184 Introduction to the Study of Science 



The bulb incloses a vacuum or is filled with nitrogen. 



Resistance depends upon the material, the temperature, the length, 

 and the diameter of a conductor. 



An electric current enters a house by one wire, does useful work 

 in lamps, heaters, and motors, and leaves by another wire. 



Electric arc lamps include luminous vapors, as those of carbon or 

 mercury. 



An electrode is the solid conductor by which an electric current 

 enters or leaves a liquid or gaseous medium. 



The color of an object depends on its nature and the nature of the 

 light which illuminates it. 



Sunlight is shown by a prism to be composed of all the colors of 

 the spectrum. 



An artificial light should be safe, hygienic, and in other ways re- 

 semble sunlight. 



REVIEW QUESTIONS 



I. What advantages do we enjoy from the use of artificial lighting? 

 2. What was the best method of lighting known in the palaces of the 

 Caesars? 3. Who invented our modern oil burning lamp? When? 

 4. (a) Who first made the use of gas for lighting possible ? (6) What 

 are the important facts in the development of -gas lighting? 5. What 

 are the chief modern systems of lighting? 



6. What improvement has made gaslighting more satisfactory? 

 7. (a) Describe the making of a Welsbach mantle. (6) What advan- 

 tage has it over the ordinary burner? 8. (a) Describe the production 

 of coal gas. (6) What important by-products result? 9. For what 

 is ammonia used? 10. Mention an instance of the production by 

 chemists of a substance formerly derived from plants. 



II. Mention several kinds of filaments for electric lamps. 12. What 

 are the advantages of tungsten? 13. Mention three metals in the 

 order of their relative conductivity. 14. (a) Mention several sub- 

 stances which are considered insulators. (6) Are they perfect non- 

 conductors? 15. Why is a little sulfuric acid put into the water 

 used in electrolysis? 16. (a) What is the effect of high temperatures 

 upon the resistance of some substances? (6) What effect has a very 

 low temperature upon the resistance of pure metals? 17. Group to- 

 gether substances that have their resistance decreased by high temper- 

 ature. 18. Mention several that have their resistance increased. 



19. What effect is made on the current which a wire carries by an 

 increase in length, other conditions remaining the same? 20. Which 

 permits more current to pass, a fine wire, or a coarser wire, other con- 



