202 APPLIED SCIENCE 



a conductor. Thus the fuse protects the conductor from 

 being overheated by excess current. The fuse, which con- 

 sists of a piece of soft metal, such as an alloy of lead, is 

 soldered to copper terminals, so shaped that they may be 

 clamped. 



249. Circuit-Breaker. When a large volume of current 

 is used it is necessary to have a device known as a circuit- 

 breaker, as fuses are sometimes too slow in action. A circuit- 

 breaker is practically a switch, which, when the current 

 exceeds a certain amount, automatically opens by means of 

 the pressure of a spring regulated by a coil through which the 

 current passes. When the current becomes greater than a 

 certain amount, the coils attract an iron rod attached tc 

 a trigger and release it. This trigger comprises a spring 

 vvhich acts upon the switch. One current-breaker is used 

 for each generator. 



Questions 



1. What is the commercial method of generating electricity? 



2. What are the principal parts of a dynamo? 



3. Explain the principle of a dynamo. 



4. The strength of a current depends upon what factors? 



5. What is an induced current? 



6. How may the direction of an induced current be determined? 



7. What is a commutator? 



8. What are armature brushes? 



9. Describe the action of a dynamo. 



10. Name and describe the different kinds of dynamos. . 



11. What is the difference between alternating and direct cur- 



rents? 



12. What is an electric motor? 



13. Describe the different kinds of electric motors. 



14. Explain the expression " efficiency of a dynamo," 



