Computer hard disk and its read/write head. The hard disk (partially seen circle centred at lower centre) is a magnetic medium that computers store data on. The polarity of the magnetism corresponds to the ones and zeros of binary code. The hard disk rotates and a read/write head (at the end of the metal arm at upper left) moves radially above the disk to access all parts. An electromagnet in this head is used to alter the disk's magnetism, writing data onto it. Electromagnetism is also used to read the data from the disk.