Noun 1. ignition system - the mechanism that ignites the fuel in an internal-combustion engine

ignition

electrical distributor, distributer, distributor - electrical device that distributes voltage to the spark plugs of a gasoline engine

in the order of the firing sequence

electrical system - equipment in a motor vehicle that provides electricity to start the engine and ignite the fuel and operate the lights and windshield wiper and heater and air conditioner and radio

ignition coil - an induction coil that converts current from a battery into the high-voltage current required by spark plugs

ignition switch - switch that operates a solenoid that closes a circuit to operate the starter

magneto, magnetoelectric machine - a small dynamo with a secondary winding that produces a high voltage enabling a spark to jump between the poles of a spark plug in a gasoline engine

mechanism - device consisting of a piece of machinery; has moving parts that perform some function

spark coil - an induction coil used to create sparks

spark gap - a component of an ignition system; consists of two shaped electrodes and the space between them

spark plug, sparking plug, plug - electrical device that fits into the cylinder head of an internal-combustion engine and ignites the gas by means of an electric spark

Ignition system

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The ignition system of an internal-combustion engine is an important part of the overall engine system that provides for the timely burning of the fuel mixture within the engine.

All conventional petrol (gasoline) engines require an ignition system. The ignition system is usually switched on/off through a lock switch, operated with a key or code patch.

Contents[hide]

1 History o 1.1 Magneto system o 1.2 Switchable systems o 1.3 Battery operated ignition

2 Modern ignition systems o 2.1 Mechanically timed ignition o 2.2 Electronic ignition

3 Engine management 4 See also

5 References [edit]

HistoryThe earliest petrol engines used a very crude ignition system. This often took the form of a copper or brass rod which protruded into the cylinder, which was heated using an external source. The fuel would ignite when it came into contact with the rod. Naturally this was very inefficient as the fuel would not be ignited in a controlled manner. This type of arrangement was quickly superseded by spark ignition, attributed to Karl Benz, a system which is generally used to this day, albeit with sparks generated by more advanced circuitry. Early low-speed stationary engines often used a moving contact which protruded into the cylinder. This contact was quickly closed and reopened at the precise instant, producing a spark across the contacts, generated by a coil.

[edit]

Magneto system

The simplest form of spark ignition is that using a magneto. The engine spins a magnet inside a coil, and also operates a contact breaker, interrupting the current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are connected directly from the magneto output. Magnetos are not used in modern cars, but because they generate their own electricity they are often found on small engines such as mopeds, lawnmowers, snowblowers, chainsaws, etc. where there is no battery; also on the small engine's ancestor, the stationary "hit or miss" engine of the early twentieth century; on older gasoline or distillate farm tractors before battery starting and lighting became common; and also in aircraft piston engines, where their simplicity and self-contained nature confers a generally greater reliability as well as lighter weight in the absence of a

battery and generator or alternator. Aircraft engines usually have multiple magnetos to provide redundancy in the event of a failure. Some older automobiles had both a magneto system and a battery actuated system (see below) running simultaneously to ensure proper ignition under all conditions with the limited performance each system provided at the time.

[edit]

Switchable systems

The output of a magneto depends on the speed of the engine, and therefore starting can be problematic. Some magnetos incorporate an impulse system, which spins the magnet quickly at the proper moment, facilitating easier starting at slow cranking speeds. Some engines, such as aircraft but also the Ford Model T, utilized a system which relied on non rechargeable dry cells, (like large flashlight batteries, not what are usually thought of as automobile batteries today) to start the engine or for running at low speed; then the operator would manually switch the ignition over to magneto operation for high speed operation. In order to provide high voltage for the spark from the low voltage batteries, however, a "tickler" was used, which was essentially a larger version of the once ubiquitous electric buzzer. With this apparatus, the direct current passes through an electromagnetic coil which pulls open a pair of contact points, interrupting the current; the magnetic field collapses, the spring-loaded points close again, the circuit is reestablished, and the cycle repeats rapidly. The rapidly collapsing magnetic field, however, induces a high voltage across the coil which can only relieve itself by arcing across the contact points; while in the case of the buzzer this is a problem as it causes the points to oxidize and/or weld together, in the case of the ignition system this becomes the source of the high voltage to operate the spark plugs. In this mode of operation, the coil would "buzz" continuously, producing a constant train of sparks. The entire apparatus was known as the Model T spark coil (in contrast to the modern ignition coil which is only the actual coil component of the system), and long after the demise of the Model T as transportation they remained a popular self-contained source of high voltage for electrical home experimenters, appearing in articles in magazines such as Popular Mechanics and projects for school science fairs as late as the early 1960s. In the UK these devices were commonly known as trembler coils and were popular in cars pre-1910, and also in commercial vehicles with large engines until around 1925 to ease starting.

