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A bar magnet is a rectangular object that has a magnetic field. It is usually made of iron or steel, but it can also be made of any ferromagnetic substance or a ferromagnetic composite. This type of magnet is almost always permanent, meaning that it will retain its magnetic field for a significant period of time without the use of a supplied electric current.
Each end of a bar magnet is called a pole — one is north and the other south. When freely suspended, the magnet will align itself so that the end of its northern pole points towards the Earth’s magnetic North Pole. This works in the exact same manner as a compass needle, which itself uses or is a magnet. If the magnet has one end painted red, that end is traditionally the north pole.
Bar magnets are usually made of ferromagnetic materials, which are elements that can naturally have a magnetic field. They include cobalt, iron, and nickel. Some magnets are made of composite materials that combine ferromagnetic materials with other substances such as aluminum, clay, or resin.
Magnets have uses based on their magnetic attraction. This attraction draws other ferromagnetic materials to the magnet, or the magnet towards them. It can be used to pick up small ferromagnetic items such as screws and metal shavings, as a “magnetic stirring rod” on a laboratory hotplate, and to hold papers and other items to the sides of refrigerators, among many other tasks.
Like other magnets, the magnetic field generated by a bar magnet can damage electronic equipment. When placed near a hard drive, computer disk, or even a video cassette, it can cause the data stored by the magnetic particles on the item to be damaged. Magnets placed on the sides of speakers or other sound devices can cause disruptions in the sound or even permanent damage. One placed on the side of a computer can result in the system being deactivated, and may even make it completely inoperable.
Bar magnets have been used for years to demonstrate magnetic fields and magnetic lines of force. One demonstration is to pour metal shavings (sometimes called filings) onto a piece of paper that is suspended above a magnet. Over time, the filings will align themselves into arcs going outward from the magnet’s north and south poles. Another demonstration involves attempting to touch the north poles of two different weak magnets together to show how the poles repel each other. Cutting a bar magnet in half will produce two magnets, each with a north and south pole, though excessive force or heat from cutting can demagnetize the bar instead.