This Troper's parents were trying to get their digital camera working. They handed it to me, and I noticed that the batteries were in the wrong way around - positive where negative was supposed to be and vice versa. So I told them that I just had to "reverse the polarity", flipped the batteries around and voila!

I did the same thing with a small fan that had the battery in backwards. See, it does work!

Inverted with my father, who found a wall-wart for a piece of electronics and managed to fry ''three'' identical copies of that piece of electronics before I insisted he use ''my'' wall-wart before trying the same with my copy. Turns out the wall-wart was the right size but the wrong polarity. I suspect he felt slightly stupid after I examined it and immediately figured that out. The three fried copies were all destroyed and had to be thrown out. Definitely ''not'' always TruthInTelevision--sometimes it causes serious problems. '''''Always''''' check the polarity before using a wall-wart that did not come with the whatever-it-is you want to plug in. And the voltage, because TimTaylorTechnology doesn't work either.

Wait, isn't wall-mart a store?

It's a wall-''w''art, and in this case, it's an AC plug.

Not quite an example, since it involves changing the polarity for a completely different application, and it's going to be a bit technical. I had gotten a receiver for the 20 meter ham radio band, and tried using the shield on a piece of coaxial cable as a simple wire antenna. I found that hooking the center conductor to the ground connection on the receiver improved reception dramatically. After I got my license to transmit on that band, I found that the same piece of wire - hooked up the same backwards way - worked passably well as a transmission antenna.\\ When coaxial cable is used as feed line, the center conductor is used to carry signal and the shield is connected to ground to reduce or prevent interference. I had deliberately reversed it to make an antenna.

This Troper was making a coilgun for his project in the final years of high school. After buttloads of trouble assembling the device (TroubleMagnet), it still wouldn't work. Eventually, it came down to "reverse barrel", stuff flies out the right end, with right timing.

This Troper received a rather old hodge podge of Erector Set parts from his Paternal Grandfather. It had a motor and battery compartment as part of the collection. I took a wire wheel and some parts and made a hand held fan but it was blowing the wrong way. My solution? Reverse the Polarity! Switching the batteries around the "wrong" way caused it to go from keeping my hand cool to blowing where I pointed it.