Identifying aluminum by paramagnetic properties

All this info is based on my experiences using a neodymium block magnet. I have not tried it with lesser strength magnets.

A strong magnet can help to identify silver based on its diamagnetic properties. Moving a strong magnet very closely over the surface of a silver dollar will cause it to shift slightly. It's most noticeable if you hold the silver dollar in your hand so you can feel the slight interaction. I have used this to buy a silver medal at a flea market (for much less than its real value, I've got to brag.) The effect is slight, but has been used to my advantage at least once.

I assumed this was unique to silver, but it turns out the magnetic response of non-ferrous metals is not as simple as my ignorance would imply.

For some reason, while recently sorting aluminum heat sinks I passed a magnet very closely over the thicker solid surface of the aluminum and was surprised at the strong interaction. It is noticeable with both cast and extruded.

Not really sure how to use this information yet, so I'm just throwing it out there for ideas. One area I think I might be able to use it is to identify brass vs. colored cast hardware. Flea market and yard sellers usually don't like their items hit with a grinder before I buy their stuff. Although I have a lot of experimenting to do, it seems that cast aluminum (paramagnetic) interacts more strongly than brass (copper and zinc are both diamagnetic) to the magnet. I need to dig through my brass box and test different sized pieces. It's immediately obvious that thicker pieces of metal interact more strongly than thinner pieces. I suspect the presence of ferrous metals would completely overwhelm the ability to detect the paramagnetic interactions.

Anybody else ever used this? If nothing else, I have a new cool thing to keep the grandkids busy for 3 minutes while working in the shop.




Just a bit more info:
Magnetic Type for all the elements in the Periodic Table

I think what you are detecting is the interaction when generating eddy currents in the metal by moving the magnet past it. Since this is electricity generation, it takes energy to do it, and you are detecting the force required as you pass your magnet over the metal. It should work with any metal. The interaction will be greater with stronger magnets, a closer pass to the metal with the magnet, and higher speed of the pass.

Unfortunately, it works with any conductive metal.

I still remember a college course demonstration of this. The had a pendulum made of aluminium sheet and a magnet at the bottom of the swing. The instructor pulled the pendulum up on one side of its swing and let it swing down past the magnet. It would stop dead once it hit the magnetic field.



Jon.

My education was a bit deficient in electromagnetism, but Wikipedia has a good entries for diamagnetic and paramagnetic. It seems to me that the paramagnetic interaction is more noticeable than the diamagnetic interaction. I like the pendulum demo idea. I may just have to add this to my Hmmmmm file for some point in the future.