Glossary of Magnet Terminology
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.
Anisotropic – 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.
See our article All About Magnetization Direction for more info.
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.
Coercivity – 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.
Curie Temperature (Tc) – TThe properties of all magnetic materials change when they are heated to a particular temperature. The Curie temperature (Tc), or Curie point, is the temperature at which the atomic structure of magnetic material is changed and the object becomes demagnetised. Once heated to, or passed, the Curie point the magnetic domains of the material are released and become randomised and ‘self keepering’, resulting in permanent magnetic damage. As a result, the magnet will not emit any external magnetic fields.
Demagnetization Curve – The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.
Demagnetization Force – A magnetizing force, typically in the direction opposite to the force used to magnetize it in the first place. Shock, vibration and temperature can also be demagnetizing forces. Generally, neodymium magnets cannot be re-magnetised once their magnetic properties have been lost.
Dimensions – The finished size of a magnet, including all surface treatments such as coatings and platings.
Dimensional Tolerance – An allowance, given as a permissible range, in the nominal dimensions of a finished magnet. The purpose of a tolerance is to specify the allowed leeway for variations in manufacturing.
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.
Electromagnet – A magnet consisting of a solenoid with an iron core, which has a magnetic field only during the time of current flow through the solenoid.
Ferromagnetic Material – A material that either is a source of magnetic flux or a conductor of magnetic flux. Most ferromagnetic materials have some component of iron, nickel, or cobalt.
Gauss – Unit of magnetic induction, B. Lines of magnetic flux
per square centimeter in the C.G.S. system of measurement. Equivalent to lines per square inch in the English system, and webers per square meter or tesla in the S.I. system. 10,000 gauss equals 1 tesla.
Gauss meter – An instrument used to measure the instantaneous value of magnetic induction, B, usually measured in Gauss (C.G.S.). Also called a DC magnetometer.
Gilbert – The unit of magnetomotive force, F, in the C.G.S. system.
Hysteresis Loop – A four quadrant graph, showing magnetising force relative to resultant magnetisation of a permanent magnet material as it is successively magnetised to its saturation point, then demagnetised, magnetised in the reverse polar direction and then finally re-magnetised.
When the cycles are complete, this four quadrant graph will be a closed loop which illustrates the magnetic characteristics of the magnetic material under test. Magnetically hard materials have a larger area inside the loop which denotes the level of magnetic energy. Magnetically soft materials lose magnetism when the magnetising field is removed and therefore these have very small areas inside the loop. The second quadrant within the four quadrants (+X and -Y) is the most important of the four curves and is known as the demagnetisation curve.
Induction, (B) – The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the C.G.S. system of units.
Intrinsic Coercive Force (Hci) – Indicates a materials’ resistance to demagnetization. It is equal to the demagnetizing force which reduces the intrinsic induction, Bi, in the material to zero after magnetizing to saturation; measured in oersteds.
Irreversible Losses – Partial demagnetization of the magnet, caused by exposure to high or low temperatures, external fields, shock, vibration, or other factors. These losses are
only recoverable by remagnetization. Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields.
Isotropic Material – A material that can be magnetized along any axis or direction (a magnetically unoriented material). The opposite of Anisotropic Magnet.
Keeper – 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.
Kilogauss – One Kilogauss = 1,000 Gauss = Maxwells per square centimeter.
Magnet – A magnet is an object made of certain materials which create a magnetic field. Every magnet has at least one north pole and one south pole. By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet. This is an example of a magnetic dipole (“di” means two, thus two poles).
If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole. If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole. No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole. It has not been shown to be possible to end up with a single North pole or a single South pole which is a monopole (“mono” means one or single, thus one pole).
Magnetic Circuit – Consists of all elements, including air gaps and non-magnetic materials that the magnetic flux from a magnet travels on, starting from the north pole of the magnet to the south pole.
Magnetic Field (B) – Magnetic induction, also known as flux density is the number of lines of magnetism in each square centimetre of pole area. The total number of magnetic field lines penetrating each 1cm x 1cm pole area is called the magnetic flux density (also known as magnetic induction). Flux density is measured in Gauss, or Tesla (10,000 Gauss = 1 Tesla).
Magnetic Field Strength (H) – Magnetizing or demagnetizing force, is the measure of the vector magnetic quantity that determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point; measured in Oersteds.
Magnetic Flux – Is a contrived but measurable concept that has evolved in an attempt to describe the “flow” of a magnetic field. When the magnetic induction, B, is uniformly distributed and is normal to the area, A, the flux, Ø = BA.
Magnetic Flux Density – Lines of flux per unit area, usually measured in
Gauss (C.G.S.). One line of flux per square centimeter is one Maxwell.
Magnetic Induction (B) – The magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and the resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.
Magnetic Line of Force – An imaginary line in a magnetic field, which, at every point, has the direction of the magnetic flux at that point.
Magnetic Pole – An area where the lines of flux are concentrated.
Magnetomotive Force (F or mmf) – The magnetic potential difference between any two points. Analogous to voltage in electrical circuits. That which tends to produce a magnetic field. Commonly produced by a current flowing through a coil of wire. Measured in
Gilberts (C.G.S.) or Ampere Turns (S.I.).
