I'd like to mention that whereas relays are not
commonly polarized there is a class of relays built
which _do_ respond differently to polarity across
These are usually latching relays - they stay in the
last state they were driven to rather than "dropping out"
when voltage is removed from their coils.
Latching relays may have a permanent magnet inside
which can hold them in either position and the
electromagnetic coil counters or aids the permanent
magnet field to switch them.
The big advantage is that the relay stays in the
same state on loss of power and only require a power
pulse to activate rather than steady current.
[noparse][[/noparse]there are also mechanically operated latching types
which have no permanent magnet inside].
Note that there are two coil latching types which
do not require polarity reversal but stay in the
state last activated by one or the other coil.
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