HUMIDITY RECORDERS 239 



ni'cessiiry to devise nictluHls by which \vc mij;ht si-curt- such 

 information. 



The subject of hygrometry has long been one of llie problems lu 

 which various investigators have given attention and the results of 

 their work are a matter of record. 



Thus it has been recognized ' that, because of its ease of manipula- 

 tion and its accuracy if suitable precautions are observed, the venti- 

 lated psychrometer is a suitable instrument for use in humidity 

 measurements. 



Consitleration of the various types of hygrometers, commercially 

 available, indicated however, that none would be suitable if reliable 

 continuous records were to be secured. The use of simple wet bulb 

 — dry bulb hygrometers would require practically constant attendance 

 if frequent observations were made, and the results would not be 

 accurate unless arrangements were made to circulate the air o\er 

 the wet bulb. A pen recorder of the circular chart type to record 

 wet and drj- bulb temperatures had been used during one summer 

 in a telephone central office where the humidity conditions were se\ere, 

 but the results secured were not considered reliable because of the un- 

 satisfactory method used to ventilate the wet bulb, as well as the 

 sluggishness of the recorder due to pen friction on the chart. 



Considerable experience in the laboratory with a recording hair 

 hygrometer also had shown that, in addition to the inaccuracies to 

 which hair hygrometers are commonly subject, the friction in the 

 lever mechanism and between the pen and the chart made the instru- 

 ment too erratic to be considered of possible use in the work being 

 undertaken. Accordingly, a study was made to determine the 

 possibility of developing apparatus which would overcome the troubles 

 inherent in such recorders. 



Development of a Recording Hygrometer 



A promising method, developed by D. T. May of the Bell System 

 Laboratories and operated successfully in the laboratory, consisted 

 in the use of accurate and matched mercury thermometers, the stems 

 of which were contained in a camera which would enable the heights 

 of the mercury columns to be photographed upon a roll of sensitized 

 paper. Arrangements were made for shifting the paper between 



' I'. S. Weather Bureau Psychrometric Tables for Obtaininf; the Vapor-Pressure, 

 ReJative Humidity and Temperature of the Dew-Point from Readings of the Wet 

 and Dry Bulb Thermometers, by C. S. Marvin. 



Proceedings of the Physical Society of London, Feb. 15, 1922. The Measurement 

 of Atmospheric Humidity, by Sir Napier Shaw. 



