INTRODUCTION
The
human aspiration of exchanging feelings and ideas is the mother of inventions
regarding communication systems. The scientific progress of a country is
assessed by its capabilities in communication systems. The various means of
communication include video signals, sound signals, conversation, correspondence,
telephone, radio (wireless), television etc. Out of these telephone, radio and
television are studied in electronics.
1.1. 1.1 SOUND
Sound
is an energy which we can experience with our ears. Objects vibrating in air or
any other medium produce sound waves. A medium is essential for the propagation
of sound waves such as air, water or any other thing but the propagation of
sound waves is not possible in vacuum. Sound is propagated in the form of waves
of frequencies ranging from 20 cps to 20 kilo cps (or 20 Hz to 20 kHz).All
waves below or above this
range are inaudible.
The
velocity of sound waves in dry air at 0C is 332 metres per second and in water
it is 1435 metres per second.
1.2.
TELEPHONE
Sound
cannot be propagated in air directly beyond few hundred metres. In the year
1876 Graham Bell invented a means of communication by which two persons could
talk with each other even being at a distance of few hundred kilometres. The
means is called telephone. 'Tele' means 'at a distance' and 'phone' means conversation'.
Thus a system enabling two distant persons to have a conversation between them
is called telephone.
In
the telephone communication each telephone holder has a handset, which consists
of microphone and a headphone. Microphone converts sound waves into equivalent
electrical waves and the later are carried upto long distance through cables. The
headphone of the handset at the distant person converts electrical waves back
to sound waves. In this way, two distant persons are able to talk with each
other. There also exists a telephone exchange to establish a wire communication
between the two distant persons.
1.3.
WIRELESS
The
use of telephone is limited for establishing a communication between two
distant places at ground. Therefore, vehicles, ships and aeroplanes etc. require
such a communication system which is cordless or wireless. In the year 1895 Marconi
invented the wireless system. In the start, only Morse code signals could
be exchanged with
the help of wireless system but radio-telephone system was developed within a
short time. In the year 1915 Bell Co. established a radio-telephone service
between Washington (America)and Paris (France).Later on, the radio-telephony
system was known as Radio-communication system. This system is wireless because
no cable etc. is required to be laid between the two distant places.
Now
a days, wireless communication systems are being used for establishing
communication between ships, aeroplanes, space crafts etc. and land controlling
stations.
1.4.
RADIO
Wireless
communication system is also known as Radio communication system. In this
system, Morse and sound signals are exchanged between two systems by employing
radio waves. Radio waves are electromagnetic in nature and are capable of being
propagated upto inlinite distances in the space, These waves were invent.
the famous
scientist Henrich Hertz. Hence, the radio waves are also known as Hterizian
waves. The frequew of these waves is beyond the audible limit, i.e., 20 kilo
Hertz to 3x 106 Mega Hertz.
Sound
waves are audible but are unable to be propagated upto long distances in the
space. Radio wava. are inaudible but are able to be propagated upto infinite
distances in the space. Thus, sound waves can propagate upto infinite distances
in the space by superimposing them on radio waves. The equipment performing
this function is called a transmitter. Simple radio chain system is explained
in Fig 1.1.
 |
| Fig 1.1 Radio chain |
Second part of
the radio chain is the receiver. This equipment receives radio waves by its
aerial from the space and then separates audio signals from the radio waves and
finally converts audio signals into sound waves. In this way, a news bulletin, music
or a speech programme broadcasted by a radio station can be heard by the public
with the help of radio receivers all around the radio station.
1.5.
TELEVISION
Radio
communication systems have made it possible to transmit and to receive sound
signals upto long distances. This system has been developed further for the
transmission and reception of video signal alongwith sound signals. In this
way, Television is a means of communication by which transmission and reception
of movie films and sound is possible upto long distances by employing radio
waves.
Now
a days, radio and television are not only the means of communication but are
also the means of entertainment. In the 20th century, such a rapid progress has
been made in the field of radio and television that only within 50 years of the
establishment of first radio telephony service in 1915,the man has developed even
colour television.
1.6.
ELECTRONICS
The
study of radio, television, record player and changer, tape recorder, video
recorder, radar, transmitters and computers etc.is done under the vast branch
of science called Electronics. Presently, the progress and prosperity of a
country is assessed on the base of its capabilities in the field of electronics.
Electronics is being utilised in each and every field of industry and human
life. In this way, the branch of
Science which
deals with the motion and applications of electrons and ions through materials
and through space is called electronics. The science based on the behaviour of
electrons is called Electronics.
1.7.
SAFETY PRECAUTIONS
An
engineer or a mechanic must necessarily observe safety precautions while
operating, repairing or maintaining electrical and electronic equipments. Failing
to it, he may have to suffer heavy physical loss and moreover loss of life. The
principal safety precautions are as under:
(i)Do not touch
any electrical or electronic equipment unless it is not ensured that no
unwanted current is flowing through the equipment's body (i.e. the equipment's
body is not live).
(ii)Always use
rubber shoes, matting or dry-wooden furniture for isolating yourself from an
earth contact while working on electronic equipments.
(iii) Do not put
a Main-switch to ON unless it is ensured that no mechanic is working on the
main line.
(iv)Always be
acquainted with the theoretical details before starting a practical work. Collect
necessary tools, equipments, circuits etc. and display them on the table as per
your convenience.
(v) Always
disconnect the equipment to be repaired up from the main line by pulling the
plug top and not by pulling the cord.
