There is no + and - on AC current. That is the reason it is called "alternating". What you are referring to are the hot and neutral (or ground) leads. The black wire is hot and the neutral is white. The hot side should be the SHORTER slot on the plug. I always check with a multimeter just to make sure that no one previously screwed it up! Just check each slot, one at a time, grounding one probe to the wall plate screw. Happy Tunes!
