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Magnet Frequently Asked Questions


FAQ

Answers to many Frequently Asked Questions on magnets and magnetism.

For a more technical treatment of these issues, please see our Magnetics 101.

The Basics

Magnet Power

Measuring Strength

Magnetic Poles

Magnet Materials

Magnetic Properties

Magnetic Assemblies

Magnets and Temperature

Machining Magnets

Handling Magnets

Other Questions

[ top ] Brief History

The ancient Greeks and Chinese discovered that certain rare stones, called lodestones, were naturally magnetized. These stones could attract small pieces of iron in a magical way, and were found to always point in the same direction when allowed to swing freely, suspended by a piece of string, or floating on water. Early navigators used these magnets for the first compass to help them determine their direction while at sea.

The name MAGNET comes from Magnesia, a district in Thessaly, Greece where it is believed that the first lodestone was mined.

Over the years magnets have evolved into the high strength materials we have today. It was discovered that by creating alloys of various materials we could create similar effects to those found in the lodestone rocks, and increase the level of magnetism. It was not until the 18th century that the first man-made magnets began to be created, and progress in creating stronger magnetic alloys was very slow until the 1920s when Alnico (an alloy of nickel, aluminum and cobalt) was formulated. Ferrites were created in the 1950s and the Rare Earths in the 1970s. Since then, the science of magnetism has exploded exponentially and extremely powerful magnetic materials have made possible miniature and powerful devices.

[ top ] What is a magnet?

Magnets can be made by placing a magnetic material such as iron or steel, in a strong magnetic field. Permanent, temporary and electromagnets can be made in this manner.

The atoms forming materials that can be easily magnetized such as iron, steel, nickel, and cobalt are arranged in small units, called domains. Each domain, although microscopic in size, contains millions of billions of atoms and each domain acts like a small magnet. If a magnetic material is placed in a strong magnetic field, the individual domains, which normally point in all directions, gradually swing around into the direction of the field. They also take over neighboring domains. When most of the domains are aligned in the field, the material becomes a magnet.

Before magnetization


After magnetization

[ top ] What does a magnet do?

Magnets do the following things:

  • Attract certain materials - such as iron, nickel, cobalt, certain steels and other alloys;

  • Exert an attractive or repulsive force on other magnets (opposite poles attract, like poles repel);

  • Have an effect on electrical conductors when the magnet and conductor are moving in relation to each other;

  • Have an effect on the path taken by electrically charged particles traveling in free space.

Based on these effects, magnets transform energy from one form to another, without any permanent loss of their own energy. Examples of magnet functions are:

  1. Mechanical to mechanical - such as attraction and repulsion.
  2. Mechanical to electrical - such as generators and microphones.
  3. Electrical to mechanical - such as motors, loudspeakers, charged particle deflection.
  4. Mechanical to heat - such as eddy current and hysteresis torque devices.
  5. Special effects - such as magneto-resistance, Hall effect devices, and magnetic resonance.

[ top ] What are permanent magnets made of?

Modern permanent magnets are made of special alloys that have been found through research to create increasingly better magnets. The most common families of magnet materials today are ones made out of Aluminum-Nickel-Cobalt (Alnicos), Strontium-Iron (Ferrites, also known as Ceramics), Neodymium-Iron-Boron (Neo magnets, sometimes referred to as "super magnets"), and Samarium-Cobalt. (The Samarium-Cobalt and Neodymium-Iron-Boron families are collectively known as the Rare Earths.)

[ top ] How are magnets made?

Modern magnet materials are made through casting, pressing and sintering, compression bonding, injection molding, extruding, or calendaring processes.

[ top ] What is a temporary magnet?

Soft iron and certain iron alloys, such as Permalloy (a mixture of iron and nickel) can be very easily magnetized, even in a weak field. As soon as the field is removed, however, the magnetism is lost. These materials make excellent temporary magnets that are used in telephones and electric motors for example.

[ top ] What are electromagnets?

Electromagnets are used when really strong magnets are required. Electromagnets are produced by placing a metal core (usually an iron alloy) inside a coil of wire carrying an electric current. The electricity in the coil produces a magnetic field. Its strength depends on the strength of the electric current and the number of coils of wire. Its polarity depends on the direction of the current flow. While the current flows, the core behaves like a magnet, but as soon as the current stops, the magnetic properties are lost. Electric motors, televisions, maglev trains, telephones, computers and many other modern devices use electromagnets.

[ top ] What are superconductors?

These are the strongest magnets. They don't need a metal core at all, but are made of coils of wire made from special metal alloys which become superconductors when cooled to very low temperatures.

[ top ] How permanent is a magnet's strength?

