Invented by Willem Einthoven in 1903, the electrocardiograph is an instrument which is used in the examination of people suffering from heart disease. It indicates how serious their illness is and maintains a check on recovery subsequent to a heart attack. In essence, it detects the amount of electricity that passes through the heart muscle as it pumps blood through the body.

Einthoven's successful invention earned him a Nobel prize in 1924, and stemmed from the discovery by two German scientists in the mid-19th century that a frog's heart creates an electronic current.