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BELL SYSTEM TECHNICAL JOURNAL 



these two resistances will indicate directly the relati\e humidity 

 corresponding to the dry and wet bulb temperatures. The circuit 

 arrangement by means of which this is accomplished is shown in 

 Fig. 10, and the mechanism of the recorder emi)lo\ing it, is shown in 

 Fig. 11. 



The recorder circuit contains three Wheatstone bridges with one 

 battery and galvanometer which are transferred in rotation from 



Fig. 10 — Circuit of Direct Reading Recorder 



each bridge to the next by the commutator antl rela\s shown in the 

 circuit. The three bridges are arranged so that they remain at their 

 last positions of balance until mechanically connected to the balancing 

 mechanism of the recorder by the electric clutch associated with each 

 bridge whose operation also is controlled by the commutator. The 

 first of these bridges, designated the "dry bulb bridge," contains 

 the dry resistance thermometer and mechanically associated with 

 its slide wire contact is a second slide wire contact operating upon 

 a slide wire resistance arm in the third bridge, designated as the 

 "humidity bridge." The second of these bridges, designated as 

 the "wet bulb bridge," contains the wet resistance thermometer, 

 and mechanically associated with its slide wire contact is a second 

 slide wire contact operating upon a second slide wire resistance arm 

 of the "humidity bridge." 



The consecutive balancing of the "tlr>- bull) bridge" and "wet bulb 

 bridge" accordingly sets off resistances upon the two slide wire resist- 

 ance arms of the "humidity bridge" proportional respectively to the 

 temperature differences described in the second preceding paragraph. 



