CHAP, iv.] ELECTRICITY AND MAGNETISM. 191 



235. Second Law of Electrostatics. The force 

 exerted between two charges of electricity (supposing them 

 to be collected at points or on two -small spheres), is 

 directly proportional to their product, and inversely pro- 

 portional to the distance' between them. This law, dis- 

 covered by Coulomb, and hence sometimes called Cou- 

 lomb's Law, was briefly alluded to (on page 16) in the 

 account of experiments made with the torsion - balance ; 

 and examples were there given in illustration of both parts 

 of the law. We saw too, that a similar law held good for 

 the forces exerted between two magnet poles. Coulomb 

 applied also the method of oscillations to verify the 

 indications of the torsion-balance and found the results 

 entirely confirmed. We may express the two clauses of 

 Coulomb's law, in the following symbolic manner. Let 

 /stand for the force, q for the quantity of electricity in 

 one of the two charges, and q for that of the other 

 charge, and let d stand for the distance between them. 

 Then, 



(i.) f is proportional to q x q'> 



and (2.) /is proportional to JL 



These two expressions may be combined into one ; 

 and it is most convenient so to choose our units or 

 standards of measurement that we may write our symbols 

 as an equation : 



236. Unit of Electric Quantity. If we are, how- 

 ever, to write this as an equality, it is clear that we 

 must choose our unit of electricity in accordance with 

 the units already fixed for measuring force and distance. 

 All electricians are now virtually agreed in adopting a 

 system which is based upon three fundamental units : 

 viz., the Centimetre for a unit of length; the Gramme 

 for a unit of mass; the Second for a unit of time. All 



