91. Directions. (A) Turn the poker end for end (See [Exp. 47]); repeat the striking, and test again the pole produced at the lower and north end of it.
(B) Now hold the poker horizontally in the east and west line, and pound it.
(C) Test for poles. Has this strengthened or weakened the poker magnet?
92. Discussion. Dipping the poker places it nearly in the same direction as that taken by the earth's lines of force. The magnetic influence of the earth acts to advantage upon the poker, by induction, only when the poker is properly held.
It no doubt occurs to the student that the end of a magnetic needle which points to the north is really opposite in nature to the north magnetic pole of the earth. The N pole of a needle, then, must be in reality a S pole to be attracted by the earth's N pole. It has been agreed, for convenience, to call the N-seeking pole of a magnet its N pole.
93. Natural Magnets. Nearly all pieces of iron become more or less magnetized by the inductive action of the earth's magnetism. Your poker was slightly magnetized at the start, perhaps, from standing in a dipping position.
Induction takes place along lines of force. In northern latitudes the earth's lines of force have a dip to the north.[35] You should now see why the greatest effect was produced upon the poker when it, also, was made to dip.
Parts of machinery, steel frames of bridges and buildings, tools in the shop, and even certain iron ores, become polarized by this inductive action. These might all be called natural magnets. Magnetic iron ore, called lodestone, is referred to, however, when speaking of natural magnets. Lodestone was used thousands of years ago to indicate N and S, and it was discovered, later, that it could impart its power to pieces of steel when the two were rubbed together.
EXPERIMENT 49. To test the effect of twisting a wire held north and south in the earth's magnetic field.