WTF is Degaussing?

The modern office has become a sea of LCD monitors. It’s hard to believe that only a few years ago we were sitting behind Cathode Ray Tubes (CRTs). People have already forgotten the heat, the dust, and the lovely high frequency squeal from their flyback transformers.

Image by Søren Peo Pedersen via wikipedia
Image by Søren Peo Pedersen via wikipedia

There was one feature of those old monitors which seems to be poorly understood. The lowly degauss button. On some monitors it was a physical button. On others, it was a magnet icon on the On Screen Display (OSD). Pressing it rewarded the user with around 5 seconds of a wavy display accompanied by a loud hum.

But what exactly did this button do? It seems that many never knew the purpose of that silly little button, beyond the light-and-sound show. The truth is that degaussing is rather important. Not only to CRTs, but in many other electronic and industrial applications.

 

Of Shadow Masks and Aperture Grilles

Close up of a shadow mask by Rauenstein via Wikipedia
Close up of a shadow mask by Rauenstein via Wikipedia

A CRT has quite a few components. There are three electron guns as well as steering and convergence coils at the rear (yoke) of the tube. The front of the tube has a phosphor-coated glass plate which forms the screen. Just behind that glass is a metal grid called the shadow mask. If you had enough money for a Sony screen, the shadow mask was replaced by the famous Trinitron aperture grille, a fine mesh of wires which performed a similar function. The shadow mask or aperture grille’s  job is to ensure that the right beams of electrons hit the red, green, or blue phosphor coatings on the front of the screen.

This all required a very precise alignment. Any stray magnetic fields imprinted on the mask would cause the electron beams to bend as they flew through the tube. Too strong a magnetic field, and your TV or monitor would start showing rainbows like something out of a 1960’s acid trip movie. Even the Earth’s own magnetic field could become imprinted on the shadow mask. Simply turning a TV from North to East could cause problems. The official term for it was “Color Purity”.

magnet-trickThese issues were well known from the early days of color TV sets. To combat this, manufacturers added a degaussing coil to their sets. A coil of wire wrapped around the front of the tube, just behind the bezel of the set. When the set was powered on, the coil would be fed with mains voltage. This is the well-known ‘fwoomp and buzz’ those old TV sets and monitors would make when you first turned them on. The 50 Hz or 60 Hz AC would create a strong moving magnetic field. This field would effectively erase the imprinted magnetic fields on the shadow mask or aperture grille.

Running high current through the thin degaussing coil would quickly lead to a fire. Sets avoided this by using a Positive Temperature Coefficient (PTC) thermistor in-line with the coil. The current itself (or a small heating coil) would heat up the PTC, causing resistance to increase, and current through the coil to drop. After about 5 seconds, the coil was completely shut down, and the screen was (hopefully) degaussed.

As time went on monitors became embedded systems. The PTC devices were replaced by transistors controlled by the monitor’s main microcontroller. Monitor manufacturers knew that their sets were higher resolution than the average TV set, and thus even more sensitive to magnetic fields. Users are also more likely to move a monitor while using it. This lead the manufacturers to add a degauss button to the front of their sets. A push of the button would energize the coil for a few seconds under software control. Some monitors would also limit the number of times a user could push the button, ensuring the coil didn’t get too hot.

Holding a magnet near the front of a black and white (or a monochrome ‘green screen’) CRT created visible distortion, but no lasting damage. Mid-century hackers who tried the same trick with their first color TV quickly learned that the rainbow effect stayed long after the magnet was moved away. In extreme cases like these, the internal degaussing coil wouldn’t be strong enough to clear the shadow mask.

Commercial degaussing coil
Commercial degaussing coil

When all else failed, a handheld degaussing coil or wand could be used. Literally waving the magic wand in front of the screen would usually clear things up. It was of course possible to permanently damage the shadow mask. Back in 2007, I was working for a radar company which had been slow to switch to LCD monitors. Being a radar shop, we had a few strong magnetron magnets lying around. One of these magnets was passed around among the engineers. Leaving the magnet under your monitor overnight would guarantee rainbows in the morning, and a shiny new LCD within a few days.

