School Science Lessons
(UNPh29.1)
2024-07-25
Magnets, Compass
Contents
29.1.0 Magnets
29.2.0 Classes of magnetism
29.3.0 Compass
29.4.0 Magnetic field of the Earth
29.5.0 Magnetism and temperature
29.6.0 Magnetic permeability
35.14 Magnetism test, (minerals), (Geology)
29.1.0 Magnets
Experiments
29.1.1 Artificial magnets, ferrite magnets, "Alnico", "Alcomax"
29.1.2 New magnetic poles in broken magnet
29.1.3 Buzzing magnets
29.1.4 Compass needle, north-seeking pole, south-seeking pole
29.1.5 Cut an iron wire magnet
29.1.6 Freely-suspended magnet
29.1.7 Magnet assortment, natural magnets, artificial magnets
29.1.8 Magnets on a pivot
29.1.9 Natural magnets
29.1.10 Simple compass needle
29.1.11 Magnetized test
29.1.12 Magnetic poles, magnetize wire, freely suspended magnets
9.4.8 Horseshoe magnet and copper wire
9.4.9 Horseshoe magnet and conducting rails
29.2.0 Classes of magnetism, domains, magnetic permeability
29.2.1 Classes of magnetism, domains
29.2.2 Diamagnetism
29.2.4 Ferromagnetism
29.2.5 Ferrite magnets
29.2.6 Hematine
29.2.7 Paramagnetism
29.2.8 Separate crystals using parmagnetism, (Geology)
29.2.9 Permanent magnets
Experiments
29.2.10 Test diamagnetic and paramagnetic crystals
29.2.11 Store bar magnets
29.2.12 Hanging magnets and inverse square law
29.3.0 Compass
29.3.1 Ship's compass, Points of the compass
36.6.1 Find north with a watch compass
37.44 Navigation data used by a ship at sea
29.4.0 Magnetic field of the Earth
29.4.1 Magnetic fields
29.4.2 Magnetic field of the Earth
29.4.3 Magnetic compass, Simple compass needles
29.4.4 Magnetic dip, magnetic inclination
29.4.5 Magnetic North Pole
29.4.6 North pole of magnet pointing magnetic north
29.4.7 True north and magnetic north
29.5.0 Magnetism and temperature, Curie point
29.1.6.1 Curie point, Curie temperature
29.1.6.3 Meissner effect
29.1.6.2 Thermomagnetic motor
29.1.1 Artificial magnets, ferrite magnets, "Alnico", "Alcomax"
See diagram 29.1.1.0: Stored bar magnets.
See diagram 31.79x: Horseshoe electromagnets.
Alnico magnets
Look for low-cost artificial magnets in discarded loudspeakers, telephone receivers and other equipment.
Artificial magnets have different shapes, e.g."Alnico", horseshoe magnet, pairs of bar magnets with a soft iron keeper, cylindrical magnets.
Store artificial magnets in pairs in a box, north to south, and south to north.
Be careful! Keep magnets away from computer diskettes (floppy discs) and colour television screens.
"Magnetic Stones" is a toy containing powerful hematine magnets made of synthetic haematite.
Ferrite magnets are made from synthetic ferrimagnetic ceramic compounds, containing mainly haematite, Fe2O3 or magnetite, Fe3O4.
Ferrite allows stronger magnetic fields than an iron core and it is an insulator, so no induced currents can occur in the core.
29.1.2 New magnetic poles in broken magnet
Experiment
1. A magnet attracts nails.
Break it and note that the broken pieces have formed new magnetic poles.
A broken magnet still exhibits north and south poles.
2. Break a magnetized steel wire in half.
Test both ends of each broken portion.
The magnetism found on each side of the break has opposite polarity.
Break off a very small piece of the wire magnet and test it with iron filings.
The smallest piece of the wire is a magnet with opposite poles.
Magnets always have two poles.
29.1.3 Buzzing magnets
'Buzzing magnets", 'Magna Buzz", "Singing Stones', "Singing Magnets", "Rattle Snake Egg Magnets" (toy products)
Toy shops may sell them as two finely-polished, oval-shaped synthetic haematite magnets (barium-strontium ferrite (Ba, Sr)Fe12O19), each about 4.5 cm long, smoke grey in colour, smooth and shiny.
Experiment
Hold them in your hand and toss them in the air for a buzzing or clicking sound when they come together and bounce off each other.
When they come together they produce a sharp, loud buzz that rises in pitch.
The poles of the magnets are oriented along one of the minor axes of the ellipsoid, not the major axis.
They are very strong magnets, so keep them away from video screens, credit cards and computers!
29.1.4 Compass needle, north-seeking pole, south-seeking pole
See: Magnetic fields, Magnetic, needle and stand, (Commercial).
See diagram 29.164.1: Magnetized steel strip.
See diagram 29.164.2: Magnetized sewing needles.
See diagram 32.163.2: Plotting compass in a matchbox.
1. Magnetize a sewing needle by stroking it with a bar magnet.
Make a simple compass by the following methods:
Push the magnetized needle through cardboard and suspend it on a thread, push the needle through the projections of a cloth-covered button, attach the needle to a strip of cardboard and balancing it over an inverted test-tube supported on a long pin.
Label the end of the magnet that tends to point north.
2. Make another simple compass needle by the following methods:
Push two magnetized sewing needles through the holes of a large press stud and balancing it on the end of a needle pushed into a cork, push a magnetized needle through thin cardboard and suspend it on a thread inside a glass jar.
3. Compare the north direction shown by a plotting compass with the directions shown by the simple compass needles.
A compass needle is marked "N" at one end.
This end points towards the north magnetic pole so it is the "north-seeking pole" of the magnet.
The other end is the "south-seeking pole".
29.1.5 Cut an iron wire magnet
Experiment
Cut in half a magnetized steel wire.
Test both ends of each broken portion.
The magnetism found on each side of the break has opposite polarity.
Cut off a very small piece of the wire magnet and test it with iron filings.
The smallest piece of the wire is a magnet with opposite poles.
29.1.6 Freely-suspended magnet
See diagram 29.167: Suspended magnet.
Experiment
1. Use loops of cotton to suspend two magnets freely.
Bring each pole of the two magnets close to, but not touching, each other.
Show that like poles repel and unlike poles attract.
2. Use a thin string to suspend a bar barnet so that it is free to rotate horizontally.
The end of the bar magnet that points toward the north geographic pole of the Earth, when the magnet is suspended is called freely, is called the north-seeking pole or north pole of the magnet.
The end of the bar magnet that points toward the south geographic pole of the Earth, when the magnet is suspended freely, is called the south-seeking pole or south pole of the magnet.
29.1.8 Magnets on a pivot
See diagram 29.2.3.1: Magnet on a pivot.
Experiment
1. One magnet is centred on a pivot.
Hold one end of the second magnet near one end of the magnet on the pivot.
Repeat with the opposite end.
2. Place one magnet on a pivot and use the other to attract or repel the first magnet.
Place a magnet in a cradle then use a second magnet to attract and repel the first.
Show interaction between bar magnets.
Show magnetic attraction / repulsion.
Show the lines of force.
29.1.9 Natural magnets
A form of magnetite, iron (II, III) oxide, called lodestone, acts as a magnet when freely suspended.
It was probably first discovered in China, where they used it for the first magnetic compasses.
29.1.10 Simple compass needle
See diagram 31.67.1: Simple compass needles.
