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I have an old CRT connected to laptop as a secondary display. As you know, when CRT turns on it degausses itself; remember that sound when you turn it on, or force degaussing through menu.

CRTs have a copper, or often in the case of cheaper appliances, aluminum, coil wrapped around the front of the display, known as the degaussing coil. Tubes without an internal coil can be degaussed using an external hand held version. Internal degaussing coils in CRTs are generally much weaker than external degaussing coils, since a better degaussing coil takes up more space. A degauss causes a magnetic field inside the tube to oscillate rapidly, with decreasing amplitude.

I searched everywhere but couldn’t find if degaussing has effects on nearby hard drives? Is it dangerous to have CRT and laptop close (about 7-8 inches) ?

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    Related information: What is the tolerable magnetic field (in Tesla) for a hard disk? – John1024 Nov 15 '14 at 18:09
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    Your second sentence is correct, but the enclosure isn't the reason. A Faraday cage doesn't do much about magnetic fields, but rather electric fields and the electric component of EM fields. To shield against magnetic fields requires special, expensive alloys such as "mu-metal". It was used in lab-grade CRT oscilloscopes. It's much too expensive to use in hard drives. And so-called "magnetically shielded" speakers (safe to use next to a color CRT TV) were made not with shielding but by designing the pole pieces to "close" the magnetic field outside of the gap. – Jamie Hanrahan Nov 16 '14 at 20:49
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    10/15 years ago monitors would sit directly on desktop cases (putting the distance at less than 7 inches) completely happily. I'm sure that wouldn't be the case if there were these kinds of issue. – Chris Nov 17 '14 at 16:32
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    If you have am incredibly sensitive lid closing reed switch (which is magentic, not an older physical one) then degaussing the monitor could potentially put your laptop to sleep, or turn off the screen for a second or two! (I know one of my old Compaq laptops used to do this!). It's not dangerous, just a mild inconvenience! – George Nov 18 '14 at 22:10
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    @Chris You did see my answer, didn't you? – a CVn Nov 19 '14 at 9:42
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It takes a tremendous field gradient to flip the magnetic domains on a hard drive. The hard drive can do it because the heads are so close to the surface and the gaps are so small. The magnets inside the drive's spindle motor and arm actuator produce a stronger field than will the external coil... but because of the motor designs, they don't put a high field gradient near the surfaces.

That's theory.

I have a bulk tape eraser left over from my days of owning quarter-inch reel-to-reel audio recorders. It pulls 8.5 amps from 120 VAC, which is far more current than an entire CRT monitor uses, let alone the degaussing coil inside it. (Magnetic field strength is proportional to current.) And besides having a stronger basic field strength, its magnetic field is far more concentrated than that of a degaussing coil (since the latter has no pole pieces).

Some time ago I had a stack of 18 too-small-to-be-useful laptop hard drives (4.3 GB). There being no market any more for such small drives, I decided to try an experiment.

Keep in mind that hard drives include an embedded servo signal (created by so-called "low level formatting" at the factory) which is essential to the drive's operation. If that is weakened too much, not only is data unrecoverable, the drive is toast.

So I tried to get the degausser to affect these hard drives.

It did not, not even a little. Even after thorough attempts at degaussing, holding the eraser's pole pieces against both sides of the drive and using a "wiping" movement, even though the poor drives were vibrating madly from the 60 Hz field... all 18 drives' entire surfaces were still perfectly readable and writeable afterwards. (n.b.: It doesn't take long to run a read/write/read surface scan on 4.3 GB!)

4.3 GB HDs are a much more primitive technology than modern HDs. But more recent hard drives require an even higher field gradient to flip the domains. (That's because the domains are smaller, packed closer together... they would self-erase themselves if it was easy.) If a device that deliberately makes a highly concentrated magnetic field, designed to erase magnetic media, can't affect those old drives, I mightily doubt that a CRT TV or monitor's degaussing coil can affect a modern multi-TB drive at all.

