[Proc. Roy. Soc. Victoria, 24 (N.S.), Pt. II., 1911.] 



Art. XVI. — The Ft^ycJivcmietric Formula. 



By E. F. J. LOVE, M.A., D.Sc, F.R.A.S. 

 (Lecturer on Natural Philosophy, University of Melbourne) 



AND 



G. SMEAL, B.Sc. 



(Governuient Research Scholar). 



(With Plate XLYIIL). 

 [Read 14th September, 1911.] 



The foniiula employed by Regnault for the wet-and-dry 

 bulb hygrometer, or psychrometer, has been generally adopted, 

 viz. : 



X = f — A B ft 



t')^ 



where x and /' are respectively the actual pressure of water- 

 vapour in the air, and the saturation pressure at the tempera- 

 ture /' of the wet-bulb. But Ekholmi, of Sweden, has sug- 

 gested modifying this formula by the insertion of a " hygros- 

 copicity factor" r], to allow for a. supposed diminution of the 

 maximum pressure at the wet-bulb due to the hygroscopic 

 nature of the material covering it. The formula as modified 

 becomes : — 



X = /;/ - A B (/ - /'). q < \. 



Firstly, it may be stated that it seemed to us, on a careful 

 examination of Ekholm's paper, that the grounds for the 

 theory are very slight. Its importance, however, is great 

 enough to justify a careful discussion. It appears to have been 

 suggested by the well-known difficulties attending the use of 

 the psychrometer at temperatures below zero, when the wet- 

 bulb becomes coated with ice. In these circumstances it is 

 necessary to use a value of A different from that used when 

 the water on the bulb is fluid, and observation confirms the 



1. Ekholm, " Ueber das Psychrometer," Arkiv. fiir Mat. Astron. och Fysik, Stock- 

 holm, 1908, B(l. 4. 