The magneto on the Model T (built into the flywheel) differed from modern implementations by not providing high voltage directly at the output; the maximum voltage produced was about 30 volts, and therefore also had to be run through the spark coil to provide high enough voltage for ignition, as described above, although the coil would not "buzz" continuously in this case, only going through one cycle per spark. In either case, the high voltage was switched to the appropriate spark plug by the timer mounted on the front of the engine, the equivalent of the modern distributor. The timing of the spark was adjustable by rotating this mechanism through a lever mounted on the steering column.

Ignition coil

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Bosch ignition coil

An ignition coil (also called a spark coil) is an induction coil in a automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs.

This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine.

In modern ignition systems, there is not an ignition distributor and ignition is instead electronically controlled and firing voltage is achieved by one of the following ways: coil-on-plug or waste spark. In the coil-on-plug system each spark plug has its own individual coil sitting right on top of it. It is a so-called Direct Ignition (DI) module. In a "waste spark" system two spark plugs share the same coil. In this system both spark plugs attached to the coil are fired simultaneously. While one cylinder is toward the end of its compression stroke, its "piston pair" is nearing the end of its exhaust stroke, and both plugs are fired. Although the spark plugs are manufactured identically, if re-installed after extended use, they should go back into the cylinder from which they came from. This is because the electrons align themselves favorably with one direction of current flow. While one plug receives positive potential (voltage) at the wire side, the piston pair receives negative potential. This negative potential is what causes the second plug to fire instead of the voltage just going straight to ground after firing the first one. (cars use the conventional theory as opposed to the electron theory of current flow) The waste spark system is better than having a single coil, and cheaper than coil-on-plug.

The disruptive discharge Tesla coil is an early predecessor of the "ignition coil" in the ignition system. Tesla also gained U.S. Patent 609250, "Electrical Igniter for Gas

Engines", on August 16, 1898. It used the principles of the ignition coil used today in automobiles. A. Atwater Kent, in 1921, patented the modern form of the ignition coil.

Flywheel

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Spoked flywheel

Simple flywheel in motion. Constructed based on drawings by Leonardo da Vinci

A flywheel is a heavy rotating disk used as a repository for angular momentum. Flywheels resist changes in their rotation speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source such as a piston-based, (reciprocating) engine, or when the load placed on it is intermittent (such as a piston-based pump). Flywheels can also be used by small motors to store up energy over a long period of time and then release it over a shorter period of time, temporarily magnifying its power output for that brief period. Recently, flywheels have become the subject of extensive research as power storage devices; see flywheel energy storage.

A momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters.

The kinetic energy stored in a rotating flywheel is

where I is the moment of inertia of the mass about the center of rotation and ω (omega) is the angular velocity in radian units. A flywheel is more effective when its inertia is larger, as when its mass is located farther from the center of rotation either due to a more massive rim or due to a larger diameter. Note the similarity of the above formula to the kinetic energy formula E = mv2/2, where linear velocity v is comparable to the rotational velocity, and the mass is comparable to the rotational inertia.

The flywheel has been used since ancient times, the most common traditional example being the potter's wheel. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating into rotary motion.

Video of a simple flywheel in motion (file info)

o Constructed based on drawings by Leonardo da Vinci

o Problems viewing the video? See media help.

In the world of venture capital, the term "flywheel" is used to represent the recurrent, margin-generating heart of a business

Modern ignition systems[edit]

Mechanically timed ignition

Distributor cap

Most four-stroke engines have used a mechanically timed electrical ignition system. The heart of the system is the distributor which contains a rotating cam running off the engine's drive, a set of breaker points, a condenser, a rotor and a distributor cap. External to the distributor is the ignition coil, the spark plugs, and wires linking the spark plugs and ignition coil to the distributor.

