ELECTRICITY OF THE ATMOSPHERE 



133 



conductor, for the whole conductor is discharged at once. 

 The strain in the electric field is greatest around the points 

 of an object; hence a discharge takes place very easily 

 from points, but less readily from blunt portions. When 

 the spark passes, the air is heated, and its rapid expansion 

 and contraction produce a sharp explosion. 



146. Storing a Charge; the Leyden Jar. Apparatus 

 for storing frictional electricity (Fig. 121) is made of two 

 conductors (metal foil) separated by a non-conductor, 

 such as glass. If one conductor is 

 connected with a source of positive 

 charges, while the other conductor 

 is connected with the earth, the 

 negative charge of the second con- 

 ductor is held on the side of the 



metal next to the glass, but the positive 

 charge of the second conductor is repelled 

 to the earth. If the earth connection is 

 then removed, we have a large positive 

 charge on one side of the glass, and a 

 correspondingly large negative charge 

 on the other side. 



FIG. 121. 

 An Electric Condenser. 



FIG. 122. 



The Leyden Jar; an 

 Electric Condenser 

 and Storing Appa- 

 ratus. 



The most common form of the apparatus is 

 the Leyden jar (Fig. 122) ; this consists of a glass 

 bottle, covered to the same height, inside and out- 

 side, with metal foil. If we charge the inside 

 coating with frictional electricity, and then almost connect the two 

 foils by some conductor, a strong spark discharge takes place. 



147. Electricity of the Atmosphere. -Benjamin Frank- 

 lin showed that a lightning discharge was only a large 



