16 Electrostatics Physical Principles [OH. i 



V. Electrification by Induction. 



19. Let us suspend a metal rod by insulating supports. Suppose that 

 the rod is originally uncharged, and that we bring a small body charged 

 with electricity near to one end of the rod, without allowing the two bodies 

 to touch. We shall find on sprinkling the rod with electrified powder of the 

 kind previously described ( 13), that the rod is now electrified, the signs of 

 the charges at the two ends being different. This electrification is known as 

 electrification by induction. We speak of the electricity on the rod as an 

 induced charge, and that on the originally electrified body as the inducing or 

 exciting charge. We find that the induced charge at the end of the rod 

 nearest to the inducing charge is of sign opposite to that of the inducing 

 charge, that at the further end of the rod being of the same sign as the 

 inducing charge. If the inducing charge is removed to a great distance 

 from the rod, we find that the induced charges disappear completely, the rod 

 resuming its original unelectrified state. 



If the rod is arranged so that it can be divided into two parts, we can 

 separate the two parts before removing the inducing charge, and in this way 

 can retain the two parts of the induced charge for further examination. 



If we insert the two induced charges into the vessel of the electroscope, 

 we find that the total electrification is nil : in generating electricity by 

 induction, as in generating it by friction, we can only generate equal 

 quantities of positive and negative electricity ; we cannot alter the algebraic 

 total charge. Thus the generation of electricity by induction is in no way 

 a violation of the law that the total charge on a body remains unaltered, 

 except in so far as it is removed by conduction. 



20. If the inducing charge is placed on a sufficiently light conductor, we 

 notice a violent attraction between it and the rod which carries the induced 

 charge. This, however, is only in accordance with Coulomb's Law. Let us, 

 for the sake of argument, suppose that the inducing charge is a positive 

 charge e. Let us divide up that part of the rod which is negatively charged 



ABC C' B'A' 



f ) 



FIG. 3. 



into small parts AB, BC,..., beginning from the end A which is nearest 

 to the inducing charge /, in such a way that each part contains the same 

 small charge e, of negative electricity. Let us similarly divide up the part 

 of the rod which is positively charged into sections A'B', B'C', ..., beginning 



