Magnets on the work bench

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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.

Yes, but there have been other "advances". E.g.,

(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?

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I've posted this before...

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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

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Thanks, Jim, I'll play around with that. The function looks like it oughta be the right shape.

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Quick and easy solution, get the platters out and microwave them. Takes about 15 seconds, works like a bulk eraser, then melts them.

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'.

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.