28, 29] Maxwell's Laws 23 



Law IV. The repulsion between two small bodies charged respect- 

 ively with e and e f units of electricity is numerically equal to the 

 product of the charges divided by the square of the distance. 



These are the forms in which the laws are given by Maxwell. Law I, it 

 will be seen, includes II and III. As regards the Definition and Law IV, 

 it is necessary to specify the medium in which the small bodies are placed, 

 since, as we shall see later, the force is different when the bodies are in air, 

 or in a vacuum, or surrounded by other non-conducting media. It is usual 

 to assume, for purposes of the Definition and Law IV, that the bodies are in 

 air. For strict scientific exactness, we ought further to specify the density, 

 the temperature, and the exact chemical composition of the air. Also we 

 have seen that when the electricity is not insulated on small bodies, but is 

 free to move on conductors, the forces of Law IV must be regarded as acting 

 on the charges of electricity themselves. When the electricity is not free to 

 move, there is an action and reaction between the electricity and matter, so 

 that the forces which really act on the electricity appear to act on the bodies 

 themselves which carry the charges. 



REFERENCES. 



On the History of Electricity : 



Encyc. Brit. Qth Ed. Art. Electricity. Vol. 8, pp. 324. 

 On the Experimental Foundations of Electricity : 



FARADAY. Experimental Researches in Electricity, by Michael Faraday. London 

 (Quaritch), 1839. ( 11691249). 



COULOMB. "Construction et Usage d'une Balance Electrique..." and six other 

 memoirs. Memoires de 1'Acad. Royale des Sciences, 1785,6,7,8,9, or Col- 

 lection de Memoires relatifs a la Physique, publies par la Societe frangaise 

 de Physique. Tome I. Paris (Gauthier-Villars), 1884. 



FRANKLIN. Experiments and Observations on Electricity made at Philadelphia in 

 America, by Dr Benjamin Franklin. Collected works of Benjamin Franklin 

 by Jared Sparks. Vol. 5, pp. 171483. Boston (Hilliard, Gray and Co.), 

 1837, or Phil. Trans. Roy. Soc., London, 1756, 1760, eta 



CAVENDISH. The Electrical Researches of the Hon. Henry Cavendish, F.R.S. 

 (Edited by Prof. Clerk Maxwell). Cambridge (Univ. Press), 1879. Intro- 

 duction by Maxwell, and " Thoughts concerning Electricity " ( 195216). 



On the Modern View of Electricity : 



J. J. THOMSON. Electricity and Matter. Westminster (Constable and Co.), 1904. 

 Chapter iv. 



