112 HOW WE USE HEAT 



This demonstration shows very clearly that heat causes expan- 

 sion. Contraction of substances is brought about when heat is 

 withdrawn from them. On a hot day a metal drawbridge after 

 being opened refused to come together again. Why? The draw- 

 bridge tender squirted a stream of cold water upon the bridge and 

 it went back into place. Why? 



How Temperature Is Measured. We have seen that 

 heat makes things warmer, and the higher the tempera- 

 ture of a body, the more heat it contains. But heat and 

 temperature are not the same thing. Temperature 

 is the intensity of heat and is measured in units called 

 degrees. This measurement makes use of the principle 

 that matter when heated expands, and when cooled 

 contracts. The thermometer is made of a glass tube 

 having a very fine bore. A bulb at one end of the tube 

 is filled with mercury or colored alcohol. Since heat 

 makes the liquid expand, it will rise in the small bore in 

 the tube as it gets warm and thus indicates the degree of 

 heat which is marked on the scale. 



Thermometer Scales. There are two thermometer 

 scales, the Fahrenheit marked F., used by the weather 

 bureaus and in everyday life, and the Centigrade marked 

 C., used in scientific work and most foreign countries. 

 The freezing point of water is 32 on the Fahrenheit scale 

 and zero on the Centigrade, while the boiling point of 

 water at sea level is 212 on the Fahrenheit scale and 

 100 on the Centigrade. 



Heat Causes Changes in the State of Matter. Another 

 thing that heat does is to change the form of different 

 substances. The quantity of heat energy possessed by 

 the molecules of water determines whether water is in the 

 solid, liquid, or gaseous state. The molecules in ice have 

 the least energy. Since heat is taken from warmer objects 

 to change ice to water, ice is used in our refrigerators. 

 Molecules of liquid water have more energy than molecules 



