406 HENRY A. ROWLAND 



The energy of electricity at rest is the quantity multiplied by half the 

 potential ; or of a current, it is the strength of current multiplied by the 

 electro-motive force, and by the time ; or for all attractive forces varying 

 inversely as the square of the distance, Sir William Thomson has given 

 the expression 



TF/**' 



where R is the resultant force at any point in space, and the integral is 

 taken throughout space. 



These last three kinds of energy are already measured in absolute 

 measure and hence their ratios are accurately known. The only ratio, 

 then, that remains is that of heat to one of the others, and this must be 

 determined by experiment alone. 



But although we cannot measure f, g, h in general, yet we can often 

 measure off equal amounts of energy of these kinds. Thus, although we 

 cannot predict what quantities of heat are produced when two atoms of 

 different substances unite, yet, when the same quantities of the same 

 . substances unite to produce the same compound, we are safe in assuming 

 that the same quantity of chemical energy comes into play. 



According to these principles, I have divided the methods into direct 

 and indirect. 



Direct methods are those where & is converted directly or indirectly 

 into a, c, d, or e, or vice versa. 



Indirect methods are those where some kind of energy, as g, is con- 

 verted into &, and also into a, c, d, or e. 



In this classification I have made the arrangement with respect to 

 the kinds of energy which are measured, and not to the intermediate 

 steps. Thus Joule's method with the magneto-electric machine would 

 be classed as mechanical energy into heat, although it is first converted 

 into electrical energy. The table does not pretend to be complete, but 

 gives, as it were, a bird's-eye view of the subject. It could be extended 

 by including more complicated transformations; and, indeed, the sym- 

 metrical form in which it is placed suggests many other transformations. 

 As it stands, however, it includes all methods so far used, besides many 

 more. 



In the table of indirect methods, the kind of energy mentioned first is 

 to be eliminated from the result by measuring it both in terms of heat 

 and one of the other kindsof energy, whose value is known in absolute 

 or mechanical units. 



