Instructables

How do I know the grade of a magnet?

I salvaged two identical neodymium magnets from an old CD rom drive. They're really strong and I suspect it to be neodymium, but is there any way to ascertain the "grade" of this magnet? Neodymium grades go like N35, N33,  etc...

first4magnets11 months ago
Hi!

The only definitive way to determine the grade of a neodymium magnet is to use a hysteresis graph testing machine to test the magnet.

If you had a Gaussmeter, which themselves can be very expensive, you could test a known-grade magnet and then test the Gauss level of your magnet and compare the two values. Depending on the increase/decrease of the known magnet you could make a best guess.

On our Tech Centre we have a page dedicated to neodymium magnet grades which you might find useful!

You might also find our glossary of magnet terms useful.
The grade numbers for neodymium magnets (such as N33, N35, etc.) are exactly equal to the number of sheets of standard weight A4 printer paper (8.5 by 11 inches, approx 5 grams per sheet) that magnet can hold against the vertical side of a steel file cabinent or refrigerator.

For example, if your magnet can pin 35 pages of printed pages to the fridge door, then that means it is a grade N35 magnet.

;-)
While that seems reasonable, is that for a given width/surface area of magnet, or does the weight/grade take that into account?

A flat wide magnet of the same grade as a long narrow magnet would have more holding power for the more surface area in closer proximity to the metal.
I was making a little joke there, but I followed the text of what I wrote with a winking smiley emoticon (semicolon, dash, close parenthesis), so that means I'm not technically lying, or breaking any rules,  by doing that.
;-)

Giving credit where credit is due, the joke is not wholly my own.  I remember there was some electronics surplus monger.  I think it was either Electronic Goldmine or BGMicro.  They were selling some surplus neodymium magnets, and of course they didn't have the exact specs either, but they'd tell you how many copies of the their current printed catalog this magnet could stick to the side of a file cabinet.

But you know, which specification is more useful? Supposing you knew that your old drive magnet was truly a grade N35 magnet, or that the maximum magnitude of its remanent field (Br) at a point on its surface was exactly 1.1 tesla, or maybe even 1.2 tesla at a point inside the magnet, supposing there was any way to get your Hall effect probe inside the solid metal, or infer the magnitude of the field that was in there.  Or suppose you knew how much of a coercive laboratory field Hc (i.e. the field you control) of 10^4 ampere*turns/meter was necessary to drag B back to zero, or even that other number, whatchallit, the point on the B-H curve of of maximum B times H (so called energy product).

Then what would you do with that information?  It's just a bunch of numbers. I mean unless you dream about vector fields and B-H curves, it might just be more impressive to know how many paper catalogs you can stick to the side of a file cabinet.

Then you know, e.g. two catalogs stuck to the side of a file cabinet.  Well, that's a pretty dang strong magnet! 
;-)

Of course, for those who want to know the technical truth of these strong magnets, and how they are graded, I was looking for some good links for this.   The Wikipedia article on "Neodymium Magnet"
http://en.wikipedia.org/wiki/Neodymium_magnet
had a reference to this document:
http://www.china-magnet.net/txing/NdFeB.htm
which is basically just a table of numbers.

And this page,
http://www.kjmagnetics.com/blog.asp?p=magnet-grade
does a decent job of how magnet grades are related to those numbers, i.e. numbers from the magnetic material's B-H curve.  This page also includes a pretty good explanation of what a B-H curve is, under the heading labeled "Appendix: What is a BH Curve?".
great...many more hours about to be sunken into wikipedia. :)

Sometimes a wink is 'and now you know'...now I know I need to know more.
Sorry about that.  Which smiley should I be using? for to say,  "This is a joke.  Please do not take this seriously."

Would the tongue sticking out smiley,

:-P

be a better choice?
much better ;)
Yeah right. I have a bridge for sale you might find interesting.....
There is no way to determine the grade unless they have been marked by the manufacture. If you contact the manufacture of the CD-ROM they may have that information.