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



I have learned from Professor Thomson that the length of the 

 needle of my galvanometer is so small in comparison with the 

 diameter of the coil, that no sensible error could have ai'isen from 

 taking the tangents of the deflections as the measure of the cur- 

 rents traversing the coil. The amount of the correctiou that I have 

 applied is, however, too trilling to affect materially the numerical 

 results arrived at. Professor Thomson has kindly allowed me 

 to describe the arrangement by means of which he has recently 

 effected a very valuable improvement in the tangent galvanometer. 

 The coil he employs is represented by PI. XI. fig. 9. It consists 

 of two concentric circles of fiat copper wire, the outer one being 

 12 inches, the inner 9 inches in diameter. It is furnished with 

 three terminals, a, b and c, which can be readily connected with 

 a battery, or other apparatus, by means of the usual clamps. It 

 will be seen that if the current pass from a to c, it will traverse 

 the outer circle ; that if it pass from b to c, it will traverse the 

 inner circle ; and that if it pass from a to b, it will traverse both 

 circles in opposite directions. The diameters of the circles being 

 in the proportion of 4 to 3, it is obvious that the effect of a con- 

 stant current on the needle will in the three arrangements be re- 

 spectively proportional to 3, 4 and 1. So that by making two 

 circles of unequal force oppose one another, Professor Thomson 

 obtains the means of measuring powerful currents accurately, 

 without needlessly increasing the size of the galvanometer. 



I observe with pleasure that Dr. Woods has recently arrived 

 at one of the results of the paper, viz. " that the decomposition 

 of a compound body occasions as much cold as the combination 

 of its elements originally produced heat," by the use of an elegant 

 experimental process described in this Magazine for October 1851. 

 I ought, however, to remark, that previous to the year 1843 

 I had demonstrated " that the heat rendered latent in the elec- 

 trolysis of water is at the expense of the heat which would other- 

 wise have been evolved in a free state by the circuit*," a pi'O- 

 position, which Professor Thomson has shown to be an inevitable 

 consequence of the dynamical theory of heatf. 



I have the honour to remain, Gentlemen, 



Yours very respectfully, 

 Acton Square, Salford, JAMES P. JoULE. 



March 30, 1852. 



Actioni contraria semper et aequalis est reactio. — Newton. 



1. My object in the present memoir is to communicate to the 

 Academy of Sciences an inverse method for ascertaining the 



* Philosophical Magazine, S. 3. vol. xxiii. p. 263. 

 t Memoir on the Dynamical Theory of Heat, § 18. 



