Let's assume that you're asking about
coaxial cables, which are cables that are
used for cabled internet connections, cable TV
signals, and the transmission of some sounds.
These types of cables are usually thin, long
tubes made of a metallic center surrounded by
an insulator, which is in turn surrounded by
another metallic layer, and then finally
wrapped in a plastic jacket on the outside.
The insulating layer is usually plastic or air;
the second metallic layer serves as a shield.
These three inner layers are designed with
specific materials and diameters to allow
electromagnetic waves to transmit at the fastest
rate possible while keeping loss at a
minimum (such as loss due to interference of
outside electromagnetic fields with the
wave inside the cable; you can imagine the
feedback noise on a microphone as a manifestation
of such interference).
The outer-most jacket protects the cable
from damages , such as those that can come
from UV light, rodent damage, chemical reactions
caused by rain and other substances, and so on.
This defined structure of four layers for coaxial
cables allows us to minimize the reflection of
the electromagnetic wave back to the source
while allowing us to twist and shape the cable
moderately, to suit our needs, whereas other types
of cables with less protecting/insulating layers
such as the twin-lead types of cables cannot be
shaped as conveniently.
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