I used a multimeter in a galvanic testing between two metals. I connect one to metal A and the another end to metal B. Then I immersed the 2 metals in a salt solution, securing that they are apart in the solution. It gave me a negative current reading, then when I switched the multimeter probes, it became positive. What does the sign of current indicates?
The negative sign is simply indicative of how you've inserted your multimeter probes (which are labelled for polarity, or should be) into the galvanic battery circuit. In one case, the multimeter "sees" electrons flowing from the positive terminal to the negative one (hence the negative sign); in the other case, the opposite is true.