001

1. Electrical and magnetic physical
phenomena and effects used to
obtain measurement and control
information.
Lecture 3

3. Electric and Magnetic Phenomena
Electric and magnetic phenomena are related and have many practical applications.
As a basis for understanding this concept:

4. What is Electricity?
Electricity is probably the most critical aspects in each and everyday actions associated
with human being lifestyle. This is basically the property or even the condition throughout
which it’s practical application is employed for a lot of uses within the everyday exercises.
Electricity can probably be said as being the qualities involving specific subatomic particles
just like electrons as well as protons which could produce any kind of attractive or even
repulsive forces. This is a common property as a result of the presence of charges.
The fundamental unit associated with charges is established because of the protons as
well as electrons. The proton is positively charged as well as an electron is definitely
negatively charged along with both collectively generate an attractive force or perhaps
repulsion between the two. The mobility involving electrons within the substances results
in charges as well as the movement of these charges by means of any metallic substances
produce electricity. The existence of electricity can be simply identified throughout
various phenomena such as lightning. Electricity could be the collection of natural
phenomena linked to existence as well as the movement of electrical charge. Electricity
provides a wide selection of well-known consequences, for example, lightning, fixed
electricity, electromagnetic induction and also electric energy. Additionally, electrical
energy enables the actual development in addition to reception associated with
electromagnetic radiation for example radio waves.

5. What is Magnetism?
Magnetism can be described as a form of physical phenomena which might be mediated
simply by magnetic fields. Electric currents, as well as the magnetic moments associated
with elementary particles, produce some sort of magnetic field, which in turn works
upon some other currents along with magnetic moments. Just about every material is
usually influenced to some degree because of a magnetic field. Probably the most
recognizable effect is usually upon permanent magnets, that have continual magnetic
moments brought on by ferromagnetism.
The majority of materials would not have permanent moments. Many are drawn to a
magnetic field (paramagnetism); another medication is repulsed because of a magnetic
field (diamagnetism); some others have a very more complicated connection which has
a utilized magnetic field (for example twist glass behavior along with
antiferromagnetism). Materials which might be negligibly impacted by magnetic fields
are called non-magnetic elements. Included in this are copper mineral, light weight
aluminum, fumes, as well as plastic. Simply one particular type of magnetism had been
recognized in the last times, the magnetism that is generated by the actual iron
magnets.

6. However, many qualities, as well as attributes with the magnetic property, have
been located during the many years which implemented. Just about all materials
on our planet are a few precisely what affected by the magnetic field just like many
are captivated in the direction of this magnetic field as well as some repulsed
because of it. There are numerous elements that happen to be negligibly impacted
by this magnetic field and they are generally referred to as non-magnetic
substances

7. Key Differences between Electricity and Magnetism
The key differences between Electricity and Magnetism are discussed as under:
 The electric field has nature created all around the electric charge whereas the
magnetic field has a nature created by the moving electric charge, not a static
one.
 Units of the electric field are Newton per coulomb or sometimes it is
expressed as volts per meter whereas the magnetic field has the units, Gauss
or Tesla
 An electric field has the force proportional to the electric charge whereas the
magnetic field has forced proportional to the charge and speed of electric
charge
 An electric field is either monopole or dipole but the magnetic field is always
dipole
 Electric field movement in the electromagnetic field is perpendicular to the
magnetic field whereas magnetic field movement in the electromagnetic field
is perpendicular to the electric field

8. Difference between Magnetism and Electromagnetism
Definition of what Magnetism and Electromagnetism mean
 Both are fundamental concepts in physics that deal with magnetic forces. Magnetism
is defined as a force or a property that can cause two objects to attract or repel each
other due to the motion of electric charges. It is a physical phenomenon that is closely
associated with magnets and magnetic fields.
 Electromagnetism, on the other hand, is the branch of physics that deals with the
study of electromagnetic force and deals with electricity and magnetism and the
interaction between them. It is the phenomenon that describes the interaction
between electric fields and magnetic fields.
Property of Magnetism and Electromagnetism
 Magnetism and electromagnetism are essentially sides of the same coin because a
changing magnetic field creates an electric field and vice-versa. Magnetism refers to
the magnetic properties of objects that have the tendency to attract or repel each
other. It is caused by a simple permanent magnet.
 Electromagnetism, on the other hand, refers to the properties which govern the rate
at which an object responds to absorption or emission of electromagnetic radiations.
It is a temporary magnet which creates a magnetic field when electric charges pass
through it. Electricity and magnetism are the two fundamental aspects of
electromagnetism.