If a magnet is stored away from power lines, other magnets, high temperatures, and other factors that adversely affect the magnet, it will retain its magnetism essentially forever.

[ top ] Will magnets lose their power over time?

Modern magnet materials do lose a very small fraction of their magnetism over time. For Samarium Cobalt materials, for example, this has been shown to be less that 1% over a period of ten years.

[ top ] What might affect a magnet's strength?

The factors can affect a magnet's strength:

  • Heat
  • Radiation
  • Strong electrical currents in close proximity to the magnet
  • Other magnets in close proximity to the magnet
  • (Neo magnets will corrode in high humidity environments unless they have a protective coating.)

Shock and vibration do not affect modern magnet materials, unless sufficient to physically damage the material.

[ top ] How does a magnet's strength drop off over distance?

The strength of a magnetic field drops off roughly exponentially over distance.

Here is an example of how the field (measured in Gauss) drops off with distance for a Samarium Cobalt Grade 18 disc magnet which is 1" in diameter and ½" long.

field at distance
Distance, x
Field at
Distance x
0.063
2,690
0.125
2,320
0.188
1,970
0.250
1,660
0.313
1,390
0.375
1,160
0.438
970
0.500
810
0.563
680
0.625
580
0.688
490
0.750
420
0.813
360
0.875
310
0.938
270
1.000
240

[ top ] What is the governing equation for field strength relative to distance?

For a circular magnet with a radius of R and Length L, the field Bx at the centerline of the magnet a distance X from the surface can be calculated by the following formula (where Br is the Residual Induction of the material):

There are additional formulae that can be used to calculate the field from a rectangular magnet and magnets in other configurations. Use our On-Line Calculators

[ top ] Can a magnet that has lost its magnetism be re-magnetized?

Provided that the material has not been damaged by extreme heat, the magnet can be re-magnetized back to its original strength.

[ top ] Can I make a magnet that I already have any stronger?

Once a magnet is fully magnetized, it cannot be made any stronger - it is "saturated". In that sense, magnets are like buckets of water: once they are full, they can't get any "fuller".

[ top ] How do you measure the strength or power of a magnet?

Most commonly, Gaussmeters, Magnetometers, or Pull-Testers are used to measure the strength of a magnet. Gaussmeters measure the strength in Gauss, Magnetometers measure in Gauss or arbitrary units (so its easy to compare one magnet to another), and Pull-Testers can measure pull in pounds, kilograms, or other force units. Special Gaussmeters can cost several thousands of dollars. We stock several types of Gaussmeters that cost between $400 and $1,500 each. Helmholtz Coils, search coils and permeameters are also used to make sophisticated measurements of magnets.

[ top ] If I have a Neo magnet with a Br of 12,300 Gauss, should I be able to measure 12,300 Gauss on its surface?

No. The Br value is measured under closed circuit conditions. A closed circuit magnet is not of much use. In practice, you will measure a field that is less than 12,300 Gauss close to the surface of the magnet. The actual measurement will depend on whether the magnet has any steel attached to it, how far away from the surface you make the measurement, and the size of the magnet (assuming that the measurement is being made at room temperature). For example, a 1" diameter Grade 35 Neo magnet that is ¼" long, will measure approximately 2,500 Gauss 1/16" away from the surface, and 2,200 Gauss 1/8" away from the surface.

[ top ]What is the strength of the Earth's magnetic field?

The surface field strength of the Earth is about 0.5 gauss, but it varies by as much as 10% depending on the strength of the crustal field. A range from 0.85 to 0.45 can be found across the globe. Geomagnetic storms can cause changes of between 1% to 5% that last from hours to a day or so.

[ top ]What are Magnetic Poles?

Magnetic Poles are the surfaces from which the invisible lines of magnetic flux emanate and connect on return to the magnet.

[ top ] What are the standard industry definitions of "North" and "South" Pole?

The North Pole is defined as the pole of a magnet that, when free to rotate, seeks the North Pole of the Earth. In other words, the North Pole of a magnet seeks the North Pole of the Earth. Similarly, the South Pole of a magnet seeks the South Pole of the Earth.

[ top ] Can a particular pole be identified?

Yes, the North or South Pole of a magnet can be marked if specified.

[ top ] How can you tell which is the North Pole if it is not marked?

You can't tell by looking. You can tell by placing a compass close to the magnet. The end of the needle that normally points toward the North Pole of the Earth would point to the South Pole of the magnet.

[ top ] How do lines of magnetic flux behave?

Lines of force are three-dimensional, surrounding a bar magnet on all sides.

Like poles repel and unlike poles attract. When opposite poles of a magnet are brought together, the lines of force join up and the magnets pull together.

When like poles of a magnet are brought together, the lines of force push away from each other and the magnets repel each other.

