19-22] The Fundamental Conceptions of Electrostatics 17 



from the further end, and such that each of these parts contains a charge -f e 

 of positive electricity. Since the total induced charge is zero, the number of 

 positively charged sections A'B', B'G', ... must be exactly equal to the 

 number of negatively charged sections AB, BC,.... The whole series of 

 sections can therefore be divided into a series of pairs 

 AB and A'B' ; BC and B'C' ; etc. 



such that the two sections of any pair contain equal and opposite charges. 

 The charge on A'B' being of the same sign as the inducing charge e, repels 

 the body 7 which carries this charge, while the charge on AB, being of the 

 same sign as the charge on /, attracts /. Since AB is nearer to / than A'B', 

 it follows from Coulomb's Law that the attractive force ee/r 2 between AB 

 and J is numerically greater than the repulsive force ee/r 3 between A'B' and 

 /, so that the resultant action of the pair of sections AB, A'B' upon / is an 

 attraction. Obviously a similar result is true for every other pair of sections, 

 so that we arrive at the result that the whole force between the two bodies 

 is attractive. 



This result fully accounts for the fundamental property of a charged body 

 to attract small bodies to which no charge has been given. The proximity of 

 the charged body induces charges of different signs on those parts of the 

 body which are nearer to, and further away from, the inducing charge, and 

 although the total induced charge is zero, yet the attractions will always out- 

 weigh the repulsions, so that the resultant force is always one of attraction. 



21. The same conceptions explain the divergence of the gold- leaves of 

 the electroscope which occurs when a charged body is brought near to the 

 plate of the electroscope or introduced into a closed vessel standing on this 

 plate. All the conducting parts of the electroscope gold-leaves, rod, plate 

 and vessel if any may be regarded as a single conductor, and of this the 

 gold-leaves form the part furthest removed from the charged body. The 

 leaves accordingly become charged by induction with electricity of the same 

 sign as that of the charged body, and as the charges on the two gold-leaves 

 are of similar sign, they repel one another. 



22. On separating the two parts of a conductor while an induced charge 

 is on it, and then removing both from the influence of the induced charge, 

 we gain two charges of electricity without any diminution of the inducing 

 charge. We can store or utilise these charges in any way and on replacing 

 the two parts of the conductor in position, we shall again obtain an induced 

 charge. This again may be utilised or stored, and so on indefinitely. There 

 is therefore no limit to the magnitude of the charges which can be obtaiued 

 from a small initial charge by repeating the process of induction. 



This principle underlies the action of the Elect rophorus. A cake of resin 

 is electrified by friction, and for convenience is placed with its electrified 

 j. 2 



