TERRESTRIAL
MAGNETISM
AND
THE DIRECT
INDICATING
COMPASS
By
Eysan Tugce Ozkan
Index
Index ................................................................................................................................... ii
Introduction ......................................................................................................................... 1
1. SubChapter One: Terrestrial Magnetism.......................................................... 2
The Magnet and Magnetic Field ................................................................................. 2
Magnetic Poles ............................................................................................................ 2
Magnetization Methods .............................................................................................. 3
Demagnetization Methods .......................................................................................... 4
Magnetic – Non-Magnetic Materials .......................................................................... 4
Hard Iron – Soft Iron................................................................................................... 4
Terrestrial Magnetism & Magnetic Variation............................................................. 5
Magnetic Dip .............................................................................................................. 6
Directive Force............................................................................................................ 6
Changes in Earth Magnetism ...................................................................................... 7
1.
Secular Change .................................................................................................. 7
2.
Unpredictable Changes ...................................................................................... 7
2. Subchapter Two: The Direct Indicating Compass ........................................... 8
Magnetic Compass ...................................................................................................... 8
Direct Reading Magnetic Compass ............................................................................ 8
Vertical Card Compass ............................................................................................... 8
Compass Requirements ............................................................................................... 8
1.
Horizontality ...................................................................................................... 9
2.
Sensitivity .......................................................................................................... 9
3.
Aperiodicity ..................................................................................................... 10
Direct Reading Compass Errors................................................................................ 10
1.
Liquid Swirl ..................................................................................................... 11
2.
Acceleration error ............................................................................................ 11
3.
Turning Error ................................................................................................... 12
4.
Deviation ......................................................................................................... 13
Bibliography ........................................................ Hata! Yer işareti tanımlanmamış.
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Introduction
One of the most interesting facts of our lives is magnetization and magnetism. Before
starting this article, in which we examine this mysterious structure, which finds application in
most of the electronic devices, the subject will be more understandable if we talk about the
magnetic attraction field of our earth.
The working principle of compasses is based on the existence of the earth's magnetic
field of attraction.
In this study, firstly magnet, magnetism and magnetic attraction field of the earth were
mentioned, and then the working principle of the compass was examined.
1. SubChapter One: Terrestrial Magnetism
The Magnet and Magnetic Field
A magnet is an object or material that produces a magnetic field. Magnets are usually
attached to some earth metals, such as iron or nickel, by magnetism, by creating a magnetic
field.
It is known that it was made by William Gilbert in the science book "De Magnet"
published in the 1600s, and the phenomenon of "Magnetism" was first introduced in this
scientific study published in England. Gilbert described the earth as a magnet in his book and
later works and showed the compass needle in the magnet as the earth's magnetic pole.
Magnetism, which studies magnetic force, magnetic field, and magnet, is also interested
in the effects and interactions of these phenomena in the electrical system. After the discovery
of the working principle of magnets and their relationship with electricity and magnetism,
Electromagnetism has also been among the subjects studied and researched. Magnetic currents
and the materials in which these magnetic currents take place are also in the main research
topics of magnetism.
Magnets can be formed in 3 ways;
•
natural magnets that exist spontaneously in nature,
•
artificial magnets
•
electromagnetic magnets created by electrical energy
The presence or absence of magnetism in materials is related to the rotation of the
electrons that make up the matter around their own axis, as well as the rotation around the
atomic nucleus. In this way, a magnetic field is created around an electron rotating around its
axis. The total attraction force created by all the electrons that make up the material
determines the magneticness of the material. If the number of electrons rotating in one
direction is different from the number of electrons rotating in the other direction, the material
shows magnetic properties. If the number of electrons rotating in both directions is equal,
these materials do not show magnetic properties and cannot create a gravitational field.
Magnetic Poles
Magnets have two poles, north and south. While these opposite poles attract each other,
the same poles repel each other. This is how magnetic poles are formed.
The areas where the magnet is most effective are these poles. The endpoints, where their
magnetic properties are best demonstrated, show repulsive and attractive properties. Each
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magnet contains 2 of these poles. It is impossible to get a single pole by smashing magnets. Each
piece will form 2 poles again.
The poles of the magnet are also called plus and minus. The positive end can be called
the anode and the negative end can be called the cathode. Apart from this, the most commonly
used form of magnet is referred to with north south poles. Two opposite poles must be brought
together to form magnetic poles. The magnetic field created by these poles appears to be
imaginary.
