USING ELECTRICITY ABOUT THE HOME 



257 



In the past few years a new type of lamp has been 

 developed which is known as the sodium vapor lamp. 

 Its greatest importance lies in the fact that it is very 

 efficient. In this type of lamp one unit of electricity 

 will produce a light intensity equal to that which 

 requires two and one-half units in an ordinary lamp. 



The sodium vapor lamp finds its greatest use in 

 outdoor illumination for highways, tennis courts, and 

 buildings. It is not probable that it will be used to 

 any great degree for indoor illumination because it 

 gives a brilliant yellow light that does not contain 

 the other colors of the spectrum. Things with color 

 and persons take on peculiar appearances under the 

 sodium vapor lamp. 



How do electric motors work? Electric motors 

 change electrical energy to mechanical energy. Much 

 of the use of electricity about our homes is made pos- 

 sible because electricity can be made to produce mo- 

 tion. Electric vacuum cleaners, washing machines, 

 ironers, sewing machines, and other devices depend 

 for their operation upon electric motors. Outside the 

 home we find motors used to drive street cars and to 

 turn the wheels of a great many industries. The elec- 

 tric motor has made the work of mankind much easier. 



Courtesy General Electric Company 



FIG. 409. SODIUM VAPOR LAMP 



The motor accomplishes a change of energy ex- 

 actly opposite to that which occurs in the generator, 

 in which energy of motion was changed to electricity. 

 In the motor the energy of electricity is changed to 

 motion. The generator and motor are not greatly dif- 

 ferent, as many generators can be run as motors if 

 they are supplied with electrical energy, and many 

 motors can be used as generators if their armatures 

 are turned. 



Joseph Henry discovered a way to produce mo- 

 tion from electricity about one hundred years ago. 

 Though his simple toy was only a rocker, he saw 

 the possibilities of the modern motor. The illustration, 

 Figure 408, is taken from Henry's notes, and in his 

 drawings g and f are two batteries, each connected to 

 wires leading to the armature b, which is an electro- 



magnet. The batteries are so connected that when the 

 ends op are connected with the battery g, current will 

 flow in one direction through the armature, and when 

 the ends qr are connected with battery f, current will 

 flow in the opposite direction, thus reversing the polar- 

 ity of the electromagnet each vibration. Placed under 

 the poles of the electromagnet are permanent magnets 

 c and d. These alternately attract and repel the poles 

 of the armature, thus giving to it a seesaw motion. 



FIG. 410. SMALL DIRECT-CURRENT MOTOR WITH DRY CELLS 



The modern electric motor is similar in construc- 

 tion to the generator, having field coils of electromag- 

 nets, an armature, brushes, and a commutator if it 

 is used on direct current. The picture, Figure 410, 

 shows the field coils, armature, commutator, and 

 brushes of a small direct current motor. 



As current from the dry cell flows through the ar- 

 mature and field coils, they become electromagnets 

 with separate magnetic fields. The current direction 

 in the armature is reversed by the commutator each 

 half revolution, thus reversing its polarity. The push 

 and pull between the like and unlike poles of these 

 electromagnets causes the armature to turn. 



Exercise. Trace to its remotest source the mechani- 

 cal energy secured from an electric motor. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 23 

 Clement, Collister, and Thurston, Our Surroundings, Chap. 



15 and 16 (part) 



Hunter and Whitman, Science in Our Social Life, Unit 7 

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



Chap. 24 

 Pieper, and Beauchamp, Everyday Problems in Science, Unit 



14 



Powers, Neuner, and Bruner, Man's Control of His Environ- 

 ment, Unit 5 



Skilling, Tours through the World of Science, Tour 10 

 Van Buskirk and Smith, The Science of Everyday Life, Chap. 



17 



Watkins and Bedell, General Science for Today, Chap. 25 

 Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 5 (part) 



Wood and Carpenter, Our Environment: How We Use and 

 Control It, Topic 9 



Special references 



Collins, The Book of Electricity 

 Darrow, Masters of Science mid Invention 

 Lunt, Everyday Electricity 



