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BELL SYSTEM TECHNICAL JOURNAL 



function of this measuring device. The object is to observe the change 

 in voltage experienced by the upper plate of the level compensator 

 condenser when incoming signals are changed from steady marking to 

 dots. The circuit provided for this purpose is essentially a vacuum- 

 tube electrostatic voltmeter of the conventional type. 



Acknowledgment 



The advances in the art which have been described in the foregoing 

 pages have taken place over a number of years and cover a considerable 

 variety of subjects. Much of the material is to be found only in test 

 reports and unpublished memoranda. Since such work is, of ne- 

 cessity, the product of the cooperation of many minds, it is imprac- 

 ticable in most cases to apportion credit equitably and the author 

 must therefore confine himself to a general expression of indebtedness 

 to his associates. 



APPENDIX 

 LEVEL COMPENSATOR THEORY 



The theory of the level compensator may best be considered in two 

 stages, first taking the steady marking condition, and second the 

 condition where signals are being received. 



Steady Marking Condition 

 Figure 25^ shows the essential elements involved when the steady 

 current /o flows through the receiving relay. Figure 2SB is the 



F"ig. 25 — Theory of level compensator. .1. Simplified circuit. 

 B. Equivalent circuit. 



equivalent of the grid circuit shown in heavy lines on Fig. ISA, on the 

 assumption that the grid-filament space is effectively a switch which 

 turns the grid current on or off when the grid goes positive or negative, 



