"There's a huge evil magnet that will tear you apart if you have any metal on you" sounds much easier to grasp and less likely to lose the listener's attention. Then, when you have them listening: "It can grab you from outside the room and hurl you into the machine where the evil magnet lives! Any metal, be it coins, necklaces, pins in your bones, belt buckles, bra wiring, dog tags. Anything can be the end of you, be damn sure you don't have any metal on you."

Oh, wait, you still want them willing to go near the machine? That complicates things a bit ;)