516 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



(5) By a common measure of capacity. The two measure- 

 ments can be effected by the methods given (26 and 40). The 

 ratio between the two measurements will give v 2 . This method 

 would probably yield very accurate results. 



Part V. — Electrical Measurements derived from the five ele- 

 mentary Measurements ; and Conclusion. 



47. Electric Potential. — The word " potential/' as applied by 

 G. Green to the condition of an electrified body and the space 

 surrounding it, is now coming into extensive use, but is perhaps 

 less generally understood than any other electrical term. Elec- 

 tric potential is denned by Professor W. Thomson as follows*. 



"The potential, at any point in the neighbourhood of or 

 within an electrified body, is the quantity of work that would 

 be required to bring a unit of positive electricity from an infi- 

 nite distance to that point, if the given distribution of electricity 

 remained unaltered." 



It will be observed that this definition is exactly analogous to 

 that given of magnetic potential (10), with the substitution of 

 the unit quantity of electricity for the unit magnetic pole. 

 (Analogous definitions might be given of gravitation-potential, 

 heat- potential; and every one of these potentials coexist at every 

 point of space quite independently one of the other.) In an- 

 other paper f Professor Thomson describes electric potential as 

 follows : — " The amount of work required to move a unit of elec- 

 tricity, against electric repulsion, from any one position to any 

 other position, is equal to the excess of the electric potential of 

 the first position above the electric potential of the second 

 position." 



The two definitions given are virtually identical, since the po- 

 tential at every point of infinity is zero, and it will be seen that 

 the difference of potential defined in the second passage quoted 

 is identical with what we have called the electromotive force 

 between the two points (16 and 27). 



When, instead of a difference of potentials, the potential simply 

 of a point is spoken of, the difference of potential between the 

 point and the earth is referred to, or, as we might say, the elec- 

 tromotive force between the point and the earth. 



The potential at all points close to the surface and in the in- 

 terior of any simple metallic body is constant ; that is to say, no 

 electromotive force can be produced in a single metallic body by 

 mere electrical distribution ; the potential at the body may there- 



* Paper read before the British Association, 1852. Vide Phil. Mag. 1853, 

 p. 288. 



t Paper read before the Royal Society, February 1860. Vide Proceed- 

 ings of the Royal Society, vol. x. p. 334, and Phil. Mag. S. 4 vol. xx. p. 323. 



