288 HEAT OF ELECTRICITY. SECT. XXV111. 



quite independent of the extent of the edge, the area being the 

 same ; for Sir W. S. Harris found that the electrical intensity 

 of a charged sphere is the same with that of a plane circular 

 area of the same superficial extent, and that of a charged 

 cylinder the same as if it were cut open and expanded into a 

 plane surface. 



The same ahle electrician has shown that the attractive force 

 between an electrified and a neutral uninsulated body is the 

 same whatever be the forms of their unopposed parts. Thus 

 two hemispheres attract each other with precisely the same 

 force as if they were spheres ; and as the force is as the number 

 of attracting points in operation directly, and as the squares of 

 the respective distances inversely, it follows that the attraction 

 between a mere ring and a circular area is no greater than that 

 between two similar rings, and the force between a sphere and 

 an opposed spherical segment of the same curvature is no greater 

 than that of two similar segments, each equal to the given 

 segment. 



Electricity may be accumulated to a great extent in insulated 

 bodies, and so long as it is quiescent it occasions no sensible 

 change in their properties. When restrained by the non-con- 

 ducting power of the atmosphere, its tension or the pressure it 

 exerts is proportional to the coercive force of the air. If the 

 pressure be less than the coercive force, the electricity is 

 retained ; but the instant it exceeds that force in any one point 

 it escapes, and that more readily when the air is attenuated or 

 saturated with moisture, for the resistance of the air is pro- 

 portional to the square of its density, but the inductive action 

 of electricity on distant bodies is independent of atmospheric 

 pressure. The power of retaining electricity depends also on 

 the shape of the charged body. It is most easily retained by a 

 sphere, next to that by a spheroid, but it readily escapes from a 

 point, and a pointed object receives it with most facility. 



The heat produced by the electric shock is proportional to 

 the square of the quantity of electricity discharged, and is so 

 intense that it fuses metals and volatilizes substances, but its 

 intensity is not felt to its full extent on account of the shortness 

 of its duration. It is only accompanied by light when the 

 electricity is obstructed in its passage through substance. 



Electrical light when analysed by a prism differs very much 