Experiment
1. Magnetize a sewing needle by stroking it with a bar magnet.
* Make a simple compass by pushing the magnetized needle through cardboard and suspending it on a thread inside a glass jar,
* Push the needle through the projections of a cloth-covered button,
* Attach the needle to a strip of cardboard and balancing it over an inverted test-tube supported on a long pin.
Label the end of the magnet that tends to point north.
See diagram 31.67.2: Two magnetized sewing needles.
2. Make another simple compass needle by pushing two magnetized sewing needles through the holes of a large press stud and balancing it on the end of a needle pushed into a cork.
3. Compare the north direction shown by a plotting compass with the directions shown by the simple compass needles.
A compass needle is marked "N" at on end.
This end points towards the north magnetic pole so it is the "north-seeking pole" of the magnet.
The other end is the "south-seeking pole".
29.1.1.7 Magnetic force and magnetic field lines (lines of force)
See diagram 29.1.1.7: Magnetic field lines (lines of force).
Experiment
Place a glass plate over a bar magnet and sprinkle iron filings over the glass.
Tap the glass plate a few times to overcome friction and the iron filings arrange themselves in a pattern of arcs from one pole of the magnet to the other.
This pattern is a map of the unseen magnetic force that permeates space.
Magnetic fields show the effect a magnet would have in that region.
Magnetic fields are usually represented by drawing magnetic field lines in the same way that electric field may be shown.
The closer the magnetic field lines the stronger the magnetic field.
Magnetic field lines point away from the north poles towards the south poles in closed loops that never cross over.
29.1.7 Magnet assortment, natural magnets, artificial magnets
The most common natural magnets are a form of magnetite, iron (II, III) oxide, called lodestone, that acts as a magnet when freely suspended.
Lodestone was common in Magnesia in the Kingdom of Lydia, an ancient kingdom now in western Turkey.
Previously, a lodestone was supposed to have magical properties, because lodestone attracts small nails.
Two pieces of magnetite in paper stirrups come to rest on the magnetic meridian.
Magnetite was probably first discovered in China and was used for the first compasses.
Look for low cost artificial magnets in discarded loudspeakers, telephone receivers and other equipment.
Artificial magnets have different shapes, e.g."Alnico", horseshoe magnet, pairs of bar magnets with a soft iron keeper, cylindrical magnets, C-magnets, U-magnets, "Alcomax" magnets, and powerful magnets.
Store artificial magnets in pairs in a box, north to south and south to north.
Experiment
1. List all the different kinds of magnets, 1.1 in the laboratory, 1.2 in the home, 1.3 in a motor car.
2. Suspend a large lodestone in a cradle with the south pole painted white.
Use a bar magnet is used to show attraction and repulsion.
29.1.11 Magnetized test
See diagram 29.1.1.4: Like and unlike poles of a bar magnet.
Experiment
1. Two bars look alike, but one is a magnet and the other is not a magnet.
With two similar bars of iron one magnetized, use the end of one to lift the middle of the other.
2. Many iron and steel objects are magnetized without you knowing it.
Detect this magnetism with a compass.
If a rod is magnetized, it must, like the compass needle, have a north pole and a south pole.
The rule of magnets is that two unlike poles attract and two like poles repel.
So one pole of the needle will be attracted to the end of the rod and the other repelled.
If the rod is not magnetized, both poles of the needle are weakly attracted to the end.
Collect objects made of paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin.
Test each object with a magnet to see which are attracted to the magnet and which are not.
Bring a soft iron wire and hard steel or piano wire near a compass needle to see if it is affected by a magnetic field.
3. Bring the north poles of 2 bar magnets near each other.
Feel the force of repulsion between them.
Reverse one magnet and feel the force of attraction.
Reverse the other magnet and feel the force of repulsion when the 2 north poles come together.
29.1.12 Magnetic poles, magnetize wire, freely suspended magnets
Magnets always have two magnetic poles.
If a bar magnet is broken in two each half has two poles.
Magnetism is the strongest at the poles of a magnet.
Use a bar magnet, a horseshoe-shaped magnet, a magnetized needle and other magnetized objects.
Immerse them in fine iron filings then take them out.
Note that most filings are at the poles.
Scatter iron filings or iron powder over every part of a magnet and note that some filings slip off the magnet and some filings are attracted to its poles.
Experiment
1. Magnetize a piece of iron wire or a needle by rubbing with a bar magnet.
Find its poles with iron filings.
Cut into two the magnetized iron wire or needle with pliers then test it with filings again.
Each piece still has two poles.
Cut each piece into two parts again then test them with filings.
Each small piece has two poles.
No matter how short the remaining wire is, it has two poles.
Outside magnetic effects can cause the domains in a ferromagnetic material to act strongly in one direction to magnetize the substance temporarily in soft iron and permanently in hard steel.
Soft iron is used in electromagnets so the magnetism can be "turned on" and "turned off".
Permanent magnetism is necessary for compasses and permanent magnets, but heating or hammering can destroy it.
2. Use a 6 cm length of steel wire or piano wire.
Draw one end of a steel magnet along it once only and in one direction from end to end.
Lay the wire on a piece of paper then test for magnetism by sprinkling iron filings over it.
The iron filings are not attracted equally along its whole length.
They call the areas of strongest attraction the "magnetic poles".
3. To isolate a magnetic pole pass a long magnetized knitting needle through a cork and float it on water.
4. Make a freely suspended magnet.
Use loops of cotton to suspend two magnets freely.
Bring each pole of the two magnets close to, but not touching, each other.
Show that like poles repel and unlike poles attract.
29.2.1 Classes of magnetism, domains, magnetic permeability
See: Magnets (Commercial).
See diagram 29.173.4: Magnetic domain models.
Magnetic domain models show two types of arrays: 1. Non-coplanar (random orientation) and 2. Coplanar, to show domains.
Magnetism is classified as diamagnetism, paramagnetism, ferromagnetism and ferrimagnetism.
1. Ferromagnetism occurs in the ferromagnetic materials, iron, cobalt, nickel, and ferrite metallic oxides when groups of atoms, called domains, have the same directions of spin and parallel alignment of of the magnetic moment of neighbouring atoms.
Ferromagnetic substances have large magnetic permeability and hysteresis.
When you think of magnetic materials, you probably think of iron, nickel or magnetite.
Unlike paramagnetic materials, the atomic moments in these materials exhibit very strong interactions.
These interactions are produced by electronic exchange forces and result in a parallel or anti parallel alignment of atomic moments.
2. Ferrimagnetism is weaker than ferromagnetism, because of anti-parallel alignment of neighbouring atoms or ions having weaker magnetic moments.
In some ionic compounds, e.g. oxides, including magnetite, more complex forms of magnetic ordering occur, because of the crystal structure.
29.2.2 Diamagnetism
29.6.0: Magnetic permeability.
Almost all substance contain some degree of diamagnetism.
Diamagnetism occurs when a substance is weakly affected by a strong magnet, e.g. Bi (the strongest metal diamagnetism), Hg, Ag, Cu, diamond, graphite, water and superconductors.
Diamagnetic materials have a very small negative magnetic susceptibility.
They are magnetized in a direction opposite to the applied magnetic field, so magnetic permeability < 1.
However, the effect is hardly noticed, except in magnetic superconductors.
Diamagnetic materials are repelled by a magnetic field.
The atoms of diamagnetic substances have no net magnetic moments.
All the orbital shells are filled so there are no unpaired electrons.
Diamagnetism is a fundamental property of all matter, although it is usually very weak.
It is caused by the non-cooperative behaviour of orbiting electrons when exposed to an applied magnetic field.
