CHAP, i.] ELECTRICITY AND MAGNETISM. 21 



portion of the conductor, and may be examined at 

 leisure. 



If the conductor be not insulated on glass supports, 

 but placed in contact with the ground, that end only 

 which is nearest the electrified body will be found to be 

 electrified. The repelled electricity is indeed repelled 

 as far as possible into the earth. One kind of elec- 

 tricity only is under these circumstances to be found, 

 namely, the opposite kind to that of the excited body, 

 whichever this may be. The same effect occurs in this 

 case as if an electrified body had the power of attracting 

 up the opposite kind of electricity out of the earth ; though 

 the former way of regarding matters is more correct. 



The quantity of the two electricities thus separated by 

 induction on such a conductor in the presence of a 

 charge of electricity, depends upon the amount of the 

 charge, and upon the distance of the charged body from 

 the conductor. A highly electrified glass rod will 

 produce a greater inductive effect than a less highly 

 electrified one ; and it produces a greater effect as it is 

 brought nearer and nearer. The utmost it can do will 

 be to induce on the near end a negative charge equal 

 in amount to its own positive charge, and a similar 

 amount of positive electricity at the far end ; but usually, 

 before the electrified body can be brought so near as to 

 do this, something else occurs which entirely alters the 

 condition of things. As the electrified body is brought 

 nearer and nearer, the opposite kinds of electricity on 

 the two opposed surfaces attract one another more and 

 more strongly and accumulate more and more densely, 

 until, as the electrified body approaches very near, a spark 

 is seen to dart across, the two electricities thus rushing 

 together to neutralise one another, leaving the induced 

 charge of positive electricity, which was formerly repelled 

 to the other end of the conductor, as a permanent charge 

 after the electrified body has been removed. 



2O. We are now able to apply the principle of 



