382 BELL SYSTEM TECHNICAL JOURNAL 



The work on the vacuum tube repeater during 1912 and 1913 was sum- 

 marized in the 1913 report on Work Order 7655 as follows: 



"... The result has been the ability to construct an Audion amplifier to give dis- 

 tortionless amplification between desired limits of current input; to give outputs of 

 energy far above any value that is normally met in telephony; to act as a potential 

 or as a current transformer with the ability when connected two or more in series 

 of giving current amplifications of as large amounts as 50 times or more; to present 

 to the circuits between which it works practically constant impedance. The 

 present form of the Audion gives practically perfect repetition and amplification of 

 currents delivered to it." 



It is of interest that on February 18, 1913 a laboratory demonstration of 

 the promising possibilities of the audion- type repeater was made for Presi- 

 dent Vail and other executives on a 900-mile non-loaded artificial 104 mil 

 open-wire line. This was a one-way test having several repeaters in tan- 

 dem, and the tubes did not have a high vacuum, due to limitations of the 

 then available apparatus in the laboratory. The fiirst use of the high 

 vacuum tube amplifier on commercial circuits was on October 18, 1913 

 when a 22-repeater installed at Philadelphia was placed in service on a 

 New York-Baltimore loaded cable circuit. 



Further developments resulted in improved amplifiers becoming avail- 

 able in the transcontinental line when Mr. Vail first talked over it in July 

 1914, and when commercial service started the following January. The 

 important over-all service characteristics of the line, including the part 

 played by the repeaters, are considered in a subsequent section of this 

 story under the heading, "The First Transcontinental Circuits." 



Repeater Circuits 



The development work on repeater circuits for the transcontinental 

 project had for its basic theoretical background a classical mathematical 

 analysis of the relations between line impedance irregularities and repeater 

 gains in two-way repeater circuits, reported by Dr. G. A. Campbell in May 

 1912. The study included the currently used two-way, one-repeater circuit 

 (21-circuit), and the two-way, two-repeater circuit (22-circuit), which had 

 not been commercially used, although it had been invented by W. L. 

 Richards in 1895, long before the availability of a commercially usable 

 repeater element. In a preliminary report dated March 7, 1912,^ Campbell 

 had recommended the 22-circuit on the basis of its greater stability and un- 

 restricted flexibility, as subsequently discussed. The March 7 memo- 

 randum also discussed the four-wire repeater circuit which later became very 



^ A complete copj^ of this memorandum is given in the last item listed in the attached 

 Bibliography. 



