38 VETERINARY HYGIENE 



Diagrammatically the various scales may be represented thus :- 



Boiling point of 

 water. 



Melting point of 

 ice. 



100 



v 



1 



53 

 U 



80 



373 



273 



Maximum and Minimum Thermometers. The maximum 

 thermometer automatically gives a record of the highest temperature 

 reached during an interval of time. The thermometric substance 

 is usually mercury, and the thermometer is hung horizontally. One 

 method of recording is by means of a constriction near the bulb. 

 When a fall of temperature occurs the mercury thread is broken 

 at the constricted portion of the tube and reading of the end of the 

 thread farthest from the bulb gives the maximum temperature. 

 Another form utilises an air bubble in the mercury thread, this 

 bubble performing the same function as the constriction in the 

 pattern described above. The minimum thermometer gives, as its 

 name implies, the lowest temperature reached during an interval of 

 time. The thermometric substance is alcohol or some such spirit, 

 and with a fall of temperature the liquid contracts and draws with 

 it a small index immersed in the liquid. A later expansion does 

 not move the index as the liquid flows past it. 



FIG. 2. Maximum and Minimum Thermometers. 



Thermographs are self-registering thermometers giving varia- 

 tion of temperature with time. 



HUMIDITY refers to the amount of aqueous vapour present in 

 the atmosphere. The existence of vapour pressure can be shown 



