46 THE HUMAN MOTOR 



In another form it can be written 

 T EQ = , 



and signifies that the internal energy of a body in which heat is 

 transformed into work has not varied. In a steam engine a body 

 of water is converted into steam, which moves a piston, after 

 which the water is brought back to its initial temperature, the 

 piston also returning to its original position, the cycle of the 

 operation being completed. The principle of Equivalence, in 

 virtue of which 



T = EQ 



presumes therefore a completed cycle of operations. 



The authoritative demonstration of this fact is due to an 

 experiment by Edlund ; work T is expended in pulling a metal 

 wire by means of a weight P. The wire evolves heat to the 

 amount of q calories. Allow it to return to its original length by 

 removing the weight and it will evolve q' calories ; and we see 

 that : 



T T 

 -? = r ' 



A return to the initial state, and an absence of variation of the 

 internal energy is therefore necessary. 



But if the cycle is not completed, if the body does not ,?o back 

 to its initial state, its internal energy will have undergone a varia- 

 tion U. Then the equivalence between the variation U and the 

 work T on the one side and the heat Q on the other will be 



T-f- U 



= Q ( in calories )- 



In the experiment of Edlund it was 



T 



If, by the aid of chemical reactions, a body could produce 

 heat and work there would be an equivalence between the 

 variation of its internal energy U and the external mechanical 

 and thermal phenomena, the heat being either evolved or absorbed. 

 From which : 



U = T db EQ. 



Chemists have made an interesting application of this law of 

 equivalence : they burn an organic substance in a closed and 

 perfectly rigid vessel and the gases due to the combustion not 

 being able to expand, there will be no work : 



T = o. 

 Therefore : 



U = EQ: 



