37 2 Note j : idea of electric potential due to Cavendish 



It would appear from the observed properties of gases that the mutual 

 action between two molecules is insensible at all sensible distances. As the 

 molecules approach, the action is at first attractive, but soon changes to a 

 repulsive force of far greater magnitude, so that the general character of the 

 encounter depends mainly on the repulsive force. 



On this theory, the elasticity of the gas may still be said in a certain sense 

 to arise from the repulsive force between its molecules, only instead of this 

 repulsive force being in constant action, it is called into play only during the 

 encounters between two molecules. The intensity of the impulse is not the 

 same for all encounters, but as it does not depend on the interval between the 

 encounters, we may consider its -mean value as constant. The average value 

 of the force between two molecules is in this case the value of the impulse 

 divided by the time between two encounters. Hence we may say that the 

 force is inversely as the distance between the molecules, and that it acts between 

 those molecules only which encounter each other. 



For an earlier investigation by Cavendish of the properties of an elastic 

 fluid, see Note 18. 



NOTE 7, ART. 101. 



[On the idea of Electric Potential, as introduced by Cavendish : see p. 18.] 



Here Cavendish endeavours to fix a precise meaning to the terms "positively 

 and negatively electrified," terms which he found current among electricians, 

 but not well defined. The meaning which he here fixes to them, and which he 

 afterwards makes much use of, is equivalent to the meaning of the modern 

 term potential, as used by practical electricians. The idea of potential as used 

 by mathematicians is expressed by Cavendish in his theory of canals of in- 

 compressible fluid. 



In the "Thoughts concerning Electricity," and in the unpublished papers, 

 degrees of electrification are spoken of. These degrees of electrification are 

 measured in the experimental researches by means of electrometers of different 

 kinds, and since he has compared the indications of his electrometers with the 

 degrees of electrification required to make a spark pass between the balls of 

 Lane's discharging electrometer, we may express all these measurements in 

 modern units, though Cavendish's original electrometers no longer exist. 



I have not been able to trace the idea of electric potential in the work of 

 yEpinus, so that Cavendish seems the first to have made use of it. The relation 

 between the charge of a body and the degree of its electrification is the main 

 object of Cavendish's experimental researches, and the results of his work were 

 expressed in the material form of a collection of coated plates, each of which 

 had a capacity equal to that of a sphere of known diameter. 



The leading idea in the great experimental work of Coulomb seems to be 

 the measurement of the charges of the different bodies of a system and of parts 

 of these bodies. Perhaps the most valuable of Coulomb's many contributions 

 to experimental physics was the measurement of the surface-density of the 



