CHAPTER XVII 

 MEASUREMENTS OF HEAT 



You learned in the preceding chapter that thermometers 

 and calorimeters are instruments for the measurement of 

 heat. A thermometer measures temperature, which is 

 the degree or intensity of heat, while a calorimeter measures 

 its amount. Both instruments are extremely useful. 



The Thermometer. The most familiar form of ther- 

 mometer is the mercury thermometer, to which you gave 

 some attention when studying the barometer (see page 

 58). It consists of a glass bulb and a tube of very small 

 caliber arising from the bulb (see Fig. 55). Mercury fills 

 the bulb and rises and falls in the tube as variations in 

 the intensity of heat cause it to contract or expand. 



Since we have noted that increase of heat is associated 

 with increase of the activity of molecules, it does not 

 surprise us to find that this involves expansion of the sub- 

 stances heated. But changes of temperature produce 

 other effects besides expansion and contraction. They 

 may change the state of a substance, as from solid to liquid 

 to gas, as in the case of water. Or they may produce 

 changes in its quality which are not so obvious. Thus, 

 in tempering steel, the quality given to the steel depends 

 upon the temperature changes to which it has been sub- 

 jected. All such changes are due to changes in the nature 

 of the molecules, 



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