240 



BELL SYSTEM TECHNICAL JOURNAL 



exposures, and a small exhaust blower was provided for circulating 

 the air over the wet bulb. The whole apparatus was controlled 

 electrically by a clock and was arranged to record the wet and dry 

 bulb temperatures at any desired time interval. When the com- 

 plete record roll had been exposed it was removed and upon develop- 

 ment showed the thermometer readings from which the corresponding 

 humidities could be found in the psychrometric tables. While this 

 t\pe of recorder would no doubt have enabled accurate informa- 

 tion to be obtained, it had two inherent objections. These were, 

 first, the bulkiness of the complete equipment which had to be placed 

 at the location where the conditions were to be determined and. 



WWvvWW 



WtT BULB D«y BULB 

 THUMOMtTtft TnCRMOMCTCR 



rig. 1 — IJridge Circuit of Difference Recorder 



second, the thermometers could not be read because their stems 

 were within the camera box, and therefore, the humidities and tem- 

 peratures measured could not bo ascertained initil the record had 

 been developed. 



t Accordingh', at this time, consideration was gi\en to a type of 

 mechanism which would produce a visible record upon a chart con- 

 tinuously a\'ailable for observation by the operator. It was found 

 that the Leeds & Northrup automatic recorder had been in com- 

 mercial use for some time for the measurement of furnace temperatures, 

 by means of thermocouples in conjunction with an automatically 

 adjusted potentiometer circuit. The same t\pe of recorder also had 



