This magnetism will induce a current in the coil. These flashes of current represent the data on the disk. This is read mode. They are incredibly tiny. When the PC is shut down, they are auto parked on a designated area of the disk, so they will not be damaged during transport.
Unchanged polarity is interpreted as a digit zero.
The magnetic disks are typically made of aluminum. There are also experiments with disks made of glass. The disks are covered with an ultra thin magnetic coating. With improved coating technologies, an increasing number of micro magnets can be placed on the disk. Currently, there are more than 2000 tracks per inch disk radius.