CHAPTER XVII 

 HEAT AND LIGHT FROM ELECTRIC CURRENTS 



210. Questions for Discussion. 1. Which is cheaper to operate, a 

 tungsten or a carbon lamp? Why? 2. Why are electric-lamp wires 

 covered with rubber and cloth? 3. What are the sparks sometimes 

 observed between the trolley wire and wheel ? 4. Why are electric wires 

 often put in metal pipes ? 5. Why are glass insulators used on telephone 

 and telegraph lines? 6. Are electric-heating devices expensive to use 

 as compared with devices which use coal and gas ? 7. How should dry 

 cells be connected for a doorbell circuit? 8. How does a push button 

 operate the electric bell? 9. Is there any danger in having electric 

 wires in a house? 10. How is a flashlight battery made? 11. What 

 is a fuse? Why is it used? 12. Why are ammeters used on auto- 

 mobiles? 13. How can telephones be protected from damage due to 

 lightning? 14. What is a short circuit? Is a short circuit dangerous? 

 15. How does the watt-hour meter aid in finding the cost of electric 

 energy? 16. What does electric energy cost in an ordinary residence? 

 in a store? 17. How is ^an electric iron constructed? 18. Why is a 

 spark plug used in a gas engine ? 19. How is a motion-picture machine 

 constructed? 20. How is the electric current supplied to street lamps? 

 21. How is an electric fire-alarm system constructed? 



211. Light and heat. Until a few decades ago artificial 

 light which was used in such places as homes and factories 

 was produced by a flame from oil, gas, or a candle. These 

 sources of light still have extensive uses, but light by use of 

 electricity has become more common than other kinds of 

 artificial light. In the older forms of artificial light it is readily 

 noticed that heat as well as light is produced. In order that 

 the light may be at its maximum, the heat that is lost must 

 be at its minimum. Some substances may be heated to a tem- 

 perature at which they will give off light without melting or 

 burning, while other substances which burn while giving off 



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