323 



copper of best electric conductivity. Its specific resistance will be 

 about 7 X 1 6 , and its specific heat about * 1 . The value we must use 

 for Joule's equivalent will be 32'2 times the number 1390, which 

 Joule found for the mechanical value in foot-grains of the thermal unit 

 Centigrade, since the absolute unit of force, being that force which 

 acting on a grain of matter during a second of time generates 1 foot 



per second of velocity, is of the weight of a grain in middle 



oL ' L 



latitudes of Great Britain. Thus we find 

 J=44758. 



Hence the expression for the rate in degrees Cent, per second, at 

 which the temperature begins rising in a copper conductor, is 



6)' 



313 xlO 8 . 



I have found the electromotive force of a single cell of Daniell's to 

 be about 2'3 X 10 6 British absolute units* ; and if we suppose of 



ft 



this to go to each foot of the conductor in question, we shall have 

 2-3 2 xl0 12 5-29 X10 12 



(7) 



and therefore the expression for the rate of heating becomes 



Now, by using a sufficiently large single cell, we may make the 

 electromotive force, E, between S and T', be as little short as we 

 please of the whole electromotive force of the cell. We might then, 

 in testing by equality, with a standard and a tested conductor each 

 three inches or so long, and using a single cell, have nearly as much 

 as half the electromotive force of one cell acting per quarter foot of 

 these conductors, or two cells per foot. Hence if either is of best 

 conductive copper, its temperature would commence rising at the 

 rate of 4 X 169 or 676 Cent, per second. It would be almost im- 

 possible to work with so high a heating effect as this. But if we 

 use only -^ th of the supposed electromotive force, that is to say -^th 

 of a cell per foot of the copper conductor, the rate of heating will be 

 reduced to yj^, that is to say, will be 6'76 per second. By using 



* Proceedings of the Royal Society, February 1860. 



