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'[OT]: VHS tape, a way to erase it.'
2001\02\21@075336 by Leo De Palo

picon face
Hi PIC'er friend,

I have been a strange request to build a device able to erase the video VHS
tape.

I have in mind to build  a coil of 15 cm. of diameter and using a PIC, to
generate a continuos frequency sweep.

Passing the tape into this coil, i suppose the tape will be erased.

Wat do you think about the idea??

Wich range of frequency should be used to erase the tape??


Tahnks in advance and regards

Leo

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2001\02\21@080150 by Andy Faulkner

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why not just use a magnet



--- Leo De Palo <spam_OUTtenagliaTakeThisOuTspamBIGFOOT.COM> wrote:
{Quote hidden}

=====

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2001\02\21@080826 by t F. Touchton

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part 1 3061 bytes content-type:text/plain; charset=us-asciiMost degaussing coils operate at 60Hz (simplicity).  What one needs to remember
is that the flux density must be great enough to re-align the particles (I think
more than 1 weber... but going from memory on reel to reel tape and may be
wrong), and should decay in nature (which can be accomodated by moving the tape
away from the coil).  The center of the coil will have the greatest flux
density... so it is most efficient to pass the tape through the coil.  Recording
tape is spec'd as to the flux density required to erase it... you can use
Biot-Savart law to calculate coil geometry, turns, and driving current to
accomplish this.


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why not just use a magnet



--- Leo De Palo <.....tenagliaKILLspamspam.....BIGFOOT.COM> wrote:
{Quote hidden}

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Tel +44 778 896  6087.....................UK Dial 0778 896 6087

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part 2 3020 bytes content-type:application/octet-stream; (decode)

part 3 154 bytes
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2001\02\21@083124 by Roman Black

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Leo De Palo wrote:
>
> Hi PIC'er friend,
>
> I have been a strange request to build a device able to erase the video VHS
> tape.
>
> I have in mind to build  a coil of 15 cm. of diameter and using a PIC, to
> generate a continuos frequency sweep.
>
> Passing the tape into this coil, i suppose the tape will be erased.
>
> Wat do you think about the idea??
>
> Wich range of frequency should be used to erase the tape??
>
> Tahnks in advance and regards
>
> Leo


Leo, I have a Radio Shack bulk tape eraser. It's AC
mains powered and erases any tape or disc in about 3
seconds. If you lay floppies flat on the table you can
erase 50 in a number of seconds. I've used it for
years.:o)
-Roman

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2001\02\21@092200 by Leo De Palo

picon face
> why not just use a magnet

Because the magnet only degrade the quality of the image but do not erase
the tape!

{Quote hidden}

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2001\02\21@092835 by Richard Sloan

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A strong enough magnet and especially one using AC current will erase.

R.

 >>  > why not just use a magnet

 >>  Because the magnet only degrade the quality of the image but do not erase
 >>  the tape!

 >>  >
 >>  > --- Leo De Palo <KILLspamtenagliaKILLspamspamBIGFOOT.COM> wrote:
 >>  > > Hi PIC'er friend,
 >>  > >
 >>  > > I have been a strange request to build a device able
 >>  > > to erase the video VHS
 >>  > > tape.
 >>  > >
 >>  > > I have in mind to build  a coil of 15 cm. of
 >>  > > diameter and using a PIC, to
 >>  > > generate a continuos frequency sweep.
 >>  > >
 >>  > > Passing the tape into this coil, i suppose the tape
 >>  > > will be erased.
 >>  > >
 >>  > > Wat do you think about the idea??
 >>  > >
 >>  > > Wich range of frequency should be used to erase the
 >>  > > tape??
 >>  > >
 >>  > >
 >>  > > Tahnks in advance and regards
 >>  > >
 >>  > > Leo
 >>  > >
 >>  > > --
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 >>  >
 >>  >
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 >>  > Pyrosync International http://www.pyrosync.co.uk
 >>  >
 >>  > Tel +44 778 896  6087.....................UK Dial 0778 896 6087
 >>  >
 >>  > Fax +44 870 132 3275.....................UK Dial 0870 132 3275
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 >>  >
 >>  >
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2001\02\21@102852 by Ken Gasper

picon face
I too have a Radio Shack bulk tape eraser and it works great.  They make
two different models, one for cassette tapes and another for VHS tapes
because the VHS tapes are much larger and take a bigger magnet.  They are
not rated for continuous duty however and do get very warm.  I think my
casette tape eraser has a 1 min ON and something like 5 to 30 min OFF duty
cycle.  I think the one I have sells for around $25US.





