SOME EFFECTS OF HEAT 65 



effect. On every hand expansion and contraction due to 

 changes in temperature must be taken into account. The 

 ends of steam pipes are allowed to be free and are never 

 attached firmly. The ends of the spans of long iron bridges 

 are placed on rollers. In places where there are considerable 

 ranges of temperature concrete sidewalks are cut into squares 

 instead of being laid as continuous solid surfaces. When 

 iron tires are fitted to wagon wheels they are first heated 

 and then placed on the wheels and allowed to cool. Tele- 

 phone wires are tighter in winter than 

 in summer. For this reason they are not 

 stretched taut when put up. 



Experiment 21. Heat a metal compound FIGURE 27 



bar. It bends over on one side. The more 

 the bar is heated the more it bends. (Figure 27.) The two 

 metals do not expand at the same rate. 



Various solids and liquids expand and contract at different 

 rates. Platinum expands and contracts at almost the same 

 rate as glass. When platinum and glass are fused together 

 they expand and contract almost as one substance. For 

 this reason, in the manufacture of incandescent lamps, plati- 

 num is the only substance that can be used to pass through 

 glass to carry the electrical current to the filament within. 

 Other metals contract either more rapidly than the glass 

 and thus let air into the bulb, or more slowly and thus 

 break the glass. One reason why mercury is used in ther- 

 mometers is that it changes rapidly in volume with changes 

 in temperature. 



Different parts of the same substance will expand at 

 different rates according to the amount of heat applied. 

 When experienced housewives wash glasses in hot water, 

 they do not dip them slowly ; they plunge them in quickly 



