CONQUEST OF DISTANCE BY WIRE TELEPHONY 379 



The theoretical principles of the device were studied to provide a back- 

 ground for straightening out initial design kinks, and to provide suitable 

 auxiliary apparatus for associating the amplifier with the telephone circuit. 

 Considerable difficulty was encountered in securing a sufficiently long life 

 of the mercury tube to permit experimental use. The first field experiments 

 occurred late in December 1912 at Philadelphia, on loaded circuits in the 

 New York-Washington cable. During the next two years a number of 

 other experimental installations were made. Sentimentally significant 

 experiments were made on the transcontinental line in the late spring of 

 1915, using three and sometimes four repeaters in tandem. In some re- 

 spects the over-all transmission performance was good, but it was not so 

 generally satisfactory as with the vacuum tube repeaters. Considerable 

 difficulty was encountered in starting and maintaining the mercury-arc 

 amplifiers. 



By the end of 1913, it was becoming apparent that the high vacuum tube 

 amplifier was destined to become the leading type. As a matter of fact, 

 the development work on the mercury arc device began to slow down late 

 in 1912, soon after the work started on the improved audion, as subsequently 

 discussed. The mercun,* arc repeater was never used in commercial service. 

 The experimental installations were dismantled during 1915, and the de- 

 velopment case was officially closed in October 1916. 



The success that was quickly achieved in the development of a satisfac- 

 tory high vacuum tube repeater, as described in the following pages, leaves 

 unanswered the question as to whether the mercury arc repeater could have 

 been developed to become a thoroughly satisfactory voice-frequency am- 

 plifier. Its more complicated structure, its need for more complicated and 

 more expensive auxiliarx' devices for associating it with working telephone 

 circuits, and the greater difficulties in operation and maintenance were 

 serious handicaps. Also, it was more limited with respect to repeater gains, 

 and with respect to working-frequency band. The device as developed was 

 used only as a voice-frequency amplifier. 



(b) The High Vacuum Tube Amplifier 



Arnold's research and development work on thermionic amplffiers started 

 in November 1912 soon after a laboratory demonstration on October 30 

 and 31 by Lee deForest of the amplifying properties of his Audion tube, 

 which for several years had been extensively used as a detector for wire- 

 less telegraph signals. The Audion as submitted was a much simpler 

 device than the mechanical and mercury arc amplifiers. It consisted of an 

 evacuated glass tube containing three elements: (1) a filament which emits 

 electrons when heated by an external "A" battery, (2) a metal-plate elec- 

 trode which attracts and collects these electrons when maintained at a 