Magnetic poles also provide color discrimination. Mostly, the north pole is represented
by red, while the south pole is represented by the blue color.
Magnetization Methods
One of the following methods can be used to produce magnetism in an unmagnetized bar
of iron:
•
Stroking the bar repeatedly in the same direction with one end of a magnet, leaving the
end of the bar last touched by the red end of the magnet as a blue pole.
•
By aligning an iron bar with the lines of force of a magnetic field and vibrating or
pounding it. The major source of aircraft magnetism is agitation during manufacturing (in
the earth's magnetic field).
•
By inserting the specimen into a solenoid (a cylindrical coil of wire conveying a direct
current). This is the most satisfying way because the current running through the coil
creates a concentrated magnetic field along the coil's axis, allowing a high degree of
magnetism to be induced in the iron. (It should be noted that the quantity of magnetism
that may be produced is restricted since the iron gets magnetically 'saturated' at a certain
level.). The induced magnetic polarity would be reversed if the current flow was
reversed.
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Demagnetization Methods
Three ways of removing most or all of the magnetism from a magnetized item are listed
below. Demagnetization occurs naturally over time. The speed of the process depends on the
material, the temperature, and other factors. The most commonly used method to demagnetize
material before, or after magnetization for magnetic particle inspection is the use of a reversing
and decreasing applied magnetic field.
•
Shock: A magnetized bar of iron can be placed at right angles to the earth’s magnetic
field and hammered.
•
Heat: If the specimen is heated to about 900°C, it loses its magnetism, and this does not
return as the specimen cools.
•
Electric Current: The component is placed inside a solenoid carrying alternating
current, the amplitude of which is gradually reduced to zero. The strong alternating
magnetic field produced by the alternating current keeps reversing the direction of
magnetization (that is the polarity of the magnetism) in the specimen. Not only is the
polarity being reversed, but the intensity of magnetization is being reduced as the current
is reduced. The specimen’s magnetism is very quickly reduced to zero or very nearly
zero.
Magnetic – Non-Magnetic Materials
Magnetic materials are ‘ferrous’ metals iron and steel, steel being iron alloyed with
substances such as carbon, cobalt, nickel, chromium, and tungsten. These metals are called
‘ferromagnetic’ and in an aircraft they may be magnetized and produce deviation in the aircraft’s
compasses. Many materials used in aircraft construction are non-magnetic and do not affect the
compass. Substances that are negligibly affected by the magnetic field are known as nonmagnetic substances. These are copper, aluminum, gases and plastic. Also, pure oxygen exhibits
magnetic properties when cooled to a liquid state.
Examples of such non-ferrous substances are aluminum, duralumin, brass, copper,
plastic, and paint.
Hard Iron – Soft Iron
The words hard and soft do not refer to the physical properties of the material but to their
magnetic characteristics.
Hard Iron: Hard iron is iron that is difficult to demagnetize once magnetized. Hard iron
magnetism is said to be ‘permanent’, meaning that the material, typically steel containing cobalt
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or chromium, remains magnetized for an indefinite period after it has been removed from the
magnetizing field.
Soft Iron: Soft iron is iron that is easily magnetized and demagnetized with a small
change of magnetic field. Soft iron magnetism is called ‘temporary’ (or ‘transient’ or ‘induced’)
the substance being easy to saturate magnetically with only a weak magnetizing field but
retaining little or no magnetism when the field is removed. Nearly pure iron behaves in this way.
Some materials exhibit magnetic characteristics which lie somewhere between those of
hard iron and soft iron. These substances can be magnetized but this ‘sub-permanent’ magnetism
is lost partly or wholly over a period of time.
Terrestrial Magnetism & Magnetic Variation
The earth acts as if a massive permanent magnet were located at its center, producing a
magnetic field throughout its surface. The poles of this hypothetical earth-magnet do not lie on
the earth's spin axis, resulting in magnetic fluctuation due to this lack of symmetry. The magnetic
poles are not stationary and are currently shifting at a rate ranging from 6 to 25 NM each year.
The north magnetic pole is migrating more quickly than the south magnetic pole. The north
magnetic pole is currently positioned north of Alaska at 86°N 153°W, and the south magnetic
pole is located south of Australia at 64°S 136°E.
Magnetic variation is the angle between the horizontal plane and an angle between
magnetic north (the earth's magnetic field lines in the direction of a magnetized north end of the
compass, corresponding to the direction of the needles) and true north (with a meridian towards
the geographic North Pole). This angle changes depending on the position on the Earth's surface
and changes over time.