The power source is a lead-acid battery, kept charged by the car's electrical system, which generates electricity using a dynamo or alternator. The engine operates contact breaker points, which interrupt the current flow to an induction coil (known as the ignition coil).

The ignition coil consists of two transformer windings sharing a common magnetic core -- the primary and secondary windings. An alternating current in the primary induces alternating magnetic field in the coil's core. Because the ignition coil's secondary has far more windings than the primary, the coil is a step-up transformer which induces a much higher voltage across the secondary windings. For an ignition coil, one end of windings of both the primary and secondary are connected together. This common point is connected to the battery (usually through a current-limiting resistor). The other end of the primary is connected to the points within the distributor. The other end of the secondary is connected, via the distributor cap and rotor, to the spark plugs.

The ignition firing sequence begins with the points (or contact breaker) closed. A steady current flows from the battery, through the current-limiting resistor, through the coil primary, across the closed breaker points and finally back to the battery. This steady current produces a magnetic field within the coil's core. This magnetic field forms the energy reservoir that will be used to drive the ignition spark.

As the engine turns, so does the cam inside the distributor. The points ride on the cam so that as the engine turns and reaches the top of the engine's compression cycle, a high point in the cam causes the breaker points to open. This breaks the primary winding's circuit and abruptly stops the current flow through the breaker points.

Without the steady current flow through the points, the magnetic field generated in the coil immediately begins to quickly collapse. This rapid decay of the magnetic field induces a high voltage in the coil's secondary windings.

At the same time, current exits the coil's primary winding and begin to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed.

The ignition coil's secondary windings are connected to the distributor cap. A turning rotor, located on top of the breaker cam within the distributor cap, sequentially connects the coil's secondary windings to one of the several wires leading to each engine's spark plugs. The extremely high voltage from the coil's secondary – often higher than 1000 volts -- causes a spark to form across the gap of the spark plug. This, in turn, ignites the compressed air-fuel mixture within the engine. It is the creation of this spark which consumes the energy that was originally stored in the ignition coil’s magnetic field.

Except that more separate elements are involved, this distributor-based system is not greatly different from a magneto system. There are also advantages to this arrangement. For example, the position of the contact breaker points relative to the engine angle can be changed a small amount dynamically, allowing the ignition timing to be automatically advanced with increasing revolutions per minute (RPM) and/or increased manifold vacuum, giving better efficiency. This system was used almost universally until the late 1970s, when electronic ignition systems started to appear.

[edit]

Electronic ignition

The disadvantage of the mechanical system is the use of breaker points to interrupt the low voltage high current through the primary winding of the coil; the points are subject to mechanical wear where they ride the cam to open and shut, as well as oxidation and burning at the contact surfaces from the constant sparking. They require regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. Electronic ignition (EI) solves these problems. In the initial systems, points were still used but they only handled a low current which was used to control the high primary current through a solid state switching system. Soon, however, even these contact breaker points were replaced by an angular sensor of some kind - either optical, where a vaned rotor breaks a light beam, or more commonly using a Hall effect sensor,

which responds to a rotating magnet mounted on a suitable shaft. The sensor output is shaped and processed by suitable circuitry, then used to trigger a switching device such as a thyristor, which switches a large flow of current through the coil. The rest of the system (distributor and spark plugs) remains as for the mechanical system. The lack of moving parts compared with the mechanical system leads to greater reliability and longer service intervals. For older cars, it is usually possible to retrofit an EI system in place of the mechanical one. In some cases, a modern distributor will fit into the older engine with no other modifications.

Other innovations are currently available on various cars. In some models, rather than one central coil, there are individual coils on each spark plug. This allows the coil a longer time to accumulate a charge between sparks, and therefore a higher energy spark. A variation on this has each coil handle two plugs, on cylinders which are 360 degrees out of phase; in the four cycle engine this means that one plug will be sparking during the end of the exhaust stroke while the other fires at the usual time, a so-called "wasted spark" arrangement which has no drawbacks. Other systems do away with the distributor as a timing apparatus and use a magnetic crank angle sensor mounted on the crankshaft to trigger the ignition at the proper time.

During the 1980s, EI systems were developed alongside other improvements such as fuel injection systems. After a while it became logical to combine the functions of fuel control and ignition into one electronic system known as an engine management system.

