Page 159 - Physics 10_Float

INFORMATION TECHNOLOGY

17.4 T R A N S M I S S I O N O F E L E C T R I C A L
SIGNAL THROUGH WIRES

Alexander Graham Bell in 1876 made a simple telephone
model to send voice in the form of electrical signal from one Mouthpiece
place to another. It consists of a metal reed, an electric coil,
and a vibrating diaphragm. Modern telephone also uses
Earpiece
diaphragms to turn voices into electrical signal that are
transmitted over phone lines. Telephone system has two
parts: the mouthpiece and the earpiece (Fig.17.3).
The mouthpiece and receiver contain carbon granules and a
thin metal diaphragm. When we speak into the mouthpiece,
the sound vibrations also vibrate the diaphragm. A slight
vibration of the diaphragm compresses the carbon and thus Telephone Dial
an electrical current can flow through the wire. Fig.17.3: Telephone diagram
This process is reversed at the other end of the line by the
receiver. The electrical current flowing through an
electromagnet in the receiver produces a varying magnetic Interesting information
field. This magnetic field attracts the thin metal diaphragm in The speed of sound in air is just
the receiver, causing it to vibrate. This vibration of the 1246 km per hour and it
diaphragm produces sound waves. cannot go far away from its
source. Therefore, it is
c o n v e r t e d i n t o
17.5 TRANSMISSIONS OF RADIOWAVES electromagnetic wave so that
THROUGH SPACE they can be sent to far off areas
with the speed of light.
Electrical signals representing information from a
microphone, a TV camera, or a computer can be sent from
Do you know?
one place to another place using either cables or radiowaves. Radiowaves are electromagnetic
Information in the form of audio frequency (AF) signals may waves and they travel with the
be transmitted directly by cable. However, in order to send speed of light. Marconi has the
information over a long distance, it has to be superimposed distinction that he transmitted
the first radio signal through the
on electromagnetic waves.
Antenna air.

Microphone

Modulator Amplifier
A.A RF
Transmitter

Not For Sale – PESRP 159