Touching a battery-powered AM radio's exposed circuit board with damp fingers turns the radio into a synthesizer by adding your body as a variable resistor

A radio contains the same basic modules as an analog synthesizer: oscillators, amplifiers, filters, and frequency shifters. When you touch the exposed solder side of the circuit board with lightly moistened fingers, your skin’s resistance (variable under pressure and moisture) effectively adds resistors and capacitors in parallel to existing components. This can re-tune oscillators, create feedback paths between gain stages, or patch sections of the radio into unusual combinations. The result—squeals, motorboating, noise, or silence—depends on touch location and pressure. The primary requirement is that the device is battery-powered: touching any AC-powered circuit board this way risks death. This technique is the direct inspiration for the Cracklebox and Buchla’s touch keyboards.

Examples

Open a cheap AM radio, set to dead band, lick fingertips, touch the solder side of the circuit board. Find spots where touch causes oscillation. Tune the dial while touching to explore different sounds. Place in a cigar box with the solder side exposed as a permanent instrument.

Assessment

What is actually happening electrically when your finger changes a radio’s sound? Why must this technique be used only with battery-powered circuits?