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Magnet Facts and Measurements

  • Magnet Strength Measurements (B)--The units for measuring the field strength (flux density) of a magnet are Gauss or Tesla. 1 Tesla = 10,000 Gauss. The Earths magnetic field is on the order of 1 Gauss. There are different ways to classify and measure field strength:
    • 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 magnets 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.
    • Br (residual flux density): The maximum flux a magnet can produce, measured only in a closed magnetic circuit. Our figures for each magnet are provided to us by the magnet manufacturer. They are a good way to compare magnet strength...but keep in mind that a magnet in a closed magnetic circuit is not doing any good for anything except test measurements.
    • B-H Curve: Also called a hysteresis loop, this graph shows how a magnetic material performs as it is brought to saturation, demagnetized, saturated in the opposite direction, then demagnetized again by an external field. The second quadrant of the graph is the most important in actual use--the point where the curve crosses the B axis is Br, and the point where it crosses the H axis is Hc (see below). The product of Br and Hc is BHmax. If we have these measurements available, they are provided to us by the magnet manufacturer--very complicated and expensive equipment is needed to plot a B-H curve.
      A sample B-H curve:
  • Magnet Quality (BHmax): The quality of magnetic materials is best stated by the Maximum Energy Product (BHmax), measured in MegaGauss Oersted (MGOe). This is because the size and shape of a magnet and the material behind it (such as iron) have a large effect on the measured field strength at the surface, as does the exact location at which it measured. All of our Nickel-plated NdFeB magnets are grade N35 (BHmax=35 MGOe) and all of our Gold-plated NdFeB magnets are grade N45 (BHmax=45 MGOe). This gives about a 5% difference in strength, and a 150% difference in is wise to balance your magnet strength needs by cost too. Other magnets are measured the same way -- a grade 8 ferrite magnet (grade C8) has BHmax=8 MGOe.
  • Coercivity (Hc): This measures a magnets resistance to demagnetization. It is the external magnetic field strength required to magnetize, de-magnetize or re-magnetize a material, also measured in Gauss or Tesla.
  • Curie Temperature (Tc): This is the temperature at which a magnet material loses its strength, permanently. Another useful number (if available) is Tmax, the recommended maximum operating temperature. Above Tmax (around 266 deg. F for most NdFeB magnets) a magnet will start ot lose its power, and at Tc all power is lost.
  • Corrosion: NdFeB magnets are susceptible to corrosion. The Fe in the name stands for Iron, and it rusts! Many of our magnets come with a Nickel, Zinc, Gold or Epoxy coating to protect them from moisture. If the coating is damaged (frequently the case with our surplus magnets) the magnet could rust if exposed to water or humidity. If this is a concern to you, you can easily add another layer of prtection by dipping the magnet in epoxy or plastic coating.
  • Drilling and Machining: Neodymium magnets are by nature very hard and brittle. Although they can be cut, drilled and machined, it should ONLY be done by folks who are experienced with ceramics. If the magnets get over about 260 deg F, they will lose their magnetism permanently. They are flammable, and it is not difficult while grinding or machining to get them (or the chips and dusts from cutting) so hot that they ignite. If they do ignite, the fumes are toxic and the material burns very fast and hot, like Magnesium! In our experience any machining of these magnets should be done with diamond tools under lots of coolant with good ventilation and the risk of fire in mind.
  • Patents: NdFeB magnets are a relatively new invention--they first became commercially available in 1984. All NdFeB magnets are currently protected under international patent. All of our magnets are either surplus from various industries, or new, legal, licensed material.
  • Manufacturing: NdFeB magnets are complicated to manufacture. The powdered NdFeB material is packed in molds, then sintered. The non-magnetized magnets are then shaped to the correct size and plated. To magnetize them, they are placed in a very expensive machine that generates an extremely high-powered magnetic field for an instant, using high-voltage capacitor discharge and coils. The polarity of the finished magnet depends on how it was oriented in the magnetizing machine, and how the particles in the sintered mixture were oriented.

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01.NdFeB Disc Magnet, 3/8 in. x 1/8 in.
02.NdFeB Disc, 3/8 in. x 1/16 in.
03.NdFeB Disc, 1/8 in. x 1/16 in., Au
04.NdFeB Disc, 1/4 in. x 1/8 in.
05.NdFeB Disc, 1/4 in. x 1/16 in.
06.NdFeB Rod magnet, 1/4 in. x 1/4 in.
07.NdFeB Disc, 3/16 in. x 1/16 in.
08.NdFeB Disc, 1/2 in. x 1/8 in.
09.Ferrite disc, .7 in. x .2 in.
10.NdFeB Rod, 1/4 in. x 1/2 in.
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