22 Electrostatics Physical Principles [OH. i 



The modern view enables us also to give a simple physical interpretation 

 to the phenomenon of induction. A positive charge placed near a conductor 

 will attract the electrons in the conductor, and these will flow through the 

 conductor towards the charge until electrical equilibrium is established. 

 There will be then an excess of negative electrons in the regions near the 

 positive charge, and this excess will appear as an induced negative charge. 

 The deficiency of electrons in the more remote parts of the conductor will 

 appear as an induced positive charge. If the inducing charge is negative, 

 the flow of electrons will be in the opposite direction, so that the signs of the 

 induced charges will be reversed. In an insulator, no flow of electrons can 

 take place, so that the phenomenon of electrification by induction does not 

 occur. 



On this view of electricity, negative electricity is essentially different in 

 its nature from positive electricity : the difference is something more funda- 

 mental than a mere difference of sign. Experimental proof of this difference 

 is not wanting, e.g., a sharply pointed conductor can hold a greater charge of 

 positive than of negative electricity before reaching the limit at which a 

 discharge begins to take place from its surface. But until we come to those 

 parts of electric theory in which the flow of electricity has to be definitely 

 regarded as a flow of electrons, this essential difference between positive and 

 negative electricity will not appear, and the difference between the two will 

 be adequately represented by a difference of sign. 



SUMMARY. 



29. It will be useful to conclude the chapter by a summary of the 

 results which are arrived at by experiment, independently of all hypotheses 

 as to the nature of electricity. 



These have been stated by Maxwell in the form of laws, as follows : 



Law I. The total electrification of a body, or system of bodies, 

 remains always the same, except in so far as it receives electrification 

 from or gives electrification to other bodies. 



Law II. When one body electrifies another by conduction, the 

 total electrification of the two bodies remains the same; that is, the 

 one loses as much positive or gains as much negative electrification as 

 the other gains of positive or loses of negative electrification. 



Law III. When electrification is produced by friction, or by any 

 other known method, equal quantities of positive and negative electrifi- 

 cation are produced. 



Definition. The electrostatic unit of electricity is that quantity of 

 positive electricity which, when placed at unit distance from an equal 

 quantity, repels it with unit of force. 



