48 Prof. Thomson on the Mathematical Theory of 



the following i*esult8, each of which is true to five places of 

 decimals. 



b adi Jbo4 



< ne 



To compare these with Mr. Harris's measurements we may 

 calculate the value of the potential in his battery, during the 

 observations, by means of his first result, and thence find the 

 attraction for the other three cases by means of the calculated 

 values of v^F (c). Thus we have v^ x 15 = 3293, which gives 



t;«= 45-56, 

 and hence F(2-5) = 7*94, -io od riwdw 



F(2-8)=4-18, "'••' ■*«»» 



F(3)=3-00. *^' 



These numbers differ considerably from Mr. Harrises results, 

 but in the direction indicated by the considerations mentioned 

 above. 



5. The most important part of the researches of Mr. Harris 

 is that in which he investigates the insulating power of air of 

 different densities. The result at which he arrives is, that the 

 intensity necessary to produce a spark depends solely on the 

 density of the air, and not otherwise on the pressure or tem- 

 perature. He thus shows that the conducting power of flame, 

 of heated bodies, and of a vacuum, are due solely to the rare- 

 faction of the air in each case. He also shows that the intensities 

 necessary to produce a spark, are in the simple ratios of the 

 densities of the air. ,.^,, 



6. In a subsequent memoir, by the same author*, we find 

 additional experiments on the elementary principles of the theory 

 of electricity. The first series which is described, was made for 

 the purpose of testing the truth of Coulomb's law, that the 

 repulsion of two similarly charged points is inversely as the 

 square of the distance, and directly as the product of the masses. 

 In experiments of this kind in which accurate quantitative 

 results are aimed at, many precautions are necessary. Thus all 

 conducting bodies except those operated upon, must be placed 

 beyond the reach of influence, and the distance between the 

 repelling bodies must be considerable with reference to their 



* Philosophical Transactions, 1836, 



