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Re: A couple of questions for all you CD demagnetists out there.

Posted by Steve Eddy on September 5, 2002 at 20:34:36

In Reply to: Re: A couple of questions for all you CD demagnetists out there. posted by geoffkait on September 5, 2002 at 19:06:22:

Steve, the CD spins as fast as 500 RPM; do you think that might be fast enough to create a static field on the polycarbonate?

Ok, I did a little experiment.

I took an old CD and rinsed it under some running water, shook off the excess water and let it air dry.

While it was drying, I cut up some little pieces of paper from a small notepad made from rather lightweight paper. The pieces were between about 1/16th and 1/32nd inch squares and rectangles.

I took the disc and placed the edge into the pile of paper crumbs. Nothing stuck. I tried it at several points around the circumference of the disc with the same result.

Then I took the disc and rubbed it moderately about four or five times across my cat's fur. Did as before and this time it picked up a dozen or so crumbs, some of them dangling from other crumbs in a chainlike fashion.

So, this shows that it's easily possible to get a trivially demonstrable charge on the disc.

I went back and rinsed off the disc, shook off the excess and let it air dry. I tried it again and it didn't pick up so much as a single crumb.

I popped the disc into my CD player, started playing the first cut and hit pause and let it spin for about 70 minutes (rougly the maximum time on a Red Book CD). Since the disc spins fastest when reading the inner portion of the disc, keeping it paused at the beginning of the disc gives something worse than worst case.

I removed the disc and tried it on the crumbs. It didn't pick up a single one. Which isn't to say that the disc picked up absolutely no charge. Just that whatever charge there might have been wasn't sufficient to pick up a single paper crumb.

I then rubbed the disc on my cat's fur again and this time I placed some aluminum foil and pressed it onto the label side of the disc for a minute or so. I removed the foil and went back to the crumb pile where it again picked up a dozen or so pieces.

I tried the same thing with aluminum foil folded over and covering both the label and read side of the disc and got the same result as previously.

Give it a try yourself and see what results you get. If you don't have a cat, your own hair will due. Well, unless you're bald. :)

By the way, the temperature was in the low 70s and humidity in the mid 30s.

se

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