How a Watch Takes an ECG From a Single FINGER

Touch one finger to your watch, and it can draw the electrical signal of your heart.

How can a watch record your heart's electricity from your wrist and a single fingertip?

Every heartbeat starts as a tiny pulse of electricity that spreads across the muscle, and it ripples all the way to your skin. Your watch has two electrodes: one on the back touching your wrist, one on the crown. Alone, one reads nothing useful. But touch the crown with a finger from your other hand, and you complete a loop: down one arm, across your chest and heart, up the other arm. Now the watch can measure the voltage difference between the two points as the wave passes. It records about thirty seconds and traces it as a heartbeat waveform.

Your heart's electricity is real electricity, measured in tiny fractions of a volt. Your watch is not sensing your pulse this time. It is reading the actual voltage your heart muscle generates as it fires.

Follow The Explainer Vault — because everything has an explanation.