EFFECTS OF HEAT 



117 



only material which does not expand with heat is rubber, which 

 shortens; however, its volume increases. 



Experiment to Show Expansion. Through an iron or brass bar 

 drill a hole just large enough to allow a rod of the same material 

 to pass through. Heat the rod until it will no longer pass through 

 the hole. Fig. 92. 



Melting Substances. Most substances expand when melting and 

 contract when changing into a solid. However, cast iron, type metal 

 and water are examples of sub- 

 stances which contract on melting 

 and expand when solidifying. 



Difference in the Rate of 

 Expansion of Different Metals. 

 All substances do not expand at 

 the same rate. If a piece of iron 

 and a piece of brass be riveted 

 together to form a compound bar, 

 as shown in the illustration, and 

 heated, the bar will form an arc and become straight upon cooling. 



Why has the bar curved toward the iron side when hot? 



Why has the bar curved toward the brass side when cold? 



The different rates of expansion are called the coefficients of 

 expansion of the respective metals. 



Coefficient of expansion means the amount the substance will expand 

 if heated 1| F. or 1 C. 



\^_ 



WORHAL 



HOT 



PIG. 93. 



Expansion in Incandescent Lamps. One of the metals that can 

 be used to connect the filament of an incandescent lamp through the 

 glass is platinum, since platinum and glass expand practically the 

 same amount on being heated. (See table.) The platinum is 

 sealed into the glass hot; the wire and glass contract equally, 



