A non-magnetic discontinuity in a magnetic circuit (i.e. the distance between two magnetic poles), this gap often includes other materials such as brass, aluminium or paint.
A magnet which has a preferred direction of orientation so that the magnetic characteristics are optimum in one preferred direction.
This exists when the flux path external to the permanent magnet is confined within high permeability materials which contain the magnet circuit.
Coercive Force, Hc
The demagnetising force necessary to reduce observed induction B to zero after the magnet has been brought to saturation. Coercive force is measured in Oersteds or more recently A/m and kA/m.
Curie Temperature, Tc
The temperature at which a material loses its permanent magnetic properties completely and is no longer able to hold magnetism.
The second/left quadrant of the hysteresis loop, generally describing the behaviour of magnetic characteristics in actual use. Also known as the B-H curve.
A material whose permeability is very much larger than one, and which exhibits hysteresis magnetising and demagnetising characteristics. The greater the flux carrying potential, the bigger the value i.e. one to several thousands.
Magnetic flux is the condition existing in a medium subjected to a magnetising force. This value is quantified by E.M.F. (electromotive force). This measurement of force in cgs units is a Maxwell.
Leakage flux particularly associated with edge effects and leakage patterns in a magnetic circuit.
Lines of magnetic flux per square centimetre. Gauss is measured in cgs units, Maxwell lines and Webers per square metre or Tesla in the Si system.
A closed curve calculated by plotting corresponding values of magnetic induction: B on the abscissa against magnetising force H.
This is the magnetic flux per unit area of section in the applied magnetic direction of flux. This is measured in Gauss.
Intrinsic Coercive Force
This is a measure of the resistance of the magnet material to a demagnetising force. Permanent magnets with high intrinsic coercivity values are usually classified as ‘hard’ permanent magnets. Intrinsic coercive force indicates magnetic stability at high temperatures. Also see stabilisation.
This is the partial demagnetisation of a magnet material when introduced to external factors such as high/low temperatures and demagnetising fields. Losses can only by rectified by remagnetisation. However, magnets can be stabilised to prevent the variation of performance caused by irreversible losses.
A magnetic material which does not have a preferred direction of magnetic orientation and therefore can be magnetised in any direction without the loss of magnetic characteristics.
Knee of the Demagnetisation Curve
The point at which the B-H curve ceases to be linear. If the operating point of the magnet falls below the knee, the magnet will not be able to recover full magnetic potential without the application of a magnetising force.
This is the loss of magnetic flux which occurs through leakage caused by saturation or air gaps introduced into the magnetic circuit. This induces a loss of efficiency in the circuit which cannot be recovered.
Length of Air Gap, Lg
Indicates the length of the central flux path across an air gap.
A line drawn from the origin of the Demagnetisation Curve with a slope. The intersection of the -B/H curve and slope represents the operating point of the magnet. Also see Permeance Coefficient, Pc.
An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.
The total magnetic induction over a given area.
Magnetising Force, H
The magnetomotive force per unit length at any point in a magnetic circuit. This is measured in Oersteds.
Magnetomotive Force, F
This is the potential magnetic difference between any two points.
Maximum Energy Product, BH max.
There is a point at the Hysteresis Loop at which the product of magnetising force H and induction B reaches a maximum. This maximum value is called the Maximum Energy Product and is measured in Mega Gauss Oersted, MGOe.
A unit measure of magnetising force (cgs). This is equivalent to Ampere Turns per Inch (S.I.).
Permeance Coefficient, Pc
Ratio of the magnetic induction to self demagnetising force. This is also known as the ‘load line’ or operating point of the magnet.
Usually illustrated in graph format, these curves are a representation of the relationship between the attractive force exerted by a magnet on a soft magnetic workpiece and the distance between them. Pull Gap curve diagrams are useful when selecting a magnet for a particular tractive or holding application.
Reluctance is the resistance in a magnetic circuit and is related to the magnetomotive force, F and magnetic flux (R =F/ magnetic flux) where F is the magnetomotive force.