Roman Black <spamBeGonefastvidspamBeGonespamEZY.NET.AU>spamBeGonespamMITVMA.MIT.EDU> on 02/21/2001 07:32:17 AM

Please respond to pic microcontroller discussion list
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Subject:  Re: [OT]: VHS tape, a way to erase it.


Leo De Palo wrote:
>
> Hi PIC'er friend,
>
> I have been a strange request to build a device able to erase the video
VHS
{Quote hidden}

Leo, I have a Radio Shack bulk tape eraser. It's AC
mains powered and erases any tape or disc in about 3
seconds. If you lay floppies flat on the table you can
erase 50 in a number of seconds. I've used it for
years.:o)
-Roman

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2001\02\21@132308 by mike

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On Wed, 21 Feb 2001 05:02:39 -0800, you wrote:

>why not just use a magnet
Have you actually tried to bulk erase video tape with a magnet - it's
actually pretty hard, even with a neodymium magnet.
- Leo De Palo <EraseMEtenagliaspamBIGFOOT.COM> wrote:
{Quote hidden}

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2001\02\21@161132 by John La Rooy

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Roman Black wrote:
{Quote hidden}

ANY tape???
we were trying to erase some DLTs just before christmas
the MRI machine didn't work, neither did the induction furnace
we were trying to remove the formatting from the tapes :o)

john



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2001\02\21@163012 by t F. Touchton

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part 1 2531 bytes content-type:text/plain; charset=us-asciiIf the tape does not saturate, a static magnetic field will not erase it.... the
varying magnetic field will still exist.


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Roman Black wrote:
{Quote hidden}

ANY tape???
we were trying to erase some DLTs just before christmas
the MRI machine didn't work, neither did the induction furnace
we were trying to remove the formatting from the tapes :o)

john



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part 3 154 bytes
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2001\02\21@234453 by Roman Black

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John La Rooy wrote:
{Quote hidden}

Wow!! Firstly, whats a DLT? And secondly it amazes me that
you couldn't demagnetise a tape with a strong AC field.
My bulk tape eraser is a lot stronger than the degauss wands
we use on TV's. Some TVs have a metal frame that slowly gets
magnetised by the earth's field over many years. These are
hard to degauss. I use my bulk tape eraser, which always
works although it might take a lot of minutes. That's
annoying because of its low duty cycle.

Surely a strong AC magnetic field WILL demagnetise
anything if given enough time?? Hmm...
-Roman

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2001\02\22@091515 by Thomas McGahee

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Erasure is faster and more complete if the item being
erased is slowly moved through the coil while the coil
is activated. If simply left in the coil during activation
there is a good chance that some magnetization of the
item will remain. Moving the item through the coil
and then AWAY from the coil ensures that any remnant of
magnetization is very small.

Because the demagnetization coil usually has no core,
it will tend to get hot very rapidly.

Fr. Tom McGahee


{Quote hidden}

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2001\02\22@115109 by Dal Wheeler

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Maybe some active cooling could up the duty cycle?
----- Original Message -----
> Because the demagnetization coil usually has no core,
> it will tend to get hot very rapidly.
> Fr. Tom McGahee

> > works although it might take a lot of minutes. That's
> > annoying because of its low duty cycle.

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2001\02\22@123759 by Alan B. Pearce

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>Maybe some active cooling could up the duty cycle?

You mean as in liquid nitrogen or some such ??? :)

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2001\02\22@124915 by Dal Wheeler

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Nah, a current sucking peltier and a massive copper heatsink so that it will
burn your hand while operating it.  :')

Of course a "properly" designed liquid nitrogen system could do the same
thing...  ;)

{Original Message removed}

2001\02\22@134605 by Peter L. Peres

picon face
>> why not just use a magnet
>
> Because the magnet only degrade the quality of the image but do not
> erase the tape!

The magnet will wipe the tape fine if it is powerfull enough.

Peter

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2001\02\22@134814 by Sean Breheny

face picon face
LOL! I want to see the tape eraser with a huge LN2 dewar hanging off the
side of it!