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    I have nuked a hard drive with a pro video degausser of the hand held type, took about 30 seconds. I couldnt be sure that it wiped the platters because when it finnaly failed it seemed to be based on demagnitising the magnets used for the voice coil and motors. It was not "easy" by any means, I had tested it a few times, and put it back in the computer and it was still working, finnaly a full round for the degausser took it out of operation. – Psycogeek Nov 15 '14 at 19:12
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    What's the current rating on that degausser? – Jamie Hanrahan Nov 15 '14 at 19:21
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    @Psycogeek: I would the most probable problem would be with the magnets used for head movement, since those are used to generate a relatively diffuse magnetic field over a rather large area. A drive head, by contrast, is designed to produce an extremely concentrated magnetic field over an extremely tiny area. – supercat Nov 16 '14 at 22:18
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    FYI: Most 'affordable' (+/- 300 dollar) 'magic wands' (hand-held 'de-gausers') require the platters from the disk to be removed and wiped multiple times individually. For this procedure (bare platters) one needs a magnetic field of at least 6000 oersted (or gauss, which is the same for this purpose) (Just an example, no affiliation: datalinksales.com/degaussers/v85.htm). It is a known fact that most AC (alternating magnetic field) tape-head degaussers often can't even wipe a lo-co (300 oersted) magnetic (brown) tape.. let alone a HD (let alone a modern vertical-bit HD) – GitaarLAB Nov 18 '14 at 18:19
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    Storage in an SSD is not magnetic, and there is not enough loop area in the conductors inside it for the external magnetic field to induce much current in them (not that that would be a provably complete method of wiping anyway). So a degausser isn't likely to do much to an SSD. Even if it did it would likely damage only the controller, but the data in the flash RAM itself might be intact! Try it on an old USB key sometime. To erase an SSD, just send it the SECURE ERASE command. (And hope the drive supports it correctly!) kingston.com/us/community/articledetail?ArticleId=10 – Jamie Hanrahan Nov 20 '14 at 8:45
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CRT monitors were commonly placed on top of desktop systems, with the lower parts of the tube just inches away from the hard disk. This has been done for a long time, and was common practice at least from the early 1980s right up through the early 2000s, and quite possibly longer. It became less common as tower PCs as well as TFT monitors became more common. A large reason for such usage was likely the desktop real estate requirements of separating the PC itself and the CRT monitor; it would have doubled the desktop real estate requirements compared to simply placing the monitor on top of the PC since, as also illustrated by the pictures below, the two were often of similar size.

With such desktop setups, the lower portion of the actual cathode ray tube was only a few inches from the storage devices, including the hard disk. I am not aware of this ever causing significant storage problems, and if it did, it certainly would not have been as common a practice as it was.

With this knowledge, we can answer your question

I searched everywhere but couldnt find if degaussing has effects on nearby Hard Drives? Is it dangerous to have CRT and laptop close (about 7-8 inches) ?

with a pretty certain no, this is not dangerous to the magnetic storage media. Maybe if you actually place a hard disk right on top of the monitor and have the monitor go through the degaussing process repeatedly it could potentially be a problem, but I think that would be about what it would take. Even if the distance itself was too close for comfort, the computer case being partly or completely metallic would most likely divert the magnetic field around the hard disk, rather than focus it. Even in cases of computer cases being made out of plastic (the Apple II case was plastic, but I'm not sure about the floppy disk drives), the hard disk itself is encased in metal and ultimately grounded, providing a return path for a potential induced current or voltage (within reason) and in effect forming a Faraday cage.

Below are some photos showing common such setups, in order of equipment design year. While several of these show floppy-disk-based systems, even the original IBM 5150 could have a hard disk retrofitted (in which case the hard disk replaced one of the two floppy drives, plus you needed a bigger power supply and a lot of money you didn't know what to do with), and you'd be hard pressed to run Windows 98 without a hard disk installed. These are just for illustration; there were many systems very similarly laid out physically. Also note the bottom photo; similar setups with magnetic storage media were not limited to computers!


Apple II with two FDDs and monitor
Apple II computer with CRT monitor sitting atop two floppy disk drives. Photo by Rama, CC-BY-SA-2.0. Equipment design circa 1977. Image source


IBM 5150 PC
IBM original 5150 PC. Photo by the German Bundesarkiv, photo accession number B 145 Bild-F077948-0006, CC-BY-SA. Equipment design circa 1981, photo 1988. Image source


IBM PS/2 model 25 with integrated CRT monitor
IBM PS/2 model 25 PC with integrated CRT monitor. Public domain photo. Equipment design circa 1987. Image source


Commodore Amiga 500 with CRT monitor and external floppy disk drive
Commodore Amiga 500 computer with CRT monitor, with internal floppy disk drive to the right on the computer itself (beneath the grille) and external floppy disk drive to the left of the monitor. The A500 was also commonly used with regular TVs as displays. Photo by Bill Bertram, CC-BY-2.5. Equipment design circa 1987, photo 2006. Image source


IBM Personal Computer 300PL, desktop with separate CRT monitor
IBM Personal Computer 300PL, desktop system with separate CRT monitor. CC-0 photo. Equipment circa 1998. Image source


Sharp TV/VHS combination unit
Sharp TV/VHS combination unit: a CRT TV and VHS player combined into a single device. Also notice the speaker grille immediately adjacent to the VHS casette slot. Photo by Bryan Derksen, CC-BY-SA, circa 2005. Image source

