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Our Surroundings 



of electrical pressure at that point. An electric current is produced 

 by a difference in the potential, or amount of electrical charges, 

 at the two ends of a conductor. The current flows from the point 

 of the greater potential to that of the lesser, much as water 

 flows from a higher to a lower level. 



The Leyden Jar. A condenser is a device for accumulating 

 and holding a large charge of electricity in a small space. Con- 

 densers are used in laboratories, and in a prac- 

 tical way in radio sets. The principle of the 

 condenser is illustrated by the Leyden jar, 

 named after the city in Holland where it was 

 first used. The jar is made of glass and has a 

 coat of tin foil, both inside and outside, up to 

 about two-thirds of its height. A brass rod, 

 having a knob at one end and a metal chain at 

 the other, is passed through the stopper of the 

 jar in such a way as to leave the knob outside 

 the jar and the chain inside, in contact with 

 the inside tin foil coating. There are two con- 

 ductors, the tin foil on the inside and the tin 

 foil on the outside, separated by the insulating 

 glass wall. 



The condenser is charged by connecting the 

 outer tin foil with the earth by means of a 

 wire or other conductor, or by direct contact, 

 and then giving the inside coating a charge of positive or negative 

 electricity. The inside charge attracts electricity of the opposite 

 kind from the ground and holds it to the outer coating, and repels 

 electricity of the same kind. 



A Leyden jar may be discharged by placing one end of a 

 bow-shaped metal rod, having an insulated handle at the middle, in 

 contact with the outside covering, while the other end is brought 

 near the knob of the jar. A spark will pass through the air 

 between the end of the rod and the knob, producing a crackling 

 noise. This illustrates how lightning is produced. 



Lightning. Lightning is the discharge between two clouds 

 bearing unlike electrical charges, or between clouds and oppo- 



THE LEYDEN JAR 



It stores static elec- 

 tricity. Used to illus- 

 trate lightning. 