Sean


On Thu, 22 Feb 2001, Dal Wheeler wrote:

> Nah, a current sucking peltier and a massive copper heatsink so that it will
> burn your hand while operating it.  :')
>
> Of course a "properly" designed liquid nitrogen system could do the same
> thing...  ;)
>

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2001\02\22@150113 by Harold M Hallikainen

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       The standard way to bulk erase tape is to just have a strong 60 Hz
magnetic field, then slowly back the tape away from it. The field is
often generated using something that looks like an EI transformer core,
but without the I part (allowing the flux to escape). Backing the tape
away from the coil is a lot simpler than trying to gradually back down
the field. Most bulk tape erasers I've seen just have the operator slowly
move the tape away. Some have a conveyor belt that passes the tape over
the coil, then carries the tape away. One did have a decreasing field. I
believe it was still 60 Hz and used phase control to back the field down.

Harold


On Wed, 21 Feb 2001 13:54:21 +0100 Leo De Palo <RemoveMEtenagliaTakeThisOuTspamspamBIGFOOT.COM>
writes:
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2001\02\22@150130 by Harold M Hallikainen

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       The magnet leaves a DC field on the tape. This could then magnetize
everything in your tape deck, which would then tend to erase tapes as you
play them. The standard way of erasing tape is to use a decreasing AC
field.

Harold


On Wed, 21 Feb 2001 05:02:39 -0800 Andy Faulkner <EraseMEandyf97spamspamspamBeGoneYAHOO.COM>
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2001\02\22@211056 by John La Rooy

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Roman Black wrote:
{Quote hidden}

DLT=digital linear tape. the ones we had were 20 or 40 Gigabyte i think
we upgraded our tape drives only to find that they worked fine with new
tapes
but not the $10,000 worth of tapes we used in the old drive. the
manufacturer
advised us to bulk erase the tapes. the MRI machine wiped the tapes
enough
to make the data on them unreadable, but not enough to work with the new
drive.
We finally had to send them to singapore to get them erased.

at a guess i'd say the tapes do have magnetic shielding built in

john

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2001\02\23@051015 by Roman Black

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Harold M Hallikainen wrote:
>
>         The magnet leaves a DC field on the tape. This could then magnetize
> everything in your tape deck, which would then tend to erase tapes as you
> play them. The standard way of erasing tape is to use a decreasing AC
> field.
>
> Harold


We fix a few tape decks here at the TV shop. Most
of the new (cheap) tape decks use a small
permanent magnet "erase head"!! It's truly awful.

Instead of a proper AC erase head at 19kHz,
this little white plastic "head" with a magnet in
it is mechanically raised to contact the tape
when in record mode. Some bean counter realised
they could save 20c per unit again...

Have a peak in your tape deck, if it is made
in the last few years and is not top $$ unit
it will probably have one of these. :o)
-Roman

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2001\02\23@052443 by Peter L. Peres

picon face
>LOL! I want to see the tape eraser with a huge LN2 dewar hanging off the
>side of it!

You don't need to go that far imho. A degaussing coil assembled out of the
E core sections of a transformer will do fine. This was a calssic DIY
degaussing coil design some years ago afaik.

Peter

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2001\02\23@055036 by Roman Black

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John La Rooy wrote:
{Quote hidden}

Cool! Faced with a $10,000 problem I would have built
a simple tape transport and run them over a heap
of VCR full erase heads. If it was 1/2" tape of course.

I think you could be right about the sheilding, probably
if they were in metal spools the spools were shielded.
Should'nt have taken that much energy to erase if
the tape was properly wrapped over a decent size erase
head.:o)

Wonder how Singapore did it??
-Roman

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2001\02\23@065001 by Peter L. Peres

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Modern magnetic tapes and disks do not have built-in shielding but they
use magnetic materials with such high remanence and threshhold that it
almost requires a nuke to wipe them properly. A correctly sized magnet
moved around in the right way should do it, but a coil will be cheaper.

Based on some rough calculations I'd say that a 5cm^2 degausser coil front
that will wipe VHS and data tapes at essentially zero distance should
require about 300W mains power input. Maybe more. The calculations are
based on the required power to do that in a normal erase head and its
power input. The numbers may be wildly off however as they assume that
the efficiency of both systems are the same, and they are not.

Peter

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2001\02\23@095733 by David VanHorn

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>
>Based on some rough calculations I'd say that a 5cm^2 degausser coil front
>that will wipe VHS and data tapes at essentially zero distance should
>require about 300W mains power input. Maybe more. The calculations are
>based on the required power to do that in a normal erase head and its
>power input. The numbers may be wildly off however as they assume that
>the efficiency of both systems are the same, and they are not.


You'll need about 700 gauss IIRC.

