STATIC ELECTRICITY 203 



168. Lightning. A lightning flash is simply a large spark 

 similar to that produced by the discharge of the Ley den jar. 

 When two clouds charged with electricity approach each other, 

 they may act as the coats of a huge Ley den jar, the air between 

 acting like the glass. When the charge breaks through the 

 air from one cloud to another, a lightning flash is the result. 

 Sometimes the earth and a cloud act as the two coats of a 

 jar, and then we may have a stroke of lightning. Tall objects, 

 such as chimneys, trees, and towers act as the wire did in 

 discharging the Ley den jar, and are thus often said to attract 

 the lightning. Heat lightning, often seen near the horizon 

 on summer evenings, is simply the reflection of the lightning 

 from thunder storms too distant for the thunder to be 

 heard. The Aurora Borealis, or northern lights, is now re- 

 garded as being caused by the passage of electric currents far 

 above the earth. 



Practical Exercises 



1. Why are electrical push buttons and switches usually made of 

 rubber or gutta-percha? 



2. The wires used for electric wiring in houses are covered with a 

 layer of rubber or silk thread or by both. Of what use is this? 



3. Does a charged body attract or repel one not charged? 



4. Hang up two balls of pith or cotton by means of silk thread and 

 charge from any convenient source. Thoroughly wet one of the silk 

 threads and again charge. Which retains its charge best? Why? 



5. Why could not wire be used as well as silk for suspending the pith 

 balls? 



6. When a large belt runs over a pulley, one may often draw a good- 

 sized spark from it by merely touching it with a piece of metal. Explain. 



