CONQUEST OF DISTANCE BY WIRE TELEPHONY 377 



the voice range, approximately 2200 cycles. The sensitiveness of the re- 

 peater, which was greatest at its natural frequency, was reduced by means of 

 a resonant shunt-type electrical filter of about the same critical frequency. 

 This arrangement provided more uniform frequency-amplification charac- 

 teristics and better quality. The modified repeater, however, was still not 

 entirely satisfactory with respect to the variation of amplification with 

 respect to input levels. 



The 1912 improvements produced an amplifier element that was good 

 enough for experimental use at Philadelphia in September 1912 on a loaded 

 New York-Baltimore circuit in the New York-Washington cable, using the 

 22-repeater circuit subsequently described. Somewhat later there was an 

 experimental installation on the New York-Denver loaded open-wire cir- 

 cuit, using the 22-t}'pe repeater arrangements with three repeaters in 

 tandem. Further development work on refinements and auxiliary devices 

 made the mechanical repeater fairly satisfactory from the quality, volume, 

 and life standpoints, and several commercial installations were made durmg 

 1913 and 1914, including a number of points along the Boston-Washington 

 cable, initially at Philadelphia, to improve and protect the service along 

 this route. 



The improved repeaters, code 3A, were used for a few days in the initial 

 3-repeater service (at Pittsburgh, Omaha, and Salt Lake City) on the 

 transcontinental line in January 1915, as alternatives for vacuum tube re- 

 peaters installed at the same points. The New York-San Francisco service 

 with the mechanical repeaters was fairly satisfactory, but not as good as 

 that with the vacuum-tube repeaters, which were retained in service. 



The inferior characteristics of the improved mechanical repeater, relative 

 to the vacuum tube repeater, led to restrictions in its general use. The 

 principal disadvantages of the mechanical repeater w^ere those previously 

 commented upon. These were such as to become more and more serious 

 with increasing lengths of circuits, involving increasing numbers of repeaters 

 in tandem. Moreover, even under most favorable operating conditions, 

 the maximum repeater gain was well below that obtainable with the vacuum 

 tube device. The inertia of its moving parts restricted its frequency range 

 application to voice-frequency telephony. Within a few years after the 

 opening of the transcontinental line no more installations were made, and 

 vacuum tube repeaters were substituted in old installations. That is to say, 

 the vacuum tube repeater soon became the standard. 



New Types of Repeater Elements 

 {a) The Mercury Arc Repeater 



The early theoretical survey of the possibilities of developing essentially 

 inertialess telephone repeaters focussed attention on gaseous discharge 



