CONDUCTION 25 



plain their direction and force, but one very definite cause 

 is the unequal heating of the surface of the earth. 



13. Conduction. A poker used in stirring a fire becomes 

 hot and heats the hand grasping the poker, although only 

 the opposite end of the poker has actually been in the fire. 

 Heat from the fire passed into the poker, traveled along it, 

 and warmed it. When heat flows in this way from a warm 

 part of a body to a colder part, the process is called conduc- 

 tion. A flatiron is heated by conduction, the heat from the 

 warm stove passing into the cold flatiron and gradually heat- 



ing it. 



In convection, air and water circulate freely, carrying heat 

 with them ; in conduction, heat flows from a warm region 

 toward a cold region, but there is no apparent motion of any 

 kind. 



Heat travels more readily through some substances than 

 through others. All metals conduct heat well ; irons placed 

 on the fire become heated throughout and cannot be grasped 

 with the bare hand; iron utensils are frequently made with 

 wooden handles, because wood is a poor conductor and 

 does not allow heat from the iron to pass through it to the 

 hand. For the same reason a burning match may be held 

 without discomfort until the flame almost reaches the hand. 



Stoves and radiators are made of metal, because metals 

 conduct heat readily, and as fast as heat is generated within 

 the stove by the burning of fuel, or introduced into the 

 radiator by the hot water, the heat is conducted through 

 the metal and escapes into the room. 



Hot-water pipes and steam pipes are usually wrapped 

 with a non-conducting substance, or insulator, such as asbestos, 

 in order that the heat may not escape, but shall be retained 

 within the pipes until it reaches the radiators within the 

 rooms. 



