If someone is found innocent of a crime, and then new evidence comes to light proving her guilt, which protection applies?

The protection is called double Jeopardy its a procedural defense that prevents an accused person from being tried a second time on the same or similar case. As described in the U.S Supreme Court in its unanimous decision  one of its earliest cases dealing with double jeopardy," the prohibition is not against being twice punished, but against being out twice in jeopardy".  

yes, since there is a positive and a negetive if the positive and negitive wasn't there wouldn't a flow electricity in it.