9. Phenomena involved in Magnetism and Electromagnetism
 Electricity induced magnetism is called electromagnetism. It is the phenomena
associated with magnetic and electric fields and their interaction with one another.
 The key difference between the two is that magnetism refers to the phenomena
associated with magnetic fields or magnetic forces, whereas the term
electromagnetism is the type of magnetism produced by electric current, and is
associated with both magnetic fields and electric fields. The ability of certain
objects to attract towards magnets is called magnetism whereas electromagnetism
is the phenomena concerned with electromagnetic forces.
Summary of Magnetism verses Electromagnetism
Magnetism and electromagnetism are essentially two sides of the same coin that
differ in their phenomena. While magnetism refers to the phenomena associated with
magnetic fields or magnetic, forces, electromagnetism is the phenomena associated
with both magnetic fields and electric fields. Since the dawn of the human race,
magnetism has become a fascinating yet equally diversified topic of interest that has
opened up new possibilities for scientific and technological developments. Electricity
induced magnetism is called electromagnetism, which is the branch of physics that is
concerned with the relation between electricity and magnetism.

10. Electromagnetic Induction
When a DC current pass through a long straight conductor a magnetising force, H
and a static magnetic field, B is developed around the wire.
If the wire is then wound into a coil, the magnetic field is greatly intensified
producing a static magnetic field around itself forming the shape of a bar magnet
giving a distinct North and South pole.
The magnetic flux developed around the coil being proportional to the amount of
current flowing in the coils windings as shown. If additional layers of wire are
wound upon the same coil with the same current flowing through them, the
static magnetic field strength would be increased.

11. Therefore, the magnetic field strength of a coil is determined by the ampere
turns of the coil. With more turns of wire within the coil, the greater the
strength of the static magnetic field around it.
But what if we reversed this idea by disconnecting the electrical current from
the coil and instead of a hollow core we placed a bar magnet inside the core of
the coil of wire. By moving this bar magnet “in” and “out” of the coil a current
would be induced into the coil by the physical movement of the magnetic flux
inside it.
Likewise, if we kept the bar magnet stationary and moved the coil back and
forth within the magnetic field an electric current would be induced in the
coil. Then by either moving the wire or changing the magnetic field we can
induce a voltage and current within the coil and this process is known as
Electromagnetic Induction and is the basic principal of operation of
transformers, motors and generators.

12. Electromagnetic Induction was first discovered way back in the 1830’s by Michael
Faraday. Faraday noticed that when he moved a permanent magnet in and out of a
coil or a single loop of wire it induced an ElectroMotive Force or emf, in other
words a Voltage, and therefore a current was produced.
So what Michael Faraday discovered was a way of producing an electrical current in
a circuit by using only the force of a magnetic field and not batteries. This then lead
to a very important law linking electricity with magnetism, Faraday’s Law of
Electromagnetic Induction. So how does this work?.
When the magnet shown below is moved “towards” the coil, the pointer or
needle of the Galvanometer, which is basically a very sensitive centre zero’ed
moving-coil ammeter, will deflect away from its centre position in one direction
only. When the magnet stops moving and is held stationary with regards to the
coil the needle of the galvanometer returns back to zero as there is no physical
movement of the magnetic field.

13. Likewise, when the magnet is moved “away” from the coil in the other
direction, the needle of the galvanometer deflects in the opposite direction
with regards to the first indicating a change in polarity. Then by moving the
magnet back and forth towards the coil the needle of the galvanometer will
deflect left or right, positive or negative, relative to the directional motion of
the magnet.
Electromagnetic Induction by a Moving Magnet

14. Likewise, if the magnet is now held stationary and ONLY the coil is moved
towards or away from the magnet the needle of the galvanometer will also
deflect in either direction. Then the action of moving a coil or loop of wire
through a magnetic field induces a voltage in the coil with the magnitude of
this induced voltage being proportional to the speed or velocity of the
movement.
Then we can see that the faster the movement of the magnetic field the
greater will be the induced emf or voltage in the coil, so for Faraday’s law to
hold true there must be “relative motion” or movement between the coil and
the magnetic field and either the magnetic field, the coil or both can move.

