OF ARTS AND SCIENCES. 153 



it to heat the water 15° during the experiment. No precaution 

 seems to have been taken as to the current passing into the water, 

 which Joule accurately investigated. Again, the water does not seem 

 to have been continuously stirred, which Joule found necessary. And 

 further. Newton's law of cooling does not apply to so great a range 

 as 15°, tin nigh the error from this source was probably small. Further- 

 more, I know of no platinum which has an increase of coefficient of 

 .0010.54 for 1° C, but it is usually given at about .00.".. 



There can be no doubt that experiments depending on the heating 

 of a wire give too small a value of the equivalent, seeing that the 

 temperature of the wire during the heating must always he higher 

 than that of the water surrounding it, and hence more heat will be 

 generated than there should be. Hence the numbers should be 

 slightly increased. Joule used wire of platinum-silver alloy, and 

 Weber platinum wire, which may account lor Weber's rinding a 

 smaller value than Joule, and Weber's value would be more in error 

 than Joule's. Undoubtedly this is a serious source of error, and I am 

 about to repeat an experiment of this kind in which it is entirely 

 avoided. Considering this source of error, these experiments confirm 

 both my value of the ohm and of the mechanical equivalent, and 

 unquestionably show a large error iu Kohlransch's absolute value of 

 the Siemens unit or ohm. 



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 chemi- 

 cal substance formed in battery or decomposed in voltameter per 

 second. Then total energy of current of energy per second is E Q, 

 where Q is the current, or cQ HJ, where //is the heat generated by 

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

 is entirely reversible, w,e 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 un- 

 stable compound like ozone. The process with a battery seems to be 