Diamagnetic substances are composed of atoms that have no net magnetic moments, i.e., all the orbital shells are filled and there are no unpaired electrons.
However, when exposed to a magnetic field, a negative magnetization is produced, but when the magnetic field is zero the magnetization is zero.
The behaviour of diamagnetic materials depends on the temperature.
Diamagnetic substances include quartz, calcite and water.
Experiment
If a very strong magnet is placed above the hand, a small magnet can be in a state of stable levitation between the first finger and the thumb.
The repulsive diamagnetic force of the finger pushes it down and the balancing diamagnetic repulsion from the thumb pushes it up.
29.2.4 Ferromagnetism
When you think of magnetic materials, you probably think of iron, nickel or magnetite.
Unlike paramagnetic materials, the atomic moments in these materials exhibit very strong interactions.
These interactions are produced by electronic exchange forces and result in a parallel or antiparallel alignment of atomic moments.
Exchange forces are very large, equivalent to a field on the order of 1000 Tesla, or approximately a 100 million times the strength of the earth's field.
The exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of two electron.
Ferromagnetic materials exhibit parallel alignment of moments resulting in large net magnetization even in the absence of a magnetic field.
The elements Fe, Ni, and Co and many of their alloys are typical ferromagnetic materials.
Two distinct characteristics of ferromagnetic materials are their spontaneous magnetization and the existence of magnetic ordering temperature.
Spontaneous magnetization
The spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume in the absence of a field.
The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic moments of electrons.
A related term is the saturation magnetization, which we can measure in the laboratory.
The saturation magnetization is the maximum induced magnetic moment that can be obtained in a magnetic field.
Beyond this field no further increase in magnetization occurs.
The difference between spontaneous magnetization and the saturation magnetization has to do with magnetic domains.
Saturation magnetization is an intrinsic property, independent of particle size, but dependent on temperature.
There is a big difference between paramagnetic and ferromagnetic susceptibility.
As compared to paramagnetic materials, the magnetization in ferromagnetic materials is saturated in moderate magnetic fields and at room temperatures.
29.2.4.5 Float magnetized needles
See: Magnetic fields, magnetic "levitation stands", (Commercial).
See diagram 29.1.1.8: Float magnetized needles.
Experiment
1. Stroke pins with the north pole of a bar magnet.
Very carefully lower the pins into water so that they float.
Note how they line up end to end.
Move the pins to make circles, north to south poles.
2. Stroke three pins many times with the north pole end of a magnet in the same direction so that their points attract each other.
Put each pin in a little paper boat made of greaseproof paper.
Put the boats in a dish of water.
The boats will line up end to end in a north south direction.
3. Rub eight needles on one pole of a magnet to magnetize them and make the sharp end of them being the same pole.
Push each needle through a cork leaving only one cm length in the cork.
Float the magnetized needles on the surface of water in a plastic bowl.
Put one pole of a strong magnet above the floating magnet needles and the floating needles will change their positions to form a certain picture.
Increase or decrease the numbers of the magnet needles, change the poles of the magnet needles, change the distance from the pole to magnet needles and observe if the shape of the picture changes.
29.2.5 Ferrite magnets
Ferrite magnets are iron-based magnets made cheaply from abundant natural resources.
They are strong and resistant to corrosion.
They have a low energy density, which does not matter for very big magnets, but is important for very small magnets.
Ferrites are mixed oxide of iron (III) oxide (Fe2O3), and another metal, used in high frequency electrical components as powders are used as a magnetic coating in audi tapes and computer floppy disks, e.g. "Mn-Zn Power Ferrite", a "soft ferrite" MnaZn(1-a)Fe2O4.
29.2.6 Hematine
Hematine, "synthetic "haematite", is used to make magnets, also called "magnetic hematite", "hematite", synthetic made from ceramic barium-strontium ferrite (Ba, Sr)Fe12O19, has magnetic field strength much larger than magnetite.
29.2.7 Paramagnetism
"Paramagnetic Putty, responds to a magnetic field, so is paramagnetic, (toy product) See 29.5.0: Magnetic permeability.
See diagram 29.173.5: Paramagnetic or diamagnetic crystals inserted between poles of a large magnet or electromagnet.
Paramagnetism refers to the tendency of the atomic magnetic dipoles in a material that is otherwise non-magnetic to align with an external magnetic field.
The alignment of the atomic dipoles with the magnetic field tends to strengthen it, resulting in a relative magnetic permeability greater than one and a small positive magnetic susceptibility.
In paramagnetism, the field acts on each atomic dipole independently and there are no interactions between individual atomic dipoles.
Paramagnetic behaviour can also be observed in magnetic materials that are above their Curie or Neel temperature.
Paramagnetism occurs when substances can produce a weak magnetic field in the same direction as that of a strong magnet, e.g. Mg, Mo, Li, Ta, W, Al.
Paramagnetic materials are slightly attracted by a magnetic field, but do not retain the acquired magnetic properties when the external field is removed.
Paramagnetic properties are caused by some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field.
The refrigerator magnet is paramagnetic.
In paramagnetic materials some atoms or ions have a net magnetic moment caused by to unpaired electrons in partially filled orbitals, e.g. iron.
However, the individual magnetic moments do not interact magnetically, and like diamagnetism, the magnetization is zero when the magnetic field is removed, because thermal motion randomizes the spin orientations.
Paramagnetism is temperature dependent (Curie law).
The temperature may be too high or too low.
However, when a strong magnetic field is applied a magnetic field forms.
Paramagnetic iron-bearing minerals include montmorillonite, nontronite, biotite, siderite and pyrite.
Paramagnetism may be strong in superconductors, the paramagnetic Meissner effect
29.2.8 Separate crystals using paramagnetism
Two magnets are placed one over the other at 4.5 cm apart with like poles facing to produce a magnetic field with a linear gradient and a minimum in the middle between the two magnets.
The crystals to be separated are suspended in a solution of paramagnetic ions and placed in a tube within the magnetic field.
The crystals to sink down to the bottom of the tube to displaces their own volume of the paramagnetic fluid upwards.
The paramagnetic fluid is attracted by the magnet, stronger closer to the face of the magnet.
The crystals sink as long as it reaches a distance above the magnet where the sinking gravitational force and the magnetic attraction on the equivalent volume of the paramagnetic fluid are balanced.
At this point, the crystals will “float” at a level in the fluid.
The strength of the sinking gravitational force depends on the density of the crystal, so different crystal forms can be separated with a difference in density as low as 0.001 g / cm3.
29.2.9 Permanent magnets
Strong permanent magnets are made from alloys of aluminium, nickel and cobalt with iron.
Alnico ™ and Alcomax ™ are two of the trade names of such alloys.
Small amounts of Cu, Ti and Nb may also be used.
The same composition may have different trade names.
"Alnico magnets" more than double the energy density of the most efficient ferrite magnets.
Magnets made from neodymium (+ some dysposium), iron and boron, "Neo" magnets, are very powerful.
However, neodymium is a rare earth available for industrial extraction mainly in China, US and Australia.
Dysposium may only be available for mining in China.
The demand for these rare earths for use in electric cars and wind turbines may not be met soon.
Permanent magnets are used in telephones, electric motors and bicycle dynamos.
29.2.10 Test diamagnetic and paramagnetic crystals
29.6.0: Magnetic permeability.
In this class of materials, some of the atoms or ions in the material have a net magnetic moment due to unpaired electrons in partially filled orbitals.
One of the most important atoms with unpaired electrons is iron.
However, the individual magnetic moments do not interact magnetically, and like diamagnetism, the magnetization is zero when the field is removed.
