246 



ELECTRICITY 



it is thought that the ancient Chinese used long pieces 

 of such ore in guiding journeys on land and sea be- 

 cause of the fact that it arranges itself in a general 

 north and south direction when suspended. The 

 Greeks also knew of this property of bars of magnetic 

 iron ore, which they called lodestones or pointing 

 stones. Today we speak of this substance as a na- 

 tural magnet. 



Only a few substances are magnetic. Since ancient 

 times it has been discovered that not only iron, but 

 also steel, nickel, and cobalt, a substance closely re- 

 sembling nickel, are magnetic. These substances can 

 be picked up with a magnet, but all other things ex- 

 cept one or two alloys 1 are not influenced when 

 brought near a magnetic field. 



In your experiment you discovered that a magnet 

 could influence a compass or another magnet without 

 touching it. A strong magnet will cause tacks and 

 other bits of iron to jump from a table top to meet it. 

 This action shows that a magnet influences things at 

 a distance. Your experiment in which you sprinkled 

 iron filings over a magnet shows the same quality. 

 The space about a magnet is spoken of as a magnetic 

 field and is filled with lines of force. The iron filings ar- 

 ranged themselves along these lines of force. The 

 stronger a magnet, the greater number of lines of 

 force it will have in its magnetic field. 



The magnetism in a magnet seems to be concen- 

 trated in two places near the ends, which are called 

 its poles. A bar magnet has a pole near each of its 

 ends, one of which points northward and the other 

 southward if the bar is suspended. They are called 

 the north-seeking and south-seeking poles of the^ mag- 

 net. Your experiment taught you a very important 

 law of magnetism that like magnetic poles push apart 

 or repel and unlike magnetic poles attract each other. 



Exercise. Plan an experiment to test whether mag- 

 nets have more than two poles, that is, whether there 

 are other points where the magnetism seems to be con- 

 centrated. Iron nails or steel needles or old bar magnets 

 may be used. 



Exercise. With a compass test the iron of a bridge. 

 It is magnetised? Infer a cause for this. Test an iron 

 fence post with a compass. Is the top magnetized? How? 

 Is the bottom magnetized? What is its polarity? Suggest 

 a possible theory to account for this. Suggestion: How 

 does the earth's magnetic field influence bodies in it 

 that are magnetic? Are all iron poles magnetised the 

 same? Suggest an experiment with a small iron bar and 

 a magnet to test your theory. Try it out. 



Refer to the drawings which you made in your ex- 

 periment and again see how the lines of force appeared 



1 Alloy: Some metals when melted together dissolve in one 

 another. When such a metallic solution cools it is called an alloy. 



Sou\W 



* Pole 



Sou 



Kjognstic. 



'o\e 



Pole. / 



FIG. 386. THE HAKTH AS A MAGNET 



when like poles and when unlike poles were placed to- 

 gether. This law explains why a compass needle arranges 

 itself in a general north-and-south direction. 



Why does a compass point northward? Sir William 

 Gilbert, an English scientist, discovered why the lode- 

 stone and other compasses point northward when he 

 found that the earth is a huge magnet with poles like 

 those of the bar magnets which you have studied. One 

 of these poles is located in northern Canada and the 

 other in the continent of Antarctica. 



You have observed from the experiments that a 

 compass needle is a magnet and that its north mag- 

 netic pole points toward the north. If you will think 

 carefully and apply 

 the law of magnetism 

 stated above, you will 

 see that the earth 

 must have a south 

 magnetic pole in 

 northern Canada and 

 a north magnetic pole 

 in Antarctica, other- 

 wise the north mag- 

 netic pole of the com- 

 pass or lodestone 

 would point south- 

 ward. Figure 386 

 shows this arrange- 

 ment clearly and should be carefully studied to fix the 

 idea in your thinking. 



Since the north pole and the earth's magnetic pole 

 in the northern hemisphere are not at the same point, 

 the compass does not point true north but magnetic 

 north. Allowance has to be made for this .when ships 

 are guided at sea by magnetic compasses. 



On his first trip to the Arctic with airplanes, Ad- 

 miral Richard Byrd found that his magnetic com- 

 passes were not reliable, and so he found it necessary 

 to use a sun compass to locate exactly the North 

 Pole. 



Exercise. Can you suggest a possible cause for the un- 

 reliability of the magnetic compasses in this instance? 



Figure 386 also shows the lines of force which are 

 about the earth because of its magnetism. These lines 

 of force are used to operate another kind of compass 

 called the earth inductor compass, which guided Lind- 

 bergh on his historic flight to Paris. 



What are electromagnets, and for what are they 

 used? Hans Christian Oersted, a Dutch scientist, dis- 

 covered in 1819 that any wire through which an elec- 

 tric current is flowing has a magnetic field about it. 

 Some years later Michael Faraday, an English scien- 

 tist, and Joseph Henry, an American scientist, dis- 

 covered that if a wire carrying a current were wrapped 



