A 



262 Mr. J. P. Joule on the Heat evolved by Metallic 



way a little practice has enabled me to estimate temperatures 

 to the tenth part of Fahrenheit's degree with certainty. 



7. In order to ascertain the heating power of a given me- 

 tallic wire, it was passed through a thin glass Fio-. 2. 

 tube, and then closely coiled upon it. The 

 extremities of the coil thus formed were then 

 drawn asunder, so as to leave a small space 

 between each convolution, and if this could not 

 be well done, a piece of cotton thread was in- 

 terposed. The apparatus thus prepared, when 

 placed in a glass jar containing a given quan- 

 tity of water, was ready for experiment. Fig. 

 2 will explain the dispositions : A is the coil 

 of wire; B the glass jar partly filled with 

 water ; T represents the thermometer. When 

 the voltaic electricity is transmitted through 

 the wire, no appreciable quantity passes from 

 it to take the shorter course through the water. 

 No trace of such a current could be detected, 

 either by the evolution of hydrogen, or the 

 oxidation of metal. 



8. Previous to each of the experiments, the 

 necessary precaution was taken of bringing 

 the water in the glass jar, and the air of the 

 room to the same temperature. When this is 

 accurately done, the results of the experiments bear the same 

 proportions to one another as if no extraneous cooling agents, 

 such as radiation, were present; for their effects in a given 

 time are proportional to the difference of the temperatures of 

 the cooling and cooled bodies ; and hence, although towards 

 the conclusion of some experiments this cooling effect is very 

 considerable, the absolute quantities alone of heat are affected, 

 not the proportions that are generated in the same time. [See 

 the table of heats produced during half an hour and one 

 hour, p. 264?.] 



9. Exp. 1. — I took two copper wires, each two yards long, 

 one of them ^th of an inch, the other ^th of an inch thick, 

 and arranged them in coils in the manner that I have de- 

 scribed (7.). These were immersed in two glass jars, each 

 of which contained nine ounces avoirdupois of water. A cur- 

 rent of the mean quantity 1 0, 1 Q*, was then passed conse- 

 cutively through both coils, and at the close of one hour I ob- 

 served that the water in which the thin wire was immersed 

 had gained 3 0, 4, whilst the thick wire had produced only 1 0, 3. 



* I place Q at the end of my degrees, to distinguish them from those 

 of the graduated card. 