Traditionally, the declination is positive when magnetic north is east of true north and
negative when west. Isogonic lines are lines where the slope on the Earth's surface has the same
constant value, and lines where the slope is zero are called agonic lines. The lowercase Greek
letter δ (delta) is often used as a symbol for magnetic declination (variation).
Magnetic declination should not be confused with magnetic inclination, also known as
magnetic dip, which is the angle that Earth's magnetic field lines make with the downward side
of the horizontal plane.
The magnetic declination in a given area can change slowly over time, possibly as little
as 2-2.5 degrees every hundred years (most likely), depending on how far it is from the magnetic
poles.
The magnet's longitudinal axis specifies the direction of the magnet meridian at the spot.
The direction of the horizontal component of the earth's field at a location on the surface
is known as the magnetic meridian.
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The magnetic variation is the angle measured in the horizontal plane between the
magnetic meridian at a place and the actual meridian at the point.
Magnetic Dip
It is the angle made by the Earth's magnetic field lines with the horizontal. This angle
varies at different points on the Earth's surface. Positive slope values indicate that the Earth's
magnetic field is pointing down at the measuring point towards Earth, while negative values are
pointing upwards.
The tilt angle is in principle the angle made by the needle of a compass held vertically,
but in practice ordinary compass needles may be counterweighted to the inclination or not be
able to move freely in the correct plane. The value can be measured more reliably with a special
instrument, typically known as a dip circle.
The 'magnetic equator,' which may be shown on a chart by a line connecting places on
the globe where the angle of dip is zero, closely follows the geographical equator (in most cases
by 10° of latitude).
When a freely hanging magnet is pushed north or south of the magnetic equator, the dip
steadily increases, reaching around 66° in the United Kingdom. The dip is 90° over the Earth's
magnetic poles, and the magnet is then vertical.
The total force T exerted by the earth's field at a given position operates in the direction
that a freely hanging magnet affected only by the earth's field takes. The overall force, angle of
dip, and magnetic variation at a position are commonly referred to as the 'magnetic elements' at
that location.
It is easier to divide this total force T into its horizontal and vertical components, H and
Z.
Directive Force
The directing force is the horizontal component H of the earth's field, which aligns the
magnetic compass needle with the magnetic meridian and so provides a directional reference.
This component approaches zero intensity when one of the earth's magnetic poles is approached,
while the value of Z approaches that of T. The magnetic sensor (compass) becomes ineffective
above the pole, with a dip of 90° and zero directional force H.
The strength of the directing force H approaches T in the area of the magnetic equator,
whereas Z, like the angle of dip, approaches zero.
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Changes in Earth Magnetism
1. Secular Change
The earth's field is not just asymmetric, but it also undergoes a number of recognized
periodic variations. The increase or decrease in the direction or strength of the earth's magnetic
field over many years. They are long-term changes. It can occur over decades or centuries. It is
usually caused by fluid motions in the liquid outer core. Changes are around 25 gamma per year.
Usually expressed in gamma units for annual averages for intensity and annual minutes for
direction.
The most notable of these are the secular changes, which are caused by the gradual
migration of the magnetic poles around the geographic poles, with a period of around 960 years.
The north magnetic pole is progressively drifting westward, which has a major impact on
magnetic variation.
2. Unpredictable Changes
A magnetic storm, also known as a geomagnetic storm, is a disturbance in Earth's
magnetic field caused by coronal mass ejections (CMEs) or solar flares from the Sun. A
magnetic storm usually begins 24 to 36 hours after a solar event, when a solar wind shock wave
reaches Earth's ionosphere. The magnetic storm then typically lasts 24 to 48 hours, although
some can last for days. The effects of a magnetic storm include disruption of communication and
navigation systems, intense auroras, damage to satellites, and during the most severe storms,
currents occur that cause blackouts and corrosion in power lines and pipelines.
Serious magnetic storms occur every ten years, and the most severe occur every century.
It occurs when energetic particles from a solar storm collide with the ionosphere and
magnetosphere, creating a string of energetic particles and disturbing the magnetic and electric
currents of the atmosphere.
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2. Subchapter Two: The Direct Indicating Compass
Magnetic Compass
"Magnetic Compass" is a device that works with the principle that magnets with the same
poles repel each other and magnets with opposite poles attract each other, taking advantage of
the magnetism of the earth and the permanent magnetism of the vehicle on which it is located,
and used for the purpose of measuring direction while navigating.