1024px-RMS_Queen_Mary_20Jun1945_NewYork
Queen Mary, showing her degaussing coil

CRTs aren’t the only devices which use degaussing coils. The term was originally coined in 1945 by Charles F. Goodeve of the Royal Canadian Naval Volunteer Reserve (RCNVR). German mines were capable of detecting the magnetic fields in a naval ship’s steel hull. Coils were used to mask this field. The Queen Mary is one of the more famous ships fitted with a degaussing coil to avoid the deadly mines.

Even mechanical wristwatches can benefit from a bit of degaussing. A watch which has been magnetized will typically run fast. Typically this is due to the steel balance spring becoming a weak magnet. The coils of the spring stick together as the balance wheel winds and unwinds each second. A degaussing coil (or in this case, more properly a demagnetizer) can quickly eliminate the problem.

A story on degaussing wouldn’t be complete without mentioning magnetic media. Handheld or tabletop degaussing coils can be used to bulk erase floppy disks, magnetic tape, even hard disks. One has to wonder if the degaussing coils in monitors were responsible for floppy disks becoming corrupted back in the old days.

So there you have it. The magic degaussing button demystified!

97 thoughts on “WTF is Degaussing?

  1. I wonder about erasing a floppy with a degaussing coil or a magnet near by. The field strength in the gap of a write head is tremendous. Can you really corrupt a disk by accident that easily?

    1. The early floppys (8 and 5.25 inch) could easily be damaged by external fields. The later, high density ones (mostly 3.5 inch), required a stronger field and were more resistant (but could still be damaged)

    2. It’s not that it would specifically wipe it clean in one shot, but just make the difference between a magnetic 1 and 0 less defined. Same thing with VHS tapes… they will self demagnetize themselves over time.

      1. A lot of people stores their VHS tapes on top of, or right next to their TV. Every time they tuned their TV on, the degausser took a little bit out of their tapes.

    3. I worked on a system where the prototypes were built in a standard PC housing, and we used the space for the 5-1/4 floppy & hard drives [under the 3-1/2 floppy drive] for a bulk 24V supply with a 10,000 uF filter cap.
      If there was a 3-1/2 floppy in the drive, the magnetic field from the transformer surge charging the cap would corrupt the data on the floppy. The fix was ridiculously simple – a piece of air conditioning duct work ‘tin’ was wrapped around the bottom and sides of the floppy drive to act as a magnetic shield.

        1. This…
          Back in the 30’s depression real Sn disposable beer cans were great for the beer and again for making stuff like soldered strip antennas stapled or nailed to barn beams and feedlines when Cu wire or even C/Fe barbed wire was too valuable.
          Now Sn(tin) is pretty hard to find as ‘junk’.
          I was never clear if they were a Sn galvanized Fe cans or all Sn.

      1. I never had corrupted disks but found a read problem with 25 prototypes that were all failing. When I went to look at the testing setup they were all lined up on a shelf next to each other. The video circuit and flyback from one was two sheets of plastic away from the floppy drive in the other. Too much electrical noise for the floppy read circuits even in the thin sheet metal of the drive housing.

        The test and solution was pretty easy. I pulled a couple of them about 6 inches apart and the tests started running clean.

        I had floppies from the days of 8″ to DSDD diskettes and laid them all over. On top of monitors or power supplies and don’t recall any problems with corruption. I had trouble with Zip disks. Maybe that is why they came and went so quickly. Anybody need a SCSI Zip drive?

    4. My father has a story of the early days of computers in the (Canadian) government. They had a user that continually had data go corrupt on their floppy disks. They would go up and check things out, find nothing wrong other then a bad disk and clean desk. Repeated calls for bad disks were starting to cause a fuss(as she was reasonably important). One day my father happened to be going past her desk returning from other business and decided to drop by to see if things were working.

      He discovered that she was a polite person who made sure that everything was ready for IT by clearing her work area and getting all the involved media laid out and ready at hand. When it was an unscheduled visit things were different, including the floppy disks being held to the (very sturdy) steel desktop PC cases using fridge magnets.

      A quick note to her about the effect of magnets on disks and the problem went away. This was before most people could have been expected to hear about this, so it was not PEBCAK.

      Lesson: Sometimes you have to sneak up on your users when they least expect you.

