and the Absolute Scale of Electric Potential. 17 



of the mechanical kinetic energy into the energy of 

 electric displacements. 



(d) The vibration of matter when arrested changes into 

 heat or some equivalent form of kinetic energy. 



(S) The vibration of electricity (i. e. light, according to the 

 view adopted in § 7 below of the electromagnetic theory 

 of light as interpreted by the doctrine of the Conserva- 

 tion of Electricity) when arrested or absorbed changes 

 to heat (unless, as shown later, the vibrations are too 

 rapid to be comparable to the periods of calorific oscilla- 

 tions). 



(e) The force with which a particle of matter acts upon 

 another particle of matter varies inversely as the square 

 of the distance between them, when the distance is varied, 

 and is of the nature of an attraction. (Law of Newton.) 



(e) The force with which a quantity of electricity (at a 

 point) acts upon a similar quantity of electricity varies 

 inversely as the square of the distance between them, 

 when the distance is varied, and is of the nature of a 

 repulsion. (Law of Coulomb.) 



5. Ponder omotive and Electromotive Forces. — The force 

 with which one quantity of electricity acts upon another 

 quantity of electricity may be regarded either as an electro- 

 motive force or as a true ponderomotive force; for it tends to 

 move electricity, and also to move matter, if matter be asso- 

 ciated with the electricity. If measured as a (ponderomotive) 

 force by any ordinary means of measuring forces, it will in 

 general be expressed by a numerical value different from 

 that in which it would be expressed as an electromotive force 

 by any of the ordinary measurements of electromotive forces. 



An instructive example of this relation between pondero- 

 motive and electromotive force is afforded by a consideration 

 of the action of a quantity of electricity at any point A upon 

 a small sphere, electrified with an electrification of the same 

 sign, at a distant point B. The action of A is a repulsion 

 exerted upon the matter of the sphere, which repulsion can 

 be measured by balancing it against the torsion of a fibre or 

 any other suitable mechanical force. But A also acts upon 

 the electricity with which the sphere at B is charged, and 

 repels it, with an electromotive force. If the sphere is moved 

 against a mechanical reaction, such as that of the torsion of a 

 fibre, work will be done ; and its value will be the product of 

 the average force into the distance through which the sphere 

 is moved. If the electricity is moved against the electric 

 reaction of some other electromotive force, work will be done 

 equally. This may be accomplished either, (i) by the elec- 



PhiL Mag. S. 5. Vol. 12. No. 72. July 1881. C 