[edit]

Engine managementIn an Engine Management System (EMS), electronics control fuel delivery, ignition timing and firing order. Primary sensors on the system are engine angle (crank or Top Dead Center (TDC) position), airflow into the engine and throttle demand position. The circuitry determines which cylinder needs fuel and how much, opens the requisite injector to deliver it, then causes a spark at the right moment to burn it. Early EMS systems used analogue computer circuit designs to accomplish this, but as embedded systems became fast enough to keep up with the changing inputs at high revolutions, digital systems started to appear.

Some designs using EMS retain the original coil, distributor and spark plugs found on cars throughout history. Other systems dispense with the distributor and coil and use special spark plugs which each contain their own coil (Direct Ignition). This means high voltages are not routed all over the engine, they are created at the point at which they are needed. Such designs offer potentially much greater reliability than conventional arrangements.

Modern EMS systems usually monitor other engine parameters such as temperature and the amount of uncombined oxygen in the exhaust. This allows them to control the engine

to minimise unburnt or partially burnt fuel and other noxious gases, leading to much cleaner and more efficient engines.

Magneto

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Jump to: navigation, searchThis article is about an engine component. For other uses of the term, see Magneto (disambiguation).

A magneto provides pulses of electrical power to the spark plugs in some petrol-powered internal combustion engines where batteries are not available, most commonly those in 2-stroke and 4-stroke engines used in small motorcycles, lawnmowers and chainsaws, as well as in most small aircraft and some racing automobiles, serving a similar function to the coil-type ignition system found in automobiles. In aircraft, typically each cylinder has two spark plugs, each driven from a separate magneto. This arrangement provides redundancy in the event of a failure of one of the magnetos, and two sparks burn more efficiently than one.

Magnetos combine the functions of a dynamo, contact breaker points and coil into one unit. The engine rotates a coil of wire between the poles of a permanent magnet to provide a basic source of electrical energy (In some variants the permanent magnet is

rotated and the coil remains stationary). On each revolution, a cam opens the contact breaker one or more times, interrupting the current, causing the voltage in the secondary winding of the coil to reach a very high value, enough to arc across the electrodes of the spark plug. Because no battery or other source of energy is required, the magneto is a rugged, reliable and self-contained solution to providing ignition of the fuel. In some modern magneto designs, an electronic switch replaces the contact breaker.

Magnet

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Jump to: navigation, searchFor other uses, see Magnet (disambiguation).

Iron filings in a magnetic field generated by a bar magnet

A magnet is an object that has a magnetic field. The word magnet comes from the Greek μαγνήτης λίθος (magnētēs lithos), which means "magnesian stone". Magnesia was an area in Ancient Greece, in present-day Manisa, Turkey where deposits of magnetite have been discovered since antiquity.

Contents[hide]

1 Introduction 2 Physical origin of magnetism

o 2.1 Permanent magnets (permagnets) o 2.2 Electronic generation of magnetism o 2.3 Electromagnets

3 Characteristics of magnetic materials o 3.1 Permanent magnets and dipoles o 3.2 North-south pole designation and the

Earth's magnetic field 4 Common uses for magnets 5 Magnetization of materials 6 Demagnetizing materials 7 Types of permanent magnets 8 Magnetic forces

o 8.1 Forces between magnets o 8.2 Magnets and ferromagnetic materials o 8.3 Magnets and diamagnetic materials o 8.4 Magnets and paramagnetic materials o 8.5 Calculating the magnetic force

8.5.1 Force between two monopoles 8.5.2 Force between two very close

attracting surfaces 8.5.3 Force between two bar magnets

9 See also 10 Online references 11 Printed references

12 External articles [edit]

IntroductionIn the modern sense,a magnet is a material that has a magnetic field. It can be in the form of a permanent magnet or an electromagnet. Permanent magnets do not rely upon outside influences to generate their field. Electromagnets rely upon electric current to generate a magnetic field - when the current increases, so does the field. Magnets are attracted to or repelled by other things. Something that is strongly attracted to a magnet is said to have a high permeability. Iron and steel are two examples of materials with very high permeability, and they are strongly attracted to magnets. Liquid oxygen is an example of something with a low permeability, and it is only weakly attracted to a magnetic field. Water has such a low permeability that it is actually repelled by magnetic fields. Everything has a measurable permeability: people, gases and even the vacuum of outer space.