        1. Yes. Yes it is. Doesn’t mean nobody actually did that ever, but it does mean 99.99% of people who “have this funny story from a while back” never actually had it happen to them.

    5. I seem to recall users keeping floppies in easy reach right under their monitor, either on top of their desktop machines or on the table for tower systems.
      I wouldn’t be surprised if that contributed to corruption. Especially for systems that were powered down often, to save electricity.

      Does anyone remember if CRTs degaussed when coming back out of standby?

      1. Some did– usually ‘newer’ ones that had the degaussing function under firmware/menu control (I had a big Nanao 21″ that would do that). On older monitors the degaussing circuit was typically a positive temperature coefficient thermistor setup that would fire once when the monitor was turned on and then would not trip again unless the monitor was turned off and the thermistor was allowed to cool down.

    6. I have destroyed my friend’s IT homework once. By fooling around with a floppy disk and a magnetic latch of my calculator lid. So yes, it can be done. I really thought the magnet was too weak to do anything with that disc.

  2. Back in the early ’90s, I had an office at my University, and there were around 16 or so separate computer monitors attached to various systems in there. One of them was a 17″ Sony Trinitron – a giant beast of a monitor that was almost twice as deep as it was wide. When I hit the degauss button on it, it would make _all_ of the screens in the room wobble, even six feet away.

    1. This reminds me of one of the lab-device-attached computers when I was in grad school. Directly on the other side of the adjacent wall was an elevator shaft. Every time the elevator went by, the CRT went wonky.

    2. I had a 21″ sony with 5 BNC’s.. and it would also do that to monitors at least 3 cubes away. It was glorious.

      Only reason I had it was the IT guy was too stupid to figure out how to hook it to his computer…”It has to be crap it doesnt have a VGA connector.”

      1. I received two of those beasts (21″ trinitron) from a friend who couldn’t figure out how to hook them up for the same reason. Thus began my addiction to multi-monitor… (and the degauss was, of course, epic)

      2. I had two 21″ HP-branded Sony Trinitron workstation monitors (And an HP 3000 UNIX workstation) that I picked up from a school district auction for I think $1 each. Sold one of the monitors for $250 on eBay (This was the late 90s) and used the $ to buy a Datsun pickup I used for dumpster diving :-P

      3. I could have gotten a gloriously huge (19″ or 21″, can’t recall) SGI/Sun monitor many years ago for all of $20, but it used the 13W3 connector (sync on green, I think) and at that point the internet wasn’t nearly as deep in terms of locating the proper video cable/adapter hardware (to get it to 5 BNCs and then to HD15), so I passed on it. Looking back, I still regret it a little.

        A few weeks later I found a more traditional 17″ monitor at the community recycling center that just needed the CRT yoke reseated on the pins… easy fix. Lasted about 8 months, then started to die, but the mfg warranty was still good(!) so I got a refurb replacement for the cost of shipping the old monitor back to them. Ended up giving that one away years later as it would not die (the degausser on that one had the most satisfying thump of any degaussing screen I ever used!). Good memories there.

        1. I had a dozen of these beautiful Sun/Sony monitors that I bought for $100 total. They had the 13W3 cables on them. I spent a day cutting off the 13W3 connectors on the end of the cable, and soldering on HD15 VGA connectors, including a capacitor in the connector shell to combine the horizontal and vertical VGA sync signals to feed the composite H/V wire bundled in the cable. Sold the monitors for $150 each after conversion. Still used one for myself until 6 months ago. Awesome CRT monitors….

  3. My parents had a TV that used a bit different technique. On power on TV charged big electrolytic capacitor at about 320V DC. On power off it was connected to degaussing coil creating a classic LC circuit. Sadly I can’t recall the make and model of TV…

      1. Can recommend the book: “Magnets: The education of a Physicist” by Francis Bitter from 1959, has a chapter about magnetic mines, degaussing ships and other magnetics during WW2. Also contains quite a lot about magnets and development.

    1. The degaussing coil was several turns of heavy conductor wire that was draped around the perimeter of the ship at/around the water line, and had a high current AC voltage passed thru it.

