Archive for the ‘Magnet information’ Category

Air gap
An ‘air gap’ is non-magnetic material, which is present between a magnet and an attracted object or between two magnets that are attracting each other.
An air gap is best described as a break in the magnetic circuit, which magnetism has to jump through to continue a circuit between north and south poles. The introduction of an air gap weakens the magnetic hold.
An air gap can be air itself or a solid non-ferrous material that does not conduct magnetism such as wood, plastic or aluminium. It could also be a thickness of paint or a surface that is very uneven. Refer to the ‘Pull-gap’ curve entry for a description of how pull strength decreases as the size of an air gap increases.
A magnet is described as anisotropic if all of it’s magnetic domains are aligned in the same direction. This is achieved during the manufacturing process and ensures that the domains are 100% orientated in the same direction to deliver maximum magnetic output. This direction is called the ‘magnetic axis’.
The alignment is achieved by subjecting each magnet to a strong electromagnetic field at a critical point during the manufacturing process, which then ‘locks’ the domains parallel to the applied electromagnetic field.
An anisotropic magnet can only be magnetised in the direction (along its magnetic axis) set during manufacture, attempts to magnetise the magnet in any other direction will result in no magnetism. Anisotropic magnets are much stronger than isotropic magnets, which have randomly orientated magnetic domains producing much less magnetism. However, isotropic magnets have the advantage of being able to be magnetised in any direction.
Axially magnetised
The term axially magnetised describes a magnet that is magnetised between two flat parallel surfaces.
Bar magnet
A bar magnet is exactly as it sounds, it is a permanent magnet which has a magnetic length which is greater than its diameter or effective diameter for rectangular bar magnets. A bar magnet has a north and a south pole typically at opposite ends of the bar

Closed circuit
A closed magnetic circuit describes an arrangement of magnetic and ferrous material which directly connects the north pole of a magnet to the south. In a closed circuit the lines of magnetic flux are allowed to flow freely from north to south and all of the magnetic flux density is retained within the closed circuit. In a closed circuit, there is no external magnetic field as all the magnetism is consumed in the circuit.

The coercivity of a magnetic field is the intensity, or energy, required to reduce the magnetisation of a magnetised (to the point of saturation) object to zero. Essentially, it measures a magnetic material’s resistance to demagnetisation. The coercivity of magnetic material is measured in Oersteds (Oe) – the higher the number, the greater the magnet’s resistance to demagnetisation.

Demagnetisation occurs when a magnet loses its external magnetic field when in open circuit.
This can be through physical stress or corrosion, through heating the magnet beyond its maximum operating temperature or by exposing the material to a strong demagnetising magnetic field.
Generally, neodymium magnets cannot be re-magnetised once their magnetic properties have been lost.

Density is a measurement of a materials mass per unit of volume. All materials have different densities and a magnet’s density can allow you to calculate its weight. The density values for the different types of magnetic material are as follows:
Neodymium magnets have a density of up to 7.5g per cm3
The density of alnico magnets vary depending on the grade from 6.9 to 7.3g per cm3
The density of samarium cobalt magnets vary depending on the grade from 8.2 to 8.4g per cm3
Ferrite magnets have a density of 5g per cm3
Flexible magnets have a density of 3.5g per cm3

Diamagnetism is a kind of magnetism that aligns itself at right-angles to the direction of an objects magnetic field and therefore has a repellent force. All materials are diamagnetic to a certain degree but only when exposed to an externally applied magnetic field. The effect is generally weak in most materials, and is completely overpowered in materials that display other magnetic characteristics. However, the effects of diamagnetism can be enhanced by introducing superconductors into the magnetic circuit.

Diametrically magnetised magnets
Cylindrical magnets are described as diametrically magnetised when their direction of magnetism is parallel to the diameter of the magnet, rather than perpendicular to the flat faces of the cylinder.

