628 SUPPLEMENTARY. 



CHAPTER XI. 



SUPPLEMENTARY. 



644. CONCLUSIONS FROM CARNOT'S PRINCIPLE. Sir W. 

 Thomson showed that the principles which serve as the basis 

 of the theory of heat that is to say, the principle of the 

 conservation of energy and Carnot's principle render it possible 

 to establish some important properties relating to electrical and 

 magnetic phenomena. 



Whenever a body loses heat or changes 'its dimensions, in 

 opposition to external forces which tend to deform' it in the 

 contrary directions, it does work. Whatever be the cycle of 

 transformation, the external mechanical work only depends on the 

 initial and final state of the body. In all cases, this mechanical 

 or calorific work corresponds to a loss of energy of the body in 

 question. 



The intrinsic or potential energy of a body is the total work 

 which it could do if it were indefinitely cooled, or if it were 

 expanded or contracted to an unlimited^ extent, according as the 

 molecular forces are attractive or repulsive. There is no means 

 of measuring this energy, nor even of knowing whether it has a 

 finite value for a limited mass ;. but we may measure the changes 

 which it undergoes, starting from a determinate condition, which 

 is taken as the normal state. 



The mechanical state of a homogeneous body which has 

 undergone any homogeneous deformation that is to say, a 

 deformation which is reproduced in the same manner in each 

 element of volume may be expressed by six independent 

 variables : for instance, the lengths of the sides and the value 

 of the angles of a parallelopipedon, or the six elements of an 

 ellipsoid, which would always contain the same portion of a solid. 



645. The potential energy E of a body for unit weight, starting 

 from the normal state, is a function of its mechanical condition 



