70 



HEAT AND ENERGY 



The only difference is in the marking of the scale of 

 degrees, as shown by the two side by side in Fig. 55. 

 The Centigrade scale is largely used in scientific work. 

 Its standard is freezing water and is marked zero (0). 

 The temperature of boiling water is marked 

 one hundred (100). The space between these 

 marks on the scale is divided into 100 

 equal parts, each called a degree (). 



The Fahrenheit scale is most commonly 

 used by us. Its zero is the temperature of 

 a mixture of ice and salt, and the boiling 

 point of water is 212. Water freezes at 

 32 above zero on this scale. 



Experiment 62. Carefully test these substances 

 (freezing water, boiling water, and ice and salt) 

 with both thermometers. Compare the tempera- 

 tures of several substances on both scales, and try 

 to discover a rule for changing one reading to the 

 same temperature on the other scale. 



79. How a Thermometer is made. A 



small tube of hairlike bore, having a bulb at 

 one end, is partly filled with mercury. The 

 air is removed, because its pressure would 

 prevent the mercury from rising, and the tube 

 is completely closed. Mercury will expand 

 when heated (see 81) and shrink when 

 cooled, so that as the temperature rises or 

 falls, the mercury moves up or down the fine 

 FIG. 55 tube. Assuming that its expansion is uni- 

 form, we may compare temperature changes by compar- 

 ing the distances that the mercury column rises or falls. 