In the presence of a field, there is now a partial alignment of the atomic magnetic moments in the direction of the field, resulting in a net positive magnetization and positive susceptibility.
In addition, the efficiency of the field in aligning the moments is opposed by the randomizing effects of temperature.
This results in a temperature dependent susceptibility, known as the Curie Law.
At normal temperatures and in moderate fields, the paramagnetic susceptibility is small (but larger than the diamagnetic contribution).
Unless the temperature is very low (<<100 K) or the field is very high paramagnetic susceptibility is independent of the applied field.
Under these conditions, paramagnetic susceptibility is proportional to the total iron content.
Many iron bearing minerals are paramagnetic at room temperature, e.g. montmorillonite (clay), nontronite (Fe-rich clay), biotite (silicate) siderite (carbonate), pyrite (sulfide). Experiments
1. Place paramagnetic and diamagnetic crystals between the ends of a large electromagnet.
2. Place small samples of bismuth, aluminium, glass, between the ends of a strong electromagnet.
3. Suspend samples of bismuth and copper (II) sulfate by threads.
4. A large horseshoe magnet attracts the copper (II) sulfate and repels the bismuth.
5. A dollar bill is attracted by a strong magnet.
6. Pull the bubble in a carpenter's level with a strong magnet.
7. Spill liquid air drops around on a sheet of paper.
Liquid oxygen sticks to the electromagnet until it evaporates.
8. Fill a test-tube with liquid oxygen.
Suspend the test-tube by a long string attached to the ceiling.
Bring a powerful magnet to the side of the test-tube.
The position of the test-tube changes due to paramagnetism.
29.2.11 Store bar magnets
See diagram 29.1.1.0: Stored bar magnets.
Permanent magnetism can be lost by hammering or heating.
Bar magnets and horseshoe magnets can lose their magnetism if you treat them roughly or if you do not store them in pairs with soft iron keepers, N pole to S pole and S pole to N pole.
A magnetized ring of iron keeps its magnetism better than a bar of iron with two magnetic poles.
So the "keepers" keep the magnetic flux in a magnetic circuit with no free magnetic poles.
Experiment
Store artificial magnets in pairs in a box, north to south and south to north.
Keep magnets away from computer diskettes and colour television screens!
29.2.1.0 Magnetic fields and forces, magnetic flux, Φ
1. Magnetic fields
Magnetic fields occur in magnetic neutron stars, magnetic resonance imaging (MRI), sunspots, bar magnets, the earth.
2. Magnetic flux
The "amount" of magnetism is called magnetic flux.
The magnetic field at a place, the magnetic flux density, vector B, is tangential to the magnetic field line drawn through that place.
F (newton) = q (coulomb) × v (volt) × B (tesla) sin a (where a = the angle between the magnetic field lines and the direction of a moving charge).
The CGS (cgs) unit for B is the gauss (G).
I G = 10-4 T.
Magnetic flux through an area is the product of the component of B perpendicular to the surface area, A, and 1. Magnetic flux = B × A Cos a (the angle between the direction of the magnetic field and the area at right angles), in weber, Wb.
If a magnetic field is perpendicular (B) to an area A in the magnetic field, magnetic flux, Φ, = B A newton metre / ampere, weber B = Φ / A, weber / metre2 = newton / ampere metre, tesla, T.
1 T = 1 weber per square metre, Wb / m2.
The SI unit of magnetic flux density = one weber per meter squared (1 Wb × m-2 ).
SI unit  = tesla,   1 T =  one kilogram per second squared per ampere (kg × s-2 × A-1 ).
1 tesla = 1T = 1 N / (A × m), where N = newtons, A = amperes and m = metres.
1 tesla = 10, 000 gauss, 1 gauss = 1 G = 10-4 T.
The magnet field on the surface of the earth is about 0.5 G magnetic field strength, H, is measured in amperes per meter, A / m magnetic flux density, B, is measured in Newton-meters per ampere (Nm / A), teslas (T).
The magnetic field can be visualized as magnetic field lines.
The field strength corresponds to the density of the field lines.
The total number of magnetic field lines penetrating an area is called the magnetic flux.
The unit of the magnetic flux is the tesla meter squared (T· m2, also called the weber and symbolized Wb).
The older units for the magnetic flux, the maxwell (equivalent to 10-8 Wb), and for magnetic flux density, the gauss (equivalent to 10-4 T), are obsolete and seldom seen today.
Magnetic flux density diminishes with increasing distance from a straight current-carrying wire or a straight line connecting a pair of magnetic poles around which the magnetic field is stable.
At a given location in the vicinity of a current-carrying wire, the magnetic flux density is directly proportional to the current in amperes.
If a ferromagnetic object such as a piece of iron is brought into a magnetic field, the "magnetic force" exerted on that object is directly proportional to the gradient of the magnetic field strength where the object is located. 3. Deflection magnetometer
The deflection magnetometer is designed to compare the strengths of two magnetic fields acting at right angles to one another.
It consists of a small magnet ns pivoted at the centre of a large circular scale graduated in degrees and a light aluminium pointer.
Any deflection of the magnet causes the pointer to move through the same angle.
The field to be tested is always placed perpendicular to the earth's horizontal component, i.e. in a magnetic east-west direction.
If the strength of the field = H oersted and the earth's horizontal component = H0 oersted, by the parallelogram of forces law, the resultant R makes an angle a with Ho.
The magnet aligns itself with this resultant field, i.e. is deflected through an angle a measured on the circular scale.
H / H0 = tan a, so H = H0 tan a.
29.2.12 Hanging magnets and inverse square law
Magnetic pole strength of bar magnet
See diagram 29.2.3.3: Deflection magnetometer.
The inverse square law of magnetism states that the force, F, between two magnetic poles varies inversely as the square of the distance, d, between them, i.e. F is proportional to 1 / distance2.
Experiment
Rotate a magnetometer until the pointer indicates 0o at one end, and the arms lie magnetic East West.
Clamp the ball-ended magnet at its centre so that ball A lies vertically above, and ball B magnetic east of the pivoted magnet, P, to give a 35o deflection.
Read both ends of the pointer to eliminate the error if the pivot is not at the centre of the circular scale.
Record the distance, d cm, of the centre of B from the pivoted magnet, P.
Repeat with ball B at the same distance from, and magnetic West of, the pivoted magnet to eliminate the error if the pivot is not at the zero marks of the linear scales.
Record the readings of both ends of the pointer.
Repeat the above procedure for values of d to give deflections between 30o and 60o.
Table 29.2.3.3 Hanging magnets 
d cm B east of P, a1 B east of P, a2 B west of P, a3 B east of P, a4 Mean a tan a 1 / d2
-
 -
 -
 -
 -
 -
 -
 -
Experiment
Draw a graph of tan a (y axis) against 1 / d2 (x axis).
The magnetic poles of a ball-ended magnet are at the centre of each ball.
Ball A has no influence on the needle since at P its field is vertical.
Assuming the inverse square law, magnetic intensity H in a horizontal direction at P caused by ball B = m / d2, m is the pole strength of B.
However, H = Ho tan a, Ho is the horizontal component of the earth's field at P and a is the angle of deflection of P.
m / d2 = (H0 tan a), so 1 / d2 = [(H0 / m) tan a].
However, Ho / m is a constant, so 1 / d2 is proportional to tan a, assuming the inverse square law.
If the graph of 1 / d2 against tan a is a straight line passing through the origin, the inverse square law is verified.
