HEAT AND EXPANSION -^ ^ '> \ /, 101 



thermometers: the Fahrenheit used in this country and 

 England for ordinary purposes, and the Centigrade used in 

 Continental countries, and by scientists. 



A thermometer consists of a cylindrical glass tube of a 

 uniform bore and diameter, sealed at one end. A fluid is 

 first placed in the tube, which then is heated until the fluid 

 expands and fills the tube, thereby driving out the air. 

 It is necessary to create a vacuum; otherwise the air would 

 prevent the fluid from expanding in the closed tube. After 

 the air has been driven out the tube is sealed. It is then 

 placed in an atmosphere of free steam representing the boil- 

 ing point of water, and next in an ice bath consisting of 

 broken pieces of ice floating in water. The positions of 

 the liquid at both of these points are marked on the tube, 

 the boiling point representing 212 F. and the freezing 

 point 32 F. The intervening distance between these two 

 points is divided into 180 divisions and each division is 

 called a degree. The Centigrade thermometer has 100 

 divisions between these two points. Mercury is especially 

 adapted for use in thermometers on account of the uni- 

 formity with which it increases in volume, and also on 

 account of its extremely high boiling point. Alcohol 

 colored with some dyestuff is used in cheap household 

 thermometers. 



102. Measurement of Temperature in Industry. Ther- 

 mometers assist us in comparing or fixing the temperature 

 of certain industrial operations. This is important, as in a 

 great many manufacturing operations it is necessary to know 

 when a certain temperature is reached. As a result a num- 

 ber of different kinds of thermometers have been invented. 

 They are all based upon the same principle as are the Fahren- 



