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The invention of the string galvanometer by Willem Einthoven in 1901 ushered in a new era of clinical investigation. Previous instruments were capable only of rudimentary measurements that were of limited utility. Advances in physiology and engineering allowed Einthoven to construct a device that was uniquely capable of recording the minute electrical currents of the heart. Early string galvanometers were massive, immovable machines. Over time, the apparatus became smaller and portable, allowing examinations at the bedside. In the decades that followed, clinicians used the now ubiquitous instrument to evaluate cardiac arrhythmias, coronary artery disease, and conduction abnormalities. The remainder of the century saw the evolution of the contemporary 12-lead electrocardiogram, with standards established regarding technique and nomenclature. Awarded the Nobel Prize in 1924 for "his discovery of the mechanism of the electrocardiogram," Einthoven's legacy endures in modern clinical medicine., Competing Interests: Disclosure: The author has no conflicts of interest to report., (Copyright © 2025 Wolters Kluwer Health, Inc. All rights reserved.)