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    Nice history lesson. +1 just for making me smile ;-) edit the bottom pic has a couple of permanent magnets right next to the main puter too. – Tetsujin Nov 15 '14 at 18:06
  • Oh .. last time I saw display-on-top was 8 years ago at school. Thanks for the answer ! I ll vote up when I get 15 rep – Nursultan Talapbekov Nov 15 '14 at 18:11
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    @NursultanTalapbekov That's what happens when you make the Network Hot Questions list. – a CVn Nov 15 '14 at 22:05
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    I saved a copy of those old computer photos for nostalgia purposes. ☺ Ah, good times. – Synetech Nov 18 '14 at 22:29
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    The Apple II floppy drives were encased in metal on the sides, top, and bottom. I'm not sure about the front and back - that might have just been heavy plastic, I don't remember. Ahh, that pic takes me back! – Doin Nov 23 '14 at 13:01
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If CRT degaussing was a true risk to magnetic media—potentially exposing a system to data loss—then the original Macintosh would be a data loss disaster waiting to happen:

enter image description here

Here is a pic of a clear cased Macintosh SE where you can see where the drives—including floppy disks—are located in relation to the CRT:

enter image description here

Ditto with the Lisa:

enter image description here

As well as the Power Macintosh 5200:

enter image description here

And let’s not forget the original G3 iMac:

enter image description here

And here is a Macintosh Color Classic:

enter image description here

And here is an inside shot of that same Macintosh Color Classic—taken from this site—showing the hard drive sitting a few inches directly below the color CRT:

enter image description here

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    Not to nitpick — but degaussing coils were typically only used in color monitors, where aligning the electron beams with the color mask was critical for correct color reproduction. They weren't generally used with monochrome monitors (like the Macintosh Lisa and SE), as fine alignment wasn't as important. – duskwuff Nov 15 '14 at 19:02
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    @duskwuff Not a problem. Inspired me to find a pic of a Macintosh Color Classic that includes an inside shot showing the hard drive inches away from the color CRT. – JakeGould Nov 15 '14 at 19:11
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    and you can see the degaus coil right there – ratchet freak Nov 17 '14 at 8:53
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Magnetic platters need far larger field strengths than an external degaussing coil produces. As a result, hard drives that are switched off are not likely to be affected from degaussing.

Operating hard drives are an entirely different piece of cake since the writing (and reading) heads pick up and focus external fields. As a result, you can damage operating hard disks. While the signal frequency of a degausser (except when switching on/off) is far too low to interact with the typical read signals, it's at least conceivable that its almost-DC part can drive the first read electronic stage into saturation, making it unable to process the real signal. That effect, however, would be temporary. The potential change on the platter is more of an issue.

Another answer stated that the casing of a hard disk drive acts as a Faraday cage: that's quite irrelevant since a Faraday cage shields electric fields but we are talking about magnetic fields here (to keep out the 50Hz magnetic field of a degaussing coil, the size of an electric cage would have to be several kilometers). Effective shielding from magnetic fields would instead require a cage of magnetically conductive material (like transformer iron plates) directing magnetic fields around the drive. I don't think that hard disk drives have significant magnetic shielding.

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With a strong enough degausser placed close enough to a hard drive, you can destroy the drive, but even with a very well aimed and strong one you have to be right on top of it. The coil on the CRT is wide, somewhat directed to the purpose it has, and on many computer monitors that whole area is shielded so not much gets out in any specific direction.

You could check the sheilding (if it even exists), know that the size of the CRT tube would require a relative strength of the magnetic field. I think in most situations even sitting on top of the case of most CRTs (for years) you couldn't get any real damage to occur, you’re 6” away should be sufficient for any situation.

If you are worried, the degauss cycle only occurs at turn-on anyway, as you probably already know. I do not think it occurs when coming out of monitor standby on most units.

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This is one of those old legends that never dies. If we travel far back into the mists of technological time (say, mid 1960s) we used tape. And we had these tape erasing machines that also did a good job straightening out the early colour TVs (I'm embellishing a lot here). Recording media back then was a LOT more sensitive to magnetic disturbance so it likely could be blanked by a hand-held degausser.

As you increase the storage density you need to DEcrease the strength of the magnetic material, otherwise it will promptly erase it's neighbors. The 1Tb drive under my desk won't be bothered by anything less than a research-grade MRI machine.

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The metal case around a hard drive is effectively one big metal Faraday cage. An alternating Magnetic field couldn't effectively penetrate it whether it's diamagnetic, paramagnetic or a ferromagnetic metal due to the induction of eddy currents in the metal itself absorbing the external magnetic field.

The bigger the AC current the warmer the case gets. Or induction furnaces wouldn't work and we'd be currently communicating via paper or stone tablets because we couldn't purify semiconductors using fractional crystallisation.

As far as the magnets within a drive are concerned. Their magnetic field is actually a closed loop where the field lines are pretty weak near the platters. The mu-metal backing on them takes care of that.

  • Sorry but "Faraday cages" don't shield much against magnetic fields (that is a common misconception).. electromagnetic fields (i.e. RF) or electric fields, yes. But to shield against magnetic fields requires alloys with much higher than usual permeability. "mu-metal" is a common choice. High-end oscilloscopes used to use mu-metal to shield the CRT. It was not used in TV sets or any but the most expensive computer CRTs because it was far too expensive. The castings in hard drives are aluminum, which has very low permeability, so magnetic fields can penetrate it easily. – Jamie Hanrahan Mar 19 '15 at 5:07

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