ELEMENTARY MAGNETISM. 267 



which you will endeavor to follow out: questions will 

 arise which you will try to answer. The same experiment 

 may be twenty different things to twenty people. Having 

 witnessed the action of pole on pole, through the air, you 

 will perhaps try whether the magnetic power is not to be 

 screened off. You use plates of glass, wood, slate, paste- 

 board, or gutta-percha, but find them all pervious to this 

 wondrous force. One magnetic pole acts upon another 

 through these bodies as if they were not present. Should 

 you ever become a patentee for the regulation of ships' 

 compasses, you will not fall, as some projectors have done, 

 into the error of screening off the magnetism of the 

 ship by the interposition of such substances. 



If you wish to teach a class you must contrive that the 

 effects which you have thus far witnessed for yourself shall 

 be witnessed by twenty or thirty pupils. And here your 

 private ingenuity must come into play. You will attach 

 bits of paper to your needles, so as to render their move- 

 ments visible at a distance, denoting the north and south 

 poles by different colors, say green and red. You may also 

 improve upon your darning-needle. Take a strip of sheet 

 steel, heat it to vivid redness and plunge it into cold 

 water. It is thereby hardened; rendered, in fact, almost as 

 brittle as glass. Six inches of this, magnetized in the 

 manner of the darning-needle, will be better able to carry 

 your paper indexes. Having secured such a strip, you 

 proceed thus: 



Magnetize a small sewing-needle and determine its 

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

 ized darning needle and suspend it by a fine silk fiber. 

 The sewing-needle, or the fragment of the darning-needle, 

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

 tion 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. Rnise your 

 needle along the strip; its oscillations, 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 experiment 

 thoroughly: you thus learn that the entire lower half of 

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