[ top ] What are the different types of magnets available?

There are 3 types of magnets: permanent magnets, temporary magnets, and electro-magnets.

Permanent magnets emit a magnetic field without the need for any external source of magnetism or electrical power. Temporary magnets behave as magnets while attached to or close to something that emits a magnetic field, but lose this characteristic when the source of the magnetic field is removed. Electro-magnets require electricity in order to behave as a magnet.

There are various different types of permanent magnet materials, each with their own unique characteristics. Each different material has a family of grades that have properties slightly different from each other, though based on the same composition.

[ top ] What are Rare Earth Magnets?

Rare Earth magnets are magnets that are made out of the Rare Earth group of elements. The most common Rare Earth magnets are the Neodymium-Iron-Boron and Samarium Cobalt types.

[ top ] Which are the strongest magnets?

The most powerful magnets available today are the Rare Earths types. Of the Rare Earths, Neodymium-Iron-Boron types are the strongest. However, at elevated temperatures (of approximately 150°C and above), the Samarium Cobalt types can be stronger than the Neodymium-Iron-Boron types (depending on the magnetic circuit).

[ top ] What does "orientation direction" mean?

Most modern magnet materials have a "grain" in that they can be magnetized for maximum effect only through one direction. This is the "orientation direction", also known as the "easy axis", or "axis".

Un-oriented magnets (also known as "Isotropic magnets") are much weaker than oriented magnets, and can be magnetized in any direction. Oriented magnets (also known as "Anisotropic magnets") are not the same in every direction - they have a preferred direction in which they should be magnetized.

[ top ]How are magnets rated?

Magnets are characterized by three main characteristics. These are known as the:

  1. Residual Induction (given the symbol Br, and measured in Gauss). This is an indication of how strong the magnet is capable of being.
  2. Coercive Force (given the symbol Hc, and measured in Oersteds). This is an indication of how difficult it is to demagnetize the magnet.
  3. Maximum Energy Product (given the symbol BHmax, and measured in Gauss-Oersteds). This is an indication of what volume of magnet material is required to project a given level of magnetic flux.

[ top ] What are the properties of commonly used magnet materials?

Here are the three important properties that characterize magnets for some of the most common magnet materials used today.

Material
Br
Hc
BHmax
NdFeB 42
13,050
12,500
42.0
NdFeB 35
12,300
11,300
35.0
SmCo 26
10,500
9,200
26.0
Alnico 5
12,500
640
5.5
Ceramic 5
3,950
2,400
3.6
Flexible
1,725
1,325
0.6

Table of Magnetic Properties

[ top ]How can I use this information?

Given a magnet size, you can estimate how much magnetic flux different materials will project at a given distance or you can use this information to compare one material to another.

Examples:

How much more flux will a Neo 35 project as compared to a Ceramic 5 of the same dimension at a given distance? Simply divide the Br of Neo 35 by the Br of Ceramic 5 (12300/3950) to get 3.1. This means that the Neo 35 would give you 3.1 times the flux a Ceramic 5 the same size would at a given distance.

Given a certain flux required at some fixed distance from the magnet, you can use this information to estimate what magnet volume will be required for different magnet materials.

For example, what volume of Ceramic 5 magnet would give the same flux as a Neo 35 magnet at a given distance? Simply divide the BHmax of Neo 35 by the BHmax of Ceramic 5 (35/3.6) to get 9.7. This means that the volume of the Ceramic 5 magnet would have to be 9.7 times that of the Neo 35 magnet to give you the same flux.

[ top ] What is a magnetic assembly?

A magnet assembly consists of one or more magnets, and other components, such as steel, that generally affect the functioning of the magnet.

[ top ] How should I assemble magnets to my device?

If a magnet needs to be fastened to a device, you can use either mechanical means, or adhesives to secure the magnet in place.

Adhesives are often used to secure magnets in place. If magnets are being adhered to uneven surfaces, you will need an adhesive with plenty of "body" so that it will conform to the uneven surface. Hot glues have been found to work well for adhering magnets to ceramics, wood, cloth, and other materials. For magnets being adhered to metal, "super-glues" can be used very effectively. (Click here for our Resource Desk and links to adhesives often used in assembling magnets.)

We can supply Flexible magnets with an adhesive already attached to the magnet: all you need to do is to peel off the liner and attach to your product.

As with all adhesive applications, it is very important to ensure that all surfaces being bonded are clean and dry before bonding.

[ top ] What are the maximum recommended operating temperatures for different magnet materials?