Remenance is the magnetic induction which remains in a magnetic circuit after the removal of an applied magnetising force. If there is an air gap in the circuit, the remenance will be less than the residual induction Br.
Residual Induction Br
This represents the maximum flux output from a given magnet material measured at the point where the Hysteresis Loop crosses the B axis at zero magnetising force.
A magnetic circuit which provides a low reluctance path for the magnetic flux. Reversible Temperature Coefficient A measure of the reversible changes in flux caused by temperature variations.
This is the condition whereby a magnet or ferromagnetic material has reached a maximum value and an increase in the appliance of magnetising force produces no increase in induction i.e. saturation flux densities for steels range from 16,000 to 20,000 Gauss.
The process where a magnet is exposed to demagnetising influences expected to be encountered in operation. The exposure to these demagnetising influences such as high or low temperatures or external magnetic fields prevents irreversible losses during actual operation.
The material has a preferred direction of magnetic orientation.
Coercive Force, Hc
The demagnetizing force, in oersteds, required to reduce the residual induction, Br, of a fully magnetized magnet to zero.
Temperature at which a material loses its magnetic properties.
A ceramic compound consisting of a mixed oxide of iron and one or more other metals. Many of them are magnetic materials.
The term “ferrous” is derived from the Latin word meaning “containing iron”. Ferrous metals are often magnetic, but not exclusively.
Unit of measure of magnetic induction, B, or flux density in the CGS system.
An instrument used to measure the instantaneous value of magnetic induction, B.
Intrinsic Coercive Force, Hci
Oersted measurement of the material’s inherent ability to resist self-demagnetization.
The material has no preferred direction of magnetic orientation, which allows magnetization in any direction.
The stored energy in a magnet, called magnet performance or magnetic energy product, is typically measured in units of megagauss-oersteds.
Magnetic Induction, B
Flux per unit area of a section normal to the direction of the magnetic path. Measured in gauss.
Maximum Energy Product, BHmax
The maximum product of (BdHd) which can be obtained on the demagnetization curve.
Maximum Operating Temperature
The maximum temperature of exposure that a magnet can forego without significant long-range instability or structural changes.
That magnetic pole which attracts the geographic North Pole.
Residual Induction, Br
Flux density, measured in gauss, of a magnetic material after being fully magnetized in a closed circuit.
What is a 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).
Amazing Magnetic Facts
Did you know that the name “magnet” was first used by the Greeks as Early as 600 B.C. for describing a mysterious stone that attracted iron and other pieces of the same material? According to one Greek legend, the name “magnet” was taken from the shepherd “Magnes” who discovered the magnetic stone by accident when his staff was mysteriously attracted to the force of the stone. Another, and perhaps more believable, theory says that the word “magnet” came from a city in Asia Minor, called Magnesia, where many of these mysterious magnetic stones were found. During the Middle Ages, this stone became known as lodestone, which is the magnetic form of magnetite.
Today, magnets are available in all sorts of shapes including discs, rings, blocks, rectangles, arcs, rods, and bars. They are made out of materials such as ceramic (strontium ferrite), alnico (aluminum, nickel, and cobalt), rare earth (samarium cobalt and neodymium) and flexible rubber-like material. Not only do the shape and material of magnets vary, so do their applications. At many companies, magnets are used for lifting, holding, separating, retrieving, sensing and material handling. You can find magnets in a car — even around your house! Magnets are used in the home to organize tools or kitchen utensils and can be found in doorbells, loudspeakers, microwaves and televisions! Business offices and schools use magnetic planning boards to display schedules and charts.
Magnets are also used in a compass to guide people if they are lost. In fact, the compass was probably the first important magnetic device discovered. Around the 12th century, someone noticed that when allowed free movement, a magnet always points in the same north/south direction. This discovery helped mariners who often had trouble navigating when the clouds covered the sun or stars.