Generally, compasses have land, ship, airplane types. The magnetic compass on the
aircraft determines and maintains the direction of flight. The plane's course is the angle between
the true longitudinal direction and the plane's axis along the meridian. It is customary to count
the route from the north side of the meridian.
Direct Reading Magnetic Compass
Also known as the “direct indicating magnetic compass”, pilot reads his direction directly
with this compass. The most common type of this compass is generally used in aircraft and its
name is vertical card compass.
Vertical Card Compass
The vertical card compass, often known as the B-type or E-type, is the most commonly
used direct reading compass and it is a new development on the previous wet compass. The term
is derived from the fact that the compass card is vertically orientated. Within the compass bowl,
this combined unit is floating in liquid. The heading can be read from the compass card thanks to
a vertical lubber line on the bowl's glass pane.
The card spins in the same direction as a turn, and the presentation appears remarkably
identical to a heading indication. Designers had to come up with a new way because there was no
liquid to dampen the vibrations. Eddy current damping, which is simply a magnet activated
through a tiny conductor that provides an opposite force, is used in vertical card compasses.
In light aircraft, it serves as the primary magnetic heading reference, whereas in bigger
aircraft, it serves as a backup compass. It is made up of a circular compass card that is joined to
the magnet assembly directly.
Compass Requirements
A pivoting magnet in a direct reading magnetic compass must be able to align itself with
the horizontal component of the earth's magnetic field and stay aligned.
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To serve it purpose a direct reading compass must possess 3 basic properties. These
conditions are defined in the following sections.
1. Horizontality
Freely suspended in the Earth’s magnetic field, a magnet system will align itself with the
direction of that field. At the magnetic equator or also known as the Aclinic line, the field
direction is parallel to the Earth’s surface; at all other places, the magnet system is titled in the
direction of total field (T), where T is the resultant of the horizontal and vertical fields.

Please note that magnetic equator is not collocated with geographic equator.
If the magnet system were allowed to align itself with the T field, it would reduce the
magnetic moment in the horizontal plane in which direction is measured.
Moment is a measure of a body tendency to rotate about a specific point or axis.
Lets say we take a magnet bar and fixed it to a pivot. As the magnet tilt, the distant from
the magnet bar north tip to the pivot point decrease, reducing the moment. The lower the
moment, the lower the magnet system ability to align itself to the earth magnetic field. Thus,
reducing its sensitivity and accuracy.
A pendulous suspension system is therefore used to reduce the magnet system’s tendency
to tilt. When the pendulously suspended magnet system tilts to align with T, the magnet system’s
center of gravity is displaced from the vertical. The magnet system’s weight forms the couple wd, which acts opposite to the z-d couple produced by the vertical component and restore the
magnet system to near horizontal.
This solution only reduces the tilt effect and not eliminating it completely. A welldesigned compass has a residual tilt of approximately +-2 degree.
2. Sensitivity
Direct reading compass must be sensitive and able to indicate the local magnetic
meridian quickly and accurately.
The following methods may increase the sensitivity:
•
Increasing the magnetic moment of the magnet system.
•
Reducing the moment of inertia of the magnet system.
•
Reducing the friction at the pivot point.
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To increase the magnetic moment, we can use a large powerful magnet. However, a large
magnet is heavy and will increase the moment of inertia. A compromise is needed between the
magnetic moment and the moment of inertia requirements. Bu using several small, light, and
powerful magnets as the magnetic sensing element of the compass a compromised is reached.
Friction at the pivot is reduced by using an iridium tipped pivot and jeweled cup. Friction
is further reduced by suspending the magnet system in lubrication fluid. Buoyancy produce by
the fluid reduces the weight of magnet assembly acting on the pivot. Furthermore, the fluid
provides lubrication to the bearing and further reduces its friction.
3. Aperiodicity
Aperiodicity measures how quickly the magnetic compass settles down to accurately
point north again after being displaced. Aperiodicity is achieved using a magnet system with a
low moment of inertia and high magnetic moment, the same compromise applied for sensitivity.
Aperiodicity also improves by filling the compass bowl with lubricating fluid. The fluid acts as
damping agent to improve aperiodicity.
Direct Reading Compass Errors
Errors associated with Direct Reading Compass are;
•
Parallax error
•
Liquid swirl
•
Acceleration error
•
Scale error
•
Turning error
•
Alignment error
•
Deviation
In this section, we will only focus on some of them.
