The electrocardiogram
Heart muscle contracts when its cells depolarise, creating small potential differences (about 1 mV) across the body. An ECG records them using electrodes on the skin, with conducting gel to reduce contact resistance and a high-impedance amplifier. The P wave shows the atria depolarising, the QRS complex the ventricles depolarising, and the T wave the ventricles repolarising.
≈ 30 min