Magnetic recording, which was invented by Paulsen a century ago, relies on two basic principles: i Magnets produce strong magnetic fields at the poles. This field can be used for reading information. ii The polarity of the magnets itself can be changed by applying external fields. This provides a possibility of writing information. Therefore, a magnetic recording device would have the following key components: a recording medium to store information, a writer head to produce localized magnetic fields for writing information, and a read sensor to convert the magnetic field from the media to electrical voltage signals reading information. There are many other components to position the head, to interpret the voltages into bits, and so on.