beecrofter writes:

A neat use for bridge rectifiers is to put in battery powered devices
so that the battery can go in either way .

Coincidentally, comp.robotics.misc has just been having a lively thread
regarding these battery contacts that allow a battery to be inserted in
either orientation, completely through the geometry of the contacts.

http://www.pcmag.com/article2/0,2817,2365995,00.asp
--
As we enjoy great advantages from the inventions of others, we should
be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)