154 REPORT—1863. 
of the potential P, entailing the measurement of an absolute force between 
two electrified bodies. This method was proposed and adopted by Weber*. 
2°. By a comparison of the measure of electromotive force. The electro- 
motive force produced by a battery, in electrostatic measure, can be directly 
weighed (43). Its electromotive force, in electromagnetic measure, can be 
obtained from the current it produces in a given resistance (28). The ratio 
of the two numbers will give the value of v. This method has been carried 
out by Professor W. Thomson, who was not, however, at the time in pos- 
session of the means of determining accurately either the absolute resistance 
of his circuit or the absolute value of the currentf. 
3°. By a common measure of resistance. We know (29 and 45) how to 
measure resistances in electromagnetic and electrostatic measure. The ratio 
between these measures is equal to v?. The measure of resistance in electro- 
static measure is not as yet susceptible of great accuracy. 
4°, By a comparison of currents. The electromagnetic value of a current 
produced by a continuous succession of discharges from a condenser of capacity 
s can be measured (18, 19). The electrostatic value of the current will be 
known if the potential to which the condenser is charged be known. The 
ratio of the two numbers is equal to v. 
5°. By a common measure of capacity. The two measurements can be 
effected by the methods given (26 and 40). The ratio between the two | 
measurements will give v*. This method would probably yield very accurate 
results. 
Part V.—EecrricAL MEASUREMENTS DERIVED FROM THE FIVE ELEMENTARY 
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. Electric potential is defined by Prof. W. Thomson as follows t: 
«‘ 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 posi- 
tive electricity from an infinite 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 
another paper§ Professor Thomson describes electric potential as follows :— 
‘The amount of work required to move a unit of electricity 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 potential at 
every point of infinity is zero, and it will be seen that the difference of 
* Pogg. Ann. Aug. 1856, Bd. 99. p. 10. Abhandlungeu der Kon. Sachsischen Gesell- 
schaft, vol. ili. (1857) p. 266. 
y t Paper read f bay the Royal Society, February 1860. Vide Proceedings of the Royal 
ociety, vol. x 
t ae read Be British Association, 1852. Vide Phil. Mag. 1853, p. 288. 
§ Paper read before the Royal Society, February 1860. Vide Proceedings of the Royal 
Society, se x, p. dd4. 
