I have been doing this about 3-4 months now and it's a great trick. It's fast, visual, there's nothing fancy in the props and everything is so fair.
I remember one time when I did this to a man (aged maybe 40+ years old) and I first let them try what happens when you drop the ring. Before that I had told that this is how people tested if someone was a witch or wizard, but I didn't tell what is supposed to happen. So, man drops the ring and HE GOT THE RING TO STAY IN THE CHAIN. The look at that man's face was priceless. His girlfriend took the ring and the chain and tried it herself and it didn't work for her. I did it once more, winked to the man saying "there are some of us still left in this earth" and left the table.
After some time some different people came to me and wanted to test their self if they were witches or wizards. :D It was awesome night.