15. FARADAY’S LAW OF INDUCTION
From the above description we can say that a relationship exists
between an electrical voltage and a changing magnetic field to which
Michael Faraday’s famous law of electromagnetic induction states: “that
a voltage is induced in a circuit whenever relative motion exists between
a conductor and a magnetic field and that the magnitude of this voltage
is proportional to the rate of change of the flux”.
In other words, Electromagnetic Induction is the process of using
magnetic fields to produce voltage, and in a closed circuit, a current.

16. So how much voltage (emf) can be induced into the coil using just
magnetism. Well this is determined by the following 3 different factors.
1). Increasing the number of turns of wire in the coil – By increasing the
amount of individual conductors cutting through the magnetic field, the
amount of induced emf produced will be the sum of all the individual loops of
the coil, so if there are 20 turns in the coil there will be 20 times more
induced emf than in one piece of wire.
2). Increasing the speed of the relative motion between the coil and the
magnet – If the same coil of wire passed through the same magnetic field but
its speed or velocity is increased, the wire will cut the lines of flux at a faster
rate so more induced emf would be produced.
3). Increasing the strength of the magnetic field – If the same coil of wire is
moved at the same speed through a stronger magnetic field, there will be
more emf produced because there are more lines of force to cut.

17. If we were able to move the magnet in the diagram above in and out of the coil
at a constant speed and distance without stopping we would generate a
continuously induced voltage that would alternate between one positive polarity
and a negative polarity producing an alternating or AC output voltage and this is
the basic principal of how a Generator works similar to those used in dynamos
and car alternators.
In small generators such as a bicycle dynamo, a small permanent magnet is
rotated by the action of the bicycle wheel inside a fixed coil. Alternatively, an
electromagnet powered by a fixed DC voltage can be made to rotate inside a
fixed coil, such as in large power generators producing in both cases an
alternating current.

18. Simple Generator using Magnetic Induction

19. The simple dynamo type generator above consists of a permanent magnet which
rotates around a central shaft with a coil of wire placed next to this rotating
magnetic field. As the magnet spins, the magnetic field around the top and
bottom of the coil constantly changes between a north and a south pole. This
rotational movement of the magnetic field results in an alternating emf being
induced into the coil as defined by Faraday’s law of electromagnetic induction.
The magnitude of the electromagnetic induction is directly proportional to the
flux density, β the number of loops giving a total length of the conductor, l in
meters and the rate or velocity, ν at which the magnetic field changes within the
conductor in meters/second or m/s, giving by the motional emf expression:

20. Faraday’s Motional emf Expression
If the conductor does not move at right angles (90°) to the magnetic field then the
angle θ° will be added to the above expression giving a reduced output as the
angle increases:

21. LENZ’S LAW OF ELECTROMAGNETIC INDUCTION
Faraday’s Law tells us that inducing a voltage into a conductor can be done by either
passing it through a magnetic field, or by moving the magnetic field past the
conductor and that if this conductor is part of a closed circuit, an electric current
will flow. This voltage is called an induced emf as it has been induced into the
conductor by a changing magnetic field due to electromagnetic induction with the
negative sign in Faraday’s law telling us the direction of the induced current (or
polarity of the induced emf).
But a changing magnetic flux produces a varying current through the coil which
itself will produce its own magnetic field as we saw in the Electromagnets tutorial.
This self-induced emf opposes the change that is causing it and the faster the rate
of change of current the greater is the opposing emf. This self-induced emf will, by
Lenz’s law oppose the change in current in the coil and because of its direction this
self-induced emf is generally called a back-emf.
Lenz’s Law states that: ” the direction of an induced emf is such that it will always
opposes the change that is causing it”. In other words, an induced current will
always OPPOSE the motion or change which started the induced current in the first
place and this idea is found in the analysis of Inductance.

