The metal detector consists of two perpendicular coils; one hooked up to a signal generator and one to an oscilloscope. A sinusoidally varying current is sent through one coil, producing a changing magnetic field. You can adjust the frequency for resonance ( made by the inherent L and C values of the coil 7130hz for this set-up) for maximum signal transfer. The coils are then adjusted so the signal observed by the oscilloscope is minimized. When a piece of metal is brought near the coils, it changes the magnetic field and the signal displayed on the oscilloscope increases.