GENERATION OF ELECTRICITY 399 



arrow; hence wire K will act as the return wire. When 4 reaches 

 the position of 3 and starts to move by magnet S, current will be 

 generated in the direction of the arrow at 3, causing the current to 

 leave by sector T which is now in the position of P, causing the 

 current to flow out over the brush X. 



The Cycle. A cycle in a dynamo occurs when a plus magnet or 

 wire on the armature passes from a plus magnet by a minus magnet 

 to a plus magnet again. For example, if there are two magnets such 

 as in Fig. 277, the passing of the wire from the north magnet by the 

 south magnet and back again to the north magnet represents one 

 cycle, that is, the wire turns around once. If the dynamo had four 

 magnets (two north and two south poles), the wire would pass 

 through two cycles in turning around once. How many cycles 

 would the dynamo have if there were sixteen plus and sixteen 

 minus poles on the dynamo? How many cycles per second would 

 the dynamo have if the wire turned around ten times in one second? 



The Cycle for Lighting. Alternating currents would naturally 

 cause lights to flicker, since the current first enters the light one way 

 and then completely reverses. A sixteen candle power (c.p.) of 

 110 volts begins to show flickering at about 30 cycles per second. 

 At 25 cycles the flickering is noticeable, and at 20 cycles it is 

 intolerable. This is true of arc lamps. It is often the cause of the 

 flickering of moving pictures which is so annoying to an audience. 

 This is corrected to some extent by enclosing the arc and keeping 

 the arc as short as possible. Difference in the hardness of the 

 carbons also assists. 



QUESTIONS 



1. Why does the hair sometimes snap when being combed with 

 a rubber comb? 



2. When would lightning strike a building? 



3. Why are sparks seen if a cat's fur is rubbed in a dark room? 



4. What kind of substances conduct electricity well? 



5. Why can not static electricity be generated from an iron rod, 

 since it can be generated from a glass or rubber rod? 



6. Who invented the lightning rod? 



7. Why are lightning rods pointed? 



