420 



HENEY A. ROWLAND 



There can be no doubt that Joule's result is most exact, and hence 

 I have given his results twice the weight of Weber's. Weber used a 

 wire of about 14 ohms' resistance, and a small calorimeter holding only 

 250 grammes of water. This wire was apparently placed in the water 

 without any insulating coating, and yet current enough was sent 

 through it to heat the water 15 during the experiment. No precau- 

 tion 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, though the error from this source was probably small. Further- 



TABLE XXXIII. 



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

 001054 for 1 C., but it is usually given at about -003. 



There can be no doubt that experiments depending on tKe heating 

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

 temperature of the wire during the heating must always be 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 plati- 

 num wire, which may account for Weber's finding a smaller value than 

 Joule, and Weber's value would be more in error than Joule's. Undoubt- 

 edly 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 in Kohlrausch's absolute value of the Siemens unit or ohm. 



