THE WATER VAPOR OF THE AIR 219 



Exercise 53. Making a Psychrometer and Determining the Relative 



Humidity 



If the laboratory has no psychrometer (Fig. 150), make one by 

 attaching two common thermometers to a piece of inch board 2 by 6 

 in. Fasten a strong cord to the upper end for whirling the psychrom- 

 eter about the head. The whirling must be done with care until 

 practice shows the best movements. Fasten a single wrapping of 

 clean muslin about one bulb. Use as in Art. 247. 

 Before using, the readings of the thermometers 

 should be compared for all temperatures, and a 

 correction card made (Art. 226, Fig. 139) for the 

 wet bulb, showing all differences between the 

 thermometers. Correct the readings before using 

 Table XI. Find the humidity: (1) On a clear day; 

 (2) a damp cloudy day; (3) in morning; and (4) in 

 mid-afternoon of a clear day. Record all results. 



249. The Hair Hygrometer. This hygrom- 

 eter shows relative humidity directly on a 

 dial, and is more convenient than the psy- 

 chrometer but not so accurate. Human hair, 

 with the oil removed, lengthens with damp- 

 ness and shortens with drying. A strand of 

 this hair is attached at one end to a rigid 

 frame, and at the other end to the circum- 

 ference of a small cylinder. A spring holds FlG - 151. Hair 

 the hair taut. A pointer is mounted on the 



end of the cylinder. As the hair changes in length with vary- 

 ing humidity it turns the cylinder and moves the pointer over 

 the dial (Fig. 151). 



250. Measuring the Moisture ; Absolute Humidity. Table 

 XII, page 220, may be used together with the relative hu- 

 midity as found from Table XI, to obtain the weight of the 

 water vapor present, expressed in grains per cubic foot. 



In the last problem (Art. 248), the temperature of the space was 

 50. Table XII shows that space at 50, if saturated (humidity 

 100 per cent.), would contain 4.076 grains of water vapor per cu. ft. 

 But in this problem the humidity was only 77 per cent.; therefore 

 the vapor present is 77 per cent, of 4.076 grains, or 3.14 grains per 

 cu. ft. 



