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Magnetism is intimately linked with electricity, in that a magnetic field is established whenever electric charges are in motion, as in the flow of electrons in a wire, or the movement of electrons around an atomic nucleus. In atoms, this invisible field consists of closed loops called lines of force that surround and run through the atom. Magnetic regions where lines of force come together densely are called north and south poles. In substances in which the magnetic fields of each atom are aligned, the magnetic field causes the entire substance to act like single magnet—with north and south poles and a surrounding magnetic field. Permanent magnets are made of substances that retain this alignment. If a magnet is cut in two, each piece becomes a separate magnet with two poles. A coil of wire wrapped around an iron core can be made magnetic by running electric current through it; the looping electrons then create a magnetic field in just the same way as the spinning electrons in individual atoms. As long as current flows, the coil remains magnetized. Such magnets, called electromagnets, are used in many devices such as doorbells and switches. The connection between electric and magnetic fields is not one of cause and effect, however. Einstein showed that both the magnetic and electric fields are part of a single electromagnetic field, described by a single mathematical object called a tensor. Observers in different reference frames will not observe the same separate values for electric and magnetic fields, but will observe identical electromagnetic tensors. Whether or not magnetic monopoles (elementary particles carrying an isolated north or south magnetic “charge,” analogous to positive or negative electric charge) actually exist remains unknown; though they are predicted by some theories, none have been detected.