Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 9 
GLASS-STEM INDICATING INSTRUMENTS 
A.—Wet and dry bulb hygrometer, the common form of this type of instrument. The right-hand 
thermometer is ordinarily the one used as a wet bulb, though either one can be so used. The central 
reservoir, which is readily detachable, must be filled with water, preferably soft or distilled, often 
enough to keep the wick fully moist at all times. Accurate readings require a brisk circulation of 
air past the wet bulb; this is usually secured, in the absence of natural circulation, by vigorous 
fanning. 
B.—Sling psychrometer. This instrument is a form of wet and dry bulb hygrometer. The air 
circulation needed to procure evaporation from the wet wick 6 is secured by whirling the entire 
instrument around the handle d; the metal sleeve c protects both the dry bulb a and the wet bulb. 
The sling psychrometer is convenient for use at the normal atmospheric temperatures of shops and 
storerooms, but has not found favor in dry kilns. 
C.—Etched-stem chemical thermometer a with metal protecting sleeve 6. This type makes a 
very satisfactory standard for the calibration of indicating and recording thermometers and hy- 
grometers. 
