ELEMENTARY MAGNETISM. 351 



darning-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 

 magnetised darning-needle and suspend it by a fine silk 

 fibre. The sewing-needle, or the fragment of the darn- 

 ing-needle, is now to be used as a test-needle, to ex- 

 amine 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 oscillations, which at first were quick, become 

 slower; opposite the middle of the strip they cease en- 

 tirely; 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 ex- 

 periment 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. 

 Supposing 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 magnet- 

 ism, while the entire upper half exhibits north mag- 

 netism. So far, then, you have determined the distri- 

 bution 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: ' 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, 