22. Likewise, if the magnetic flux is decreased then the induced emf will oppose this
decrease by generating and induced magnetic flux that adds to the original flux.
Lenz’s law is one of the basic laws in electromagnetic induction for determining
the direction of flow of induced currents and is related to the law of
conservation of energy. According to the law of conservation of energy which
states that the total amount of energy in the universe will always remain
constant as energy can not be created nor destroyed. Lenz’s law is derived from
Michael Faraday’s law of induction.
One final comment about Lenz’s Law regarding electromagnetic induction. We
now know that when a relative motion exists between a conductor and a
magnetic field, an emf is induced within the conductor.
But the conductor may not actually be part of the coils electrical circuit, but may
be the coils iron core or some other metallic part of the system, for example, a
transformer. The induced emf within this metallic part of the system causes a
circulating current to flow around it and this type of core current is known as an
Eddy Current.

23. Eddy currents generated by electromagnetic induction circulate around the
coils core or any connecting metallic components inside the magnetic field
because for the magnetic flux they are acting like a single loop of wire. Eddy
currents do not contribute anything towards the usefulness of the system but
instead they oppose the flow of the induced current by acting like a negative
force generating resistive heating and power loss within the core. However,
there are electromagnetic induction furnace applications in which only eddy
currents are used to heat and melt ferromagnetic metals.
Eddy Currents Circulating in a Transformer

24. Eddy Currents Circulating in a Transformer

25. The changing magnetic flux in the iron core of a transformer above will induce an
emf, not only in the primary and secondary windings, but also in the iron core.
The iron core is a good conductor, so the currents induced in a solid iron core will
be large. Furthermore, the eddy currents flow in a direction which, by Lenz’s law,
acts to weaken the flux created by the primary coil. Consequently, the current in
the primary coil required to produce a given B field is increased, so the hysteresis
curves are fatter along the H axis.
Eddy current and hysteresis losses can not be eliminated completely, but they can
be greatly reduced. Instead of having a solid iron core as the magnetic core
material of the transformer or coil, the magnetic path is “laminated”.
These laminations are very thin strips of insulated (usually with varnish) metal
joined together to produce a solid core. The laminations increase the
resistance of the iron-core thereby increasing the overall resistance to the flow
of the eddy currents, so the induced eddy current power-loss in the core is
reduced, and it is for this reason why the magnetic iron circuit of transformers
and electrical machines are all laminated.

26. Eddy Current Transducer
Eddy Currents
Eddy currents, also known as “Focault Currents”, are currents induced in a conductor
due to the magnetic field produced by the active coil. The conductor is placed in a
changing magnetic field and the current is produced according to the change of
magnetic field with time. The amount of eddy current produced will be more if the
field strength is greater. When there is high field strength, the conductivity of the
metal conductor increases, causing faster reversals of the field and hence more flow
of eddy currents. Eddy currents will be produced in both conditions where either the
conductor moves through a magnetic field or a magnetic field changes around a
stationary conductor. Even a small amount of the current will be produced in cases
where a small change in magnetic field intensity is experienced on a conductor.
Like other currents in a conductor, eddy currents can also generate heat, EMF, and all
types of losses. Its biggest disadvantage can be seen in a transformer, where power
loss due to this, affects the device’s efficiency. This can be reduced by reducing the
area of the conductor, or by laminating it. Since the insulator in the lamination area
stops the electrons from moving forward, they will not be able to flow on wide arcs.
Thus, they accumulate at the insulated ends and resist further accumulation of
charges. This, in turn will reduce the flow of eddy currents. The amount of currents
produced can also be reduced by using conductors having less electrical conductivity.

