Magnets on the work bench
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From: Dirk Bruere at NeoPax on 11 Feb 2010 23:07 life imitates life wrote: > On Thu, 11 Feb 2010 16:36:15 +0000, Dirk Bruere at NeoPax > <dirk.bruere(a)gmail.com> wrote: > >> Tim Williams wrote: >>> "Chris" <christofire(a)talktalk.net> wrote in message >>> news:9f794283-f986-4e41-93af-5600bd2dbe34(a)z26g2000yqm.googlegroups.com... >>>> Those are all about 1 tesla according to their table at >>>> http://www.supermagnete.nl/eng/data_table.php, which seems typical for >>>> Neodymium (-Iron-Boron) magnets, and those are 'the strongest magnets >>>> in the world' according to that web site! >>>> >>>> Tesla is the unit of flux density. The number around 40 is an 'energy >>>> product' apparently: the square of B, in Tesla, divided by mu0 for >>>> air, although the unit MegaGaussOersted must involve a strange >>>> conversion. >>> The type 50M shows a particularly high remenance of 1.40-1.46T, from a >>> magnetization of 860-995kA/m (which means, for a 1 cm thick magnet in a >>> steel fixture, you need to pulse about 10kAt around it -- 100A * 100 turns >>> let's say!). >>> >>> Assuming the B-H curve is exactly that square, the "energy product" is: >>> 1.4527M T*A/m = Wb*A/m^3 = V*s*A/m^3 = J/m^3. >>> >>> (Webers are volt-seconds, the amount of flux applied. Volts get into it >>> because integrating EMF over time gives flux, hence, V*s. Flux actually has >>> nothing to do with amps, which sounds funny for magnetism which is all about >>> amps, but it's like how current and voltage are dependent on resistance, >>> there's a connection, it's just not a direct connection.) >>> >>>> I wonder if it's significant that I ordered an even number?! Perhaps >>>> if you order three, they send you a fourth one free of charge! >>> But then you'd have to magnetize it yourself! <rimshot> >>> >>> Tim >>> >> I've known people stick floppy discs to filing cabinets with magnets. >> No harm done apparently > > > I have passed magnets over and near floppies, no problem. I would NOT > place a standing field next to one at that proximity, and leave it there, > however. That is just asking for errors. Could even re-orient some of > the hard sector flags. A big magnet washes over the whole disk, and > everything pretty much snaps back when the field is removed. A small > magnet, holding it against a steel medium behind it bothers me just > thinking about it. :-) I think I threw out the last of my floppies a couple of years back, inc the 5"1/4 ones. Temp stuff on flash and more permanent on CD/DVD -- Dirk http://www.transcendence.me.uk/ - Transcendence UK http://www.theconsensus.org/ - A UK political party http://www.blogtalkradio.com/onetribe - Occult Talk Show
From: D Yuniskis on 12 Feb 2010 01:57 life imitates life wrote: >> I'm *sure* there are semi-"photographic" >> techniques used to "image" data from damaged discs -- if the >> motivation is high enough. > > Discs with magnetic data recorded onto them exhibit absolutely no > physical or optical signifier of what bit they contain. It REQUIRES > being read magnetically with the same head type it was written with. Note that "photographic" was in quotes. I suspect there are "liquids" nowadays that can be "poured" onto media to "develop" the individual magnetic domains which could then be photographed (photomicrographed) and analyzed *graphically*. We used to do this with tape decades ago. For low density recordings (like credit cards), *you* can probably do it in your kitchen with a good magnifying glass and the right "chemicals" > It would be very hard. > > In doubt? Shoot it three times, equally spaced. > >> We used to do it with *tape* >> decades ago... > > Not on a modern hard drive. We are talking about Gigabits per lineal > inch. EVERYTHING about the read head has to be perfect to 'grab' the > data. Almost getting right down to the molecules here. If you have money and motivation, I am *sure* it can be done. (think of the sorts of folks who have "limitless funds" and the types of things that would "motivate them")