  4. I still have my good old 19″, 1600×1200 CRT monitor (guess it’s pretty huge for a CRT?), currently working as the second screen to my iMac, because why not? It still works fine! And as I read this article I pressed the degauss button, so satisfying.

  5. Those were the days. I was sitting in front of a iiyama Vision Master Pro 510 22 inch CRT. The beast grilled me with all the x-ray it had and was warming up the building so much that half Siberia started melting. In good days it had a fair image display, on bad days I hated it. Weight a ton, cost some huge bribe to get it and was overall a kwels-kid-in-the-block-toy.

  6. Had an office above a plasma physics experiment a few years back. Took me a few weeks to figure out why my monitor image suddenly jumped a few times a day every few days.

  7. My nearly beat me around the age of 6 or so when he found out I had taken a magnet and found that the TV set did weird things when I placed the magnet against the screen…. Thankfully a few on-off cycles later and the distortions cleared — Before he gave me my very own on-off cycle!!!

  8. nothing like degaussing your leatherman then passing through metal detectors at various “secure” places and watching the consternation when they realize you are still wearing it.

        1. When you said this I thought of a bolt that is used to hold down a torroidal transformer because it looks like stainless steel but is a completely non-ferric alloy.

          On the other hand, stainless steel *is* ferric so it should be able to be magnetized??

          1. I know many things, but I don’t know why high-alloy stainless is largely non-magnetic. Maybe nickel and chrome reverses the polarity? Cobalt again is used in very strong magnets.

          2. It has to do with the phases present in the steel. Usually stainless steel is austenitic which is paramagnetic. Some types of stainless steel can be martensitic which is ferromagnetic. Of course there are also other types…

          3. For this application I know only ordinary steel screws (galvanized). it does not matter for the transformer, if it is ferromagnetic. There is nearly no stray field in the center of the toroid. But you must never build a short circuit winding with this fastening bolt.

  9. I always remember the slight BZZzzt of the TV and later colour monitor made when you turned it on. When we got our new computer in the late 90s it came with a huge ~20?” CRT – When I first plugged it in, the degausser in it was so horribly loud that I ran away thinking the thing was blowing up!

  10. A little know trick was that you can use an electric drill to degauss CRTs in the event that you don’t have a degaussing ring handy. Simply hold the body of the drill sideways against the Monitor / TV and pull the trigger for a few seconds and slowly pull it away from the monitor. That trick even worked on monitors that were so far gone that a regular degausser wouldn’t save them.

      1. My fathed used to fix TVs too. He has one degauss coil made by himself, around 30cm of diameter. Well, it worked very well… I used to turn it on and he started to make circles in front of TV, getting bigger and far. After that, I turned it off.
        I guess commercial coils are smaller.

        Sorry, i wrongly reported your post.

    1. My wife found a similar hack after my oldest found out how cool it was to put a magnet on the TV. Her trick was to put a magnet in the chuck and get it spinning from a foot away from the screen, move it in and “wipe” it across the screen then pull it away. Worked like a charm.

        1. But not this one.

          Apparently that is a real magnet for teaching purposes! Drag this through the ocean during WW2 and you could have cleared a path I suppose. Then again, I’m betting they did!
          This was a great article; I didn’t know about degaussing ships, that was news to me!

  11. When I was a small child back in the late 60s my parents purchased our first color TV. It was a huge console Zenith, “the quality goes in before the name goes on”. We had the TV for about a week. My dad wanted to show me some magic and held a magnet up to the TV for a second. The colors look really cool to a 5 year old. The next day when dad came home and turned on the TV to watch the evening news. To his surprise the screen was just a distorted rainbow of awesome color. My dad knew exactly what had happened and exactly who did it. I did not get punished. I guess that was my first hacking fail.

  12. You don’t have to wonder if the degaussing coils corrupted floppies. I did the experiments.

    5-1/4 inch floppies take a weaker magnetic field to affect them. Same with Dual Density (DD) 3.5 inch floppies. Merely leaving them on top of a monitor was enough to damage them. It might take a few on-off cycle for the 3.5 inch to show damaged data.

    HD 3.5 inch floppies got their higher density partly by using tougher-to-magnetize/demagnetize ferrite. I had a small hand-held degaussing coil like the one in the article that would not cause obvious damaged, although I’d not count on that saving them. The same coil could completely erase DD 3.5 inch and any 5-1/4 inch floppies.