27. Eddy Current Transducer
This type of transducer is comparatively low in the measurement field and depends
mostly on the quality of a high alternating source which is fed to a set of coils. One coil
is called the active coil and the other provides temperature compensation
(Compensating coil) by being the adjacent arm of a bridge circuit. A conducting
material is kept close to the active coil so as to make it influenced by its absence or
presence, or, by being any closer or away. Magnetic flux is induced in the active coil
and is passed through the conductor producing eddy currents. The density of this
current will be maximum at the surface and will lessen as the depth increases. This
penetration depth can be calculated using the equation given below.
δ= 1 / f π μ σ
δ-Penetration Depth (m)
f-Frequency (Hz)
μ-Magnetic Permeability
σ-Electrical Conductivity (S/m)

28. The circuit diagram of an eddy current transducer/sensor is shown below.
Eddy Current Transducer

29. Working
The active coil is kept closer to the conducting material and both of them are
placed inside a probe. The compensating coil is kept further away from the
conducting material. The high frequency source acts as the bridge circuit and feeds
the coil across the two capacitors. The amount of eddy current produced becomes
more as the distance between the conducting material and the active coil becomes
less. This causes a change in the impedance of the active coil and thus unbalances
the bridge circuit. The bridge circuit produces an output proportional to the
amount of closeness between the conducting material and the active coil. The
output of the bridge circuit is given to a low pass filter (LPF) and then its dc output
is calculated. The high frequency allows a thin target to be used and also with this,
the frequency response becomes good up to a target frequency 1/10th the supply
frequency.
It should be noted that the diameter of the conducting material should be larger or
at least same as that of a probe. If not, the output is prone to reduce linearity. If
shafts are used as conducting materials, they should have a bigger diameter so that
their curved surfaces effectively behaves as flat surfaces.

30. Applications
Since it is a non-contact device, it is suitable for higher resolution measurement
applications. The device is used for finding out the position of an object that is
conductive in nature.
Position Measurement
Since the output of an eddy current transducer represents the size of the
distance between the probe and the conductor, the device can be calibrated to
measure the position or displacement of the target. Thus, it can be applicable in
monitoring or sensing the precise location of an object such as a machine tool.
It can also be used to locate the final position of precise equipments such as a
disk drive.
Vibrating Motion Measurement
The device is also suitable for finding the alternate positions of a vibrating
conductor. Since a contact device is impracticable for this application, a non-
contact device such as eddy current transducer is highly recommended. Thus, it
can be applicable in measuring the distance of a shaft from a reference point or
the to-and-fro movement of vibrating instruments.

31. Advantages
 Measurement of distance can be carried out even in rough or mixed environments.
 Cost-effective.
 The device is insensitive to material in the gap between the probe and the
conductor.
 The device is less expensive and has higher frequency response than a capacitive
transducer.
Disadvantages
 The result will be precise only if the gap between the probe and the conductor is
small.
 The device cannot be used for finding the position of non-conductive materials.
Another way is to connect a thick conductor onto the non-conductive material.
 There always occurs a non-linear relationship between the distance and
impedance of the active coil of the device. This problem can be overcome only by
calibrating the device at fixed intervals.
 The device is highly temperature sensitive. This can be overcome by adding a
suitable balance coil to the circuit.

33. Eddy Current Probes
Eddy current sensors belong to the category of non-contact displacement
sensors. The name-giving principle admits the distance measurement towards
conductive objects. As a speciality of this technology - the presence of non-
conductive mediums like oil, water or coolant does not affect the measurement.
This circumstance predestines the eddy current sensor for applications in the
rough industrial environment.
Furthermore eddy current sensors are perfectly suited for the observation of
dynamic events. Eddy current sensors of the TX-Series stand out with an
excellent dynamic range >100 kSa/s and resolutions in the sub-micron range.
With this premise the eddy current sensor is suitable for general motion analysis
and in automotive applications.
Besides robustness, high dynamics and high resolution the TX-Series also stands
out with a wide temperature range. With a temperature range from -60°C up to
180°C eddy current sensors are the ideal choice for applications in combustion
engines.

34. Eddy-current sensor for measuring ranges up to 10 mm, T-series

35. Functional principle
The sensing element of an eddy current sensor is the coil of an oscillating circuit. The
oscillating circuit is made up of the actual probe (inductance) and an interconnect
capacitance. The sensing electromagnetic field is emitted from the probe (coil). The
electromagnetic field induces eddy currents on the surface of conductive objects (i. e.
metallic objects). These eddy currents counteract their cause and attenuate the
amplitude of the oscillating circuit. This attenuation effect is inversely proportional to
the distance between object and sensor. The TX-System is driving the oscillating circuit
and interprets the attenuation as position.

