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BELL SYSTEM TECHNICAL JOURNAL 



until the voltage across the lamp reaches the breakdown potential 

 which, in the example shown, is about 210 volts. From this point 

 on the current increases linearly with respect to voltages in excess of a 

 value somewhat below the breakdown point. It will also be seen 



100 140 180 220 260 300 



E.M.F. -VOLTS 



Fig. 25 — Current voltage characteristic of typical neon lamp 



from the curve that the value of current depends somewhat upon the 

 direction in which the voltage is changing. In most cases, however, 

 the function comes sufficiently close to being single valued for our 

 present purposes. In view of the well-established linear corre- 

 spondence between the intensity of the illumination resulting from 

 the glow discharge and the current, it is required to so arrange the 

 circuits that the current through the lamp is at all times proportional 

 to the illumination at the transmitting terminal. 



It will be recalled that the electric signal wave as transmitted 

 through the various amplifier circuits differs fundamentally from the 

 initial luminous wave in that the direct current component has been 

 eliminated. It is necessary, therefore, to restore this component 

 before the changes in light intensity at the receiving terminal will 

 follow those at the transmitting terminal. The several factors entering 

 at this point may perhaps best be examined in terms of an elementary 

 circuit such as given in Fig. 26. In this case the neon lamp is con- 

 nected in series with the plate circuit of a vacuum tube and its 

 polarizing battery. The circuit may be considered for the present as 

 equivalent to one in which the neon tube is replaced by an ohmic 

 resistance and in which the potential of the polarizing battery is 



