254 



ELECTRICITY 



FIG. 401 



used fuse and hold it in a 

 match flame. Record your 

 observations and complete 

 the following statements 

 in your notebook. 



The fuse used in this 



experiment had a 



melting point as demon- 

 strated by holding it in a 

 When electric cur- 

 rent flows through it, a 



wire becomes because 



of the which it offers 



to the current. When a 



large current flowed in the circuit sufficient heat was de- 

 veloped to fuse. This the circuit and no more cur- 

 rent could flow until Ordinary fuse wire, as shown 



by this experiment, has a melting point. 



Experiment 157. How do electric heating devices 

 work? 



Set up a board as shown 

 in Figure 402. Tack a 

 piece of asbestos about 

 four inches square to the 

 board and drive two small 

 nails three inches apart 

 through it into the wood. 

 Lead a wire from a dry 

 cell through a switch or 

 push button to one nail and 

 another from the battery 

 to the other nail. Secure 

 a piece of fine iron wire 

 about four inches long and 

 attach the ends to the 

 nails. One strand of pic- 

 ture wire will work well 

 for this. Allow current to 

 flow for a very short time 



by closing the circuit at the push button or switch. Remove 

 the iron wire and replace it with a copper wire of about 

 the same size. Again allow current to flow. Observe an 

 electric toaster or other electrical device with an open 

 heating element. Study its construction and observe the heat- 

 ing element when it is cold. If possible, secure a piece of 

 the heating element from an old toaster or flatiron and 

 connect it between the nails in your set-up. Record your ob- 

 servations and complete the following statements in your 

 notebook. 



When current from the battery flowed in the circuit, 

 the iron wire while the copper wire .. This was 



FIG. 402 



due to the fact that 

 current than _ 



offers more resistance to the 



would be better than for use 



in an electric heating element, as shown by this experiment. 



Activity 158. How do electric lamps work? 



Secure an unfrosted electric-light bulb and study it care- 

 fully. Trace the current from the point where it enters the 

 bulb to the point where it leaves. Study the method used 

 to remove the air from the bulb. Study the filament and 

 learn how a high resistance is secured. Learn how the fila- 

 ment is supported. Is the filament made of the same material 

 as the wire which goes through the glass stem at the base? 

 Examine the way in which the glass bulb is fastened to 

 the brass shell. 



FIG. 403 



In your notebook, diagram the bulb which you studied, 

 showing the pathway of the current through the bulb. 

 Letter the diagram to identify the filament, the brass shell, 

 the air removal tube, and the contact buttons. Complete 

 the following statements. 



The filament is made of The material marked 



is an insulator between the upper contact button and the 

 . The brass shell is fastened to the globe with 



Experiment 159. How does the electric motor work? 



Suspend a bar magnet and bring another one near it so 

 that two unlike poles are together. Keep turning the poles 

 of the magnet in the hand in such a way that the sus- 

 pended magnet rotates. 

 Connect a dry-cell bat- 

 tery to a small Erector 

 motor or a St. Louis mo- 

 tor having permanent 

 bar magnets for the field. 



In your notebook com- 

 plete the following state- 

 ments. 



The rotating part of 

 this motor is different 

 from the rotating mag- 

 net in the part above in that this is an magnet while the 



one above is a magnet. The armature of this motor 



(has, does not have) a commutator. (Direct, Alternat- 

 ing) electric current is furnished by the dry cell. From 



Experiment 152 you learned that the poles of an electro- 

 magnet can be reversed by reversing the flowing in the 



coil. The commutator of the motor does this each half 

 revolution. 



How the armature poles of a motor change Set up an 

 electromagnet as in Figure 403. Bring a compass near the 

 electromagnet. Press the push button and determine the 

 polarity of the head of the nail. Reverse the current through 

 the electromagnet by reversing the battery connections. 

 Again test the polarity of the head of the nail. The nail 

 and coil represent one coil in the armature. The commuta- 

 tor of the motor reverses the current through these coils 

 each half revolution, thus reversing their polarity. 



In your notebook complete the following statements. 



Unlike poles of two magnets and like poles repel. 



The polarity of electromagnets may be reversed by 



the current flowing in them. The armature of a motor is 



an magnet. The current in the armature is . . each 



half revolution by the . This the poles of the 



armature each revolution, causing attraction and re- 

 pulsion between the poles of the magnets and arma- 

 ture and resulting in the rotation of the 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Learn how to replace the fuses in your home. 



2. Learn how an arc lamp works. 



3. Wire a miniature house system on a board with fuses, 

 lights, heating devices, etc. 



READING WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What are electric fuses for, and how are they re- 

 placed? In the same way that water flows through a 

 smooth pipe more easily than through a rough one, 

 electricity flows more easily through some things than 



