Immediate use of this discovery was made in improving radio telegraph 

 receivers. The tubes and circuits thus employed were not of types 

 that could satisfactorily be used in telephone work which required a 

 high stability of the amplifying device and freedom from distortion of 

 speech currents. However, an intensive study of the possibilities of 

 this device showed that the use of such tubes based on DeForest's 

 discovery was by far the best method of amplifying telephone currents 

 yet developed. 



Development work on vacuum tubes carried on by the Bell System 

 has included the perfection of the tubes by the use of high vacuum, 

 scientific proportions and new types of filaments to secure improved 

 efiiciency. The performance of vacuum tubes used in the Bell System 

 has been improved extensively through continued development work. 

 For example, during the last twelve years the average life of the tubes 

 used in the Bell System has been extended by a factor of 10, and, at 

 the same time, their power consumption has been reduced appreciably. 



Vacuum tubes were first applied to telephone repeaters experimen- 

 tally, and to a small degree commercially as early as 1913. One of the 

 first important uses of vacuum tube repeaters, however, was in 1915 

 in connection with the first transcontinental telephone service between 

 New York and San Francisco, a distance of approximately 3400 miles. 

 The circuit consisted of No. 8 B.W.G. open-wire copper conductors 

 loaded at eight-mile intervals and having vacuum tube telephone 

 repeaters located at Pittsburgh, Omaha, and Salt Lake City. 



Years of experience with early forms of telephone repeaters had 

 shown that the successful use of repeaters depended not only upon 

 the development of a suitable amplifier but also upon the design of 

 suitable circuit arrangements for associating the repeater with the 

 telephone line and on improved methods of line construction. An 

 important consideration is the fact that a telephone circuit must oper- 

 ate in both directions, that is, it must permit talking to be carried on 

 from either end of the circuit. A single telephone repeater element, 

 however, is inherently a one-way device, receiving attenuated currents 

 at one pair of terminals and transmitting amplified currents from the 

 other pair of terminals. The association of such one-way elements 

 with a two-way telephone circuit is not a simple matter because if any 

 considerable proportion of the amplified output current of the repeater 

 reaches the input terminals, it is again amplified and results under 

 ordinary conditions in turning the repeater into a generator of alter- 

 nating currents (an oscillator) and destroying its usefulness as a 

 repeater. 



