130 BELL SYSTEM TECHNICAL JOURNAL 



Although we are experimenting with a structure capable of trans- 

 mitting a band of one or two million cycles in width, a coaxial structure 

 capable of transmitting a band twenty million cycles in width is 

 apparently not wholly unfeasible. 



The question naturally arises whether, with interference from 

 outside sources almost wholly or entirely eliminated, it is possible to 

 allow speech currents to be attenuated to an unlimited degree before 

 the introduction of amplification to bring them back to their original 

 value. With all outside interference eliminated, however, noise 

 arising within the conductor itself sets the limit. This interference is 

 termed resistance noise or sometimes thermal-agitation noise because 

 it is a function of the temperature of the conductor. It is apparently 

 due to the continual moving around of the free electrons which exist 

 in all conductors. Our Laboratories have investigated this phe- 

 nomenon and determined its characteristics. This resistance noise 

 varies in amount with the resistance of the conductor and with the 

 temperature. It is uniformly distributed over the whole frequency 

 range from lower voice frequencies up to the highest frequency which 

 we have considered using. One ready means of observing this phe- 

 nomenon is to provide an amplifier covering the voice range, with its 

 input connected across the resistance, and to listen on the output of 

 the amplifier with a telephone receiver. If the amplifier has an 

 amplification of about 140 db, the noise heard in the telephone receiver 

 is about as loud as would be heard in the receiver were it connected 

 directly ^across the output from a telephone substation. 



To prevent this thermal or resistance noise from being noticeable 

 in a telephone conversation, we must limit the amount of amplification 

 used at any one point in a long system, even though it were perfectly 

 shielded, to an amount considerably less than 140 db. These con- 

 siderations have led us to conclude that for a long circuit with many 

 amplifiers distributed along the route, the amount of amplification at 

 any one point should not exceed about 70 db. 



The amount of amplification involved in present-day telephone 

 circuits is illustrated by the 4-wire cable circuit, in which amplifiers 

 are located in each pair at intervals of about 50 miles (80 kilometers) 

 and each amplifier is set to give an amplification of about 25 db, or a 

 power amplification of 300 times. For such a circuit between, say. 

 New York and Chicago, a distance of about 900 miles (1450 km.), 

 the total amplification is about 500 db, or a power amplification of 10^". 

 It is obviously necessary that these amplifiers must be made very 

 stable so that the cumulative variations in the many amplifiers may 

 not make it impossible to obtain the required degree of overall stability 



