Magnets and CDs

Will leaving a magnet near a CD cause any damage?

The reason I ask is that there's a list of items not to leave near the 601s I've got my brother and I just want to be sure CDs are ok.

Possibly a stupid question I know. I've never noticed any damage before.

 

It shouldn't matter. CDs are (I think, somehow) photographically cut, putting tiny holes into metal film. Then sandwiched into the plastic. Putting it near a magnet wont change where the holes are.

 

Deleted ....because there seems to be people who want argue for no reason

 

Thats assuming demagnetisers work eh?

 

Standard CD's are not cut or burned but moulded (in that sense they are very like vinyl pressings), the data is physically in the plastic and cannot be erased by a magnet. The data side is coated with a reflective layer, that layer is covered with a protective lacquer and the label is printed on top. As the data layer is on the label side 'top' of the disc when putting them down anywhere they should be placed label side up as any damage on the top will likely ruin the CD.

 

No, it isn't. Very few printing inks contain iron, and even those that do are in the form of chemical compounds, with iron in a chemically bound form, and they exhibit no ferromagnetic properties (ability to be permanently magnetised) whatsoever. Consider: sodium is a soft metal that reacts violently with water, chlorine is a highly dangerous greenish gas, but together they form sodium chloride, common salt, something you sprinkle without hesitation on your food, something that you wouldn't do with the separate elements. The iron oxide commonly known as rust can't be picked up with a magnet. Even a colourant as iron-rich as Prussian Blue (Fe4[Fe(CN)6]3) can't be magnetised.

There are magnetic inks, but these are highly specialised materials used for particular applications and they're not cheap.

In other words, CDs are quite safe near magnetic fields, as there's nothing magnetic about them (polycarbonate, aluminium, acrylic). Just down the corridor from me is an NMR (nuclear magnetic resonance) machine with a magnet so powerful that watches and people with pacemakers aren't allowed anywhere near it. I'd be quite happy to store my CDs in there.

 

It might make them sound better!

 

The pits are made in a polycarbonate base layer which is then coated with vaporised aluminium under conditions of high vacuum. A protective acrylic layer is then applied (aluminium oxidises at astounding speed under normal conditions and all reflectiveness would disappear literally in seconds).

 

Doubt it. As you say yourself, the pits don't move. And the Densen DeMagic drink coaster did precisely nothing for them.

 

He he - you can ask PDO about that - I've had 8 of their CDs replaced due to oxidisation.

Also, this is what plagues many early laservision videodiscs (DiscoVision in the USA, and Laservision in the UK). Witness the purple meanies and crackle pop crackle audio on my 1982 "Raise the Titanic" for a good example of this!

 

You mean the one about which Lord Grade famously said that it would have been cheaper to lower the Atlantic?

 

Deleted ....because there seems to be people who want argue for no reason

 

I've got the answer I was after. I'll now leave you to argue out any "spin-off" points aMOngst yourselves.

Ta

 

Zanash,

I don't know why you mention optical isomerism, for it has no relevance to the current discussion. It has to do only with covalent compounds of carbon being "left-handed" or "right-handed" in space because of the tetrahedral nature of carbon. So, no, optical isomerism does NOT take place in iron compounds in a magnetic field. I think you've got the wrong term.

Fe3O4 is the double oxide magnetite, and is one case of an iron compound being ferromagnetic. Magnetite is not used in normal printing inks.

Again be careful to make a distinction between the reaction of compounds to a magnetic field and the ability to be magnetised. The NMR technique I mentioned earlier is for detection not of Iron but of hydrogen and the environments in which hydrogen atoms are found. It's a powerful analytical tool for organic (and very non-magnetic) compounds. This is completely different from endowing something with magnetic properties. The technique you mention in the link is of this type. Note that the ferrous atom becomes paramagnetic; this means that the element acts as a magnet for as long as the magnetic field is present. Once the magnetic field is taken away, the element returns to being non-magnetic.

No, I got your point just fine. I'm just saying that there is no basis for the alleged magnetism.

 

Deleted ....because there seems to be people who want argue for no reason

 

I only dismiss them if they're plainly wrong - which this one is. It is completely scientific to dismiss ideas that are plainly wrong.

Sorry, but again rotation in magnetic fields has precisely zilch to do with optical isomerism, or indeed with isomerism of any kind. I handle this stuff all the time (many fragrance molecules are optical isomers) and am surrounded by, and talk all the time with, experts in the field. Our patent applications are full of it. I know what I'm talking about here.

If by "any" you mean a quantity that will have an effect, yes, I am. I naturally can't rule out there being a single atom of iron in there somewhere, but I think you would agree that this would make no difference, the way your ingesting a single molecule of hydrogen cyanide will not kill you - you wouldn't even notice. Unless catalysis has changed drastically since I did polymer science, not even the catalyst residues for the polymers utilise those metals - and even then catalytic amounts are so tiny that they would have no effect.

No hesitation required. There is no magnetic effect. Period.

 

this is wonderful, and so good mannered given that, like the best of ZG argument, so little is at stake. But I secretly worry that someone will enter the fray, claiming to know James Clerk Maxwell personally or that the beneficial/harmful effect of M@n@ is due to the electro-thermal current that is generated by its mass or that some power conditioners are truly worth their salt as they effectively prevent entropy in the molecular structure of the signal in the audible range.

did like the 'lowering of the Atlantic' lol, reminded me of some upgrade paths!

 

Do you work at doctor evil's lair?

 

or CERN research?
I have a mate works there

 

And as tones has pointed out, the hearsay is completely wrong, because it's making claims that are patently untrue. A CD is about as likely to become magnetised as a sugar cube - and even if it were possible to produce a ferromagnetised CD made out of iron or something, there's no particular reason to expect the magnetisation to have any audible effect.

Well, it's true to say that the changes they reported were due to an entirely different effect.

Acknowledging a fact established through evidence, such as "polycarbonate cannot be magnetised", is hardly an "extreme position"; the "unscientific" part would be ignoring anything that doesn't fit with something you'd like to believe to be true. You're implicitly trying to keep open the possibility that an aluminium / polycarbonate disc might somehow pick up some kind of magnetisation, and that moreover this magnetisation might through some undefined means exert an audible effect when the disc is played. This proposal requires overturning an awful lot of what we know about materials science and magnetism, amongst other things.

You can manipulate anything with a strong enough magnetic field, because of the very weak but ubiquitous diamagnetism (there's a famous video of a levitating frog somewhere on the internet). That still doesn't have anything to do with 'geometric forms', optical isomerism or CD players though!

If you think they're applicable to the discussion, bring 'em on and we'll see what we can do.

Edit: and BTW, NMR instruments cost several hundred thousand pounds, the preamp sections handle signals at much lower levels than you find in audio applications, and the limiting factor when using them is generally the intrinsic noise floor of the electronics. Strangely, however, nobody uses special support platforms, interconnects, exotic mains filtering or anything of that kind; you can get "cryoprobe" instruments, where the thermal noise is suppressed through liquid helium cooling, but they're about twice as expensive again, and even then the noise floor is still a problem. A possible unexploited market for accessory manufacturers? Of course, for a scientific application you'd have to prove it worked, but hey, that'd be easy, right?