EFFECT OF HEAT ON VOLUME 



57 



a day. In stringing telegraph, telephone, and other wires, 



the temperature must also be considered. If drawn too tight 



on a hot day the wires may shorten and snap on a cold one. 



Long runs of steam pipes have to have " expansion joints" at 



intervals to take up the extra length of the pipe when heated. 



In late fall and early spring, plants are often killed by being 



"heaved by the frost." That is, during a thaw, 



water accumulates about them and in freezing 



expands and lifts them out of the ground. In 



the same way, foundations may be damaged if 



they are not carried down into the earth below 



the frost line. Expansion caused by the frost 



breaks up rocks into soil and makes plant 



growth possible. Land is often plowed late in 



fall to permit of this mellowing action of the 



frost. Since solids and liquids differ from gases 



and do not expand alike, it is often necessary 



to make a nice choice of materials in order to 



insure the proper working of our machines. 



Invar, a compound of steel and nickel, often 



called nickel-steel, expands very little with 



changes of temperature and is therefore an 



ideal substance from which to make measuring 



instruments, clock pendulums, scales, and the 



like. If pendulums were made of ordinary ^ FlG ' 20 ;~ 



* . * Compensated 



metals they would be too long in summer and mercury pendu- 

 cause the clock to run slow. In winter with the l " m : [ Black and 



Dctvis.) 



same pendulum the clock would run too fast. 

 In good clocks, the pendulum is often made of two different 

 metals with different rates of expansion so that one counter- 

 acts the effects of the other. Alcohol and mercury, on the 

 other hand, rapidly change in volume with changes in tem- 

 perature and thus are useful in thermometers. In electric 

 light bulbs the current must be carried to the filament by 