36. Eddy current principle

37. Sensors and temperature behaviour
The eddy current sensor must resist hardest conditions. Eddy current sensors are
utilised at high temperatures and high pressure, within oil or coolant and in
strong electromagnetic fields - and various combinations. In particular operation
at high temperatures presents enormous challenges to materials and processing
technology. As for example - every eddy current sensor has to go through a 12h
temperature treatment before the final calibration is carried out. The sensors are
specified for operation between -60°C and 180°C. Within this temperature range
typical temperature coefficients are ±0.05 % of MR/K. Within the technically
relevant range between ambient temperature and 120°C - typical for oil
lubricated engine components - the temperature coefficient is at ±0.03 % of
MR/K.

38. Temperature coefficient as a function of the position and temperature

39. When it comes to extreme applications like the observation of an operating disc
brake we also provide sensors with an integrated water cooling system.
All sensors fulfil the protection class IP68. In applications at high pressure and
aggressive mediums we also provide custom-made sensors with ceramic components
and further protection features. Modern and highly functional assemblies quite often
require compact sensors. Smart solutions are our speciality - we provide custom-
made sensors at low quantities and if necessary individual items.
The ultra-compact CM-series from eddylab are eddy current sensors, excellent for
use in harsh industrial environments under high pressure and temperature
conditions. The ceramic housing can be used as the pressurised component, there is
no extra housing needed. All CM sensors are shielded. The combination of shielded
coil and ceramic housing guarentees an universal use in limited space of machine
parts. Measuring of lubrication gap of crankshafts is a typical application for the CM
series.

40. Ultra-compact ceramic sensors, CM-series
TX-Driver

41. eddyMOTION
eddyMOTION is an analysis- and configuration tool for windows used in conjunction
with the TX-Driver. Communication is based on the USB. The targeted area of
application is the visualisation and documentation of mechanical motion and the on-
site linearisation of eddy current sensors.
Triggering a tuning fork

42. eddyMOTION as analysis tool
eddyMOTION is made up as a universal analysis tool for the USB based data stream from
the TX-Driver. The requirements in signal analysis can be of various nature - therefore
eddyMOTION is structured in several modules. The different modules can be used to
monitor fast and slow motion. The measured data can be displayed in the time- and in the
frequency domain. The underlying sampling rates are 22.5 kSa/s in the two channel
version and 38 kSa/s in the single channel version.
The oscilloscope is the ideal tool for the analysis of dynamic events. As the name suggests
- the module oscilloscope is mostly similar to a classical oscilloscope. Handling this module
is fairly easy for everyone with basic experience in oscilloscopes. The functional range of
an oscilloscope is ideally suited for the observation of eddy current motion. eddyMOTION
replaces the voltage (classical oscilloscope) with the position from the TX-Driver. It is
basically possible to visualise periodic and non-periodic motion. In the trigger mode data
acquisition is event based. A typical example for this mode is the storage of the signal
before and after passing a threshold value in a defined time window. The figure below
shows the event based data acquisition of a tuning fork while triggering it. Another useful
function is AC-coupling. This feature displays the variation of the position instead of the
absolute position (the position value oscillates around zero). This function is in particular
useful for the visualisation of vibration with small amplitudes. Measurements on the
captured data can be taken. These are the frequency, the amplitude and the maximum
and minimum values. Captured data can be exported as picture or text-file.

43. https://smartfutures.org.uk/will-eddy-current-sensors-replace-inductive-sensors-and-switches/
Will eddy current sensors replace inductive sensors and switches?

44. Both eddy current sensors as well as inductive switches and displacement sensors
each have their respective advantages when measuring position and displacement
of objects in harsh environments, says Glenn Wedgbrow, Business Development
Manager at Micro-Epsilon UK.
However, recent advances in eddy current sensor design, integration, packaging and
overall cost reduction, have made these sensors a much more attractive option then
hitherto, particularly where high linearity, high speed measurements and high
resolution are critical requirements,
In order to appreciate the inherent advantages of eddy current sensors relative to
inductive switches and displacement sensors, it is important to first understand the
operating principle of both types.
Most exciting inherent features are:
A very high measurement frequency of up to 5 kHz.
High resolution, down to 0.5 µm.
High linearity and temperature stability.

