The Propagation of Heat 183 



120. From everyday experience we know that the 

 end of a poker placed in a hot fire soon becomes red- 

 hot, and that the other end often reaches such a high 

 temperature that it is too hot to hold, although the 

 hand may be held for some time at a distance from 

 the fire equal to the length of that portion of the poker 

 outside the fire without becoming uncomfortably hot. 

 Heat evidently travels along the material of the poker 

 in such a way that the end becomes hotter than the 

 air situated at the same distance from the fire. 



EXPERIMENT 65. Obtain two thin rods of different 

 metals of similar diameter. A knitting-needle and a 

 length of stout copper wire will serve if no better 

 materials be available. Support the rods on a sheet 

 of asbestos millboard placed upon the top of a tripod 

 stand in such a way that one end of each may be heated 

 by the same Bunsen flame. Arrange that the heating 

 shall be as nearly as is feasible the same for each rod. 

 Test the temperatures attained by the other ends of 

 the rods from time to time by rough comparative 

 methods such as the following : 



(a) By touching the ends with the fingers, find 

 out which first becomes too hot to be held with comfort. 



(6) By placing the end of a wax-taper at each 

 end in turn, find which first reaches the temperature 

 at which the wax melts. 



(c) Place the head of a match at each of the 

 ends to discover whether either is hot enough to ignite 

 the match. If neither becomes sufficiently hot to do 

 this, move the match-head along the rods to find that 

 point on each which has become hot enough to bring 

 this about. 



If rods of copper and iron be made use of, as sug- 

 gested, the copper will be observed to become hot much 



