252 



ELECTRICITY 



are known as the step-up type and those used to lower 

 the voltage step-down. 



The working of a transformer is easily understood 

 if one keeps in mind some of the things already 

 learned about magnetism and electricity. You will re- 

 call that every electric current has a magnetic field 

 about it and that if magnetic lines of force cut across 

 conductors a current will be set up in the conductor. 

 The amount of the current depends partly upon the 

 number of conductors cut by the lines of force. You 

 must learn one new fact about magnetic fields to 

 understand fully the operation of the transformer. 

 The magnetic field about a direct current is steady 



while the one about 

 an alternating cur- 

 rent is continually 

 moving. As the cur- 

 rent flows in one di- 

 rection the magnet- 

 ic field builds out 

 around the conduc- 

 tors, and then as the 

 current reverses it 

 stops for a moment 

 and builds up again 

 as the current flows 

 in the opposite direc- 

 tion. This keeps the 

 magnetic lines of 

 force always moving. 

 In the transformer 

 two coils are wound 

 on an iron core as 

 shown in Figure 399. 

 The one which leads 

 from the generator is 

 called the primary 

 winding and the 



other the secondary winding. In each type of trans- 

 former there are more turns of wire in the high volt- 

 age winding than in the low voltage winding. As the 

 current flows in the primary winding its magnetic 

 lines of force surge through the iron core and cut the 

 coils in the secondary and either raise or lower the 

 electrical pressure, depending upon the number of 

 turns of wire in it. 



Exercise. Carefully study the diagram shown In Fig- 

 ure 399. Which is the primary coil? Which is the sec- 

 ondary coil? Predict the effects -when the push button 

 is closed. Is this a step-up or a step-down transformer? 

 Give evidence to support your statements. 



One of the most difficult problems which the power 

 companies have had to solve is transmitting the elec- 

 trical energy from the generating station to the home 

 or industry where it is consumed. Generating stations 



Courtesy General Electric 



FIG. 400. LARGE TRANSFORMER WITH 

 SHELL REMOVED 



are, as a rule, placed as near the source of natural 

 energy as possible. This is particularly true in water- 

 power plants and is usually the case where coal is the 

 source of the energy. In the latter case, transporting 

 the fuel great distances is costly, and transporting the 

 electricity over wires is simpler. 



When the place of consumption of the electrical 

 energy is a considerable distance from the generating 

 plant, step-up transformers, which raise the voltage 

 of the electricity usually to about 66,000 volts, are 

 used. It is not unusual to use much higher voltages in 

 transmitting electricity over such lines, which are 

 known as high tension or power lines. Where the 

 high tension lines are to be tapped for using the elec- 

 tricity, they are led to a substation which contains 

 step-down transformers to reduce the voltage to the 

 required point. 



There is less loss of energy in the transmission 

 line when the voltage is high because as the voltage is 

 raised the current in amperes is reduced in propor- 

 tion. Since line losses are due mostly to the heat 

 energy that results from the resistance of the wires, 

 and since resistance is less when the current is less, 

 it is much more economical to transmit the electricity 

 at high voltage and low amperage. 



Ordinarily the substations that receive the high 

 voltage currents from the generating station reduce 

 the line voltage from 66,000 volts, or higher, to about 

 2,200 volts. From these substations the current is 

 carried about the city or town at the reduced voltage. 



Twenty-two hundred volts is still an extremely high 

 and dangerous voltage to handle. In fact, such a volt- 

 age is almost always fatal to one who touches a wire 

 carrying it. It is obvious, then, that this high volt- 

 age could not possibly be brought into our homes for 

 general use. The appliances in our homes are de- 

 signed to operate at one hundred ten volts ; it is there- 

 fore necessary further to reduce the voltage from the 

 twenty-two-hundred line voltage to the house voltage 

 of one hundred ten. This is accomplished by what is 

 known as a "line" or distribution transformer. These 

 transformers may be seen as iron casings usually 

 placed on poles in various parts of a city or town. In 

 these transformers the voltage is stepped down from 

 2,200 to 110 and then delivered to the homes. 



REFERENCES FOR FURTHER STUDY 



Texts 



Caldwell and Curtis, Science for Today, Chap. 22 



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



16 (part) 

 Hunter and Whitman, Science in Our World of Progress, 



Unit 7; Science in Our Social Life, Unit 7 

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



Chap. 23 

 Pieper and Beauchamp, Everyday Problems in Science, Unit 



14 



