14 



A YEAR IN SCIENCE 



and heated until it expands sufficiently to slip on the 

 wheel. When it cools it contracts, and then fits the 

 wheel closely. 



In construction work, when two pieces of steel are 

 riveted together, the rivets are used red hot. When 

 they cool they contract and thus draw the two pieces 

 of steel closer together. 



While we generally notice the expansion of a body in 

 one direction only, that is in its 

 length, it must not be forgotten that 

 a body expands in three dimensions. 



The increase in length of a sub- 

 stance is known as its linear expan- 

 sion. The expansion or contraction 

 of different substances is not equal 

 for the same change in temperature. 



If a bar, made of two different 

 metals tightly welded or riveted to- 

 gether lengthwise, is heated, it bends. 

 This is because one of the metals 

 is expanding more rapidly than the 

 other. If the bar is then cooled, it 

 again becomes straight. In this in- 

 stance the one side is contracting 

 faster than the other. 



A steel wire which measures ^ 

 mile on a cold winter day will gain 25 inches in length 

 on a warm summer day. Under the same conditions an 

 aluminum wire would gain about 50 inches. 



Fig. 7. A bar 

 which is. made of 

 two different met- 

 als, bends when it 

 is heated, because 

 one of the metals 

 expands more rap- 

 idly than the other. 