The SI unit of magnetic field strength is the tesla, and the SI unit of total magnetic flux is the weber. 1 weber = 1 tesla flowing through 1 square meter, and is a very large amount of magnetic flux.

[edit]

Physical origin of magnetism[edit]

Permanent magnets (permagnets)

All normal matter is composed of particles such as protons, neutrons and electrons, and all of these have the fundamental property of quantum mechanical spin. Spin gives each one of these particles an associated magnetic field. Because of this, and the fact that the average microscopic piece of matter contains huge numbers of these particles, it would be expected that all matter would be magnetic. Even antimatter would have magnetic characteristics. However, everyday experience shows that this is not the case.

Within each atom and molecule, the spin of each of these particles is highly ordered as a result of the Pauli Exclusion Principle. However, there is no long range ordering of these spins between atoms and molecules. Without long range ordering, there is no net magnetic field because the magnetic moment of each one of the particles is cancelled by the magnetic moment of other particles.

Permanent magnets are special in that long range ordering does exist. The highest degree of ordering exists within magnetic domains. These domains can be likened to microscopic neighbourhoods in which there is a strong reinforcing interaction between particles, and as a result, a great deal of order. The greater the degree of ordering within and between domains, the greater the resulting field will be.

Long range ordering (and the resulting strong net magnetic field) is one of the hallmarks of a ferromagnetic material.

[edit]

Electronic generation of magnetism

Electrons play the primary role in generating a magnetic field. Within an atom, electrons can exist either individually or in pairs within any given orbital. When they are paired, the individuals in that pair always have opposite spin — one up, one down. The fact that the spins have opposite orientation means that the two cancel one another. If all electrons are paired, no net magnetic field will be generated.

In some atoms, there are electrons that are unpaired. All magnets have unpaired electrons, but not all atoms with unpaired electrons are ferromagnetic. In order for the material to become ferromagnetic, not only must there be unpaired electrons present, but those unpaired electrons must interact with one another over long ranges such that they are all oriented in the same way. The specific electron configuration of the atoms (as well as the distance between atoms) is what leads to this long range ordering. The electrons find that they can exist in a lower energy state if they all have the same orientation.

[edit]

Electromagnets

An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops. This coil is known as a solenoid. When electric current flows along the coil, a magnetic field is generated around the coil. The orientation of this field can be determined via the right hand rule. The strength of the field is influenced by several factors. The number of loops determines the surface area of interaction, the amount of current determines the amount of activity, and the material in the core determines electrical resistance. The more loops of wire and the greater the current, the stronger the field will be.

If the coil of wire is empty in the center, it will tend to generate a very weak field. Different ferromagnetic or paramagnetic items can be placed in the center of the core with the effect of magnifying the magnetic field, for example an iron nail. In addition, soft iron is commonly used for this purpose. The addition of these types of materials can result in a several hundred- to thousand-fold increase of field strength.

At long distances, magnetic fields obey an inverse square law. This means that the field strength is inversely proportional to the square of the distance from the magnet.

In the case of an electromagnet in contact with a flat metal plate, the force needed to separate the two will be greatest if the two surfaces are machined as flat as possible. The flatter the surfaces, the more points of contact between them, and the smaller the magnetic circuit's reluctance to the magnetic field.

Electromagnets find uses in many places, ranging from particle accelerators, to junkyard cranes, to magnetic resonance imaging machines. There are also specialized applications that involve more than a simple magnetic dipole, such as the quadrupole magnets used to focus particle beams.

If an electromagnet is strong enough, the magnetic force between neighbouring loops of wire can cause the electromagnet to be crushed by its own magnetic field.

[edit]

Characteristics of magnetic materials[edit]

Permanent magnets and dipoles

All magnets have at least two poles: that is, all magnets have at least one north pole and at least one south pole. The poles are not a pair of things on or inside the magnet. They are a concept used to discuss and describe magnets. In the image at the top of this page, the poles look like specific locations, because the highest surface intensity of the field occurs at the poles, but this does not mean that they are specific locations.

