212 BELL SYSTEM TECHNICAL JOURNAL 



The application of these various measures served to reduce the total 

 noise manifest at Key West and Havana by a factor of from 40 to 

 60 db so that as now operated the noise from all other sources is of the 

 same order of magnitude as that picked up in the submarine cable 

 itself, which is extremely small at the high frequencies. It approaches 

 that of the Johnson effect '" or so-called resistance noise, in which the 

 conductor itself acts as a source of voltage fluctuations which are 

 distributed uniformly over the whole frequency spectrum. The 

 Johnson effect presents a definite lower level limit to all communication 

 circuits. 



Future Possibilities 



The present arrangements do not represent the ultimate possibilities 

 in communication facilities which the new cable affords. As previ- 

 ously noted, if traffic requirements continue to grow, so that more 

 facilities are required, a wider frequency range may be employed and 

 additional channels obtained. Certain further development work will 

 be required and it is believed that at least three more telephone 

 channels can be provided. In addition, if telegraph circuits are re- 

 quired, carrier telegraph systems may be operated in place of one or 

 more of the telephone channels. For example, if the ultimate capacity 

 of the cable is a total of six telephone channels, a possible arrangement 

 would be to employ two of these telephone channels for carrier tele- 

 graph circuits. The cable may then carry simultaneously a total of 

 four telephone messages and 24 or more two-way telegraph messages. 



1" "Thermal Agitation of Electricity in Conductors," J. B. Johnson, Physical 

 Review, July 1928, p. 97-109. 



