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. Raise 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 
