CHAPTER XIX 

 THE MAGNETIC EFFECTS OF ELECTRIC CURRENTS 



230. Questions for Discussion. 1. How is a compass constructed? 

 How is the mariner's compass constructed ? 2. What kinds of materials 

 are attracted by a magnet? 3. How are telephone and magneto mag- 

 nets used? 4. What kind of a magnet is used in an automobile am- 

 meter? in a battery-testing ammeter? in a watt-hour meter? in a 

 telephone ringer? 5. Do magnets lose their magnetism? 6. What is 

 needed to make a magnet that will operate on the current from a dry- 

 cell battery? 7. Who invented the electromagnet? 8. What kinds of 

 magnets are used in a telephone receiver? 9. How is a telegraph 

 sounder made? 10. How are electric bells connected? 11. How does 

 an annunciator work? 12. What are the necessary parts of an electric 

 motor? 13. How is electricity used in operating street cars? 14. What 

 kinds of electric circuits are dangerous? 15. What is the difference 

 between a direct-current motor and an alternating-current motor? 

 16. How is a motor started? 17. Why do most large motors have a 

 "starting box"? 18. What are the advantages of electric motors? 

 the disadvantages? 19. What is the difference between a dynamo and 

 a motor ? 20. How are electric currents generated at the power plant ? 

 21. Why are transformers used? 22. How is an induction coil made? 

 23. How is electricity brought to a residence? 24. How is a "door 

 opener" made? 25. How may buildings be protected from lightning? 

 26. How is a toy transformer made ? 



231. Permanent magnets and their uses. If a bar magnet 

 is covered with small nails or iron filings and carefully 

 lifted, most of the adhering metal (fig. 112) will usually, 

 though not always, be near the ends of the magnet. The 

 parts in which the attraction is the strongest are called the 

 magnetic poles. 



If a bar magnet is suspended horizontally by a piece of 

 thread at some distance from other magnets and iron, and 

 left undisturbed, it will take a definite position, pointing 

 224 



