488 Mr. J. P. Joule on the Heat disengaged 



ought to indicate currents whose ratio is as 4 to 1 . Experiments 

 of this kind having been repeated, with proper precautions against 

 any alteration in the intensity of the battery cells, it was found 

 that o ^dth had to be added to the tangent at a deflection of 55°, 

 T £-th at 82°, &c. 



My thermometers were constructed by a method very similar 

 to that employed by Regnault and Pierre*. The calibre of the 

 tube was first measured in every part by passing a short column 

 of mercury along it. The surface of the glass having then been 

 covered with a thin film of bees-wax, the portions of tube pre- 

 viously measured were divided into an equal number of parts by 

 a machine constructed for the purpose. The divisions were then 

 etched by means of the vapour of fluoric acid. Two thermome- 

 ters were employed in the present research, in one of which the 

 value of each space was y^j, in the other ^^ of a degree Centi- 

 grade. A practised eye can easily estimate the tenth part of each 

 of these spaces ; consequently I could by these thermometer's 

 observe a difference of temperature not greater than o, 005. 



The voltaic pile that I made use of was one of very large di- 

 mensions, each cell being 2 feet high and 5 inches in diameter. 

 The internal arrangements of the cells were similar to those of 

 the ordinary pile of Daniell. 



5. I shall now proceed to describe my experiments on the heat 

 evolved by currents traversing metallic wires. The apparatus 

 used consisted of a wire of pure silver, 8 metres long and about 

 - 6 of a millimetre in thickness, coiled upon a thin chimney- 

 glass, the several coils being prevented from touching one an- 

 other by means of silken threads. The ends of the silver wire 

 were connected metallically with two thick copper wires, the ends 

 of which dipped into cups of mercury. The coil, thus mounted, 

 was immersed in a jar of tinned iron, capable of containing two 

 pounds and a half of water. In order to prevent, as well as pos- 

 sible, the influence of the surrounding atmosphere in raising or 

 depressing the temperature of the water, the sides and bottom of 

 this jar were made hollow by soldering two jars of unequal mag- 

 nitude within each other. Fig. 4 represents a section of this 

 double can, aa being the hollow part between the internal and 

 external cans. The positions of the coil, thermometer and stirrer, 

 are also shown in the same figure. 



At 7 o'clock a.m., Sept. 4, 1844f, having filled the jar with 

 2| lbs. of distilled water, I immersed the coil of silver wire into 



* Annates de Chimie et de Physique, 1842, vol. v. p. 428, note. 



t My object in being so particular as to the dates of the experiments, 

 was to eliminate the effects of any variation in the intensity of the earth's 

 magnetism. In the subsequent series of experiments I have not always 

 thought it necessary to mention these dates, but I have nevertheless used 

 the same precaution in all of them. 