To understand the concept of pole, it can be imagined that a row of people who are all facing the same direction and standing in line. While there is a "face" end of the line and a "back" end of the line, there is no one place where all of the faces are and all of the backs are. The person at the front of the face end has a back; and the person at the back end has a face. If the line is divided into two shorter lines, each one of the shorter lines still has a face end and a back end. Even if the line is pulled completely apart so that there are just individuals standing around, each one of the individuals still has a face and a back. This can continue without end.

The same holds true with magnets. There is not one place where all of the north or south poles are. If a magnet is divided in two, two magnets will result--and both magnets will have a north and a south pole. Those smaller magnets can then be divided, and all of the resulting pieces will have both a north and south pole. In most instances, if the material continues to be broken into smaller and smaller pieces there will be a point where the pieces are too small to retain a net magnetic field. They won't become individual north or south poles though; instead, they will just lose the ability to maintain a net field. Some materials, however, can be divided down to the molecular level and still maintain a net field with both a north and a south pole. There are theories involving the possibility of north and south magnetic monopoles, but no magnetic monopole has ever been found.

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North-south pole designation and the Earth's magnetic field

A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet.

The Earth's current geographic north is thus actually its magnetic south. Confounding the situation further, magnetised rocks on the ocean floor show that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be backward at some time in the future.

Fortunately, by using an electromagnet and the right hand rule, the orientation of the field of a magnet can be defined without reference to the Earth's geomagnetic field.

To avoid the confusion between geographic and magnetic north and south poles, the terms positive and negative are sometimes used for the poles of a magnet. The positive pole is that which seeks geographical north.

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Common uses for magnets

Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.

Credit , debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individuals financial institution and connect with their account(s).

Common televisions and computer monitors: The vast majority of TV's and computer screens rely in part on an electromagnet to generate an image--see the article on cathode ray tubes for more information. Plasma screens and LCDs rely on

different technology entirely.

Loudspeakers and microphones: Loudspeakers actually rely on a combination of a permanent magnet and an electromagnet. A speaker is fundamentally a device to convert electric energy (the signal) into mechanical energy (the sound). The electromagnet carries the signal, which generates a changing magnetic field that pushes and pulls on the field generated by the permanent magnet. This pushing and pulling moves the cone, which creates sound. Not all speakers rely on this technology, but the vast majority do. Standard microphones are based upon the same concept, but run in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as

well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law). This voltage in the wire is now an electric signal that is representative of the original sound.

Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.

Transformers : Transformers are devices that transfer electric energy between two devices that are electrically disconnected via magnetic coupling.

Chucks : Chucks are used in the metalworking field to hold objects. If these objects can be held securely with a magnet then a

permanent or electromagnetic chuck may be used. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.

Magic : Naturally magnetic Lodestones as well as iron magnets are used in conjunction with fine iron grains (called "magnetic sand") in the practice of the African-American folk magic known as hoodoo. The stones are symbolically linked to people's names and ritually sprinkled with magnetic sand to reveal the magnetic field. One stone may be utilized to bring desired things to a person; a pair of stones may be manipulated to bring two people closer together in love.

[edit]

Magnetization of materialsFerromagnetic materials can be magnetised in the following ways:

Placing the item in an external magnetic field will result in

the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the earth's magnetic field and which are subject to vibration (eg frame of a conveyor) have been shown to acquire significant residual magnetism.

Placing the item in a solenoid with a direct current passing through it.

Stroking - An existing magnet is moved from one end of the item to the other repeatedly in the same direction.

Placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools. This can be done by laying the magnet in a North-South direction in the Earth's magnetic field. The magnet is not very strong; however, the effect is permanent.

[edit]

Demagnetizing materials

Permanent magnets can be demagnetized in the following ways:

Heating a magnet past its Curie point will destroy the long range ordering.

Contact through stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.

Hammering or jarring will destroy the long range ordering within the magnet.

A magnet being placed in a solenoid which has an alternating current being passed through it will have its long range ordering disrupted, in much the same way that direct current can cause ordering.

In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.

[edit]

Types of permanent magnets

A stack of ferrite magnets Rare Earth or

Neodymium magnets, - some of the most powerful permanent magnets

Samarium-Cobalt magnets

Ceramic magnets Plastic magnets Alnico magnets

[edit]

Magnetic forcesMagnetized items interact with other items in very specific ways.

