For example, an alternator, which is a type of electrical generator used in automobiles, operates by rotating electromagnets on a shaft past coils of stationary wire conductors, which consequently generates large amounts of electrical current in those conductors. Because electromagnetism works two ways, a motor can become a generator, and vice versa. When applied to magnetic storage devices, this two-way operation of electromagnetism makes it possible to record data on a disk and read that data back later. When recording, the head changes electrical impulses to magnetic fields, and when reading, the head changes magnetic fields back into electrical impulses.