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1. Liquid Swirl
Liquid swirl happen due to the fluid viscosity, it tends to stick to the compass casing
and compass card. Thus, when the aircraft is turning, the lubrication fluid will turn with the
aircraft and drag the compass card along.
Various liquids, including alcohol, have been used. The main properties required of a
compass liquid are:
• Low coefficient of expansion
• Low viscosity
• Transparency
• Low freezing point
• High boiling point
• Non-corrosiveness (CAE Oxford Aviation Academy, 2014)
2. Acceleration error
Acceleration errors occurs when aircraft is positioned away from magnetic equator
and when accelerating in easterly and westerly direction.
Acceleration errors happen due to magnetic dip. The greater the distance of the
compass from the magnetic equator, the greater is the angle of dip. Magnetic dip causes the
center of gravity to be displaced to the equator side of the pivot.
When aircraft accelerate, the action force act on the pivot while the reaction force act
on the center of gravity. Reaction force and the distance of center of gravity produce a
turning moment which turn the magnet sensing system and produce inaccurate compass
reading. The turning of the magnet sensing system creates an illusion that the aircraft is
turning while accelerating.
The opposite happens when aircraft decelerates.
➢ When accelerating on East/West direction compass indicate a turn toward
nearer pole,
➢ When decelerating on East/West direction compass indicate a turn toward
further pole,
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3. Turning Error
The root cause of the turning errors is the same as the acceleration errors described
before, namely that the center of gravity of the magnet system is displaced to the equator side
of the pivot. Turning errors are observed to be most pronounced during turns thoroughly
north and south.
When an aircraft turn action force is acted towards the center of the turn. Since the
direct reading compass center of gravity is displaced from its pivot the reaction force will act
on the CG away from the center of turn. Moment produced by R and d turn the magnet.
While an aircraft in Northern Hemisphere is turning through North, turning error will
cause the compass indication to lag.
While still in Northern hemisphere aircraft turn through south, turning error will
cause the compass indication to lead.
Turning errors are generally more significant than acceleration errors due to
following reasons;
•
Turns are generally greater magnitude which results in a greater displacement of
the magnet assembly.
•
Turns occur more often and are likely to last longer than linear acceleration.
How does liquid swirl affect turning errors?
When an aircraft turns, the liquid turn in the same direction as the aircraft. When
the compass card turns in the same direction as the aircraft, the swirl will add to the
turning error.
When the compass card is turning in the opposite direction to the aircraft, liquid
swirl will reduce the turning error.
➢ Turning through nearer pole compass reading lag and swirl increase turning
error,
➢ Turning through further pole compass reading lead and swirl decrease
turning error,
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4. Deviation
The iron/steel components of the airplane cause deviation. It's the angle formed by the
local magnetic meridian and the orientation of the compass magnets.
If the north-seeking (red) ends of the magnets point east of magnetic north, the deviation
is called easterly (or plus). The deviation is considered to be westerly if the north-seeking
endpoints point west of magnetic north (or minus).
Because deviation varies by heading, it must be assessed across a number of distinct
headings. This is commonly accomplished by swinging a compass (which is fully covered in the
chapter on aircraft magnetism).
The residual deviation is recorded on a compass deviation card, which is placed in the
aircraft once the deviation has been decreased as much as practicable.
Normal flight circumstances should be replicated as closely as possible throughout the
swing, with engines operating, electrical/radio services turned on, and the aircraft in a level flight
attitude.
It is evident that no ferromagnetic things, such as tools or watches, should be put near the
compass because this will cause unknown quantities of deviation. Furthermore, within the
loading restrictions, ferromagnetic cargoes should be placed as far away from the compass as
possible. In the case of extremely large ferromagnetic weights, a compass swing may be required
prior to flying with the load aboard.
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CONCLUSIONS
The ability to attract is known as magnetism. It can relate to the attraction to iron and
other metals in electric currents and magnets, or to another form of attraction, such as when
individuals desire to be near one other. Some magnets attract, while others repel, due to different
types of magnetism.
Compasses, which were invented based on the magnet principle, are an indispensable part
of our life today. plays an important role in aviation. it may cause some errors from time to time,
but the benefits are obvious as a result of proper use.
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Bibliography
CAE Oxford Aviation Academy. (2014). INSTRUMENTATION ATPL GROUND TRAINING
SERIES.
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