TRANSFER OF HEAT 21 



times as rapidly as iron. A flat iron placed on a stove 

 becomes heated throughout; cooking utensils become 

 very hot; stoves heat quickly from the burning fuel 

 within them ; etc. 



If a test tube full of water is held in a flame so that 

 the water boils at the top, it will be some time before 

 the tube will be hot enough to pain the hand by which 

 it is being held. A bar of metal of the same size would 

 be hot before the tube of 

 water even felt warm. As 

 compared with metals, water 

 is a poor conductor of heat. 



Iron utensils frequently 

 have wooden handles because 

 wood is a poor conductor and 

 does not allow heat to pass' 

 quickly through to the hand. Fig:. 11. when a test tube 



full of water is heated at One 

 AsbestOS is another poor COn- end, ^tak es sometimejjefore 



ductor. It is frequently used becomes warm - 



to wrap hot water pipes and steam pipes in order that 



the heat may not escape. 



If the hand is placed in water, which is the tempera- 

 ture of the air around it, the water seems much colder 

 than the air. This is because water is a better conductor 

 than air and so takes the heat out of the hand more 

 rapidly. 



Fireless cookers. The fireless cooker depends upon 

 the principle of non-conduction. One vessel is placed 

 inside another. The space between the two is filled 



