ELECTRIC DISTRIBUTION. 221 



handles. Electrify the globe ; bring it near the electroscope to be 

 sure that it is electrified. Place the hemispheres upon the globe. 

 Remove them quickly, being careful that their edges do not touch 

 the sphere after the first separation. Bring first one shell and then 

 the other near the electroscope ; they are electrified. Bring the 

 globe itself near the electroscope. It is no longer electrified. Delicate 

 manipulation is needed to make the experiment successful. You 

 will fail, perhaps, more times than you succeed. But when the 

 experiment is successful, it is instructive. The apparatus is called 

 Biot's hemispheres. 



(6.) Charge with electricity a hollow sphere having an orifice in 

 the top. Bring a proof-plane, made by fastening a disc of gilt pat>er 

 to a long thin insulating handle, into contact with the outer surface 

 of the sphere. The proof plane is charged by the sphere, as may be 

 shown by bringing it near an electroscope. Discharge the proof - 

 Diane and bring it into contact with the inner surface of the sphere. 

 Remove it carefully without allowing it to touch the sides of the 

 orifice. Bring it to the electroscope. It is not charged. (Fig. 158.) 



(c.) Vary the experiment by the use 

 of Faraday's bag. This consists of a 

 conical bag of linen, supported, as 

 shown in Fig. 159, by an insulated metal 

 hoop five or six inches in diameter. A 

 long silk thread extending each way 

 from the apex of the cone enables the 

 experimenter to turn the bag inside- 

 out without discharging it. Whichever 

 surface of the linen is external, no elec- 

 tricity can be found upon the inside of 

 the bag. Nothing can be more conclu- 

 sive than this. 



(d.) Vary the experiment by the use p IG . 159. 



of a hat suspended by silk threads. 



Notice that the greatest charge can be obtained from the edges ; 

 less from the curved or flat surface ; none from the inside. 



Note. This rule does not apply to an electric current. A hollow 

 wire will not conduct electricity as well as a solid wire of the same 

 diameter. Electricity may be drawn to the inside of a hollow con- 

 ductor by placing there an insulated body oppositely charged. 



359. Distribution of Electricity on the 

 Surface. Experiments show that when a sphere is 

 charged, the electricity is evenly distributed over the sur- 



