THE TRANSFERENCE OF HEAT 



89 



FIGURE 43 



rents were seen to be developed. The heated and expanded 

 air and water rose. Here again the heat was transferred 

 by conduction, but it was helped by the 

 upward movement of the heated water 

 and air. These upward movements of 

 the water and the air are known as con- 

 vection currents. The efficiency of the hot 

 water and hot air furnaces which heat our 



houses is 

 due to the 

 convection 

 currents. 

 We shall 

 find later 



that if it were not for con- 

 vection currents there would 

 be no winds nor ocean cur- 

 rents. 



Whether we heat a test 

 tube of water from above or 

 from below, the heat is car- 

 ried by conduction from one 

 molecule to another. But 

 when we heat it from below, 

 the process is hastened by 

 convection currents. 



If an incandescent lamp 

 (Figure 45) is turned on and 

 the hand held a little dis- 

 tance from the glass bulb, 

 the hand will be warmed, 

 although the glass bulb itself 



FIGUEE 44. HOT WATER 

 FURNACE 



As the water in the boiler begins to 

 heat, convection currents are set 

 up. Cold water, which is heavier, 

 flows from the radiators down into 

 the boiler and forces warmer water 

 up into the radiators. As long as 

 fire is maintained in the furnace, 

 there is constant circulation. Since 

 water expands under heat, an over- 

 flow tank must be provided to 

 prevent explosion of the pipes or 

 boiler. 



