18 THE NATVRE-STIDY REVIEW [6:i-jan., 1910 



Statement of the law worked out by the class. 



V. Further study of poles. 



Effect of breaking a magnetized needle into two, 



four, eight pieces. 

 Effect of placing one pole of a magnet in contact 



with the middle of soft iron bar. 



VI. Temporary and permanent magnets. 

 VII. Theory of magnetism. 



Illus. experiment: Fill test-tube partly full of iron 

 filings; cork; turn on side and distribute evenly; 

 magnetize by drawing one pole of magnet from 

 center out to end and other pole from center out 

 to other end. Test with compass and suspended 

 needle. Shake and test again. Explain. 



Use this to illustrate what happens to a needle when a 

 magnet is drawn over it. (See "Principles of 

 Physics" by Gilley, page 391.) 

 [Note: These children had some notion of the molecular 

 theory gained from their fall study of osmosis in plants. Other- 

 wise that would have to be given here.] 



For diagrams see "First Course in Physics" by Milli- 

 kan and Gale, p. 233. A good plan is to cut little 

 cardboard oblongs making one pole blue and ar- 

 range them as in an unmagnetized and as in a mag- 

 netized bar. This will require much discussion, 

 but it is well worth while. 



Go back over previous experiments and see how this 

 theory helps us to explain many curious things in 

 the behavior of magnets. 



Work out the difference between a theory and a law. 

 VIII. Study of magnetic fields and lines of force. 



Experiment by placing a bar magnet on sheet of paper 

 and putting a magnetized sewing needle suspended 

 by an untwisted silk thread near north-seeking pole 

 and gradually moving it in a half-circle around to 

 the south-seeking pole. Mark the path of the 

 needle on the paper. 



Map field of force by using iron filings (see any text 

 book.) Draw field of force as shown by filings. 



Again use the suspended needle carrying i t from 

 north-seeking to south-seeking pole above the bar 

 magnet and get the image of lines of force passing 



