430 BELL SYSTEM TECHNICAL JOURNAL 



This apparatus ^ was then demonstrated to officials of the Western 

 Union Telegraph Co. and with the cooperation of their engineers tests 

 extending over a period of about a year were carried on at Rock- 

 away Beach on several of the cables entering that station. It was 

 shown in these experiments that while the vacuum tube amplifier 

 together with suitable distortion-correcting networks would permit 

 a considerable increase in the simplex speed (limited only by the 

 interference present in the cable), the duplex speed was limited by the 

 imperfect balance between the cable and the artificial line, and the 

 increased sensitivity and signal shaping ability of the amplifier were 

 of little value under the conditions then obtaining. Serious efforts 

 were made to utilize the current limiting properties of vacuum tubes 

 in conjunction with differentiating and integrating networks in re- 

 ducing the effect of the unbalance on the signal and some successful 

 results were attained.'" 



By 1920 the research leading to the development of the permalloy 

 loaded cable had progressed to a point where it was evident that a 

 new type of high speed cable amplifier would be required and the 

 investigations were continued with this end in view. After the 

 solution of numerous difficulties an amplifier was produced which was 

 capable of correcting almost any variety of signal distortion which 

 might be caused by a loaded cable. An amplifier of this type was 

 tested on a trial length of 120 miles of loaded cable laid in a loop out 

 of Devonshire Bay, Bermuda, and found to be generally satisfactory. 

 The amplifier was then redesigned in a form suitable for commercial 

 use and two amplifiers were built and installed at Rockaway Beach 

 and Fayal in readiness for the New York-Azores loaded cable. They 

 were put into successful operation and the predicted speed of 1,500 

 letters per minute was demonstrated within an hour after the cable 

 had been released by the electricians of the ship which laid it. 



Circuit Arrangements of Signal Shaping Vacuum Tube 



Amplifier 



The electrical requirements of a cable signal shaping amplifier 

 suitable for use on high speed loaded cables may be briefly stated as 

 follows. It must take an input signal having components as low as 

 one half millivolt and as high as possibly ten volts in amplitude, and 

 correct the distortion by amplifying the weaker components much 

 more than the stronger, at the same time making any necessary phase 



9 See U. S. Patents to R. C. Mathes No. 1,311,283, July 29, 1919, No. 1,426,755, 

 August 22, 1922, No. 1,493,216, May 6, 1924 and No. 1,586,821, June 1, 1926; 

 also Canadian Patent No. 207,231, January 4, 1921, granted to B. W. Kendall. 



i" D. K. Gannett and M. Kirkwood, U. S. Patent No. 1,483,172. 



