Pseudo-scientific detector used to identify a selected card

This trick was a gift from friend and magician Tony Middleton in 2015. Tony said that he saw Peter Killworth do this trick at a Cambridge Pentacle Club evening. Tony asked him where it came from and Peter said he had built it. Peter agreed to make one for Tony, and this is it. The magician has one of a number of cards selected and then lays them down on the table. The wrist strap can be put on the helper, or could be worn by the magician who is holding the helper’s hand. When the hand passes over the chosen card, the meter goes off screen.