[ 347 ] 



LIII. On the Heat produced by an Electric Discharge. 

 By Professor W. Thomson. 



To the Editors of the Philosophical Magazine and Journal. 



2 College, Glasgow, 

 Gentlemen, April 19, 1854. 



IT has been pointed out by M. Clausius, in a letter addressed 

 to you and published in the last Number of your Magazine, 

 that the first discovery of the true relation between the genera- 

 tion of heat in the discharge of a Leyden phial and the quantity 

 of the previous charge is not, as I had stated it to be, due to 

 Joule, but that it had been given in a paper published about 

 three years eai-lier by Riess. I may be allowed to explain, that," 

 in making the statement in question, I considered the law of that 

 relation as an evident corollary from the great principle, that 

 the whole heat generated in any discharge of electricity is exactly 

 the equivalent in thermal energy for the mechanical value of the 

 electrical charge which is lost*. There is no doubt who is the 

 discoverer of this, and the originatoi-, in your most valuable 

 Magazine, of the theory of mechanical equivalence among the 

 electric, chemical, magnetic, frictional, and pneumatic develop- 

 ments of energy, which has within the last two or three years 

 attracted so many investigators. 



The mere law, that the heat generated by the discharge of a 

 Leyden phial or battery is proportional to the square of the quan- 

 tity of electricity in the previous charge, is not, as I inadvertently 

 stated, due to Joule; neither is it, as M. Clausius seems to sup- 

 pose, due to Riess. Becquerel, I find, in his Traitede I'Electricite 

 (vol. iii. p. 150, published in 1835, or two years earlier than the 

 paper referred to by M. Clausms), enunciates it quite explicitly 

 as having been established by " Cuthbertson and others, who 

 had used electrometers in measuring the calorific action of the 

 discharge of a battery." Mr. Joule, too, although in his first 

 publication he only referred to the researches of Snow Harris 

 which had recently appeared in the Philosophical Transactions, 

 remarks in a subsequent paper (On the Heat disengaged in che- 

 mical Combinations, Phil. Mag. June 1852), "that Brooke and 



* The application of this principle to the discharge of a Leyden phial 

 shows that the whole heat generated must be equal to - . ^Q- ~, if J de- 

 note the mechanical equivalent of the thermal unit, Q the amount of the 

 charge, t the thickness of the glass, I its specific inductive cajjacity, and S 

 the area of either side of the coated surface; a conclusion which wants no 

 other experimental verification than such as may be considered desirable 



for verifying that I - — is the true expression for the capacity of a Leyden 

 phial. 