(vi)Always
discharge high voltage filter capacitors after opening the equipment and before
starting repairs by short circuiting the capacitor terminals with a piece of
wire.
(vii)Do not start
a repairing job unless you are tensionless or otherwise an accident may happen.
While repairing any equipment your motto should be-May you not be able to
repair up the equipment but the same must not be damaged any more by you.
1.8.
FIRST AID
The
possibilities of an accident are greatly reduced by observing safety
precautions, even though an accident may happen any time. When phase wire is in
mechanic's hand and his body comes in contact with earth contact then a current
starts to flow through his body. It may cause breathing suppression, reduction
in pulse count, blisters
on the body or even death due to heart failure. The following first aid should
be provided if an accident has occurred:
(i)Put the
Main-switch to OFF so as to release the victim from electric contact.
(ii)If the victim
is unconscious or blisters have developed on his body then immediately arrange
for a doctor.
(iii)Start the
following first aid procedure till the doctor is available:
(a)Loosen or remove victim's
shoes, clothes etc. but take care that blisters are not broken up.
(b)Protect
the victim from cold by covering his body with a blanket.
(c)If the victim's breathing
appears to be suppressed then remove the crowd from his surroundings. If the victim
is in a room then open all the doors and windows so as to enable him to breathe
in fresh air.
(d)Remove artificial
teeth,tobacco,etc.from victim's mouth and start artificial respiration
proce-dure for restoring his normal breathing.
1.9.
ARTIFICIAL RESPIRATION
There are
following three principal methods of artificial respiration:
1. Sylvester's Method-Lay down the victim on his back
on a bed. Put a pillow beneath his shoulders so that his chest rises up
slightly. Now sit at your knees near the victim's head. Spread victim's arms
beyond his head and then fold his arms at his chest. Apply some pressure for
2-3 seconds and then again spread his arms beyond his head. Repeat the whole
procedure 10-12 times per minute till the normal breathing is not restored.
 |
| Fig 1.2 Sylvester's Method |
2. Schaffer’s Method - Lay down the victim on his
chest on a bed. Spread his arms towards his head and rest his head over his
arms folded together. Now sit near victim's hips and knee over his back with your
both palms placed near his lowest ribs and your fingers remain spread outwards.
Apply some pressure for 2-3 seconds by straightening your arms and then release
the pressure by returning yourself in original position. Repeat the whole
procedure 10-12 times per minute till the normal breathing is not restored.
 |
| Fig 1.3 Schaffer's Method |
3. Mouth to Mouth Resuscitation
- Lay down the
victim on his back. Put a pillow beneath his shoulders so that his head is
tilted backwards. Open the victim's mouth, clean it and place your open mouth on
his mouth. Blow vigorously into patient's mouth and allow his chest to expand
while his nostrils are kept shut. Remove your mouth for a moment to allow the
returning air to escape. Repeat the whole procedure 5-10 times per minute and
then check that his normal breathing has restored or not. If not then continue resuscitation.
 |
| Fig 1.4 Mouth To Mouth Resuscitation Method |
1.10.
FIRE EXTINGUISHING
An
electric short circuit or any other reason may cause a fire in a factory or
workshop. Fire extinguishers are used for fire fighting. These equipments are
installed in such a manner in the workshop that they may be located at first
sight.
There
are following three types of extinguishers:
1.
Carbon Dioxide Extinguisher - This type of extinguisher is
used for fighting the fire in cotton, cloth or wood etc. It consists of a
container filled with aqueous solution of Sodium Bicarbonate (NaHCO,). A sealed
glass bottle filled with dilute Sulphuric Acid is placed in the Sodium
Bicarbonate solution in such a way that it can be broken up by a gentle push on
the screw fitted over it.When Sulphuric Acid comes in contact with Sodium
Bicarbonate, it produces Carbon Dioxide gas and water mixture.
2
NaHCO3+H,SO4->Na,SO4+2H,0+2CO,
The
shower of Carbon Dioxide gas and water extinguishes the fire quickly. Carbon
Dioxide gas cuts the supply of Oxygen to the fire and water reduces the
temperature of burning goods. This instrument is also known as Soda-Acid
extinguisher.
2. Foam Type Extinguisher - This type of extinguisher is similar
to Carbon Dioxide type of extinguisher. In addition, the solution of Sodium
Bicarbonate also contains a foam producing red-oil.
Carbon Dioxide
gas and the oil together form a layer of foams over burning goods which cuts
their supply of Oxygen and as a result the fire is extinguished. This type of
extinguishers is very useful in extinguishing fire in kerosene and petrol.
3. C.T.C. Extinguisher - This type of extinguisher
consists of a container filled with Carbon Tetra Chloride (CCl,).When CCl, solution
is sprayed over fire with the help of a compressor then liquid carbon particles
being heavy in weight get deposited over the burning goods and cover them all
around. Thus, the supply of Oxygen is cut down and the fire is extinguished, this
type of extinguisher is very useful in extinguishing fire in electric cables
etc.
Besides
above explained extinguishers a water stream or shower is also used for
extinguishing fire spread widely. This whole system is fitted on a truck. It
consists of a large water tank and an electric operated compressor for producing
water shower. This system is not suitable for extinguishing fire in oil, diesel,
petrol and electric live cables. Oil etc, being light in weight rises over the
water layer and continues to burn, Ordinary water is a conductor of electricity
hence it is unsuitable for extinguishing fire in live electric cables.
