If you're referring to the wall plugs with integral transformers and
rectifiers, commonly known as "wall warts", with the round output plug with
a hole in the centre - I'm not sure about those sold in America, but here in
the UK, there doesn't seem to be any standard. Usually the inner pin recess
is positive, and the surrounding sheath is negative, but sometimes it's
reversed. Often they print a small diagram on the housing or the plug to
show this, but I usually test them with a multimeter just to make sure.
Some of the general purpose replacement models have an option to reverse the
polarity, along with a voltage selector, presumably because of the lack of
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