TRANSFER OF HEAT 



81 



move about, convection is limited to liquids and gases. 

 The direction of movement in convection is upward and 

 downward, warmer particles rising and cooler ones 

 falling. As any portion of a fluid body becomes heated 

 it expands, that is, its particles are farther apart ; 

 thus the heated portion becomes lighter than the cooler 

 parts of the fluid around it. Of course gravity will then 

 pull the heavier parts downward, and the lighter heated 

 portion will be forced upward. 



Experiment 75. Heat a can of water. Before it is entirely 

 warmed through test its temperature at different depths. 



92. Uses of Convection. Fluids, especially gases, are 

 such poor conductors that they can only be heated very 

 slowly by conduction. In fact, dry air is 

 so poor a conductor that it would hardly 

 carry heat at all ; we should have to live 

 constantly in very cold air, if it were not 

 for convection. The rise of heated air 

 from a lamp chimney (Fig. 61), which 

 may be easily noted, shows us how readily 

 air may be set in motion ; and in just the 

 same way the warm air above a stove 

 or other heater rises and is spread about. 

 Similarly the water in a kettle is quickly 

 heated by convection ; the warmer parts, 

 constantly rising to the top as they be- 

 come heated, allow the colder portions 

 to receive heat at the bottom. Fig. 62 

 shows how the rise of warm air above a fire keeps it 

 supplied with a good draught of fresh air from below. 



FIG. 61 