  13. aperture grille! Thank you for identifying that. I have a buddy who makes art with slumped glass. He was experimenting with the thick front glass from CRT monitors. He ‘popped’ an old Sony TV tube and found a cool metal frame with a row of thin tight wires stretched across it. He threw it on the scrap metal pile and it made an awesome sound so he brought it to me. I added a piezo pickup and amped it. Wow what an ungodly scifi racket it made!

      1. Unfortunately, I no longer have it. The humidity caused a patch of rust on one end of the wires. It added a bit of buzz, so I left it alone. A few days later some of the ‘wires’ had snapped where the rust was. again, they rattled against the intact wires and made a cool sound so I did nothing. A few days after that I went out to the garage and the whole thing had snapped- all the wires were broken. I imagine that as the wires snapped it increased tension on the remaining wires causing them to eventually snap, increasing the tension even more, rinse and repeat. My only regret is that I didn’t get to hear it when it finally cut loose. It must have been glorious!
        Maybe I’ll find another Trinitron and set aperture grille up outside with a webcam- kind of like a noisy version of that pitch drop experiment that’s been running forever.

  14. I did software for one of the very first 19 inch colour monitors sold for Macs (the unix kernal code) … one day the screen just went haywire, I looked up, a very long truck was driving used BART rails down the street

  15. Hmmm some history form an old fart!

    Degaussing wasn’t a “thing” for black and white TV’s but it was used in electronics earlier than color TV.

    We had tape players very early on. These were the reel to reel tape players that were long before cassette tapes existed.

    Magnetic tape (including the later cassette tape) would eventually start to magnetize the tape head and this resulted in loss of sound quality so the tape heads we “demagnetized”.

    There are tools for demagnetizing tape heads and there is also a degaussing wand to demagnetize TV (CRT) screens.

    So about now you are probable asking – what is the difference between degaussing and demagnetizing? or are they the same thing?

    Yes and no. degaussing is to remove magnetism – not necessarily to nil. Demagnetizing is to remove all magnetism.

    The degaussing coils of a CRT are able to correct for more minor amounts of magnetism. They tend to average out magnetism but not remove magnetic bias completely. It’s kind of like an oily hand print on a window, if you smear the oil over the total surface it looks clean but there is no less oil in total.

    On the other hand a degaussing wand will remove all magnet bias – ie demagnetize.

    For the internal coils – the applied voltage reduces as the PTC heats up.

    By contrast here is how to use a wand. Stand 2m away and press the on button. Walk over to the set and wave it around the edge of the screen in a circular manner then continue this pattern as you *slowly* move back to 2m away where you can release the power button.

    A hand wand has much more power. This means that it can drive the tube into magnetic saturation. As you move away the effective magnet force reduces as an inverse square relationship to the distance away. So by driving the tube into saturation and then decreasing the alternating magnetic force until it has no effect, will leave the tube at the zero bias (demagnetized) state.

    Alternate theory:-

    Degaussing became a thing in the 1960’s. It’s rumored that colored iron filings were used with experimental magnetic fields to create many of the psychedelic patterns that we associate with this era.

    Around the same time an experimental perception enhancing drug named lysergic acid (LSD) became commonly used in the perception that it was intelligence enhancing.

    It made it’s way into pop culture with songs like “Lucy in the sky with diamonds” being a code for LSD.

    Unfortunately this drug was not intelligence enhancing and many who know those who experimented with this in the 60’s can tell they the experimenters did not return in a meaningful way. They are physically here but somehow they are still mentally lost in the 60’s.

    This presented a huge challenge for society as the mental health costs alone would be insurmountable.

    In the 70’s it was discovered that these people lost in the 60’s could function normally if they were occasionally exposed to images from their past drug experiences and hence the government mandated that all CRT’s have a psychedelic mode and these people were then often seen fleeting from one monitor to the next during work.

    Now that time has passed, all these people have been promoted out of harms way and retired so we didn’t need this same psychedelic mode on LCD.

  16. On magnets VS computers. Circa 1998 I was working in a computer shop. One morning I come in and the boss says there’s a PC in the back for me to fix. The customer called him up the night before and he went there after hours to meet the owner.