[edit]

Forces between magnets

If two magnets are brought close enough together, their fields will begin to interact in the following ways:

If the magnets' north poles are brought together, the magnets will repel one another (like poles repel)

If the north pole of one magnet is brought to the south pole of the other magnet, the magnets

will attract one another (opposite poles attract)

[edit]

Magnets and ferromagnetic materials

If a magnet is brought close enough to a ferromagnetic material (that is not magnetized itself), the magnet will strongly attract the ferromagnetic material regardless of orientation. Both the north and south pole of the magnet will attract the other item with equal strength.

[edit]

Magnets and diamagnetic materials

By definition, diamagnetic materials weakly repel a magnetic field. This occurs regardless of the north/south orientation of the field.

[edit]

Magnets and paramagnetic materials

By definition, paramagnetic materials are weakly attracted to a magnetic field. This occurs regardless of the north/south orientation of the field.

[edit]

Calculating the magnetic force

Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets.

[edit]

Force between two monopoles

The force between two magnetic monopoles is as follows:

[1]

where

F is force (SI unit: newton) m is pole strength (SI unit: weber) μ is the permeability of the intervening medium (SI unit: tesla meter per ampere) r is the separation (SI unit: meter).

Since magnetic monopoles are only a theoretical construction, this equation does not describe a physically realisable arrangement. It is stated here because it is the simplest possible calculation of magnetic forces. In reality, one of the more complex formulae given below will be more useful.

[edit]

Force between two very close attracting surfaces

[2]

where

A is the area of each surface, in m2, and B is the flux density between them, in teslas

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Force between two bar magnets

The force between two identical cylindrical bar magnets placed end-to-end is given by:

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where

B0 is the flux density at each pole, in T, A is the area of each pole, in m2, L is the length of each magnet, in m, R is the radius of each magnet, in m, and x is the separation between the two magnets, in m

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See also electromagnet electromagnetism electromagnetic field

neodymium magnet diamagnetism magnetic dipole magnetic monopole magnetism molecular magnet paramagnetism single-molecule

magnet magnet therapy

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Online references HyperPhysics E/M ,

good complete tree diagram of electromagnetic relationships with magnets

Magnet Education and Understanding Commercial Magnets useful companies who have many helpful magnet equations

Maxwell's Equations and some history...

Detailed Theory on Designing a Solenoid or a Coil Gun

Magnet University Resources from the fundamental theory of magnetism to advanced applications of magnetic materials.

G2 CONSULTING A textbook style resource with information from the fundamental theory

of magnetism and electromagnetism, to advanced applications of magnetic devices.

Contact breaker

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Breaker arm with contact points at the left. The pivot is on the right and the cam follower is in the middle of the breaker arm.

A contact breaker (or "points") is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of spark-ignition internal combustion engines.

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PurposeThe purpose of the contact breaker is to interrupt the current flowing in the primary circuit of the ignition coil. When this occurs, the collapsing current induces a high voltage in the secondary winding of the coil, which has very many more turns. This causes a very large voltage to appear at the coil output for a short period - enough to arc across the electrodes of a spark plug.

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OperationThe contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the piston's compression stroke. The contact breaker is usually mounted on a plate that is able to rotate relative to the camshaft operating it. The plate is rotated by a centrifugal mechanism, thus advancing the timing (making the spark occur earlier) at higher revolutions. This gives the fuel time to burn so that the resulting gases reach their maximum pressure at the same time as the piston reaches the top of the cylinder. The plate's position can also be moved a small distance using a small vacuum-operated servomechanism, providing advanced timing when the engine is required to speed up on demand. This helps to prevent pre-ignition (or pinging).

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Disadvantages of contact breakersSince they open and close several times every turn of the engine, contact breaker points and cam follower suffer from wear - both mechanical and pitting caused by arcing across the contacts. This latter effect is largely prevented by placing a capacitor parallel across the contact breaker - this is usually referred to by the more old fashioned term condenser by mechanics. As well as suppressing arcing, it helps boost the coil output by creating a resonant LC circuit with the coil windings. A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise, needs regular adjustment of the dwell (contact) angle, and at higher revolutions, its mass becomes significant, leading to poor operation at higher engine speeds. These effects can largely be overcome using electronic ignition systems, where the contact breakers are retrofitted by a massless sensor device.

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