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Basically, the reed
switch consists of two
springy metal strips
(reeds) sealed in a
glass tube, filled with
an inert gas which
prevents corrosion.
The switch is activated
by an external magnetic
field.
This field can be
provided by bringing a
permanent magnet close
to the switch, or by
passing current through
a coil which is wrapped
around the switch.
It can be used as part
of an intruder detection
system, with a magnet
fixed to a door and the
switch fitted to the
door frame.
It can also be used as a
high speed relay. Reed
relays require much
lower operating current
than normal relays
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