vantage of the amplifying characteristics of a variable resistance 

 telephone transmitter and combined in one instrument, in refined 

 form, the fundamental elements of a telephone receiver and a 

 transmitter. The attenuated telephone currents entering the receiver 

 side of the device caused the vibration of a diaphragm which, in turn, 

 actuated a variable resistance element which transmitted amplified 

 currents to the next section of telephone line. Figure 9 shows a 

 cross-section of the amplifying element of this repeater, commonly 

 known as the mechanical repeater. Repeaters of this type were used 

 in commercial service for a number of years but since development 



CARBON CHAMBER OF 

 TRANSMITTER BUTTON 



Fig. 9 — Cross-sectional drawing of a mechanical type of telephone repeater. 



work up to the time of the application of vacuum tubes to telephone 

 repeaters had not overcome the fundamental difficulties of distortion, 

 gain limitations and instability of the mechanical repeater, its use 

 was gradually discontinued after the introduction into the telephone 

 plant of the vacuum tube repeater. Other arrangements such as 

 variation of the field current of a generator to produce corresponding 

 variations in the armature voltage and the electromagnetic control of 

 gaseous devices were tried out but were never successfully applied in 

 any important degree to telephone circuits. 



In 1906, Lee DeForest demonstrated before the American Institute 

 of Electrical Engineers that the discharge between a hot cathode and 

 a plate of a thermionic tube can be controlled by a third electrode. 



16 



