has been carried on, with little intermission, for more than two 

 years. Mr. Robert Davidson, Mr. Charles A. Smith, and other 

 friends have also given much valuable assistance during the greater 

 part of this time, in the different experimental investigations of 

 which results are now laid before the Royal Society. Only nu- 

 gatory results were obtained until recently from multiplied and 

 varied experiments both on copper and iron conductors; but the 

 theoretical anticipation was of such a nature that no want of expe- 

 rimental evidence could influence my conviction of its truth. About 

 four months ago, by means of a new form of apparatus. I ascertained 

 that resinous electricity carries heat with it in an unequally heated iron 

 conductor. A similar equally sensitive arrangement showed no re- 

 sult for copper. The second hypothesis might then have been ex- 

 pected to hold ; but to ascertain the truth with certainty I have 

 continued ever since, getting an experiment on copper nearly every 

 week with more and more sensitive arrangements, and at last, in 

 two experiments, I have made out with certainty, that vitreous elec- 

 tricity carries heat tvith it in an unequally heated copper conductor. 



The third hypothesis is thus established : a most unexpected con- 

 clusion I am M'illing to confess. 



I intend to continue the research, and I hope not only to ascer- 

 tain the nature of the thermal effects in other metals, but to deter- 

 mine its amount in absolute measure in the most important cases, 

 and to find how it varies, if at all, with the temperature ; that is, to 

 determine the character (positive or negative) and the value of the 

 specific heat, varying or not with the temperature, of the unit of 

 current electricity in various metals. '*t' 



■AM, 



§ II. On the Law of Thermo-electric Force in an unequally heated '^^ 

 circuit of tivo Metals. ::• 



A general relation between the specific heats of electricity in two 

 diflferent metals, and the law of thermo-electric force, in a circuit 

 composed of them according to the temperatures of their junctions, 

 was established in the communication to the Royal Society of Edin- 

 burgh referred to above, and was expressed by an equation* which 

 may now be simplified by the thermometric assumption 



(jx denoting Carnot's function, J Joule's equivalent, and t the tempe- 

 rature measured from an absolute zero, about 273^° Cent, below the 

 freezing-point,) since this assumption defines a system of thermometry 

 in absolute measure, which the experimental researches recently made 

 by Mr. Joule and myself establish as not differing sensibly from the 

 scale of the air- thermometer between ordinary limits. The equ^,*] 

 tion, when so modified, takes the following form : — ^^'l 



F=j{|(s-T)+j;;.(i-i)*}. "^ 



where ^ denotes the excess of the specific heat of electricity in the 

 * See Proceedings R.S.E. Dec. 1851, or Philosophical Magazine, 1852. -• «»rt 



PhiL Mag, S. 4. Vol. 8. No. 49. July 1854. F 



