How data is stored on a disk ?

        You may remember from science projects that one magnet can be used to make another.For example you can make a magnet by taking an iron bar and stroking it in one direction with a magnet.The iron bar becomes a magnet itself,because its iron molecules align themselves in one direction.Thus the iron bar becomes polarized,that is its ends have opposite magnet polarity.You also can create a magnet by using electrical current to polarize a piece of iron.The process results in an electromagnet,you can control the polarity and strength of an electromagnet by changing the direction and strength of the current.
       Magnetic storage devices use a similar principle to store data.Just as a transistor can represent binary data as 'on' or 'off' the orientation magnet field can be used to represent data.A magnet has one important advantage over a transistor.that is,it can represent 'on' and 'off' without a continual source of electricity .
       The surface magnet disks and types are coated with millions of tiny iron particles so that data can be stored on them.Each of these particles can act as a magnetic field when subjected to an electromagnet.The read/write hands of a magnet disk or type drive contain electromagnets that generate magnet fields on the iron on the storage medium as the head passes over the disk or tape.
      To read data from a magnet surface,the process is reversed.The read/write head passes over the disk or tape while no current is flowing through the electromagnet.The head possesses no charge,but the storage medium is covered with magnet field,which represent bits of data.The storage medium charges the magnet in the head,which causes a small current to flow the head in one direction or the other,depending on the field,s polarity The disk or tape drive senses the direction of the flow as the storage medium passes by the head, and the data is sent from the read/write head into memory.