DEVICES USED FOR HEATING 



131 



steam. What physical change takes place in the boiler? What 

 is this change called? Does it absorb or give out heat 

 energy? What physical change takes place in the radiators? 

 What is this change called? Does it absorb or give out heat 

 energy? 



In the boiler, water is changed to . and heat is (ab- 

 sorbed, given out) In the radiator, is changed to 



This process is known as , and heat is (absorbed, 



given out) as it goes on. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



How is heat transferred from place to place? In the 

 first topic of this unit we were concerned chiefly with 

 a study of how heat energy is produced. We will now 

 be interested in learning how heat may be conserved, 

 controlled, and transmitted. In fact, a clear under- 

 standing of how to conserve heat depends upon a 

 study of the ways of transmitting heat. In general we 

 may say that there are three different methods of heat 

 transmission. They are called conduction, convection, 

 and radiation. 



Conduction. Heat always tends to pass from warmer 

 objects to cooler objects that are in contact with them 

 or from one end of an object to the other when one 

 end is warmer than the other. Everyone has experi- 

 enced this in everyday life. The handle of a silver 

 teaspoon placed in a cup of hot cocoa or coffee soon 

 becomes warm. When one end of an iron poker is held 

 in a fire the other end becomes heated by the flowing 

 of heat energy from the fire through the metal. The 

 passing of heat through these substances from molecule 

 to molecule is called conduction. It is explained in the fol- 

 lowing manner. The molecules of all substances are 

 continually vibrating. The silver teaspoon in the hot 

 liquid or the iron poker in the fire is raised in tempera- 

 ture ; this really means that the molecules are vibrat- 

 ing at a greater rate of speed. These more rapidly 

 moving molecules bump against those next to them, 

 making them vibrate faster. In this way the increased 

 molecular motion is transmitted to the other end of 

 the metal and produces the rise in temperature. 



Substances vary greatly in their ability to conduct 

 heat. Some conduct heat quickly while others resist 

 the flow of heat. In general, metals are better conduc- 

 tors than any other substances. Solids as a class are 

 better conductors than liquids, while liquids are better 

 conductors than gases. 



Light and porous substances such as felt, cork, and 

 asbestos are poor conductors. In fact, most of our 

 best insulating materials are porous. This is due to the 

 fact that most of their bulk is air, which is one of the 

 best insulating materials known. Heat travels through 

 a piece of aluminum nearly ten thousand times as 

 readily as it passes through air. The following table 

 gives lists of good, fair, and poor conductors. Study 



GOOD CONDUCTORS 



FAIR CONDUCTORS 



POOR CONDUCTORS 



them carefully; perhaps you can add other substances 

 to the lists. 



Convection. If you have studied the topic of weather 

 you are perhaps familiar with the fact that heat 

 travels in currents by convection. These currents, you 

 will recall, are set up in gases or liquids because of a 

 difference in temperature. The heated gas or liquid 

 expands and becomes less dense than the substance 

 around it. The lighter gas or liquid as a result is then 

 forced upward by the heavier matter, thus forming 

 currents. In this manner heat is distributed, the 

 heated particles themselves moving in the form of 

 currents. We shall find that convection plays an im- 

 portant part in several of the heating systems em- 

 ployed in heating our homes. 



Radiation. Transmission of heat by conduction and 

 convection requires the presence of matter. Hot 

 bodies, however, also radiate heat, that is, they send 

 out waves of heat in straight lines which are able to 

 pass through a perfect vacuum. 



The heat we receive from the sun comes to us by 

 radiation. These radiant waves travel across the 

 93,000,000 miles of intervening space at the enormous 

 speed of 186,264 miles per second. 



Radiant heat waves are peculiar in that they pass 

 through some substances such as air and glass with- 

 out heating them to any appreciable extent. These 

 substances are transparent to radiant heat waves. 

 When heat waves strike a substance through which 

 they cannot pass, they make the molecules in the sub- 

 stance vibrate faster, thus raising the temperature of 

 the matter. 



It has been found by experiment that substances 

 with rough surfaces absorb the radiant heat better 

 than substances with smooth or polished surfaces. In 

 general, good absorbers are good radiators, and poor 

 absorbers are poor radiators. Also dark-colored sub- 

 stances are better absorbers of heat than light-colored 

 objects. Why is the thermos bottle silvered inside and 

 out? Why does it have a vacuum space between its 

 walls? How does it keep hot things hot and cold 

 things cold? 



What are some of the earlier methods of heating the 

 home? The problem of how better to heat and venti- 

 late the home has occupied the attention of mankind 



