The heart is a muscular pump whose contractions are driven by action potentials that start in the sinoatrial node. Einthoven built a string galvanometer, stretching a thin quartz filament in a magnetic field to measure the minute currents on the body surface. He defined “Einthoven’s triangle” by connecting both arms and the left leg, establishing standard limb leads I, II, and III. This made it possible to diagnose arrhythmias such as premature beats and atrial fibrillation non-invasively. ECGs are employed from emergency rooms to sports medicine and support public health worldwide. His work laid the foundations of modern biomedical engineering.

Until the late 19th century, cardiac bioelectricity could only be measured in animal experiments with cumbersome capillary galvanometers, far from clinical use. Einthoven designed a string galvanometer with high sensitivity and bandwidth, successfully recording the 0.5–2 mV cardiac potentials from the human body surface in real time. He placed a silver-coated quartz filament in a vertical magnetic field, optically magnified its displacement, and traced the signal continuously on paper. By analyzing the waveform he introduced the terminology of the P wave, QRS complex, and T wave to classify phases of electrical activity. Assuming an equilateral triangle formed by the right arm, left arm, and left leg, he derived the lead relationship I + III = II from vector addition. This concept later evolved into Goldberger’s augmented leads and Wilson’s precordial leads, the prototype of the modern 12-lead ECG. The project was an archetype of interdisciplinary work, integrating physiology, physics (electromagnetic induction), engineering (precision detectors), and mathematics (vector analysis). Clinically, the ECG became indispensable for localizing myocardial infarction, detecting electrolyte disturbances, and monitoring drug side effects. Today’s digital ECG analysis software and remote monitoring still rely on Einthoven’s basic model. His achievement is cited in lectures and textbooks as a classic example of how collaboration between medicine and technology fosters breakthroughs.

Einthoven refined a Wheatstone-bridge series string galvanometer, achieving a resonant frequency of 600 Hz and a sensitivity of 0.01 mm/μA. The silver-plated quartz filament had a diameter of 3 μm and length of 60 mm, suspended within a 0.12 T horseshoe electromagnet. An optical projection system recorded a 400 mm trace onto cine film, calibrated with a polar graph for voltage and time scales. By projecting the induced current vector onto a planar triangle he computed the mean electrical axis geometrically, introducing a diagnostic criterion for cardiac hypertrophy. He performed harmonic analysis to compensate frequency response, providing tables in the 1903 paper’s appendix for low-frequency amplitude attenuation. In demonstrations of premature ventricular contractions he showed the absence of a preceding P′, refuting the re-entry hypothesis and supporting an ectopic focus origin. These findings stimulated the vectorcardiography work of Lewis and Wilson, culminating in Frank’s horizontal plane leads and three-dimensional loop analyses. Correlation with post-mortem pathology revealed that anterior wall infarction produced the most pronounced ST elevation and positive T deflection in V2–V4. Einthoven proposed instrumentation standardization at the International Physiological Congress, leading to its adoption into IEC codes at the 1924 Boston meeting. His design philosophy—minimizing mechanical inertia while using optical amplification—echoes in modern fiber-optic ECG sensors.