196 BELL SYSTEM TECHNICAL JOURNAL 



upon the probable interruptions, is determined by shifting this curve 

 10 db to the left as measured by the indicated db spacing between 

 curves. 



It should be remembered that estimates based upon Fig. 8 are prob- 

 ably very approximate but the curves will at least serve to indicate 

 the trend of improvement effects. 



Acknowledgement 



Acknowledgement is due the various engineers and operators of 

 the British General Post Office and of the American Telephone and 

 Telegraph Company who were responsible for the accumulation of the 

 data used in this analysis. 



