# Stainless Steel increases B?

1. Nov 5, 2009

### Gauss20000

Why does the magnetic field increase significantly when stainless steel is placed behind a magnet?

"B (flux density): This is the measurement (in Gauss or Tesla) you get when you use a gaussmeter at the surface of a magnet. The reading is completely dependant on the distance from the surface, the shape of the magnet, the exact location measured, the thickness of the probe and of the magnet's plating. Steel behind a magnet will increase the measured 'B' significantly. Not a very good way to compare magnets, since B varies so much depending on measurement techniques."

2. Nov 6, 2009

### Bob S

There are two types of stainless steel; austenetic (300 series, non-magnetic) and martensitic (400 series, magnetic). Putting martensitic stainless steel (or any ferromagnetic material) behind a pole will reduce the reluctance of the magnetic circuit, and increase the magnetic field.

Bob S

3. Nov 6, 2009

### Gauss20000

I'm curious as to how and why the magnetic flux increases for a magnet when you place martensitic stainless steel (or ferromagnetic material) behind one of its poles. I am building a form of pyramid out of copper and am placing magnets in the apexes. It is my understanding that if I put ferromagnetic stainless steel on the outside of the apexes, this will increase the magnetic field of the magnets.

What would be stronger - magnet with a layer of copper and then the stainless steel; or the magnet with the stainless steel directly behind it so the magnet is stuck to the stainless steel?

And secondly, the increase in magnetic flux will be limited to the size of the stainless steel. So the larger the sheet of stainless steel the greater the increase in flux. Is this right?

Does this mean when I stick a simple magnet to my fridge, the magnetic flux of the magnet is immediately increased?

4. Nov 6, 2009

### Bob S

The magnetic steel will either shield or increase the magnetic field, depending on where the measurement is being made.

See discussion of reluctance and stored energy here:
http://www.femm.info/wiki/Analogies
See definition of reluctance here:
http://en.wikipedia.org/wiki/Magnetic_reluctance
If you have a permanent magnet (horseshoe magnet) with a well-defined air gap, and you bring up a piece of steel to it,you reduce the air gap, decrease the reluctance, and decrease the stored energy. This increases the B field in the remaining gap.
There often is no direct relation to the size of piece of steel.

If the magnet is pulled to the fridge, the B field close of the magnet is increased.
Bob S

Last edited: Nov 6, 2009
5. Dec 27, 2009

### keith03

I would like to know a couple of formulas (and simple explanation) of this topic. I am about to have some steel work done, but I need to squeeze a little more B from my magnets. If I want to increase the B of a circular 1"dia 1/8" thick N48 magnet, I would expect to place magnetic steel behind the magnet. I would expect to shield the B on the side I placed the steel, and increase the stregnth of B on the opposite side. How do I know how thick the steel needs to be without being saturated? What kind of increase can I expect?

Also, I know nothing of steel numbers or grades. When ordering, what is the cheapest and also most effective steel for increasing magnet stregnth? This will be a custom job, and I need to know the lingo....

6. Dec 28, 2009

### Bob S

The first question is easily answered. The area of the N48 magnet is pi R2, where R = 0.5". because the B field is continuous, the steel plate must carry the same flux (B-field times area) as the magnet itself. Soft iron saturates about at 1.2 Tesla, similar to the field from N48. At a radius R = 0.5" on the steel, the circumference times the thickness (2 pi R t) should be the same area as the magnet itself, because the flux in the steel is entirely radial. So we get

2 pi R t = pi R2, or t= R/2 = 1/4" thick.

Look at attached thumbnail of iron and stainless permeabilities. Pure iron followed by 1018 steel are much better than the two 400-series stainless steels shown.

Bob S

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