Material
Approximate Maximum
Operating Temperatures
° C
° F
NdFeB
140
280
SmCo
300
570
Alnico
540
1000
Ceramic
300
570
Flexible
100
210

The maximum temperature that a magnet may be effectively used at depends greatly on the "permeance coefficient" or Pc - which is a function of the magnetic circuit the magnet is operating in. The higher the Pc (the more "closed" the circuit), the higher temperature at which the magnet may operate at, without becoming severely demagnetized. Shown here are approximate maximum operating temperatures for the various classes of magnet material. At temperatures close to those listed here, special attention may be needed in order to ensure that the magnet will not become demagnetized.

[ top ] Why is the maximum temperature a magnet can operate at not a set value?

Magnets function at different levels of efficiency given different circuits that they operate in. The more closed the circuit the magnet is operating in, the more stable it is, and the less effect temperature will have on it.

[ top ] Can I machine magnets?

Magnets can be machined. However, hard magnet materials - as opposed to the flexible or rubber type magnet materials - are extremely difficult to machine. Magnets should be machined using diamond tools or soft grinding wheels, and in the unmagnetized state as far as possible. In general, it is best not to try to machine hard magnet materials unless you are familiar with these specialized machining techniques.

[ top ] How much does it cost to machine magnets?

The factors which determine cost to machine magnets are:

  • Quantity - the larger the quantity, the lower the cost since set-up charges must be amortized over the quantity, and special tooling can be created to machine larger quantities;

  • Material - SmCo materials are more costly to machine since they are very brittle, flexible materials are very inexpensive to machine because of their physical characteristics;

  • Shape - complex shapes are more expensive than simple shapes; and,

  • Tolerances - the closer the required tolerances, the more expensive it will be to machine the magnets.

[ top ] What can I use to block a magnetic field?

Only materials that are attracted to a magnet can "block" a magnetic field. Depending on how thick the blocking piece is, it will partially or completely block the magnetic field.

[ top ] Tips on handling and storing magnets

  • Always take care! Magnets can snap together and injure personnel or damage themselves.

  • Keep magnets away from magnetic media - such as floppy discs and credit cards - and computer monitors.

  • Store magnets in closed containers, so that they do attract metal debris.

  • If several magnets are being stored, they should be stored in attracting positions.

  • Alnico magnets should be stored with "keepers" (iron or magnetic steel plates that connect the poles of the magnet) since they can easily become demagnetized.

  • Magnets should be kept away from pacemakers!

[ top ] What are eddy currents?

These are electrical currents that are induced when a magnetic field moves in relation to an electrical conductor, which is placed within reach of the magnetic field. In turn, these eddy currents create a magnetic field that acts to stop the relative motion of the original magnetic field and electrical conductor.

[ top ] What are some good magnetic reference books?

Permanent Magnet Design handbook, by Lester Moskowitz, a 385-page book aimed at the technical layperson, price approximately $150.

Permanent Magnets and their Applications, by Dr. Peter Campbell, a 203-page book aimed at the technical person, price approximately $40.

The Driving Force, by James Livingston, a 310-page book aimed at the non-technical reader - a very well written and interesting book on the history of magnets and some of their more exotic applications. Price approximately $20.

[ top ] What do magnets cost?

The costs of different magnet materials vary significantly from one to the other. Here is an approximate guide as to what magnets cost.

Material
BHmax
Relative Cost
($ / Pound)
Relative Cost
($ / BHmax)
NdFeB
40
35
1.7
SmCo
26
60
4.9
Alnico
5
25
9.5
Ceramic
3
2
0.9
Flexible
1
1
1.0

* Note: the costs shown here are relative costs based on high volumes of magnet materials that have no special machining or other characteristics.

On a cost-per-pound basis Neodymium magnets seem very costly. However, on a cost per BHmax basis, they do not seem so costly. Often by using a more powerful magnet, the entire device that the magnet goes into can be miniaturized, yielding cost savings that favor the more powerful magnet materials.

[ top ] Are there Industry Standards for Magnets?

Yes. Standards were published by the Magnetic Materials Producers Association (MMPA) and the Magnet Distributors and Fabrications Association (MDFA). (Both of these associations are now a part of the International Magnetics Association.) Some of the publications and standards produced by them are reproduced here for your convenience.

[ top ] How do I order magnets?

To efficiently order magnets, you need to have a good idea of what you want to accomplish. Here are a few items that you will need to consider:

  • General nature of application - Holding, moving, lifting, etc.
  • Shape of magnet desired - Disc, Ring, Rectangle, etc.
  • Size of magnet desired - Diameter, length, width, height, etc.
  • Tolerances - what variation in dimensions is allowed.
  • Conditions magnet will be used in - Elevated temperature, humidity, outside, inside, etc.
  • Strength of magnet required - In pounds of holding force, Gauss, etc.
  • Magnet should cost no more than? - This will eliminate certain materials from consideration.
  • Quantities you will need.

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