94 



Introduction to the Study of Science 



which process is known as conduction. Any substance which 

 is thus heated by being in contact with a source of heat is called 

 a conductor. 



Examples of heat conductors. When the poker becomes too hot 

 to hold with the bare hand, one may use a holder of such material as 

 paper, cloth, wood, or asbestos. While we know from common 

 experience that all sorts of substances may be heated by conduction, 

 we know also that some are heated at different rates of speed and with 

 different degrees of resulting temperature in various parts. We 

 know that paper is a poor conductor, as are the other substances 

 named above. What utensils are often made with wooden handles 

 for use about the fire? Why are lid-lifters equipped with handles 

 of different material or relatively small diameter wire of the same 

 metal? What substances besides wood are used because they are 

 poor conductors of heat? 



Exercise : Relative conductivity. The differences in conductivity 

 of several common metals may be illustrated experimentally. Pro- 

 cure a six-inch length of each of the following metals, 

 all being of the same diameter as that of a telegraph 

 wire : iron, copper, brass, aluminum, lead, or other 

 available metals. They may be tested in either of 

 these ways. 



(1) Dip the metals in melted paraffin and when 

 thoroughly coated, allow them to cool. Arrange 

 them so that the ends are simultaneously heated by 

 a flame, and the rest of each rod is shielded from 

 the heat by a screen. Note the difference in the 

 rate of melting on the rods. Does this indicate the 

 rate at which each conducts heat ? 



(2) Arrange the rods, fan-shaped, upon an as- 

 bestos plate (Fig. 28), and supported high enough to 



FIG. 28. Ar- 

 rangement of metal 



rods to determine a u ow an alcohol lamp or a Bunsen burner to stand 

 conductivity 6 *"** & the flame touching the ends. After heating a 

 few moments move a match head slowly along each 

 rod toward the flame. The points at which ignition occurs indicate 

 the comparative conductivity of the metals. Classify the metals ac- 

 cording to their ability to conduct heat. 



Exercise : Water as a heat conductor. Fill a test tube nearly full 

 of water and, holding the lower part of the tube in your hand, apply 

 heat to the top of the water (Fig. 29). Does the water conduct heat 

 rapidly? Another experiment may be substituted for the above. 



