Dutch physiologist Willem Einthoven invented the electrocardiogram (ECG), a way to record the heart’s electrical activity with electrodes on the skin
Source:
https://www.newscientist.com/article/2203921-how-willem-einthoven-gave-doctors-a-window-on-the-heart/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home