Using magnet + metal to erase tape recording

I would like some advice about magnetism from the specialists in
the physics groups.

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SUMMARY

My kitchen table experiment seems to show that ferrous material
near a magnet weakens the magnetic flux of a fixed point between
the two.

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BACKGROUND

From time to time I need to erase some recordings made on tape
cassettes. I don't have a bulk eraser so I erase the tape in the
machine by recording over it in real time.

That's slow! I often get a much faster result by using a strong
neodymium magnet from a hard disk drive and wiping it over the
surface of the tape cassette. But sometimes this method does not
fully erase the tape 100%.

So I figured that I could hold the tape in front of some ferrous
metal sheet (like the stove, the side of the microwave oven, etc)
and then put the magnet ontop of the the tape in order to let the
metal sheet focus the magnetism and so in this way subject the
tape to a STRONGER magnetic field.

However I tested this idea at home using a magnetic compass and a
scrap of ferrous metal. My simplistic experiment showed that the
ferrous sheet behind the compass REDUCED the needle deviation due
to the neodymium magnet.

Is this result correct? Or is my experiment crap?

Or is it my physics which is crap because I thought the ferrous
metal on the other side of my tape (or compass) would subject it
to a stronger magnetic field.

Advice please.

Dear David Peters:

"David Peters" <[Only registered users see links. ]> wrote in message
news:Xns978829E9E270B351D7E@204.153.244.170...
....

Results are what results are.


Well, sort of. Your ferrous metal acts a little bit like a
"magnetism superhighway". Rather than amplifying local field
strength, it simply draws the field away...


Get a good "degaussing" setup.

I made one from a very long two-wire extension cord, by splitting
the cord lengthwise, then winding each wire a different direction
around a loop. Tie the loop neatly, plug in an electric drill,
and turn it on. The insides of the loop get a weak but
alternating magnetic field, suitable over some long period of
time for degaussing a color TV screen. A typical "okey" setup,
but if it is all you have...

David A. Smith

In article <Xns978829E9E270B351D7E@204.153.244.170>,
David Peters <[Only registered users see links. ]> wrote:


Whether or not the tape gets erased that way, the tape is left with a
higher level of background noise than it would have if "properly" erased
("properly" means with a high-frequency AC field).

Any kind of decent recorder erases a tape very well immediately prior to
passing it through the record head anyway.

Isaac

On 16 Mar 2006, isw wrote:




Yes I find exactly that.


The problem is that I do not want to leave these recordings on the
tapes because they are confidential. Also if I do not erase the
tape after I have transposed it then when I pick up a tape and
hear some voices I am not sure of the status of that tape. Then I
need a very careful system of organizing the tapes.

I find it far easier to pop a tape in the machine and see if there
is anything on the tape. If not then I can use the tape.

That way confidentiality is preserved and it is easier for me to
use - UNLESS I have to erase each tape in real time! :-)

Then you should permanently and irrerversibly segregate the tapes after
they have been transcribed.

Got a match?

Tom Davidson
Richmond, VA