Experiments
1. Hang two magnets horizontally and parallel.
Use the inverse square law to compute the magnetic pole strength from the length of the suspension the saturation and mass of the magnets.
2. Find the magnetic length 2L of a weak bar magnet with known polarity.
Draw the outline of the bar magnet.
Put a plotting compass in several positions near one end and mark with a pencil dot the position of each end of the compass needle.
Repeat the procedure at the other end of the magnet.
Remove the magnet.
Draw a straight line through each pair of dots, producing the lines to intersect over two small areas that are the poles of the magnet.
The distance between these poles is the magnetic length 2L of the magnet.
3. Put the magnet in the centre of the paper on the board with its south pole pointing magnetic north and its axis in the magnetic meridian.
Plot lines of force in the region of the neutral points P and Q.
When the compass is placed on these points the needle does not set in any particular direction.
Measure the distances d1 and d2 from the centre of the magnet.
If the pole strength of the bar magnet is m and its magnetic length is 2L, then the field strength H at a point distance d from its centre and on its magnetic axis produced = 4mLd / (d2-L2)2.
At the neutral points, the field H caused by the magnet is equal and opposite to H0, the earth's horizontal component.
So H0 = 4mLd / (d2- L2)2, d = average distance of neutral points P and Q from centre of magnet.
4. Recoiling magnets
* Hold two small horseshoe magnets together on an overhead projector and observe the recoil.
* Pull apart two elastic band reaction carts of unequal mass attached with an elastic band.
A stretched rubber band pulls two carts together with accelerations inversely proportional to their mass.
29.3.1 Ship's compass, Points of the compass
See diagram 8.31: Compass rose.
The English compass has Cardinal four names: N, E, S. W, Eight principal winds: N, NE, SE, S, SW, W, NW.
To box the compass is to name the 32 points of the compass in correct order.
A wind may said to box the compass if it blows from every quarter in succession.
The 32-wind compass rose with each direction 11.24o from the next compass direction (32 X 11.25o = 360o)
# Compass point, Middle azimuth o
1 North, N, 0.00o
2 North by east, NbE, 11.25o
3 North-northeast, NNE, 22.50o
4 Northeast by north, NEbN, 33.75o
5 Northeast, NE, 45.00o
6 Northeast by east, NEbE, 56.25o
7 East-northeast, ENE, 67.50o
8 East by north, EbN, 78.75o
9 East, E, 90.00o
10 East by south, EbS, 101.25o
11 East-southeast, ESE, 112.50o
12 Southeast by east, SEbE, 123.75o
13 Southeast, SE, 135.00o
14 Southeast by south, SEbS, 146.25o
15 South-southeast, SSE, 157.50o
16 South by east SbE, 168.75o
17 South, S, 180.00o
18 South by west, SbW, 191.25o
19 South-southwest, SSW, 202.50o
20 Southwest by south, SWbS, 213.75o
21 Southwest, SW, 225.00o
22 Southwest by west, SWbW, 236.25o
23 West-southwest, WSW, 247.50o
24 West by south, WbS, 258.75o
25 West, W, 270.00o
26 West by north, WbN, 281.25o
27 West-northwest, WNW, 292.50o
28 Northwest by west, NWbW, 303.75o
29 Northwest, NW, 315.00o
30 Northwest by north, NWbN, 326.25o
31 North-northwest, NNW, 337.50o
32 North by west, NbW, 354.37o
The loose wire is catapulted horizontally away from the magnetic field, due to the Lorentz force acting on the electric current in the wire.
29.4.1 Magnetic fields
Magnetic fields, magnetic poles, permanent and temporary magnets, magnetize, magnetic length
See: Magnets (Commercial).
See diagram 29.4.0: Permanent bar magnet.
1. A magnet is any object, usually made of iron, steel or iron ore, that has the property of attracting iron and aligning north-south when freely suspended.
Bar magnets are in the form of a straight bar.
Horseshoe magnets have the bar bent until the ends almost meet in one line.
Magnetic substance have the properties of a magnet.
2. Any object that can produce a magnetic field, a field of force produced by a magnet, is usually called a magnet.
Magnets are masses of a substance that can repel or attract the same substance.
3. All magnets have two poles, the magnetic north pole and the magnetic south pole.
If a magnet is broken into two pieces, each piece is a magnet with its own north pole and south pole.
4. Magnets may be temporary magnets or permanent magnets.
Temporary magnets are made of soft iron.
A magnet may magnetize a piece of soft iron to give it the temporary properties of a magnet.
External magnetic forces can induce magnetism, i.e. magnetize ferromagnetic materials.
Induced magnetism can be temporary magnetism as in the soft iron used in electromagnets or permanent magnetism as in hard steel.
Temporary magnetism lasts only if the external source of magnetism lasts.
Some solutions of salts, e.g.MnCl2, FeCl3, CoSO4, show some magnetic susceptibility using a Quincke-type glass tube.
Permanent magnets are made of steel and include the bar magnets in school laboratories and compass needles.
5.The magnetic length, 2L, is the distance between the two poles and is always less than the physical length.
6. Moving electric charges may cause magnetization, so do not carry out magnetism experiments near large masses of magnetic material or near apparatus or wires through which an electric current is passing.
Most substances are not magnetic, because the electrons in their atoms have equal numbers with opposite spin alignments.
In iron and elements near it in the periodic table, (e.g. Co and Ni), the outer electrons concerned with chemical bonding can have the same spin alignment and this can remain permanent to form permanent magnets when the substances are subjected to a strong magnetic field.
29.4.2 Magnetic field of the Earth
Magnetic declination, magnetic dip, geomagnetism
The Earth has a magnetic field inclined at about 11o to its rotational axis, with its size on the Earth's surface varying by 25 to 65 microtesla,  µT (0.25 to 0.65 gauss, G), but is usually recorded in nanotesla, nT.
1 µT =1 000 nanotesla, nT.
The cause is probably the circulating electric currents in the Earth's molten metallic core.
The Earth dies not contain a huge bar magnet!
The vertical plane containing the poles of a compass needle is called the magnetic meridian.
The angle between the magnetic meridian and geographic meridian, i.e. the angle between true north and magnetic north, is called the magnetic declination (magnetic variation, magnetic deviation, angle of declination).
The magnetic declination is recorded on all accurate maps, e.g. to tell you that magnetic north is 9o east of true north.
Experiments
1. The magnetic field strength of the magnetic field of the Earth can be given absolute values with a coil carrying an electrical current and comparing it with a known value created by a coil of known size, current, and turns.
Place a compass on a bench to point north, place a magnetic field at right angles to the compass direction to make it move towards the east or west.
If the compass needle moves to 45°, then the magnetic field of the coil is the same magnitude as the magnetic field of the Earth, but acting at right angles.
The use the Biot-Savart Law to calculate magnetic field strength due to the coil: BC = 8 μoNI/(R x 5√5), where N = number of turns on each coil, (not the total), I = current, R = radius of the coil.
The constant   μo = permeability of free space (4πx10-7 Tm/A).
Place two Helmholz coils on the bench and place a compass between them on a stand so that it is in the centre of the coils.
Use plastic water pipe coils made of clear plastic, 109 mm diameter, (so R = 0.045 m).
Wind 25 turns of wire on each coil (so N = 25).
Orient the coils so their axis is east-west, i.e. at 90° to the compass needle.
2. Places with the same magnetic declination are shown on maps as isogonic lines.
It may also be shown as an angle at lookouts that people visit to see surrounding countryside.
If the compass needle points east of true north that place has an easterly declination and if west of true north it has a westerly declination.
