Fun puzzle. It seems like the disarming mechanism could be a little better thought out. "Touch this ring with the magnetic bar to lower the acid wall" just seems to be an odd way to disarm the acid wall. Either way, I love unconventional traps that make players think.
The player reaches through the acid wall, removes the stone, and gently lifts one end of one of the bars closer to the acid wall. The player can then place any metal object on her side of the acid wall to see if the bar and object pull towards each other. Since the acid wall doesn't negate the magnetism, they can test it without passing any object through the acid.