700 turns at 1A would get you there. Wind a solenoid big enough to put the
tape in, with about an inch of extra on the ends.  The flux through the
center of a solenoid is uniform until you put the tape in, but that's ok,
because the flux going through the tape is what you want.

Watch that collapsing field though :)

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2001\02\23@105914 by Simon Nield

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david:
>Wind a solenoid big enough to put the
>tape in, with about an inch of extra on the ends.


wind that coil in sections, power the coils from phase sequenced sine wave sources, and you even
have an auto load / eject feature. i mean rail gun.

Regards,
Simon

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2001\02\24@090512 by Peter L. Peres

picon face
>You'll need about 700 gauss IIRC.
>
>700 turns at 1A would get you there. Wind a solenoid big enough to put
>the

1A at 220V is 220W (ok, I know it's a coil). This is slightly less than
the 300W I came up with. However I seem to remember that high remanence
materials require more like 1400 Gauss and more but I am not sure. 700
Gauss is Iron ?

The idea is not to make an airwound coil, instead it's a transformer for
~300W with only the primary wound and the E cores all assembled from the
same side. This yields a large magnet with a respectable field across the
open ends of the Es. I have one of these at work and they almost work (too
little power spec to erase VHS tapes and floppies, but everyhting else is
ok).

Am I wrong if I say that one cannot erase a higher magnetization material
using pole pieces made of a lower magnetization material ? I think that a
strong enough field cannot be achieved ?

Which probably leaves the aircoil mounted in a power oscillator at ~1MHz.
I can see an use for a PL509 I have here ...

Peter

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2001\02\24@093434 by Roman Black

flavicon
face
Peter L. Peres wrote:
{Quote hidden}

I think my radio shack bulk tape eraser is about 400W.
The construction is boxed, but an E-core laminated former
(with the "I" part missibng) and a big coil would be
about perfect to fit in that box, and would explain the
flat bottom surface that goes onto the tape! :o)

It gets real hot, duty cycle 10:1 from memory. Works great!
I have demagged many things our "proper" degaussers would
not touch.

My magnetic theory is sketchy, but surely ANY ac magnetic
field will demag things, if given enough time? I'm always
having to demag TVs that were magnetised by the earth's
magnetic field over ten+ years. Surely it's a field/time
thing?? Curious.
-Roman

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2001\02\24@102255 by David VanHorn

flavicon
face
>
>My magnetic theory is sketchy, but surely ANY ac magnetic
>field will demag things, if given enough time? I'm always
>having to demag TVs that were magnetised by the earth's
>magnetic field over ten+ years. Surely it's a field/time
>thing?? Curious.
>-Roman


No. You have to get above the threshold, or nothing happens.

I need to brush up on the terminology, but that's the basics.
Tape has a min flux value. There's some microscopic variation around that
number, but for lo-coercivity tape at 300 gauss, a field of 250 is totally
harmless.


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2001\02\24@140219 by Robert Rolf

picon face
Roman Black wrote:
>
> Peter L. Peres wrote:
> >
> > >You'll need about 700 gauss IIRC.
> > >
> > >700 turns at 1A would get you there. Wind a solenoid big enough to put
> > >the

> My magnetic theory is sketchy, but surely ANY ac magnetic
> field will demag things, if given enough time? I'm always
> having to demag TVs that were magnetised by the earth's
> magnetic field over ten+ years. Surely it's a field/time
> thing?? Curious.
> -Roman

Nope. It's instantaneous once one has crossed the flux threshold for
the magnetic domains.

The 'long term' magnetizing effect is due to Brownian motion (you know,
that random jostling that molecules do) wherein the the molecules settle
back in line with the external bias field after being disturbed by the
random motion of heat.

The simple proof is the way one can magnetize a ferrous rod by simply
holding it in north/south alignment, and banging it repeatedly with
a hammer. The shock wave disrupts the molecules, and they align
themselves
as they settle back to a lower energy state.

This is also why materials loose their magnetic field when heated to the
Curie point. The Brownian motion become total (without melting).

As an aside, in the early days of magnetic tape, video tape machines
were horrifically expensive, so to duplicate multiple copies
(as for a commercial) a custom built machine recorded a mirror image
onto high coercivity tape. This master tape was then run in a loop
through
a pair of rollers that were heated to the Curie point of the lower
coercivity tape and thus a contact print of the tape was made.
Multiple copies of the commercial would spool up on the takeup reel,
and then spooled off and cut into individual reels for distribution
(this is back in the days before it was uneconomic to use satellite
distribution).