From: JosephKK on 14 Feb 2010 12:22 On Thu, 11 Feb 2010 09:35:36 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Wed, 10 Feb 2010 14:06:37 -0700, D Yuniskis ><not.going.to.be(a)seen.com> wrote: > >>Hi Jan, >> >>Jan Panteltje wrote: >>> On a sunny day (Wed, 10 Feb 2010 13:49:55 -0700) it happened D Yuniskis >>> <not.going.to.be(a)seen.com> wrote in <hkv5h1$em9$1(a)speranza.aioe.org>: >>> >>>> Jan Panteltje wrote: >>>>> Just got myself some small magnets, these are not very strong, >>>>> specified as about 1.2 Tesla IIRC, but anyways, >>>>> had to take some stuff apart, and had them on the work bench. >>>> <grin> We don't allow magnets in the house (way too much magnetic >>>> media!) >>> >>> My floppies are far away in the attic, next to the CRT monitors... >>> The CRT stuff was why I kept magnets away. >> >>I think the only floppies I now have left are those for the Unisite. >>I'm more concerned with disks, mag tape, video tape and audio tape >>(the last two just until I can finish transcribing everything to >>digital format) > >I have a quite old HP logic analyzer (came with probes, LOL) that >boots off a floppy. Took a bit of doing to back it up, but I did it. > That was likely to be an old LIF disk, 256 byte sectors instead of the PC normal of 512 byte sectors. Not to mention many other differences. > >> >>>>> Normally screws are all over the place, but with these magnets everything just jumped to it :-) >>>>> Very easy, also used the magnets to magnetise my screwdrivers... >>>>> Works much easier. >>>> Only works with ferrous screws. Someday you'll find yourself wondering >>>> why a particular screw "doesn't stick" (even if only for a moment). >>>> Then, you'll find a more universal way to hold screws on your >>>> screwdriver! ;-) >>> >>> Glue? >> >>There are some mechanical devices that can do this. I used to use >>a magnetic screwdriver until encountering painted/plated nonferrous >>screws (and wondering why it wouldn't "stick"). The same holds >>true for nylon, of course. Also, when working in tight quarters >>in a ferrous box, screws tend to "lean" off the screwdriver >>towards the wall of the box -- since mounting holes are often >>close to the edges of a box.
From: JosephKK on 14 Feb 2010 12:30 On Thu, 11 Feb 2010 23:57:57 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: >life imitates life wrote: >>> I'm *sure* there are semi-"photographic" >>> techniques used to "image" data from damaged discs -- if the >>> motivation is high enough. >> >> Discs with magnetic data recorded onto them exhibit absolutely no >> physical or optical signifier of what bit they contain. It REQUIRES >> being read magnetically with the same head type it was written with. > >Note that "photographic" was in quotes. I suspect there are >"liquids" nowadays that can be "poured" onto media to "develop" >the individual magnetic domains which could then be photographed >(photomicrographed) and analyzed *graphically*. We used to do this >with tape decades ago. For low density recordings (like credit >cards), *you* can probably do it in your kitchen with a good >magnifying glass and the right "chemicals" > >> It would be very hard. >> >> In doubt? Shoot it three times, equally spaced. >> >>> We used to do it with *tape* >>> decades ago... >> >> Not on a modern hard drive. We are talking about Gigabits per lineal >> inch. EVERYTHING about the read head has to be perfect to 'grab' the >> data. Almost getting right down to the molecules here. > >If you have money and motivation, I am *sure* it can be done. >(think of the sorts of folks who have "limitless funds" and the >types of things that would "motivate them") Good old MagnaSee ferrofluids. They are still in business by the way. Things are a bit different nowadays; way back then tape densities were 800, 1600, and 5680 FCI 7 or 9 tracks on 7/16 wide base. Modern disk is over 50,000 FCI and 1200+ TPI.
From: JosephKK on 14 Feb 2010 12:53
On Thu, 11 Feb 2010 13:13:11 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Thu, 11 Feb 2010 14:15:21 -0600, Tim Williams wrote: > >> I find a lot of >> magnetized rod or bobbin type inductors in monitors. They are used to boost >> the horizontal deflection supply (to allow for different sweep rates on the >> flyback transformer). > >I think you'll find they're used to compensate for non-linear dI/dt in the >horizontal deflection coils caused by coil resistance. Back when i was still interested in TV the multipole ring magnets on the neck of the CRT were called "purity rings". The field strengths were small and they were used adjust the collimation of the three electron beams to go through the shadow mask in the right direction. |