MEASUREMENTS OF HEAT 



129 



CENT. PAHH. 



But to go back to the thermometer, and the expansions 

 and contractions which it records. If increase of heat 

 causes expansion of substances in general, then we may 

 expect that the glass of the thermometer will expand 

 when the mercury expands, and contract when it contracts. 

 And it does. Evidently, if the glass ex- 

 panded and contracted at the same rate as the 

 mercury, the thermometer would be value- 

 less. But it does not. The glass expands 100 

 and contracts only one-seventh as much as w 

 the mercury. Hence, what a thermometer M 

 actually records is the di/erence between the 70 

 expansion and contraction rates of mercury w 

 and glass, and its value as a recording in- 80 " 

 strument depends on the fact that there is * 

 such a great difference between these rates. 80 



In the manufacture of thermometers mer- 20 

 cury is poured in at the top of the tube * 

 (then open) until it fills the bulb and part of 

 the tube. Then heat is applied until the - 10 

 mercury has expanded to the top, and the " 17-78 

 top is closed by melting (fusing) the glass 

 at that point. Then the bulb is surrounded 

 by finely broken ice. When the mercury 

 has stopped falling (contracting) the point 

 at which it stands is marked on the glass. 

 That point is called the freezing-point the freezing-point, 

 that is, of pure water; different substances have different 

 freezing-points. The next thing is to mark the boiling- 

 point. This seems a perfectly simple thing to do until 

 you remember that the boiling-point of water is not al- 

 ways the same. You recall that water boils at a much 



FIG. 55. The mer' 

 cury thermom~ 

 eter. 