Direction of magnetisation
Magnets can be specified and ordered to be magnetised across any axis, allowing them to be used to different effect. The direction of magnetism determines which side of the magnet the north and south poles appear. This has to be specified before manufacture as, for example, an anisotropic rectangular magnet can only be magnetised in one of the three possible directions.

Unlike permanent magnets, the magnetic field exerted by an electromagnet is produced by the flow of electric current. The magnetic field disappears when the current is turned off.
Typically, an electromagnet consists of many turns of copper wire which form a solenoid
When a DC electric current flows around the solenoid coil, a magnetic field is created. If an iron core is inserted into the bore of this solenoid, then magnetism is induced into it and it becomes magnetic, but immediately becomes nonmagnetic when the current stops flowing.
Ferromagnetism is the strongest form of magnetism and is the only form that creates forces so strong that they can be noticed by human hands. A ferromagnetic substance is strongly attracted by a magnet.

Magnetic flux is the number of lines of magnetism travelling from a magnetic pole. The CGS unit of measurement for ‘flux’ is Maxwells and the SI unit is Webers.

Named after the famous German mathematician and physicist Carl Friedrich Gauss, the Gauss is a unit of measurement for magnetic flux density.
1,000 Gauss is 1,000 lines of magnetism in each cm2 of pole area.

Gauss meter
A gauss meter is used to measure the flux density (Gauss) of a magnet. A gauss meter has a hall probe, which when placed onto a magnetic pole will display the number of lines of magnetism within each cm2 of pole area.

There are a number of different types of magnet, neodymium, samarium cobalt, ferrite and alnico, for example.
Each type of magnet is manufactured in a number of different grades. The term grade defines the chemical characteristics of the material and its magnetic properties. Each grade of material, depending upon its core elements and how it is manufactured will have different magnetic properties.

The Gilbert (G) is a unit to quantify magnetomotive force named after William Gilbert who was an English scientist and physician born in 1544 and is credited by many as being the father of electricity and magnetism. An alternative measure for magnetomotive force is ampere-turns (At); the Gilbert (G) is a slightly smaller unit than ampere-turns. To convert from ampere-turns to Gilberts multiply by 1.25664.

Horseshoe magnet
The most recognisable style of magnet, a horseshoe magnet is a permanent magnet, usually made from alnico material. In most cases a horseshoe magnet will have a north pole on one of its tips and the south pole on the other. Horseshoe magnets are typically stronger than bar magnets as their pull is doubled when attached to a piece of ferrous material that spans both its poles, therefore creating a closed circuit.
High field gradient magnet
High field gradient magnets have the highest clamping forces in direct contact with ferrous material, but the weakest pull through larger air gaps.

Irreversible losses
Partial demagnetisation can be caused by exposure to high temperatures, external magnetic fields, shock or vibration. When exposed to certain conditions a magnet will regain any magnetism lost, however, in extreme situations the magnet will lose a percentage of its magnetism that won’t be recovered, known as irreversible losses. An example is exposing a magnet to temperatures exceeding its maximum operating temperature.

A magnet made of magnetically isotropic material has no preferred direction of magnetism and has the same properties along either axis. During manufacture, isotropic material can be manipulated so that the magnetic field is applied in any direction. Neodymium magnets are anisotropic due to their strength, on the other hand, flexible magnets are usually isotropic allowing all of the magnetic field to be exerted from one side of the sheet.
A keeper is a steel bar or disc placed between and attached to opposite poles of a magnet to allow all the magnetism to flow from one pole to the other. The keepered magnet will appear completely non-magnetic until the keeper is removed. Keepers were needed for old alnico magnets to preserve magnetism in these low coercivity magnets. This is useful if magnets need to be airfreighted and stray magnetism needs to be contained. Neodymium, samarium cobalt and ferrite magnets do not need to be keepered to protect their magnetism, however they are sometimes keepered to make them safer to handle.

A material, or magnet is defined as magnetised when it exerts a magnetic field, either because of its interaction with an electromagnet or another permanent magnet.

