INDUCTION. 47 



ent sizes, one placed outside of the other, and having 

 the electrified globe for their common center. 



Since the surfaces of spheres are to each other 

 as the squares of their radii, and since the radii meas- 

 ure the distances from the center, the surfaces are to 

 each other as the squares of their distances from the 

 center. 



But as each surface embraces all the lines of force, 

 the intensity of force on equal surface areas of the two 

 spheres would be in the inverse ratio of their entire sur- 

 faces ; and hence would vary inversely as the squares 

 of their distances from the center. 



Hence, electric induction varies inversely as the square 

 of the distance. 



Practically the conditions of the supposed case are 

 never exactly fulfilled ; but that does not affect the 

 correctness of the principle, which is the same in elec- 

 tricity as in light and radiant heat. 



CYLINDER ELECTRIFIED BY INDUCTION. The effect 

 of induction may be further illustrated by an insulated 

 cylinder of conducting material, placed between two 

 spheres of similar material, one insulated, and the other 

 connected with the earth by a chain, as shown in Fig. 

 6 ; the cylinder having mounted on it three pith-ball 

 electroscopes, connected with it by conductors. 



If the insulated sphere, A, be positively electrified, 

 electricity will be repelled by induction from the end of 

 the cylinder next A to the end next B. And since B 

 is connected with the earth, the electricity accumulated 

 on the end of (7, next to it, will repel to the earth from 

 B an amount equal to the positive charge on A. 



Hence the pith ball next A, being negative and A 

 positive, is attracted by A, while the one next B, being 



