<hardware, storage> (Or "hard disk drive", "hard drive", "floppy disk drive", "floppy drive") A <a href="">peripherala> device that reads and writes <a href="">hard disksa> or <a href="">floppy disksa>. The drive contains a motor to rotate the disk at a constant rate and one or more read/write heads which are positioned over the desired <a href="">tracka> by a servo mechanism. It also contains the electronics to amplify the signals from the heads to normal digital logic levels and vice versa. In order for a disk drive to start to read or write a given location a read/write head must be positioned radially over the right track and rotationally over the start of the right sector. Radial motion is known as "<a href="">seeka>ing" and it is this which causes most of the intermittent noise heard during disk activity. There is usually one head for each disk surface and all heads move together. The set of locations which are accessible with the heads in a given radial position are known as a "<a href="">cylindera>". The "<a href="">seek timea>" is the time taken to seek to a different cylinder. The disk is constantly rotating (except for some <a href="">floppy diska> drives where the motor is switched off between accesses to reduce wear and power consumption) so positioning the heads over the right sector is simply a matter of waiting until it arrives under the head. With a single set of heads this "<a href="">rotational latencya>" will be on average half a revolution but some big drives have multiple sets of heads spaced at equal angles around the disk. If seeking and rotation are independent, access time is seek time + rotational latency. When accessing multiple tracks sequentially, data is sometimes arranged so that by the time the seek from one track to the next has finished, the disk has rotated just enough to begin accessing the next track. See also <a href="">sector interleavea>. The disks may be <a href="">removable disksa>; floppy disks always are, removable hard disks were common on <a href="">mainframesa> and <a href="">minicomputersa> but less so on <a href="">microcomputersa> until the mid 1990s(?) with products like the <a href="">Zip Drivea>. A <a href="">CD-ROMa> drive is not usually referred to as a disk drive. Two common interfaces for disk drives (and other devices) are <a href="">SCSIa> and <a href="">IDEa>. <a href="">ST-506a> used to be common in microcomputers (in the 1980s?). (1997-04-15)

Disc integration, also known in integral calculus as the disc method, is a method for calculating the volume of a solid of revolution of a solid-state material when integrating along an axis "parallel" to the axis of revolution. This method models the resulting three-dimensional shape as a stack of an infinite number of discs of varying radius and infinitesimal thickness.