At some places there is no difference in the direction of true north and magnetic north.
However, magnetic declination variation changes slowly with time and it also varies at different longitudes, and in the same place at different times of the year.
The magnetic dip (magnetic inclination) is the angle between the horizontal and a suspended bar magnet.
So a suspended bar magnet held in an aeroplane flying over the magnetic north or south pole dips straight down, because the angle of the magnetic dip is 90o.
So the magnetic poles are the two places where the planet's magnetic field points vertically downwards.
At the equator the magnetic field lines of the Earth are parallel.
The magnetic poles move so that the north magnetic pole was in Canadian territory, but is now outside it at about 5.9N 147.0W.
Magnetic poles have reversed about 170 times as shown by the study of palaeomagnetism, with the last reversal about 900, 000 years ago.
3. If the north pole of a compass needle by definition points toward the north magnetic pole of the Earth then it follows that the north geographical pole of the Earth is near the south pole of the magnetic field of the Earth.
However, this point of logic is not worth further mention, but some people insist that the geographic north pole of the Earth is really the south magnetic pole of the Earth's magnetic field.
4. Charged particles, electrons and protons, from the loop structures caused by the magnetic fields on the Sun, solar wind particles, interact with the terrestrial magnetic field to move in helical paths of the magnetic field lines, then collide with atoms and molecules in the Earth's atmosphere to form a glowing aurora borealis (northern lights) in the northern hemisphere and aurora australia (southern lights) in the southern hemisphere.
5. A pocket compass has a compass needle made of magnetized steel in a non-magnetic case.
It can be used to plot magnetic fields, because it shows the directions of lines of magnetic force.
Any unusual change of direction is called a magnetic anomaly, e.g. near a mountain containing iron minerals.
The study of the magnetic field of the earth in the geologic past, palaeomagnetism, shows that the earth's magnetic fields have reversed their polarity more than once.
A ship's compass contains a disc with parallel bar magnets attached underneath pivoted on a hard bearing.
The compass case floats in a liquid and is suspended so that it always remains horizontal.
The ship's compass is used fo r reading bearings and plotting courses relative to magnetic north.
Most ports have two prominent reference points to check that the ship's compass is correctly pointing to magnetic north.
The phrase "to swing a ship" refers to checking the compass deviation of a ship by swinging the ship in the smallest possible circle through the points of the compass and taking sightings on objects with known positions and comparing these sightings with the true bearings.
Sometimes the structure of the ship or even the cargo has its own magnetic properties that affect the true reading of the ship's compass.
6. The cardinal points of a compass are due north, south, east and west, i.e. in the direction of the poles, sunrise and sunset.
To "box the compass" is a nautical phrase meaning to name the 32 points of a ship's compass in correct order.
A wind that boxes the compass blow round from every direction until the starting direction.
7. The Australian Geomagnetic Reference Field model (AGRF) describes the geomagnetic field in the Australian region, updated at five yearly epochs.
Components of the Magnetic Field D, the magnetic declination (magnetic variation), is the angle between the horizontal component of the magnetic field and true north.
It is positive when the compass points east of true north, and negative when the compass points west of true north.
Declination is given in degrees and its annual change is in degrees per year.
The value of magnetic declination should be added to a magnetic compass bearing to yield the true north bearing.
F, the total field, is the strength of the magnetic field.
F is given in nanoTesla (nT) and its annual change in nT/year.
H, the horizontal field, is the strength of the horizontal part of the magnetic field.
H is given in nanoTesla (nT), and its annual change in nT/year.
X, Y, and Z are the magnetic field components in the true north, east, and vertically down directions.
This forms a standard right-handed coordinate system.
X, Y and Z are given in nanoTelsa (nT) and their annual change in nT/year.
I, the magnetic inclination, is the angle between the magnetic field and the horizontal plane.
It is positive when the magnetic field points down, as it does in the northern hemisphere, and negative when the magnetic field points up, as it does in the southern hemisphere.
Inclination is given in degrees and its annual change is in degrees per year.
29.4.3 Magnetic compass, simple compass needles
See: Compass, Magnetic Needle, (Commercial).
See diagram 29.164.1: Magnetized steel.
See diagram 29.164.2: Magnetized needles.
See diagram 32.163.2: Plotting compass.
A plotting compass is a small permanent magnetic needle pivoted at its centre, so it can swing freely in a horizontal plane inside a plastic or glass case.
The magnetic needle will turn in a magnetic field pointing along the direction of the field, i.e. the direction along which a north pole would be urged if it were free to move.
However, the direction shown by the compass needle can be affected by nearby metallic objects and power lines to cause inaccurate readings.
A compass needle is a permanent bar magnet one end of which points towards the magnetic north pole and is called the north seeking or N pole.
The other end is the south seeking or S pole.
A compass needle is marked "N" at on end.
As this end points towards the north Magnetic Pole, it is called the "north seeking pole" of the magnet.
The other end is the "south seeking pole".
Like poles repel each other and unlike poles attract each other.
A bearing is a horizontal angle fixing a direction in respect to North.
A bearing from one point to another can be fixed either on the map or in the field.
For a map bearing, the direction North is marked by meridians or grid lines.
A map bearing is either a true bearing or a grid bearing.
However, in nearly all maps are based on true north, so the grid lines run both north-south and east-west.
For a field bearing the direction North is shown by the magnetic needle, so a field bearing is a magnetic bearing, compass bearing.
Experiments
1. Make a simple compass needle.
Stroke a sewing needle many times in the same direction.
Push it sideways through a flat cork or through the centre of a circular piece of paper.
Put the needle and cork or paper in a plastic bowl of water.
The needle turns to a north-south direction.
Carefully turn the bowl in a circle.
The bowl turns, but the needle keeps pointing in the north-south direction.
2. Magnetize a sewing needle by stroking it with a bar magnet.
Make a simple compass by pushing the magnetized needle through cardboard and suspending it on a thread.
Label the end of the magnet that tends to point north with an arrow.
Use a magnetic needle on a stand or a 16 mm plotting compass and compare the direction it points to the direction of the simple compass needles.
3. Make another simple compass needle using two magnetized sewing needles pushed through the holes of a large press stud.
Balance it on the end of a needle pushed into a cork.
Repeat the experiment with this stand and other magnetized objects.
Use a half hemisphere-shaped metallic button with a smooth surface.
Place it on a piece of smooth glass.
Place a magnetized needle on the two buttonholes.
Repeat the experiment with this stand and other magnetized objects.
4. Push a magnetized needle through thin cardboard and suspending it on a thread.
Mark the end of the magnet that tends to point north.
Use a magnetic needle on a stand or a 16 mm plotting compass and compare the direction it points to with the direction of the simple compass needles.
A compass needle is marked "N" at on end.
This end points towards the north magnetic pole so you call it the "north seeking pole" of the magnet.
The other end is the "south seeking pole".
5. Rub a piece of hacksaw blade, a needle and a piece of razor blade with a pole of a bar magnet to magnetize them.
Repeat the experiment by rubbing back and forth in a single direction or in different ways.
Note whether direction of rubbing ways influences the magnetization effect.
Push a large iron nail through a cork and put a small test-tube over the point of the nail.
Place a magnetized hacksaw blade on the top of the test-tube.
Adjust its position until it balances on the test-tube.
Mark the balance point on the magnetized hacksaw blade.
Put a drop of hot wax on the top of the test-tube then quickly place the hacksaw blade on the test-tube again so that you glue the hacksaw blade on the test-tube firmly.
Bring the north pole (N pole) of a small permanent magnet close to one end of the saw blade.
