102 



HOW WE LIGHT OUR HOMES 



from thread, but this was soon replaced by a better 

 filament made from bamboo. The bulb was exhausted 

 of air to prevent the filament from burning. 



All substances that conduct electricity offer resist- 

 ance to the flow of electrical energy. There is no such 

 thing as a perfect conductor. Some of the electrical 

 energy is changed to heat energy, and when the re- 



DURNER 



CHIMN 



TIP 



FISHTAIL BURNER WELSBACH MANTLE 



FIG. 149. TYPES OF GAS LIGHTS 



sistance is very great, light may also be produced. 

 Carbon was first used because it could be heated to 

 1850 degrees centigrade, give off white light, and still 

 not melt. 



The carbon filament lamp is fast being replaced by 

 the tungsten light. Tungsten is a metal that can be 

 heated to 2100 degrees centigrade without melting and 

 in nitrogen-filled lamps to even higher temperatures. 

 The higher the temperature, the more nearly the qual- 

 ity of light approaches sunlight and the greater is 

 the amount of electrical energy changed to light. 



FIG. 150. DEVELOPMENT OF THE ELECTRIC LIGHT BULB 



Until recent times all bulbs of incandescent lamps 

 were exhausted to an almost perfect vacuum. The 

 presence of air in the bulb would burn up the fila- 

 ment. However, in lamps of this kind the filament 

 slowly evaporates and a dark coating of the metal 

 forms on the inside of the bulb. Electric lamp manu- 

 facturers now make a bulb filled with some inactive 

 gas, such as nitrogen or argon. These inactive gases 

 slow down the evaporation of the filament, making it 



FIG. 151. ELECTRIC 

 ARC LAMP 



possible to heat the tungsten filament to higher tem- 

 peratures than in the common vacuum bulb. 



Gas-filled electric lamps are now being used for 

 street lighting, and they are rapidly replacing the arc 

 lamps used for this purpose. 



Electric arc lamps, however, are 

 still used for lighting. When two 

 carbon rods are touched together 

 and a current of electricity passes 

 through them, the ends of the rods 

 become very hot hot enough, in 

 fact, to turn some of the carbon into 

 vapor. If the carbon rods are sep- 

 arated a quarter of an inch, the cur- 

 rent continues to flow across the 

 gap carried by the carbon vapor. A 

 very brilliant light which formerly 

 was used for street lighting and in 

 stereopticon lanterns and in the 

 early motion-picture machines is 

 thus produced. Figure 151 shows a 

 drawing of a carbon arc lamp. You 

 can make a simple carbon arc lamp 

 by using two lead pencils for car- 

 bons and a dry-cell battery as a 

 source of electricity. See if you can 

 plan and build such a lamp. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Lake, Harley, and Welton, Exploring the World of Science, 

 Unit 11 



Pieper and Beauchamp, Everyday Problems in Science, Unit 

 15 



Watkins and Bedell, General Science for Today, Chap. 22 



Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 1 (part) 



Special references 



Faraday, Chemical History of a Candle 

 Bachman, Great Inventors and Their Inventions 

 General Electric Lecture Service, Schenectady, N.Y., In- 

 ventor and Lamp (Lecture 8) 

 Tappan, Modern Triumphs 

 Tappan, Wonders of Science 

 Luckiesh, Lighting the Home 

 Percival, The Electric Lamp Industry 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. What the ancient methods of securing light were. 



2. How new and better methods of lighting were 

 developed as civilization advanced. 



3. An understanding of the incandescent lamp and 

 its advantages. 



TEST OF MASTERY OF THE TOPIC 



In your notebook comply with the instructions and com- 

 plete the statements. 



