LOADING FOR TELEPHONE CIRCUITS 223 



l)eing so arranged as to iiulit-ate (lircclly \hv iiaUirc oi the cliaiij^es 

 which occur when unifornil\- (listril)iited iiuliictance is added to a 

 uniform Hue initialK' haxinu, zero inductance. 



Inspection of tlie formulas shows that the adcHtion ol (H>lril)iiled 

 inductance: 



(a) Reduces tiie atteiuiation constant and the \-eh)city, i)ro\ided 

 that the ratio R 2L is less than />; in i)ractice, this limitiniL; condition 

 is approached only at very low fretjuencies which nsualh' are of 

 negligible importance in speech transmission. 



(b) Increases the impedance, and imjiroxes the power factor. 



(c) Makes the attenuation, \'elocity and impedance independent 

 of frequency over the frequency range where R is small with reference 

 to pL; in practice, this condition holds generally, except at the low- 

 voice frequencies. 



From the standpoint of the power transmission engineer, the 

 general effect of loading in reducing the attenuation losses may be 

 explained in terms of the changes in line impedance noted in (b) 

 above. These impedance changes make it possible for the loaded 

 line to transmit a given amount of power corresponding to speech 

 sounds at a higher line potential and with a (proportionately) lower 

 value of line current than is possible without the loading. In the 

 non-loaded line which is inherently a low impedance line, the series 

 dissipation losses which are proportional to the square of the line 

 current are ordinarily \ery large relative to the shunt dissipation 

 losses which are proportional to the square of the line potential. 

 Consequently, when the line impedance is increased by a suitable 

 amount, the reduction in series losses is much greater than the increase 

 in shunt losses and a substantial improvement in line efficiency is 

 obtained. The optimum impedance for minimum line losses is that 

 which results in the shunt and series losses being equal. Ordinarily, 

 it is not economical to apply a sufficient amount of loading to reach 

 this condition. 



In general, commercial power lines are electrically short in terms 

 of the wave length of the transmitted frequencies and consequently 

 the sending end impedance is very largely influenced by the receiving 

 end impedance. This allows high impedance transmission lines to be 

 obtained by using high ratio transformers at the receiving end to step 

 up the terminal impedance. On the other hand, telephone lines which 

 are of interest from the loading standpoint are electrically long and 

 the sending end impedance is practically unaffected by the terminal 

 impedance. Consequenth', the addition of series inductance to the line 

 is the most practical way of increasing the telephone line impedance. 