Magnetomotive force (mmf)
Magnetomotive force is the magnetic field produced by a coil of wire when current is passed through it. The more current that is passed through a solenoid coil and the more coils the solenoid has, the larger the magnetic field produced. A magnetomotive force is expressed in ampere-turns; a value of the amount of applied current multiplied by the number of turns in a solenoid. Alternatively magnetomotive force is sometimes measured in Gilberts.

The term material refers to the physical composition of a magnet. For example, neodymium magnets are made out of a neodymium alloy (NdFeB) material containing neodymium (Nd), iron (Fe) and boron (B).
There are five main types of magnetic material and they are:
Samarium Cobalt
Flexible magnets

Maximum Operating Temperature (Tmax)
The maximum operating temperature is exactly as it sounds, it represents the maximum temperature that a particular grade of magnet will be able to function at, before it becomes permanently demagnetised.
All permanent magnets weaken in relation to their temperature coefficient, but as long as the maximum operating temperature is not exceeded, this is fully recoverable on cooling. If the maximum operating temperature is exceeded, then the losses will not be fully recovered on cooling. Repeatedly heating a magnet above its maximum operating temperature and cooling will significantly demagnetise the magnet.
Neodymium magnets operate best in cold temperatures down to approximately -130oC. Regular neodymium magnets will maintain their magnetism in operating temperatures up to 80oC whereas different variants of neodymium magnets can operate up to temperatures of 230 oC.

Maxwell is a measurement for magnetic flux on the CGS scale where 1 Maxwell is equal to 1 line of flux. The measurement is named after James Clerk Maxwell who was a Scottish theoretical physicist born in 1831. Maxwell’s most high-profile achievement was formulating a set of equations that united electricity, magnets and optics into one consistent theory. Maxwell’s achievements were widely acclaimed as the second great unification in physics after those realised by Isaac Newton.

A neodymium magnet is the most widely used type of magnet, it is permanent magnet. Neodymium magnets are the strongest type of permanent magnet commercially available. They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as motors in cordless tools and magnetic fasteners.

The Oersted (Oe) is a measure for magnetic field strength and is named after the Danish physicist and chemist Hans Christian Oersted. In 1820, Oersted discovered the magnetic effect of electric current, contributing significantly to the study of magnetism. The Oersted is closely related to the Gauss measurement for flux density and is used to measure external electromagnetic forces usually produced in magnetisers and demagnetisers.

Open circuit
A magnet is said to be in open circuit when it is not attached to any other ferrous material, meaning that its lines of magnetic flux make their way from the north pole to the south pole through the air alone, rather than through a ferromagnetic material. Because it is more difficult for lines of magnetic flux to travel through air rather than other parts of a circuit, a magnet produces less Gauss when in open circuit.
A magnet’s orientation refers to the physical location and direction of its magnetic poles, e.g. through length, thickness, diameter, axially, radially or diametrically.

Some materials, when placed inside a magnetic field, become magnetised themselves. The permeability of a magnetic substance represents the increase or decrease of the magnetic field inside the substance compared to the magnetising field that the substance is located within. Simply put, it is the ability for a material to acquire its own magnetism or for magnetism to flow through it.
Ferromagnetic metals have the greatest permeability of all substances and will become magnetised when exposed to a magnetic field. The rate of magnetic permeability will increase until the substance reaches a point of saturation. ‘Soft’ ferromagnetic materials are easily magnetised, but once the external field is removed they lose most of their magnetism. Conversely, ‘hard’ ferromagnetic materials are difficult to magnetised, but once they are, they will remain magnetised.

Permanent magnet
A permanent magnet is a solid material that produces its own consistent magnetic field because the material is magnetised. A permanent magnet is different to an electromagnet as an electromagnet only acts as a magnet when an electric current passes through its coils.

All magnets have both a north and a south pole, usually 180 degrees apart. Polarity refers to a magnet’s magnetic orientation with regards to its poles. Opposite poles attract each other but similar poles repel.

