Disk read/write heads are the small parts of a disk drive, that move above the disk platter and transform platter's magnetic field into electrical current (read the disk) or vice versa – transform electrical current into magnetic field (write the disk). The heads have gone through a number of changes over the years.
In a hard drive, the heads 'fly' above the disk surface with clearance of as little as 3 nanometre

Nanometre

A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

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