Magnetic Reed Switches activate when they
are placed lengthwise in a magnetic field. The orientation of the
field relative to the switch does matter. Reed Switches are sealed
glass tubes with a metal "reed" entering from each end, overlapping
slightly, but not normally touching in the middle. In the presense of a
strong enough magnetic field, one reed becomes North, while the other
becomes South. This draws them together, making them touch and turns on
the lamp. Reed Switches are sensitive, fast, and come in a variety of
sizes. If many reed switches are laid out adjacent to each other, and a
magnet passed over the top of the array, the switches will activate in
sequence. This scheme can be motorized for automation.
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