Re: [Harp-L] Steel Reeds & Magnetic Harpracks

[Michelle LeFree <mlefree@xxxxxxxxxxxxxxxxxxxxx>]

Jerry Deall wrote:

> Max,
> First you can't block a magnetic field only redirect it. See the website:
> http://www.lessemf.com/mag-shld.html#274
>
> Also, as you use the magnetic mount, sliding your harp across the magnet, you are effectively magnetizing you cover plates which will make matters worse

Jerry, I'm afraid I'm going to have to disagree with you here. 
Electromagnetic (EM) fields can indeed be blocked. And, you don't need 
to slide the harp on the magnet, you just put it place and leave it.

Without going into vector calculus, Maxwell's equations and Faraday's 
law, blocking electromagnetic fields happens all the time. It's an 
effect that is part of nature's laws. When an EM field interacts with a 
metal object, its energy is dissipated in the form of electrical surface 
currents flowing onto the metal object.

A common example of blocking EM fields is with MRI medical imaging 
machines in hospitals. One of the reasons MRI scans are so costly is the 
expenses associated with preparing the site before the MRI system is 
even installed. In addition to its structural integrity, an MRI room 
must be completely shielded from external EM waves. X-rays, cosmic 
waves, radio interference all have negative impacts on the ability of 
the MRI machine to do its job. The MRI system uses a cylindrical array 
of highly sensitive EM wave detectors to detect the EM waves it 
produces. So the entire MRI room must be completely lined with a special 
metal foil that has high magnetic permeability, represented in Faraday's 
equations as the Greek Mu. Enclosing a space with metal is called 
Faraday shielding. The special metal used is called Mu metal.

It is very expensive to line a large room in its entirety with Mu 
metalfoil, but small sheets of it aren't terribly costly. You can get an 
8"x12" sheet from Amazon for $45.

http://www.amazon.com/MuMETAL%C2%AE-Magnetic-Shielding-Thick-Sheet/dp/B017ABLYRO/ref=sr_1_1?ie=UTF8&qid=1458826670&sr=8-1&keywords=mumetal

However...

Notice I mentioned "completely lined." I'm afraid that simply lining the 
inside of the cover plates won't have the end result you seek. Unless 
the comb is metal, which is only available from Seydel as its 
aluminum-combed Noble harmonica or from the after market (Blue Moon), 
the magnetic fields produced by the rack will not be dissipated by the 
comb material itself. Geometry is in your favor though, since the magnet 
is in full contact with the opposite side of the cover plate and doesn't 
extend beyond its edge. The degree to which such a lining might be 
effective I don't know but if you have a whole set of harps what give 
you problems it might be worth a try. It wouldn't cost a fortune and 
because it is only 1/100 of an inch thick so I doubt it would have too 
much of an effect on tone.

I'd give it a try, Max. If you do, please let us know how it works.

Michelle