Computer hard disk read/write head. Coloured scanning electron micrograph (SEM) of the read and write electromagnetic device (head) of the hard disk of a computer. Computers store data on magnetic media, with the polarity of the magnetism (up or down) corresponding to the one or zero of computing binary code. An electromagnet (grey block, upper left) is magnetised using electricity from the wires trailing at lower left. It is then used to alter the magnetism of the disk, writing data onto it. This device also reads data from the disk. The hard disk (seen in background) rotates. The head moves radially above the disk to access all parts. Magnification: x17 at 6x7cm size.