CHAP. i.J ELECTRICITY AND MAGNETISM. 



37 



upon their flatter parts. This distribution can be de- 

 duced from the theory laid down in Lesson 2OC., but 

 meantime we will give some of the chief cases as they 

 can be shown to exist. The term Electric Density is 

 used to signify the amount of electricity at any point of 

 a surface ; the electric density at a point is the number 

 of units of electricity per unit of area (i.e. per square 

 inch, or per square centimetre), the distribution being 

 supposed uniform over this small surface. 



(a) Sphere. The distribution of a charge over an 

 insulated sphere of conducting material is uniform, 

 provided the sphere is remote from the presence of all 

 other conductors and all other electrified bodies ; or, in 



Fig. -27. 



other words, the density is uniform all over it. This is 

 symbolised by the dotted line round the sphere in Fig. 

 22, <2, which is at an equal distance from the sphere all 

 round, suggesting an equal thickness of electricity at 

 every point of the surface. It must be remembered 

 that the charge is not really of any perceptible thickness 

 at all ; it resides on or at the surface, but cannot be 

 said to form a stratum upon it. 



(b) Cylinder "with rounded ends. Upon an 

 elongated conductor, such as is frequently employed in 

 electrical apparatus, the density is greatest at the ends 

 where the curvature of the surface is the greatest, 



where th 



