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Magnetizing cooling iron. Historical artwork of a blacksmith magnetizing a piece of cooling iron. The smith is hammering the iron, which has been aligned in a north-south direction along the Earth's magnetic field lines ("Septentrio", at upper left, means north, whilst "Auster", at centre right, means south). Other materials, including basalt rock and brick, also become weakly magnetized when cooled in the direction of Earth's magnetic field. Taken from De Magnete by William Gilbert, published in 1600.
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