THERMODYNAMICS OF ELASTIC BODIES. l6l 



upon both the intrinsic nature of the substance of the bod}^ 

 itself as well as upon the manner of application of the force 

 — and perhaps upon other causes. As examples there may be 

 cited the following facts which have been both theoretically 

 and experimentally established : — 



The heating of a body under ordinary circumstances 

 increases its elastic resistance as regards cubical compression : 



The torsional rigidity of a wire is diminished by an 

 increase of temperature, at least in the case of the com- 

 moner metals : 



A spiral spring under tension has its elastic force dimin- 

 ished by an increase of temperature : 



A bar subjected to end thrust acquires a higher coefficient 

 of elasticity when its temperature is raised : 



On the other hand, a bar or wire under tension loses some 

 of its elastic force when heated : 



A rubber band under tension acquires an increased elastic 

 force upon heating. 



In the foregoing examples it goes, of course, without say- 

 ing, that the effect of cooling is to produce the corresponding 

 inverse elastic effects. 



As before stated, a satisfactory explanation of the cause of 

 these effects is not yet forthcoming. For the present it is 

 better to accept them as experimental facts. It is seen that 

 two principal factors concerned in the varying actions are 

 first, the intrinsic nature of the material, as evidenced by the 

 steel wire and the rubber band under tension ; and second, the 

 nature of the strain, as illustrated by the steel bar under 

 compression and tension respectively. 



Corresponding to these effects of heat upon elastic force 

 are certain inverse heat effects resulting from mechanical 

 strain produced in bodies by the application of force. The 

 general thermodynamic principle underlying these is set forth 

 in the Encyclopedia Brittanica in the article " Elasticity," 

 Volume \'II, as follows : — 



" Heat is produced when a solitl is strained against, and 



