Whadda Madlab Lie Detector Kit

The lie detector works by measuring the galvanic response. This means that when a person lies, they tend to sweat. Sweat has a high sodium content, making it conductive. Dry skin has a resistance of about 1 million ohms, while the resistance of moist skin is reduced by a factor of ten or more. A person holding the probe cables alters the voltage at the upper probe cable depending on their skin resistance. The skin resistance is in parallel with R2, and since it is likely smaller or similar to R2, the voltage at the probe wire decreases as the skin resistance drops.