in Electric Induction. 409 



oumfercnee ; at the edge of this surface a narrow non-electric 

 ring could he detected. Lastly, on the greatest disc, of copper, 

 a broader ring of positive electricity was found on the anterior 

 surface itself. The negative electricity upon this surface de- 

 creased from the centre to a distance of about 1^ inch; at 1^ 

 inch there was no electricity; and at If inch there were decided 

 appearances of positive electricity, which increased in density 

 towards the circumference, and from there to the centre of the 

 posterior surface again diminished. This superficial knowledge 

 of the electric distribution upon the disc (which would suffer 

 some change in presence of the neutral sphere), is sufficient to 

 explain the action of the disc as intermediate plate. Whenever 

 a disc is placed normally between two spheres, one of which is 

 directly charged, two equal quantities of unlike kinds of electri- 

 city are elicited upon it, of which the quantity, unlike the elec- 

 tricity of the charged sphere, extends at all events over a less 

 surface than the quantity like the electricity of the sphere does. 

 "When the disc is very thin, the two electric strata lie on two 

 planes very close to one another, both of which are perpendicular 

 to the line joining the centre of the spheres ; hence the unlike- 

 named stratum will act more strongly upon the neutral sphere 

 than the like-named, because it will act at a less distance. For 

 example, let us consider the copper disc placed between the two 

 spheres. Then, acting upon the nearest point of the neutral 

 sphere, we have the whole negative electricity upon a circular 

 surface at a distance of one-third of an inch, and whose diameter 

 is 2| inches, together with a smaller quantity of positive elec- 

 tricity upon a circular surface, whose distance from the point in 

 question is only 0302 inch, but whose diameter is 3"958 inches. 

 At the same time the negative electricity is most dense at the 

 centre where it is nearest to the neutral, whilst the positive elec- 

 tricity is most dense at the circumference where it is furthest 

 from the neutral sphere. The negative electricity therefore exer- 

 cises a greater influence upon the neutral sphere than the posi- 

 tive; and the induction of the charged sphere must appear 

 diminished, as experiment plainly shows. When the thickness 

 of the disc is great in comparison to its diameter, an opposite 

 effect must ensue. By placing the thick brass disc between the 

 spheres, the whole quantity of negative electricity is confined to 

 a circular surface § rds of an inch in diameter, at a distance of 

 0*43 of an inch from the nearest point of the neutral sphere; an 

 equal quantity of positive electricity is spread over the cylindrical 

 Surface, and a circular surface whose diameter is also |rds of an 

 inch, but whose distance from the nearest point of the sphere is 

 only O'lH inch. This great difference between the distances 

 manifestly causes the action of the positive electricity upon the 