If the end is attracted, it must be the S pole of the magnetized hacksaw blade.
If the end is repelled, it must be N pole of the magnetized hacksaw blade.
Outdoor product: "Lensatic or folding map compass, for bush walker and orienteering."
29.4.4 Magnetic dip, magnetic inclination
See diagram: 29.165: Magnetic dip needle.
The directions of lines of magnetic force vary from straight vertical at the magnetic poles to to horizontal only along an irregular line near the equator, called the magnetic equator, ME.
So the north end or south end of a magnetic needle tends to dip down in areas between the magnetic poles and the magnetic equator.
However, in some compasses this tendency is reduced by ensuring that the centre of gravity of the magnetic needle is below its pivot point.
The magnetic needle may be be counter-balanced for use between certain latitudes where they are expected to be used, e.g. for between 20o and 40o.
So such a magnetic needle must be rebalanced for other latitudes.
Dip is the angle in vertical plane between the Earth's magnetic field and the horizontal.
The Earth's magnetic north pole is at 67oS, 143oE.
The Earth's magnetic south pole is at 75oN, 101oW.
At the equator the dip is about 0o, so at the equator a suspended bar magnet hangs horizontally.
At the earth's magnetic poles the magnetic dip is 90o, so in an aircraft flight passing over the north magnetic pole, a suspended bar magnet will point vertically straight down!
At London the dip from the horizontal is 67o.
At New York the dip from the horizontal is 72o.
The earth's magnetic field is about 0.2 g.
29.4.5 Magnetic North Pole
"The magnetic north pole is moving faster than predicted", the National Oceanic and Atmospheric Administration, (NOAA) said 04/02/2019.
The agency released an updated version of the World Magnetic Model, which is used to ensure safe navigation for operating around the North Pole.
The previous version, made in 2015, was supposed to last until 2020, but the NOAA had to make another sooner than expected.
This version of the World Magnetic Model will only last a year before being updated as previously scheduled.
In the 1990s, the magnetic north poll started moving from just over 9 to about 34 miles per year.
Flows in the Earth's core are responsible for magnetic north pole's unusual behaviour.
Experiments
1. Cut a 2 cm wide rectangular strip of sheet copper.
Bend the strip into an U-shape stand.
Glue the bottom of the copper stand to the middle of a piece of plywood.
Glue a protractor to the front of the stand.
Insert a pin at the centre of each end of the cork.
Insert a steel knitting needle through the centre of the cork.
Place the cork on the stand supported by the two pins.
Adjust the lengths of the knitting needle on each side of the cork until the cork balances horizontally, i.e. it is balancing about its centre of gravity.
Take the cork off the stand and magnetize the knitting needle without changing its position in the cork.
Put the cork with the magnetized needle on the copper stand again.
When the knitting needle balances again, it inclines from the horizontal line.
Measure the angle between the needle and the horizontal line.
This is the magnetic dip angle or dip.
2. Stroke two pins many times with the north pole end of a magnet in the same direction so that their points attract each other.
Push them into each end of a thin stick of foam plastic to make a dip needle.
Push a sewing needle across the middle of the dip needle to act as a pivot to balance between two drinking glasses.
If you adjust the direction of movement to a north-south direction the dip needle will dip down from the horizontal, because it will be parallel to (tangent to) the earth's magnetic field lines.
At the equator the dip needle will be about horizontal, 90o to the vertical.
At the north or south pole it will point almost straight down, vertical.
3. Push a steel knitting needle through cylindrical cork at right angles to its long axis.
Push a pin into the centre of each end of the cork to act as an axle.
Balance the cork through its axle of pins on knife edges.
Magnetize the steel knitting needle using a magnetizing coil.
Balance the cork again.
One end of the needle is pulled downwards by the earth's magnetic field.
Fix a spirit level, or a glass tube containing a bubble in water, above the knitting needle.
Use a protractor to measure the angle of dip between the horizontal spirit level and the knitting needle.
At the north or south Magnetic Pole the needle should point straight down!
At the equator the knitting needle will be about parallel to the spirit level.
29.4.6 North pole of magnet pointing magnetic north
See diagram 29.4.2: Lines of magnetic force.
Experiments
1. Draw a line AB on paper fixed to a drawing board.
Place a plotting compass on AB and rotate the board until the line lies in the magnetic meridian.
Fix the position of the board with chalk marks.
Remove the plotting compass and place a weak bar magnet of known polarity in the centre of the board with its north pole pointing magnetic north and its axis over the line AB.
Draw the outline of the bar magnet.
Put the plotting compass close to the north pole of the magnet and make pencil dots A and B at the south and north poles of the compass needle.
Move the compass until its south pole is over B.
Tap it gently to prevent the needle sticking.
Draw a dot C at the north end.
Repeat this process until the line of dots either goes off the paper or finishes up at the south pole of the magnet.
Start again at a slightly different point A1.
Do this many times on both sides of the magnet.
Remove the magnet.
Join up the dots to give lines of magnetic force and show the directions of these lines with arrows.
2. South pole of magnet pointing magnetic north.
The method used is the same as used before except that the south pole of the magnet points towards magnetic north.
29.4.7 True north and magnetic north
Magnetic variation (magnetic declination, magnetic deviation)br> Study a pocket compass or a ship's compass and compare a compass reading between two points with the same two points shown on a map.
The study of palaeomagnetism records changes in the earth's magnetic field in the past.
The polarity of the earth's magnetic field has reversed many times.
This information can be used to date old rocks. Experiments
1. Study maps, e.g. the International Chart Series (Admiralty charts) from an hygrographic office or visit lookouts to see the importance of magnetic variation.
Find where the direction of true north is shown, then use a compass to measure the angle, D, between the horizontal direction it points, true north and the geographic meridian, magnetic north.
International charts used by mariners show the magnetic variation curves for a certain base year in in degrees followed by the letter E or W to denote east or west.
For example, at Cabo Maguari at the mouth of the Amazon River, the magnetic variation is 19oW (2'W), i.e. at Cabo Maguari a magnetic compass points 19o west of geographic north, in 1990, with a further movement west of 2' per year since 1990.
2. Hammer a soft iron bar pointing north and sloping downwards towards north.
The bar becomes slightly magnetic.
Some of its particles have become aligned with the earth's magnetic field.
Hammer it again pointing east-west.
The bar loses its magnetism. 3. Convert a compass bearings to a true north bearing.
If the magnetic declination for Perth (31 57'S, 115 51'E) at 1 July 2002 is -1.6 degrees.
A compass bearing of 72 degrees in Perth converts to a true bearing of 70.4 degrees [72 + (-1.6)].
Map and compass users may require the angle between grid north and magnetic north.
Grid north differs from true north by the "grid convergence".
The MGA94 grid convergence for the Perth location above is -0.6 degrees.
A true bearing of 70.4 degrees in Perth converts to a grid bearing of 69.8 degrees [70.4 + (-0.6)].
On some topographic maps, grid convergence and magnetic declination are shown in diagrammatic form, so the signs of these values can be deduced from the diagram.
29.6.0 Magnetic permeability
See 3.7.32: Teflon , Polytetrafluoroetheylene, PTFE.
29.2.7.8 Ampère's law (permeability of free space, μ0).
Magnetic permeability, is proportional constant magnetic induction and magnetic field intensity.
For a vacuum this constant is equal to approximately 1.257 x 10-6 henry per meter (H / m) in a vacuum.
It is much greater in different substances and so is often listed as relative permeability comparing its magnetic permeability to magnetic permeability in a vacuum, symbolized as µo.