45. Benefits of eddy current
•Non-contact measurement of displacement, distance and position on
ferro- and non-ferromagnetic materials
•High resolution and temperature stability
•Temperature range -40 to 200°C and higher
•Robust and reliable sensors IP67
•Resistant to oil, dust & dirt

46. Uses of Electromagnet – Electromagnetic Relay

47. 1.A relay is an electrical switch that opens and closes under the control of
another electrical circuit.
2.The switch is operated by an electromagnet to open or close one or many sets
of contacts.
3.A relay has at least two circuits. One circuit can be used to control another
circuit. The 1st circuit (input circuit) supplies current to the electromagnet.
4.When the switch is close, the electromagnet is magnetised and attracts one
end of the iron armature.
5.The armature then closes the contacts (2nd switch) and allows current flows
in the second circuit.
6.When the 1st switch is open again, the current to the electromagnet is cut,
the electromagnet loses its magnetism and the 2nd switch is opened. Thus
current stop to flow in the 2nd circuit.

48. Uses of Electromagnet – Electric Bell

49. 1.When the switch is on, the circuit is completed and current flows.
2.The electromagnet becomes magnetised and hence attracts the soft-iron
armature and at the same time pull the hammer to strike the gong. This
enables the hammer to strike the gong.
3.As soon as the hammer moves towards the gong, the circuit is broken. The
current stops flowing and the electromagnet loses its magnetism. This
causes the spring to pull back the armature and reconnect the circuit again.
4.When the circuit is connected, the electromagnet regains its magnetism
and pull the armature and hence the hammer to strike the gong again.
5.This cycle repeats and the bell rings continuously.

50. Uses of Electromagnet – Telephone Earpiece

51. 1.An electromagnet is used in the earpiece of a telephone. The figure shows the simple
structure of a telephone earpiece.
2.When you speak to a friend through the telephone, your sound will be converted into
electric current by the mouthpiece of the telephone.
3.The current produced is a varying current and the frequency of the current will be the
same as the frequency of your sound.
4.The current will be sent to the earpiece of the telephone from your friend.
5.When the current passes through the solenoid, the iron core is magnetised. The
strength of the magnetic field changes according to the varying current.
6.When the current is high, the magnetic field will become stronger and when the
current is low, the magnetic field becomes weaker.
7.The soft-iron diaphragm is pulled by the electromagnet and vibrates at the frequency
of the varying current. The air around the diaphragm is stretched and compressed and
produces sound wave.
8.The frequency of the sound produced in the telephone earpiece will be the same as
your sound.

52. Magnetostrictive position sensors

53. Magnetostriction – the principles
In the transducer a strain pulse is induced in a specially designed
magnetostrictive waveguide by the momentary interaction of two magnetic
fields. One field comes from a moving magnet, which passes along the outside
of the transducer tube, and the other field is generated from a current pulse
which is applied to the waveguide. The interaction between these two magnetic
fields produces a strain pulse which travels along the waveguide until the pulse
is detected at the head of the transducer. The position of the moving magnet is
precisely determined by measuring the elapsed time between the application of
the current pulse and the arrival of the strain pulse. As a result, accurate non-
contact position sensing is achieved with absolutely no wear to any of the
sensing elements.

54. The characteristics
Magnetostrictive sensors combine the strengths and overcome the weaknesses of
other technologies in one measuring system. Temposonics sensors offer non-
contact absolute measurement at great accuracy and repeatability in extreme
environments. This includes areas of mechanical extremes like high shock loads
vibration and pressures. Samples of position information are available at very high
updates, allowing for realtime control. Various mechanical variants are available,
and lengths of more than 20 metres are possible. The units also come equipped
with analogue, fieldbus and Ethernet interfaces.

55. Magnetostrictive Level Measuring in Mobile Machines
https://blog.wika.us/products/level-products/magnetostrictive-level-measuring-mobile-machines/