Material Grade – Neodymium (NdFeB) magnets are graded by the magnetic material from which they are manufactured. Generally speaking, the higher the grade of material, the stronger the magnet. We find that the Pull Force of a magnet relates directly to the “N” number. Neodymium magnets currently range in grade from N35 to N52. The theoretical limit for Neodymium magnets is grade N64, though it isn’t currently feasible to manufacture magnets this strong. The grade of most of our stock magnets is N42 because we feel that N42 provides the optimal balance between strength and cost. We also stock a wide range of sizes in grade N52 for customers who need the strongest permanent magnets available.
Maximum Energy Product (BHmax) – The magnetic field strength at the point of maximum energy product of a magnetic material. The field strength of fully saturated magnetic material measured in Mega Gauss Oersteds, MGOe.
Maximum Operating Temperature (Tmax) – Also known as maximum service temperature, is the temperature at which the magnet may be exposed to continuously with no significant long-range instability or structural changes.
Maxwell – Unit of magnetic flux in the C.G.S. electromagnetic system. One maxwell is one line of magnetic flux.
Magnetization Curve – The first quadrant portion of the hysteresis loop (B/H)
Curve for a magnetic material.
Magnetizing Force (H) – The magnetomotive force per unit of magnet length, measured in Oersteds (C.G.S.) or ampere-turns per meter (S.I).
Maxwell – The C.G.S. unit for total magnetic flux, measured in flux lines per square centimeter.
MGOe – Mega (million) Gauss Oersteds. Unit of measure typically used in stating the maximum energy product for a given material. See Maximum Energy Product.
North Pole – The north pole of a magnet is the one attracted to the magnetic north pole of the earth. This north-seeking pole is identified by the letter N. By accepted convention, the lines of flux travel from the north pole to the south pole.
Oersted (Oe) – The C.G.S. unit for magnetizing force. The English system equivalent is Ampere Turns per Inch (1 Oersted equals 79.58 A/m). The S.I. unit is Ampere Turns per Meter.
Orientation – Used to describe the direction of magnetization of a material.
Orientation Direction – The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties.
Paramagnetic Materials – Materials that are not attracted to magnetic fields (wood, plastic, aluminum, etc.). A material having a permeability slightly greater than 1.
Permanent Magnet – A magnet that retains its magnetism after it is removed from a magnetic field. A permanent magnet is “always on”. Neodymium magnets are permanent magnets.
Permeance (P) – A measure of relative ease with which flux passes through a given material or space. It is calculated by dividing magnetic flux by magnetomotive force. Permeance is the reciprocal of reluctance.
Permeance Coefficient (Pc) – Also called the load-line, B/H or “operating slope” of a magnet, this is the line on the Demagnetization Curve where a given magnet operates. The value depends on both the shape of the magnet, and it’s surrounding environment (some would say, how it’s used in a circuit). In practical terms, it’s a number that define how hard it is for the field lines to go from the north pole to the south pole of a magnet. A tall cylindrical magnet will have a high Pc, while a short, thin disc will have a low Pc.
Our online Pull Force Calculator can calculate Pc for common shapes. It assumes a single magnet in free space. Other nearby magnets or ferromagnetic materials can change matters.
Permeability (µ) – The ratio of the magnetic induction of a material to the magnetizing force producing it (B/H).
The magnetic permeability of a vacuum (µo) is 4π×10-7
N/Amp2.
Pole – An area where the lines of magnetic flux are concentrated.
Plating/Coating – Most neodymium magnets are plated or coated in order
to protect the magnet material from corrosion. Neodymium magnets are
mostly composed of neodymium, iron, and boron. The iron in the magnet will
rust if it is not sealed from the environment by some sort of plating or
coating. Most of the neodymium magnets that we stock are triple plated in
nickel-copper-nickel, but some are plated in gold, silver, or black nickel, while
others are coated in epoxy, plastic or rubber.
Polarity – The characteristic of a particular pole at a particular location of a permanent magnet. Differentiates the North from the South Pole.
Pull Force – 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 – Commonly used to describe high energy magnet material such as NdFeB (Neodymium-Iron-Boron) and SmCo (Samarium-Cobalt).
Relative Permeability – The ratio of permeability of a medium to that of a vacuum. In the C.G.S. system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the C.G.S. system.
Reluctance (R)- A measure of the relative resistance of a material to the passage of flux. It is calculated by dividing magnetomotive force by magnetic flux. Reluctance is the reciprocal of permeance.
Remanence, (Bd) – The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force.
Return Path – Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux.
Reversible Temperature Coefficient – A measure of the reversible changes in flux caused by temperature variations.
Saturation – The state where an increase in magnetizing force produces no further increase in magnetic induction in a magnetic material.
Shunt – A soft iron piece temporarily added between the pole of a magnetic circuit to protect it from demagnetizing influences. Also called a keeper. Not needed for Neodymium and other modern magnets.
S.I. – Abbreviation for “Système International”. Refers to the International Standard System of units. It is also known as the MKS system.
South Pole – The south pole of a magnet is the one attracted to the south pole of the earth. This south-seeking pole is identified by the letter S. By accepted convention, the lines of flux travel from the north pole to the south pole.
Stabilization – The process of exposing a magnet or a magnetic assembly to elevated temperatures or external magnetic fields to demagnetize it to a predetermined level. Once done the magnet will suffer no future degradation when exposed to that level of demagnetizing influence.
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 – 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.
Tesla – The S.I. unit for magnetic induction (flux density). One Tesla equals 10,000 Gauss.
Weber – The S.I. unit for total magnetic flux. The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.
Weight – The weight of a single magnet
