ELECTRICITY. 375 



whenisabody When a conducting body is insulated, it 

 cwged" with retains upon its surface the electricity com- 

 eiectricity? muuicated to it, and in this condition it is 

 said to be charged with electricity. 



A .conductor of electricity can only remain electric as long as it is insulated, 

 that is, surrounded by perfect non-conductors. The air is an insulator, since, 

 if it were not so, electricity would bo instantly withdrawn by the atmosphero 

 from electrified substances. "Water and steam are good conductors, conse- 

 quently, when the atmosphere is damp, the electricity will soon be lost^ 

 which, in a dry condition of the air, would have adhered to an insulated con- 

 ductor for a long period of time. 



Thus a globe of metal supported on a glass pillar, or suspended by a silken 

 cord, and charged with electricity, will retain the charge. If, on the oon- 

 traiy, it were supported on a metallic pOlar, or suspended by a metallic wire, 

 the electricity would immediately pass away over the metaUic surface and 

 escape. 



In the experiments made with the pith balls (§ 733, Fig. 308), the silk 

 thread by which they were suspended acts as an insulator, and the electricity 

 with which they become charged is not able to escape. 



745. When electricity is communicated to 



Does clectnci- , , , ^ . . , 



ty accumulate a couductino^ bodv it Fcsidcs merely upon the 



upon the sur- ii ii 



face or the in- surfacc, and does not penetrate to any depth 



terior of bodies? ... wo. 



withm. 



_ Thus, if a solid globe of metal suspended by a 



silken thread, or supported upon an insulated 

 glass pillar, be highly electrified, and two thin 

 hollow caps of tin-foil or gilt paper, furnished 

 with insulating handles, as is represented in 

 Fig. 310, be applied to it, and then withdrawn, 

 it will be found that the electricity has been 

 completely taken off the sphere by means of the caps. 



An insulated hollow ball, however thin its substance, will contain a charge 

 of electricity equal to that of a solid ball of the same size, all the electricity ia 

 both cases being distributed upon the surface alone. 



In the case of a spherical body charged with electricity; 

 form of a body *^^ distribution is equal all over the surface ; but when th© 

 Influence iu body to which the electricity is communicated is larger in one 

 dition? direction than the other, the electricity is chiefly found at it3 



longer extremities, and the quantity at any point of its sur- 

 fece is proportional to its distance from the center. 



The shape of a body also exercises great influence in retaining electricity : 

 it is more easily retained l^ a sphere than by a spheroid or cylinder; but it 

 readily escapes from a point, and a pointed object also receives it with tha 

 greatest facility. 



