248 



ELECTRICITY 



9. Make a diagram of two bar magnets with the south 

 pole of one facing the north pole of the other. Draw the 

 lines of force about these opposite poles. 



10. Electromagnets are used in telephone . 



11. An electromagnet has an _ core. 



12. The strength of an electromagnet may be increased 

 by making turns of wire on the core or by 



13. An electromagnet its magnetism when the elec- 

 tric circuit is broken by a switch. 



14. The magnetic pole of the earth is in the south- 

 ern hemisphere. 



15. If a person were to carry a compass needle suspended 

 on a string to the magnetic pole in northern Canada, how 

 would it behave ? 



16. In which direction would the N pole of a compass 

 point if carried to northern Greenland? 



17. Suggest a way in which the steel blade of a pocket knife 

 might be magnetized. 



18. Suggest a simple experiment by which you could iden- 

 tify the poles of an unmarked bar magnet. 



19. A boy tried to make an electromagnet by winding a 

 coil of insulated wire around a copper bar. What do you 

 think the results were? Why? Suggest a way of improving 

 his electromagnet. 



20. What evidence can you give that a magnetic field 

 really exists about a magnet ? Have you ever seen the lines of 

 force in a magnetic field? 



21. Suggest a method of experiment that might prove that 

 the magnetic lines of force about a magnet surround it com- 

 pletely and do not, as your experiment would seem to indi- 

 cate, lie only in the plane of the table top. 



TOPIC 3. HOW ELECTRICITY IS GENERATED 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What did Michael Faraday discover about 

 magnetism and electricity, and how was it 

 put to work? 



2. What are the two types of electric currents? 



3. How is electrical energy generated and dis- 

 tributed on a modern scale? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Carefully study the problems to see if they in- 

 clude questions in which you are interested. 



2. You may find the following new words and 

 phrases for the first time in this topic : 



commutator a device on an electric generator to keep 

 the current always flowing in the same direction in 

 the outside circuit. 



cutting lines of force lines of magnetic force are cut 

 whenever a wire or coil of wires moves across a mag- 

 netic field at right angles to the lines of force. 



armature the turning part of a motor or generator. 



dynamo a device used to change mechanical energy to 

 electrical energy. 



turbine a machine which uses the energy of steam or 

 water to turn it and which is used to run electric 

 generators. 



magneto a simple electric generator using permanent 

 magnets for the magnetic field. 



induced current a current produced in a coil whenever 

 it breaks magnetic lines of force. 



galvanometer a device used to detect small electric cur- 

 rents. 



field coils coils of wire which are a part of both mo- 

 tors and generators. As current flows through them 

 they supply the magnetic field needed in either device. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 153. How did Faraday secure electricity 

 with a coil and magnet? 



Wind about fifty turns of bell wire into a coil about three 

 inches in diameter, leaving about ten inches of wire on either 

 end. Attach the ends to a sensitive galvanometer as shown 

 in the illustration in Figure 390. If no galvanometer is 

 available, one may be made by winding about two hundred 

 turns of double cotton-covered wire, No. 26, or No. 28, 

 around a frame which can be made from a safety match 

 box cover in the following manner. Cut the top of the cover 

 as shown in Figure 391. 



FIG. 390 



Wind the coil over the bridge B in the center as shown, 

 leaving several inches of wire on either end for connections. 

 Place a small magnetic compass in the cover under the coil 

 or suspend a small magnetized needle between the top and 

 bottom of the match box cover, as shown in Figure 391. 



When the coil is connected to the galvanometer, move 

 the coil over the end of a bar or horseshoe magnet. Note 

 any movement of the galvanometer needle and its direc- 

 tion. Remove the coil from the pole of the magnet and note 

 the direction of movement of the needle. Move the magnet 

 through the coil. Record your observations and complete 

 the following statements in your notebook. 



A movement of the galvanometer needle shows that 



through the coils. When the coil was moved down 



over the magnet the galvanometer needle moved 



1 See workbook, p. 89. 



