This feature and the choice of disk coating and the pressure loading of the head are such that, at the rotational speed of 360 rev/min, the wear on the recording surface is minimal. This runs in contact with the disk surface during read/write operations and is retracted otherwise. One major simplification in the design of the floppy disk system is the arrangement of the read/write head. The compactness and flexibility of the disk makes it very simple to handle and store, and possible for it to be sent by post. Diskettes are of three standard diameters, approximately 203 mm (8-inch), 133 mm (5 1/2-inch) and 89 mm (3 1/2-inch). As the name implies, the magnetic medium used is a flexible, magnetic oxide-coated diskette, which is contained in a square envelope with apertures for the drive spindle to engage a hole in the centre of the disk and for the read/write head to make contact with the disk. The floppy disk, while having the four elements described above, was conceived as a simple, low-cost device providing a moderate amount of random access back-up storage to microcomputers, word processors and small business and technical minicomputers. Ian Robertson, in Mechanical Engineer's Reference Book (Twelfth Edition), 1994 4.12.6 Floppy disk
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