The magnetic permeability SI units are henries per metre (H.m-1), and, newtons per ampere2 (N.A-2).
Table of the relative permeability of different substances,  μ / μ0:
1. Superconductors, μ / μ0 = 0
2. Bismuth, water, copper, sapphire, Teflon, μ / μ0 < 1
3. Vacuum, μ0 = 1, (called the permeability of free space, μ0)
4. Air, aluminium, platinum, wood, μ / μ0 >1
5. Neodymium magnet, μ / μ0 1.05
6. Pure iron, μ / μ0 5 000
7. Steel alloys, μ / μ0 100 - 4 000
8. Permalloy, μ / μ0 8 000
9.Metglas,  μ / μ0 1 000, 000
Magnetic fields of force can be shown as lines of magnetic force, magnetic flux.
You can draw lines of magnetic flux around a source of magnetism to show the magnetic field so that the tangent at any point gives the direction of the magnetic field.
The lines of magnetic flux are drawn as going from the north pole to the south pole.
A magnetic compass aligns itself in the direction of the magnetic field, i.e. a tangent to the line of magnetic force at that point.
Magnetic field is sometimes called the "flux density".
When two or more magnetic fields interact, the result is equal to the vector sum of the separate fields.
A magnetic field is a field of force that appears on magnetic poles or magnets, i.e. around a magnetic body.
Also a magnetic field appears around a current-carrying conductor and is associated with the motion of electrons in atoms.
The strength and direction of a magnetic field is measured by magnetic flux density, B, SI unit tesla, or magnetic field strength, magnetizing force, H, SI unit ampere per metre.
Magnetic flux is related to the product of the magnetic permeability of the medium and the magnetic field intensity normal to the surface, SI unit weber, CGS (cgs) unit maxwells. Experiments
1. Cover a bar magnet with a piece of stiff white paper.
Sprinkle iron firings on the paper and tap it lightly.
The iron filings line up along the lines of force from north pole to south pole.
Hold a small magnetic compass, plotting compass, above the paper.
It aligns itself to the direction of the magnetic field.
Move the compass around to see the directions of magnetic field at different places. 2. Put paper over a magnet.
Scatter iron filings on it.
Tap the paper lightly, and a pattern forms.
The curved lines of the iron filings show the direction of the magnetic force.
Make the pattern permanent by dipping paper into melted candle wax and let it cool.
Scatter iron filings on it.
Hold a hot iron over the wax after the formation of the magnetic lines.
The pattern will be fixed. 3. Place a piece of heavy paper over a bar magnet.
Sprinkle iron filings on the paper and tap gently.
The pattern shows the direction of the field of the magnet.
You can use a sheet of glass instead of the paper.
You may also plot the field with the aid of a small compass, placing it in various positions near the bar magnet and noting the direction in which the needle points.
The iron filings form themselves into lines, because each filing, being in a magnetic field, becomes itself a tiny magnet.
The north pole of each tiny magnet is attracted by a south pole of a magnet near by and the filings arrange themselves into lines.
4. The magnetic curves by sprinkling iron filings over a glass plate may be preserved indefinitely a glass is warmed on the smooth surface of a hot plate.
Put a piece of paraffin and let it spread evenly in a thin layer over the surface.
Remove the glass plate and let the surplus paraffin running off.
Form the image with iron filings that do not stick to the iron, so if the image is unsatisfactory the filings may be removed and a new figure taken.
To fix the curves, the plate of glass is again placed on the warming stove.
Cover the surface of the paraffin with white paint so the curves appear on a white background.
For a simpler process, cover one surface of stiff white paper with a layer of paraffin by warming over an iron plate, spread the filings over the cooled surface and fix them with a hot iron or gas flame.
5. "Magnetic Viewing Film" is a toy containing nickel flakes to show lines of force.
29.5.1 Curie point, Curie temperature
See diagram 29.1.6.1: Curie point.
A Curie point is the temperature at which the form magnetism or electrical behaviour of a substances changes, especially the temperature above which a ferromagnetic material becomes paramagnetic.
Iron under magnetic attraction is heated until it falls down but after cooling it is again attracted by the magnet.
For example, a soft iron wire held up by a magnet falls away when the wire is heated past the Curie point.
A length of soft iron wire heated with 110 V DC through a rheostat shows loss of magnetic properties when it passes through recalescence.
Recalescence is a sudden and temporary increase in glow and loss of heat in ferromagnetic material, as crystal structure and magnetic properties change during the cooling process.
Monel metals (nickel based alloys), have Curie points between 25oC and 100oC, depending on the alloy.
A rod of nickel is attracted to a magnet when cool, but swings away when heated.
Thermal energy produces a randomizing effect in ferromagnets at the Curie temperature (TC).
So below the Curie temperature, the ferromagnet is ordered and above the Curie temperature it is disordered.
The Curie point for iron is 770oC.
Experiment
Iron under magnetic attraction is heated until it falls away.
Upon cooling it is again attracted.
29.5.2 Meissner effect
See diagram 29.1.6.3: Meissner effect.
(F. W. Meissner, 1882-1974, Germany)
The Meissner effect occurs in a diamagnetic material, which expels all its magnetic flux when cooled below its critical temperature and a magnetic field is applied.
Such a material can show superconducting levitation.
For example a small powerful magnet can be placed over a superconductor cooled to liquid nitrogen temperature so that it "floats" in the air.
The Meissner effect is when magnets levitate over superconductors, because the magnet induces a magnetic field in the superconductor.
This effect is used in Maglev trains, e.g. in Shanghai, and in magnetic resonance imaging, MRI.
LN2, Liquid nitrogen is nitrogen in a liquid state at low temperature.
Liquid nitrogen BP at sea level is −195.79 °C.
Experiment
Place a magnet on warm superconducting disk to show how nothing happens, then remove.
Add LN2 to the Styrofoam container holding the superconducting disk.
When the boiling stops, the disk is cold.
Use the plastic tweezers to place one of the magnets on the disk.
  The magnet will float above the disk.
29.5.3 Thermomagnetic motor
Local heating of permalloy tape or nickel rings in a magnetic field will cause rotation.
Experiment
Place the rim of a wheel of Monel tape in the gap of a magnet and apply heat to one side to make the wheel turn.
Place a thin strip of magnetic alloy around the rim of a well balanced wheel in the gap of a magnet with a light focussed on a point just above the magnet.
Applied heat changes the magnetic properties and the wheel rotates.
9.4.8 Horseshoe magnet and copper wire
See diagram 9.2.6.2: Horseshoe magnet and copper wire
Place a strong horseshoe magnet on its side.
Suspend stiff copper wire between the two poles of the magnet like a trapeze.
Connect one of the flexible copper wires to a dry cell or DC. power supply then touch the other copper wire to the cell.
The copper wire trapeze will swing away from or towards the magnet, depending on the connection.
The motion is due the interaction between the magnetic field and the electric current in the trapeze.
9.4.9 Horseshoe magnet and conducting rails
See diagram 9.2.6.3: Horseshoe magnet and conducting rails
Construct a wooden frame as shown in diagram then mount two copper rails 75 mm apart across the centre of the frame.
Cut a piece of copper wire 100 mm long to lie across the conducting rails.
Mount a horseshoe magnet between the conducting rails so that the rails are a height midway between the poles of the magnet.
Connect the conducting rails to a DC power supply.
Energize the circuit and observe what happens to the copper wire conductor that lies across the conducting rails.
It will roll along the conducting rails, the direction depending on the electrical connections.