Magnets on the work bench

Hey, thanks for this!

Now, is there any chart or anything that shows the relative "strengths" of magnetic fields? Like, Jan Panteltje says, "these are not very strong, specified as about 1.2 Tesla IIRC..." and Chris says, "Since when was 1.2 T 'not very strong'?!"

So, how strong is "strong?" Is there some kind of way to relate Teslas to Gauss to whatever other measures of flux density or whatever that can be related to physical stuff, like "A 1 magnet can support one pound of plain ol' iron" or something that could be related to something I could hold in my hands?

What other measures of magnetic force are there? Teslas, Gausses, Ampere-turns, Other? (and how to they relate to physical reality?)

Thanks, Rich

Yup. That's what they make nuts, bolts and screws out of.

I believe some of the 400 series are martensitic, 300 series are austenitic.

I think he was confusing "ferrous" with "ferromagnetic".

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Actually, they're usually 316 or 18-8 if they're stainless.

I think you'll find they're used to compensate for non-linear dI/dt in the horizontal deflection coils caused by coil resistance.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

My experience is that if you don't specify 316 / 18-8, you can get all sorts of crap.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Well, ferromagnetics saturate in the 1-1.8T range (note: ferrimagnetics (i.e. ferrite) saturate lower, 0.2-0.5T).

1T = 10,000 G

T = Wb/m^2, literally the flux density. Flux (Phi) in Wb = B*A (flux density times area). Wb == V*s, which is interesting because it means you cannot have flux without having put some voltage into it at some point. And that's exactly what happens, you apply volts to an inductor to increase current, V = L*dI/dt.

Maxwell stress sigma_m = B^2/(2*mu_0).

In SI units, T^2 / (H / m) = V^2 s^2 m A / (m^4 V s) = V A s / m^3 = J / m^3, an energy density. But: J / m^3 = N / m^2 = Pa, pressure.

If you integrate the stress over all space, you'll get the total energy stored in that magnetic field. Or if you integrate it over a cross section, you get the force on that section.

If you imagine the flux lines coming out of the N pole of a magnet, looping around and returning to S, these lines carry a mechanical tension equal to this pressure, while expanding perpendicular to the lines with identical pressure. Thus, if you have a uniform 1T magnetic field from top to bottom through a 1 meter cube, there is a force of F = A*sigma_m = 1m^2 *

1T^2/(4*pi x 10^-7 H/m) = 795kN pulling the top and bottom together, and forcing the sides apart (assuming the surfaces are interacting magnetically). That's a lot of force, which is why big magnets are made with even bigger steel.

A 1cm cubic neodymium magnet with 1T on each end experiences a net force of

0, but if you stick two together, you get 1T from each magnet at the ends and 2T where they add in the middle (assuming they are actually a lot longer than 1cm, and assuming the material is linear, which certainly isn't; the internal field may not get much above 1.5T or so). The apparent attractive force is then around (10^-4 m^2) / (4*pi x 10^-7) * (1.5T^2 - 1T^2) = 99N, or about 22 lbs (lbf, I should say).

Instead of sticking magnets together, you'll get a similar answer for a sufficiently wide and thick hunk of steel, because the magnetic field from one pole is sucked into the iron, loops around and runs back through the air around the magnet, with very little fringing through the back side of the plate. The B field at the surface of the iron is then around 1T, and nearly

0 at the back side. Thin steel or small pieces will act more like extensions of the magnet,

Amp turns is magnetomotive force. Whereas a battery or generator supplies electromotive force to an electronic circuit, At supplies MMF to a magnetic circuit. Flux replaces current, and reluctance replaces resistance. A mechanical analogy would actually be more appropriate, because reluctance obviously doesn't consume power as resistance does.

Magnetization is amps per meter (or At/m to be practical and specific), and is related to flux density as B = mu_0 * H. Because mu_0 is teensy, it takes a *lot* of magnetization to make 1T in air. Very permeable materials (mu_r > 100) are a lot easier to magnetize; this is usually used to 'cheat' because, instead of brute-force magnetizing air, you magnetize a big stinking core and leave a little air gap to play around in. The required magnetization is much more reasonable, maybe 100 to 1000At.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

errors.

in, and write it back.

FLASH.

The best medium will probably be phase change "flash"

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.blogtalkradio.com/onetribe - Occult Talk Show

Standing field. Zero effect on a transformer, as well as the rest of the supply circuit, UNLESS you are moving it OVER the transformer placing the transformer inside the field loop. Still won't introduce much into it, since the transformer is a closed loop as well.

