1 62 C. M. BROOM ALL : 



cold when it is strained by yielding to, any elastic force of its 

 own, the strength of which would increase if the temperature 

 were raised. And again, cold is produced whenever a solid 

 is strained by opposing, and heat when it is strained by yield- 

 ing to, any elastic force of its own, the strength of which 

 would diminish if the temperature were raised." 



Two of the examples above cited, namely, the spiral spring 

 and the rubber band under tension, furnish the means of pre- 

 paring a simple apparatus which at first glance seems to act 

 in a manner somewhat paradoxical. 



Let there be hung from appropriate points of attachment a 

 spiral spring and a rubber band, each supporting an equal weight 

 at the same level. Assuming the two systems at the same 

 temperature and the weights at the same elevation, comparisons 

 between the two cases are easily made. Let a source of heat 

 be now brought close to the bodies. The temperature of both 

 spring and rubber will rise and after a certain time they will 

 both be at the same increased temperature. But the spiral 

 spring will have elongated and the rubber band shortened, 

 and the weights will have been respectively lowered and 

 raised. The system containing the spiral spring, which we 

 will call S, will therefore have lost potential energy, and the 

 rubber band system, R, will have gained potential energy, 

 both effects, however, resulting from the same cause. 



On the face of it this looks a little like a contradiction of 

 the law of the conservation of energy. In other words, we 

 are giving heat energy to system S, and yet after things have 

 come to a state of temperature equilil)rium we find the poten- 

 tial energy to be less. 



Again, let the heated body be now removed and the sys- 

 tems cooled to the original temperature. The two weights 

 will return to the original level, and system S will now have 

 gained potential energy as a result of taking away heat. 

 This again, at first glance, appears odd. 



Of course any difficulty in understanding the above actions 

 rests upc^n the fact thai we are not distinguishing properly 



