in terms of the product of a number 

 of factors provides a convenient means for doing this. Under these 

 conditions the large factors such as line attenuation and certain other 

 factors are determined for the individual circuits, whereas factors which 

 are close to unity can be treated approximately by tables representing 

 various types of cases rather than individual circuits. 



The convenience of treatment of circuit equations in this way for 

 telephone use has led to the use of a logarithmic measure for expressing 

 the efficiencies of telephone circuits. Although the computations can 

 often be made in terms of currents or voltages, where changes of im- 

 pedance are involved, such as inequality ratio transformers, we are, of 

 course, concerned with variations in power rather than variations in 

 either the current or voltage. The losses in a telephone circuit are 

 therefore expressed in terms of the logarithm of the ratio of input power 

 to output power. The unit is so defined that 10 transmission units cor- 

 respond to a ratio of 10, 20 transmission units correspond to a ratio of 

 100, etc. The overall efficiency of practical telephone circuits from 

 transmitter to receiver is in many cases in the order of 20 transmis- 

 sion units, that is, the circuit delivers at the receiving end one percent, 

 of the power delivered to it at the transmitting end. 



Line Efficiency 



A problem of great importance in electric transmission in all fields 

 is that of obtaining the maximum transmission line efficiency practic- 

 able within economic limits. Certain comparisons between power 

 transmission and telephone transmission would seem to be of interest. 



The losses in a unit length of transmission circuit include both re- 

 sistance losses and leakage losses, and may be represented by the for- 

 mula Pr + V^g. If both r and g are constant with variations in voltage 

 and current, it is easy to show that the maximum efficiency of trans- 

 mission takes place when the voltage and current are so adjusted that 

 these two parts of the line losses are equal in magnitude. 



Actually in a well designed power transmission circuit without 

 Corona losses V^g is very small. Hence the solution of the problem of 

 increasing line efficiency is to raise the transmission voltage, thus 

 decreasing the transmission current. This is accomplished in the 

 shorter power transmission lines by using step-up and step-down 