    I went into the back and upon seeing the computer I busted out laughing. The entire motherboard side of the case was covered in magnetic business cards. I peeled them all of then hooked it up, all fine, not a problem.

    Turned out that every other time the owner had taken it to a shop to figure out what was wrong, he’d remove all the cards, then put them back on when he took it back home. Just happened he was in a bit of a hurry that day and didn’t have time to take them off.

    I told him to put the cards on his refrigerator. Normally the steel case would shield against such small magnetic fields, but when you cover the whole thing, it makes the side of the case like a large magnet, with a field able to reach the short distance to the motherboard and mess things up a little.

    No charge for the ‘fix’.

  17. “Running high current through the thin degaussing coil would quickly lead to a fire. Sets avoided this by using a Positive Temperature Coefficient (PTC) thermistor in-line with the coil. The current itself (or a small heating coil) would heat up the PTC, causing resistance to increase, and current through the coil to drop. After about 5 seconds, the coil was completely shut down, and the screen was (hopefully) degaussed.”

    This is only partly true. The process of degaussing needs an alternating magnetic field, but the amplitude needs to decrease over time. If you would just have an alternating magnetic field and then switch it off, it would not work.

    Please update these facts in your article.

    https://en.wikipedia.org/wiki/Degaussing

  18. Great article/comments.
    Tangentially-related question, can hard disks that’ve been degaussed/demagnetized be reused/reformatted at all? They usually have a servo-track on one platter, right? So, as I understand, once that’s been destroyed there’s no way for the read/write heads to be positioned on the tracks, because there are no more “tracks”…?

      1. Was afraid of that… ‘Spose it’s *plausible* some drives could have a mode to be put in for writing that track, but even *if* such thing existed, would probably require intimate knowledge of each model that is usually a trade-secret. And, realistically, now that you mention it, writing such a servo-pattern would probably require finer control than most hard-disks have available (especially when lacking a servo-pattern to begin-with!).
        Screw the bulk-erasers, companies needing to destroy data should just invest in TNT, it’d certainly be more enjoyable… Maybe there should be a company that does it, “Bring us your data and watch it get destroyed!” Could make for an interesting team-building/field-trip day for employees. ;)

        1. I would think a company like that, would make big bucks.
          How would you like your sensitive customer data erased? Please check one or more boxes.
          *Fire (Thermite)
          *One large hole (50 cal. Through platter)
          *EXPLOSION (TNT of course)
          *Boring government approved way (Many little hole drilled and degaussing)
          *Crushed (HDD placed in mini-crusher. Is turned into a tiny cube)
          They could send you video of them destroying it as proof. And the cube could be part of an art gallery.

          1. Sounds great! The cube idea’s kinda cool. Call it “data compression” :)
            I also thought about maybe unscrewing the cover and inserting a bunch of fireworks… Colorful! Drilled-holes would probably be a faster insertion-method.

          2. Hahaha! Data compression is right! The error correction would be terrible though. Lol
            I should’ve added a microwave oven option as well. Or good ol’ sparky. Put that hard drive on top of a Tesla Coil until it’s cooked.

  19. My mum used to bring a 3.5″ floppy home from the office every day with her backups on and she used to complain that every time she made a new backup, she’d need to reformat the disk. I figured out that the magnetic clasp on her purse was wiping it every day she bought it home.

  20. There’s a lot of mysteries surrounding the Sony Trinitron aperature grill…and that faint horizontal wire about a third of the way up the screen.

    Anyway, when I spotted a bored software person who’d extracteded the long, and powerful, rod magnet from a toner cartridge and was waving it in front of his $3,000 Sony 19-inch monitor, I knew nothing good could come of this. “Look at me, I’m Mr. Wizard!” he squealed with joy.

    Anyway, as if often the case, the software guy wandered into the hardware guy’s cubicle, with a sick look on his face, wondering if I could “fix” things. Specifically, did I have a degausssing coil.

    Anyway, the degauassing coil improved things…somewhat. Apparently, he’d either permanently magnetized the grill, or quite possibly physically distorted the damn thing, because try as we might, the colors were never the same.

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