Magnet Basics FAQs
- 1.0 History
- 2.0 Magnet Basics
- 3.0 Magnetic Strength
- 4.0 Blocking Magnetic Fields
- 5.0 Magnetic Poles
- 6.0 Magnetic Flux
- 7.0 Magnetic Orientation
- 8.0 Temporary Magnets
- 9.0 Electromagnets
- 10.0 Magnetic Assemblies
- 11.0 Machining Magnets
- 12.0 Handling & Storage
- 13.0 Costs & Ordering Tips
The ancient Greeks and Chinese discovered that certain rare stones possessed mysterious and attractive properties. 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 these first "lodestones" were mined.
Over the millennia 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 lodestones, and increase the level of magnetism. It was not until the 18th century that the first man-made magnets were 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 (also known as Ceramics) 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 the myriad devices that we have today.
Certain materials, such as iron or steel, can be made magnetic by placing them in a strong magnetic field. Permanent and temporary magnets 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, swing around into the direction of the magnetizing field. When most of the domains are aligned in the field, the material becomes a magnet.
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:
- Mechanical to mechanical - such as attraction and repulsion.
- Mechanical to electrical - such as generators and microphones.
- Electrical to mechanical - such as motors, loudspeakers, charged particle deflection.
- Mechanical to heat - such as eddy current and hysteresis torque devices.
- Special effects - such as magneto-resistance, Hall effect devices, and magnetic resonance.
Based Modern magnet materials are made through casting, pressing and sintering, compression bonding, injection molding, extruding, or calendaring processes. Once manufactured, magnets often need to be further processed by grinding or other machining processes, and then assembled into a next level assembly.
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.
Modern permanent magnets are made of special alloys that have been found through research to create increasingly better magnets. Permanent magnets have a magnetic field that does not turn on and off like electromagnets. 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 Rare Earths.
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.
The surface field strength of the Earth is about 0.75 gauss, but it varies by as much as 10% depending on the strength of the "crustal field". A range from 0.85 to 0.60 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.
Will magnets lose their power over time?
Modern magnet materials 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. Otherwise, permanent magnets are just that permanent (you have to do some work to actively try and de-magnetize them).
Can a magnet that has lost its magnetism be re-magnetized?
Provided that the material has not been damaged by extreme heat, magnets can be re-magnetized back to original strength, if they have been exposed to conditions which cause them to become demagnetized.
How do you measure the strength 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 it's 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.
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 Neodymium Disc Magnet that is 1/4" long, will measure approximately 2,500 Gauss 1/16" away from the surface, and 2,200 Gauss 1/8" away from the surface.
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".
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 1/2" long.
What Is The Equation For Field Strength Relative To Distance?
For a circular magnet with a radius of R and Length L, the field at the center line 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, but the formulae get too long and complex looking to include here!
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.
Magnetic Poles are the surfaces from which the invisible lines of magnetic flux emanate and connect on return to the magnet.
What are the standard industry definitions of "North" and "South" Poles?
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.
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.
The general answer is "Predictably"!
Lines of force are three dimensional, surrounding a bar magnet on all sides.
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.
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.
Soft iron and certain iron alloys 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, for example, in telephones and electric motors.
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, which is conducted through the iron core. 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.
A magnet assembly consists of one or more magnets, and other components, such as steel, that generally affect the functioning of the magnet.
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.
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.
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 un-magnetized 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.
- 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 not 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!
The costs of different magnet materials vary significantly from one to the other. Here is an approximate guide as to what magnets cost.
* Note: the costs shown here are relative costs based on high volumes of magnet materials that have no special machining or other characteristics.
|Material||BHmax (MGOe)||Relative Cost|
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.
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
Contact Us today for answers to all of your custom magnet questions!