78 BELL SYSTEM TECHNICAL JOURNAL 



correspondingly increased gain required, it was possible to 

 obtain a working equivalent of only 12 miles of standard cable 

 as compared to the original figure of 21 miles. This means 

 that with the same amount of speech power applied at one end, 

 the power received over the non-loaded circuit is 7 times as 

 large as that formerly received over the loaded circuit. 2 



The example of the transcontinental line, above, may well bring 

 up the question as to why it is that cable circuits are loaded. This 

 is done for two reasons: In the first place, it is in general cheaper to 

 load cables than it is to make up the increased attenuation by means 

 of more repeaters. In the second place the loading lessens the amount 

 of distortion introduced by the cable circuits. In the case of open 

 wire circuits, their series inductance is sufficient to keep the distortion 

 small. 



IV. Attenuations and Corresponding Amplifications — 



Power Levels 



Owing to the fact that the weight of loading applied to the longest 

 cable circuits is very light, the attenuation of such circuits is very 

 great. A four-wire X.L.L. 19 gauge circuit 1,000 miles long has the 

 enormous line equivalent of 500 miles of standard cable. The total 

 power amplification applied to this circuit by the repeaters exceeds 

 10 47 . This amount of amplification is more than enough to talk half 

 way around the world at the equator using non-loaded No. 8 Bir- 

 mingham Wire Gauge open-wire commonly employed for handling 

 very long distance business (No. 8 B.W.G. copper weighs 435 pounds 

 per wire mile, or 120 kilograms per kilometer). 



In order to obtain an idea of how enormous this amplification is, 

 assume that no repeaters were employed and an attempt were made 

 to apply enough power at one end of the circuit to enable the normal 

 amount of speech power to be received at the distant end. The power 

 applied at the sending end would then have to be about 50 quadrillion 

 times as great as the total power which it is estimated is radiated by 

 the sun. 



While the total amount of power amplification is very great, the 



amount of amplification put in at any one point is, of course, limited. 



The maximum amount of power at a repeater point is limited partly 



by the capacity of the vacuum tubes and partly by the power carrying 



capacity of the telephone circuit, including the loading coils. (By 



power carrying capacity is here meant the ability to carry voice waves 



5 A material improvement in the telephone quality was also effected by the unload- 

 ing of the circuit. 