Typical net effect: Zero. All orientations.

A standing external field will not "saturate" the core. Saturation would be a DC standing field produced from within the transformer's closed magnetic loop. In other words, from one of the windings.

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. :-)

It was seemingly the SCSI controller and that drive pair up. I would not blame the OS or driver first. I look at standards implementations made by the controller maker, etc. Brand names matter in certain realms too. Maybe someone thought "full compliance" meant "fool compliance".

In the early days of SCSI match ups which would not work were common. I thought all that was worked out by now. Configuration can also cause problems, but that is usually with connecting to the drive at all. Once connected, and successfully formatted, which it obviously was, it should function fine. I suppose a virus could worm its way into the driver and cause the catastrophic failure you describe.

Normally the DRIVE performs the operations from within the hardware local to the drive, so the interface sends and receives cluster write commands, etc. Should still be no way to "spray bits all over the drive" or the VTOC, for that matter.

I started out with old reed contact pinball machines and worked into upright video games, so I have been around a lot of CRTs. In the case of nearly horizontally placed CRTs in such games, the orientation of the game when demagnetized and set-up and the orientation of where one places it in the arcade would determine whether it looked right or not. Turned one way, it was perfect. Turn it 90 degrees from the, however, and the colors get all funkified.

I used to be able to demag a CRT WITH a magnet. A huge speaker magnet, I would 'wash' all around the face and edges of it from about two feet away, and then flip the magnet over and do it again. Working closer, and closer, and remembering to always flip and equalize, one can remove any anomalous magnetizations that occur over time, just like the coils do automatically these days, and the big tech owned coils did in the way back when.

The orientations flip in the presence of the field, make no mistake. They simply flip back when the field is removed. The concentration of the small magnet's field when it is only about 0.040" away from the disc face, makes me think it might beat the perm figure of the disc media and flip some of the opposite oriented domains and them not return when the field is removed.

I too have yet to have seen floppies lose data, but I have seen old, wide head 360kB disc drives with discs that were written by the newer

1.2MB thinner head drives, that would not read on the older drive due to the reduced magnetic energy of the thinner track of data being read by a head expecting a much 'fuller' 'bit flip' 'signal' coming off the data as it passes by. Too many false highs and false lows.

I have magnetron magnets in the other room, that sat out here in the living room for well over a year.

Threat to whom or what? Me? Not worried. My magnetic recordings? Not worried.

Of course, we will not be seeing me wheeling any monsters in here on a hand truck any time soon either.

But I do like magnets. I would not pass one up if I saw it at a swap meet or garage sale, etc, and it was pennies on the dollar. I could quick turn it on ebay if it was too week to please me, and keep it if it is good! Or not.

Wow. Mine was all solid magnets, like a parquet floor. Two layers on each side. On the tape... bits be gone. :-)

The bullet through the platters stops all but the folks dragging heads across the drive faces by hand. The cylinder pitch would require precision alignment, and co-linear motion to the tracks. It would be a long, arduous process to get even a little data.

Sounds like a series of small, encrypted virtual volumes are in order. Then, all the bits looks random, and there is nearly no file structure to find at all.

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.

It would be very hard.

In doubt? Shoot it three times, equally spaced.

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.

Not me. I still have every hard drive I ever owned.

That includes a 10MB Tandon original HD for the XT.

Back then, it was nearly $100 per MB. Now, it isn't even $100 per TB.

W O W !!!

He sent out 12% per year checks to some of his investors.

Some of them paid taxes on dividends they never actually received!

errors.

in, and write it back.

and FLASH.

You buy one and use the piss out of it.

Mine is a Linux boot and swap drive, and it is a Windows NTFS drive as well.

I have put Linux on it several times and several flavors. I am giving it as much hell as I can.

I think they are fine, and there will be those one in a million failures that are not related to old, multi-cycled memory chip failure.

Somebody, whoever it happens to, will not be happy.

But they seem pretty sturdy, or we would already be hearing about failures in the review sites, which both VARs and IT pros at big companies follow. Essentially, the best way is to set up a big RAID server and beat the hell out of it, and make sure that it is only a mirror for an actual server. Good way to get stats on access times, etc as well. Damn thing will probably run forever.

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

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"

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")

Yuniskis

That was likely to be an old LIF disk, 256 byte sectors instead of the=20 PC normal of 512 byte sectors. Not to mention many other differences.

everything just jumped to it :-)

wondering

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,=20 1600, and 5680 FCI 7 or 9 tracks on 7/16 wide base. Modern disk is=20 over 50,000 FCI and 1200+ TPI.