The pole of a magnet is the area of a magnet which has the greatest magnetic field strength in a given direction. Each pole is either north facing or south facing.
Pull-gap curve
A pull-gap curve plots the ‘pulling power’ of a magnet in direct contact with a thick and flat piece of steel and then though a steadily increasing range of air gaps. Pull follows an inverse square law relationship with distance.
High field gradient magnets have the highest clamping forces in direct contact with ferrous material (zero air gap), but the weakest pull through steadily increasing air gaps.
Low field gradient magnets have the lowest clamping forces in direct contact with ferrous material (zero air gap), but the highest pull through steadily increasing air gaps.
A high field gradient magnet’s pull-gap curve and a low field gradient magnet’s pull-gap curve will cross over if plotted on the same graph.

Pull strength
The pull strength is the highest possible holding power of a magnet, measured in kilograms. It is the force required to prise a magnet away from a flat surface of steel when the magnet and metals have full and direct surface-to-surface contact. The grade of the metal, surface condition and angle of pull all have an impact on the pull strength.

Rare-earth metals
Rare-earth metals are categorised in the periodic table in the group known as Lanthanides. The most common elements in this category are neodymium, samarium and dysprosium. Despite the name, rare-earth elements are relatively abundant in the earth’s crust, however, they are not typically found in economically exploitable deposits and are often dispersed, deriving the term ‘rare-earth.’

When two magnets are placed close together with the same poles facing each other, e.g. north facing north or south facing south, they will always repel one another. The reason for this is because the magnetic fields being generated by each magnet are trying to flow in the same direction and when placed closed together they collide, having a repellent effect.

Shear force/sliding resistance
As a rule of thumb it is five times easier to slide a magnet than to pull it vertically off the surface of a ferrous material.
When a magnet slides on steel, the coefficient of friction is approximately 0.2 and this is how the five times is derived.
Magnets attached to a vertical steel wall will slide down the wall when only 20% of the rated pull is experienced as a load. Rubber coated magnets have a much higher coefficient of friction and therefore will resist sliding at a far higher rate because of the friction caused by the coating.
If the vertical wall is made of thin sheet steel which cannot absorb all the magnetism generated by the magnet, then the holding force will be reduced further.
Single domain particle
A particle that is so small there is no room for a magnetic domain wall. Hence the particle is a tiny but very strong magnet. All magnetic recording tapes are made using such particles.

South pole
In magnetic terms this is the specific pole of the magnet which ‘seeks’ the earths geographic South Pole. The earth’s geographic South Pole actually has a magnetic north polarity, thus greatly confusing the issue.

Stacking refers to the process of placing magnets together to increase the net pull strength . When five magnets are stacked together to make one magnet which is five times thicker, then this magnet will be substantially more powerful because of the increase in its L/d ratio (length to diameter). Once the length of the magnet exceeds the diameter of the magnet, the magnet is working at an optimum level and further additions to magnetic length will provide only small increases in performance.

Surface Field / surface gauss
The surface field strength is measured in Gauss and is the magnet’s maximum field strength taken from the magnet’s pole surface. Measurements are usually taken using a gauss meter.

Temperature coefficient (T)
Temperature coefficient is a factor that is used to calculate the decrease in magnetic flux corresponding to an increase in operating temperature. The loss in magnetic flux is recovered when the operating temperature is decreased, providing the maximum operating temperature is not exceeded. The temperature coefficient for magnetic materials are typically;
Neodymium 0.11 % per degree C rise in temperature
Alnico 0.02% per degree C rise in temperature
Ferrite 0.2% per degree C rise in temperature
Samarium cobalt 0.03 % per degree C rise in temperature
Flexible magnets 0.2 % per degree C rise in temperature

Tesla (T)
The Tesla is a unit of measurement for magnetic flux density. It is named after Nikola Tesla who was a Serbian-American inventor, engineer and physicist. One Tesla is equal to 10,000 Gauss.

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