Magnets on the work bench
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From: D Yuniskis on 14 Feb 2010 14:52 I Joseph, JosephKK wrote: > 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. Yes, but there have been other "advances". E.g., http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=12280&DID=241078&action=detail (google will turn up a variety of other techniques which may/may not be suitable to particular recording technologies.) The point is, 3 holes in the platter stack won't prevent someone who REALLY wants to find out what you're trying to hide... I wonder how hot you would have to get the materials to cause the domains to collapse?
From: Jim Thompson on 14 Feb 2010 15:07 On Sun, 14 Feb 2010 11:09:38 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sun, 14 Feb 2010 09:53:46 -0800, JosephKK wrote: > >> 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. > >That's not what he was describing. He was describing small chokes biased >towards saturation with a permanent magnet. Inductance dependent upon >magnitude and direction of current. These are for the purpose I have >described. > >Typically 10uH at zero current rising to 50uH at 5A in the "forward" >direction. In the reverse direction, inductance doesn't decrease by much, >as the core is pretty much saturated already. IOW, "forward" >amp-turns neutralize the permanent magnet bias, reverse amp turns augment >it. > >Interesting thing to model. I've posted this before... Message-ID: <c08l04psgdaeolojgu1plh8pgmf7qd6jc8(a)4ax.com> L = Lo/[1 + (I/IH)^2] Lo = Inductance at zero current IH = Current at which L falls to 1/2 of Lo This was measured and modeled (by me) at Sperry/Honeywell Space Systems Division. Seems to work well everywhere I've tried it. Just add some bias in the denominator term for "I" to account for either a current bias OR a magnetic bias. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: JosephKK on 15 Feb 2010 06:21 On Sun, 14 Feb 2010 12:52:16 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: >I Joseph, > >JosephKK wrote: >> 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. > >Yes, but there have been other "advances". E.g., >http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=12280&DID=241078&action=detail > >(google will turn up a variety of other techniques which may/may not >be suitable to particular recording technologies.) > >The point is, 3 holes in the platter stack won't prevent someone >who REALLY wants to find out what you're trying to hide... > >I wonder how hot you would have to get the materials to cause >the domains to collapse? Quick and easy solution, get the platters out and microwave them. Takes about 15 seconds, works like a bulk eraser, then melts them.
From: Michael A. Terrell on 15 Feb 2010 12:03 JosephKK wrote: > > 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. There were also 'Static Convergence Magnets' on the necks of Tri-Gun Color CRTs. Early Monochrome CRTs had an adjustable magnet used as an 'Ion Trap'. -- Greed is the root of all eBay.
From: Michael A. Terrell on 17 Feb 2010 03:36
Fred Abse wrote: > > On Mon, 15 Feb 2010 12:03:28 -0500, Michael A. Terrell wrote: > > > Early Monochrome CRTs had an adjustable magnet used as an 'Ion Trap'. > > The trick there was a "bent gun", pointing off axis, at a slight angle. > The magnet realigned the beam so that electrons went straight down the > middle. Ions didn't get deflected by the magnet, and just hit the neck > wall more or less harmlessly. > > But you knew that already :-) Yes. I learned that in the '60s when replacing CRTs. :) (I was 13) Instructions were packed with each one explaining why you no longer needed the Ion Traps. -- Greed is the root of all eBay. |