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As the title states, what is the unit measure used to detect magnetic indentations in a hard drive?

To clarify, what I mean by indentation is, what is measured to determine if a byte is a 1 or 0?

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This question seems better suited to electronics.stackexchange.com – jwodder May 22 '12 at 20:33
@jwodder, it depends on context. Where, hydroparadise, will you use this word? In a technical document? A novel? – Bobbi Bennett May 22 '12 at 21:29
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@jwodder & OP: Surely, this should have been on a tech site, if a reliable answer is sought. – Kris Jul 21 '12 at 10:43

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Usually magnetic field is measured in gauss. Others might use tesla, which is a unit much bigger than a gauss. 10000 gauss to a tesla, I think. A milligauss would be a good unit to describe a small magnetic field, though I think on a disk it is likely to be much smaller than that.

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However, I am pretty sure what is measured is the change in magnetic field, and the 1 or 0 is determined by when that changes. – Bobbi Bennett May 22 '12 at 19:02
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You're right: it's the [change of] direction of field which is measured, and the field strength is of the order of milligauss. Lower-case for the names of units, incidentally. – Andrew Leach May 22 '12 at 19:34
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Actually, what's being measured are flux reversals -- the change from north pole to south, and vice versa. A flux reversal creates a voltage spike in the read/write head; these spikes, naturally, are measured in volts. (And, strictly speaking, aren't measured directly, but are used to generate a digital value.) – Gnawme May 22 '12 at 21:02
-1 Sounds more speculative/ presumptive than authentic. I understand, thought, since this Q. ought to have been on a tech site rather than on englishSE. – Kris Jul 21 '12 at 10:41
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Magnetic Hysteresis is the measure used to determine electrical changes in the magnetic indentations on a hard drive disk. Though the tesla or gauss are units of measure for magnetism, when taken alone they cannot properly measure nor set (write) the magnetized media on-disk. The dynamics of hysteresis present a mechanism that can provide reliable basis for for measurement.

The magnetic indentations themselves are called magnetic domains.

These follow the same physical concept as magnetic depressions in the atmosphere, or in space

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