A magnet is a material or object that produces a magnetic field . A low-tech means to detect a magnetic field is to scatter iron filings and observe their pattern, as in the accompanying figure. A "hard" or "permanent" magnet is one that stays magnetized, such as a magnet used to hold notes on a refrigerator door. Permanent magnets occur naturally in some rocks , particularly lodestone , but are now more commonly manufactured. A "soft" or "impermanent" magnet is one that loses its memory of previous magnetizations. "Soft"
magnetic materials are often used in electromagnets to enhance (often hundreds or thousands of times) the magnetic field of a wire that carries an electric current and is wrapped around the magnet; the field of the "soft" magnet increases with the current.
Two measures of a material's magnetic properties are its magnetic moment and its magnetization. A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted ( paramagnetic ), or repelled ( diamagnetic ). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors , which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.
本文转自：China Industry News
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