The electrocardiograph - also known as EKG ECG - is a device designed to record the small electrical currents generated within the heart, which is the basis for diagnosing different types of heart disease. At the end of the 19th century, physiologists already knew that heartbeats produce electrical currents, but they could only measure them by placing electrodes directly on the heart muscle. It was the Dutch physician and physiologist Willem Einthoven (1860-1927) who adapted the string galvanometer, already in use for amplifying electrical signals transmitted by submarine cables, for cardiological purposes. The galvanometer that Einthoven presented in 1903 consisted of a microscopic quartz wire, a “string” vertically suspended in a strong magnetic field. As the electrical currents passed through, the string deflected and obstructed a beam of light, allowing the shadow to be recorded on photographic paper. Early prototypes of Einthoven's galvanometer were cumbersome to operate—they weighed 600 pounds, required five operators, and required patients to dip their hands and feet into buckets of cold water. But they were sensitive enough to detect the electrical impulses of the heart's contraction and relaxation. Einthoven studied normal and abnormal electrocardiograms to give doctors a framework for interpreting the results. By the 1920s, heart attacks could be diagnosed based on characteristically abnormal patterns. ECGs became lighter and more portable until, after World War II, the string galvanometer was replaced by chart recorders.