A dry electrode, which is a single-plate capacitor, is particularly adapted to be placed on a patient's scalp to detect brain waves in an EEG system. The electrode comprises a metal disk sandwiched between very thin and relatively thicker layers of ceramic. The metal disk is attached to a small disk printed circuit board that carries an amplifier to amplify the brain waves detected by the electrode. The quality of contact of the electrode with the patient is measured by applying a pulse or voltage step through the ground electrode.