If whole programs were being duplicated (too long for a loop), then
the master and image tape were wound together on to one plastic reel,
and the sandwich passed through a calibrated degausing coil.
The high coercivity tape wouldn't be erased (much) but the image tape
would pick up the magnetic imprint as the field decayed away.
The thick bases of the master & image tapes were sufficient to
keep the back side of the tape from imprinting (print-through).

See 'Magnetic Recording' circa 1970 for the interesting details.

Robert

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2001\02\24@150849 by David VanHorn

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face
>
>See 'Magnetic Recording' circa 1970 for the interesting details.


Is this the Finn-Jorgensen book?
Anyone know where I can get a copy?

Great book, the Horowitz and Hill of magtape.
However, Nothing on discontinuous media magnetised along the direction of
travel. :(
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2001\02\24@171315 by James Newton

face picon face
www.amazon.com/exec/obidos/ASIN/007033045X/jamesnewtonpers
But are your sure that is a good book? Its not well reviewed. How about
http://www.amazon.com/exec/obidos/ASIN/0521449731/jamesnewtonpers

James Newton, PICList Admin #3
.....jamesnewtonspam_OUTspampiclist.com
1-619-652-0593 phone
http://www.piclist.com

{Original Message removed}

2001\02\24@172559 by David VanHorn

flavicon
face
www.amazon.com/exec/obidos/ASIN/007033045X/qid=983053466/sr=1-1/ref=sc_b_1/105-4944617-8508707

This is the one I was referring to.
Excellent book. Mine's on order!
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2001\02\24@174726 by Robert Rolf

picon face
If memory serves, I was refering to:
"Magnetic Recording" Charles E. Lowman

http://www.amazon.com/exec/obidos/ASIN/0070388458/qid=983054488/sr=1-23/ref=sc_b_23/105-0414136-8522368

It's packed away in boxes that are throughly buried so I can't check for
certain.
It's also quite old (70's) so it's no surprise that it's out of print.

David VanHorn wrote:
>
> http://www.amazon.com/exec/obidos/ASIN/007033045X/qid=983053466/sr=1-1/ref=sc_b_1/105-4944617-8508707
>
> This is the one I was referring to.
> Excellent book. Mine's on order!

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2001\02\25@164543 by Peter L. Peres

picon face
Roman, magnetization has almost nothing to do with time. A stable magnetic
material will be magnetized and stay that way until one of two things
happes: 1. It is heated above the Curie temperature, and will loose all
magnetization or 2. An external field will induce a magnetization stronger
than the magnetic hysterezis of the material and switch the polarity of
the magnet (or not). Weller soldering irons have a thermostat that
operates according to #1 and all magnetic recording and erasure relies on
#2. Now, on instable materials, like the mild steel in TV screen chassis,
sundry effects, like repeated heating and cooling, metal fatigue, and
mechanical deformation change parts of the material from and to the
magnetizable state. This makes for some 'unexplained' magnetization. This
can come and go at whim over time.

Peter

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2001\02\26@025414 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> Roman, magnetization has almost nothing to do with time. A stable magnetic
> material will be magnetized and stay that way until one of two things
> happes: 1. It is heated above the Curie temperature, and will loose all
> magnetization or 2. An external field will induce a magnetization stronger
> than the magnetic hysterezis of the material and switch the polarity of
> the magnet (or not). Weller soldering irons have a thermostat that
> operates according to #1 and all magnetic recording and erasure relies on
> #2. Now, on instable materials, like the mild steel in TV screen chassis,
> sundry effects, like repeated heating and cooling, metal fatigue, and
> mechanical deformation change parts of the material from and to the
> magnetizable state. This makes for some 'unexplained' magnetization. This
> can come and go at whim over time.
>
> Peter


That all makes sense. But when I demag a TV picture
tube, after each "pass" of the wand i can clearly
see the magnetism on the tube get less and less until
fully demagged. Takes maybe 20 seconds, (1000 AC
demag cycles).

Some TVs with bad problems take a few minutes. It
takes magnetism/time. I can demag them in less seconds
with the wand held closer (stronger ac mag field),
or more seconds with the wand further away
(smaller AC mag field).

I think that given ENOUGH strength of magnetic
field the magnetism can happen almost instantly.
But given a weaker field the amount of molecules
that become polarised depend on the amount of time??
-Roman

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