ELEMENTARY MAGNETISM. 



needle, will be better able to carry your paper indexes. 

 Having secured such a strip, you proceed thus : 



Magnetise a small sewing-needle and determine its 

 poles ; or, break half an inch, or an inch, off your magnet- 

 ised darning-needle and suspend it by a fine silk fibre. 

 The sewing-needle, or the fragment of the darning 

 needle, is now to be used as a test-needle, to examine 

 the distribution of the magnetism in your strip of steel. 

 Hold the strip upright in your left hand, and cause the 

 test-needle to approach the lower end of your strip ; 

 one end of the test-needle is attracted, the other is 

 repelled. Raise your needle along the strip ; its oscil- 

 lations, which at first were quick, become slower; 

 opposite the middle of the strip they cease entirely ; 

 neither end of the needle is attracted; above the 

 middle the test-needle turns suddenly round, its other 

 end being now attracted. Go through the experi- 

 ment thoroughly : you thus learn that the entire lower 

 half of the strip attracts one end of the needle, while 

 the entire upper half attracts the opposite end. Sup- 

 posing the north end of your little needle to be that 

 attracted below, you infer that the entire lower half of 

 your magnetised strip exhibits south magnetism, while 

 the entire upper half exhibits north magnetism. So 

 far, then, you have determined the distribution of 

 magnetism in your strip of steel. 



You look at this fact, you think of it ; in its sug- 

 gestiveness the value of an experiment chiefly consists. 

 The thought naturally arises : 6 What will occur if I 

 break my strip of steel across in the middle ? Shall I 

 obtain two magnets each possessing a single pole?' 

 Try the experiment ; break your strip of steel, and test 

 each half as you tested the whole. The mere presenta- 

 tion of its two ends in succession to your test-needle, 

 suffices to show that you have not a magnet with a 



