in Electric Induction. 411 



tial, and the plates must also act when they occupy a side posi- 

 tion in the neighbourhood of those conductors. By examining 

 the induction upon a conducting disc, we may learn what effect 

 this action must have. A copper disc, 4*03 inches in diameter, 

 was so placed before a positively charged sphere, 087 inch in 

 diameter, that the perpendicular let fall from the centre of the 

 sphere upon the plane of the disc intersected the horizontal dia- 

 meter of the latter at a point 077 of an inch from its centre, 

 the distance between the nearest point of the sphere and the 

 disc being 026 of an inch. The following are the results of an 

 examination of the disc's horizontal diameter. The negative 

 electricity induced upon the disc's anterior surface extended to 

 a distance of 2*57 inches from that part of the circumference 

 nearest to the sphere; the last - 86 of an inch extending up to 

 the most distant part of the circumference was occupied by posi- 

 tive electricity, as was the whole diameter of the posterior surface. 

 If such a disc were placed near a positively charged ball during 

 its inductive action upon a neutral one, with its plane parallel to 

 the line joining the centres of the spheres, and its anterior cir- 

 cumference equidistant from both spheres, the negative electricity 

 upon the disc would be nearer to the induced sphere than the 

 positive. This negative electricity, therefore, would exercise a 

 superior influence upon the sphere, and the induction of the 

 originally charged sphere would appear weakened. The same 

 effect would be produced if the conducting disc were placed 

 between the spheres perpendicular to the line joining their cen- 

 tres, but without allowing its plane to be intersected by that 

 line. This last experiment was unintentionally made in all the 

 foregoing ones, in which interposed plates were employed. It 

 was before mentioned, that the induced electricity appeared to 

 be diminished at the moment when a plate was interposed be- 

 tween the two discs or spheres. With some of the non-conduct- 

 ing plates this diminution of the induced electricity was even 

 greater than the subsequent increase of the same, when the plate 

 was completely interposed. Imagine the examined copper disc 

 placed near the two spheres with its surface parallel to the line 

 joining their centres, and with its centre perpendicularly opposite 

 to the centre of that line ; the positive electricity on both sur- 

 faces of the disc will be much nearer to the induced sphere than 

 the negative electricity, so that the positive electricity will exer- 

 cise a superior influence on the neutral sphere, and the induction 

 of the directly charged sphere will be apparently increased. I 

 have made this experiment with the non-conducting plates before 

 mentioned, The discs were placed in the described position at 

 the side of two spheres, 0'60 inch distant from one another, and 

 at a distance of 026 inch from their surfaces ; in this position 



