A classroom demonstration is described that shows a method for detecting nonmagnetic metals. The demonstration uses the simple and sensitive beat oscillations technique and employs off-the-shelf equipment usually available in most physics laboratories. More than 80 years ago, the beat oscillations technique was used to design the first electronic musical instrument, the famous Theremin. The demonstration set-up also serves as a Theremin. Metal detection and the Theremin are attractive topics for student projects.