Ox THE MECHANICAL EQUIVALENT OF HEAT 421 



The experiments of Joule and Favre, where the heat generated by 

 a current, both when it does mechanical work and when it does not, 

 are very interesting, but can hardly have any weight in an estimation 

 of the true value of the equivalent. 



The method of calculating the equivalent from the chemical action 

 in a battery, or the electro-motive force required to decompose any 

 substance, such as water, is as follows: 



Let E be such electro-motive force and c be the quantity of chemical 

 substance formed in battery or decomposed in voltameter per second. 

 Then total energy of current of energy per second is EQ, where Q is 

 the current, or cQHJ, where H is the heat generated by unit of c, or 

 required to decompose unit of c. Hence, if the process is entirely 

 reversible, we must have in either case 



CHJ = E. 



But the process is not always reversible, seeing that it requires more 

 electro-motive force to decompose water than is given by a gas battery. 

 This is probably due to the formation at first of some unstable com- 

 pound like ozone. The process with a battery seems to be best, and we 

 can thus apply it to the Daniell cell. The following quantities are 

 mostly taken from Kohlrausch. 



The quantity c has been found by various observers, and Kohlrausch M 

 gives the mean value as -009421 for water according to his units (mg., 

 mm., second system). Therefore for hydrogen it is -001047. 



The quantity H can be observed directly by short-circuiting the 

 battery, or can be found from experiments like those of Favre and 

 Silbermann. 



The electro-motive force E can be made to depend either upon the 

 absolute measure of resistance, or can be determined, as Thomson has 

 done, in electro-static units. In electro-magnetic units it is 



Absolute Measure 



Siemens. Ohms. according to my 



Determination. 



After Waltenhof en 11-43 10-90 10-80 XlO 10 



" Kohlrausch 39 11-71 H'17 11-07X10 10 



After Favre, 1 equivalent of zinc developes in the Daniell cell 23993 

 heat units; 



. / E 



38 Fogg. Ann., cxlix, 179. 



39 Given by Kohlrausch, Pogg. Ann., cxlix, 182. 



