Your whole body conducts. So how does an ECG see your heart?

The full narration of the video.

Your whole body conducts electricity yet your left and right hands sit at different voltages. For an ECG, clips go on your wrists and ankles and a row of little pads goes across your chest. They all sit on the same body. One conductor should read the same everywhere, so what is the machine actually measuring?

Start with a calm tub of bathwater. Stand a ruler at each end. The water is level, both rulers read the same, and the difference is zero. Now give the water a push toward one end.

The water heaps up in front, dips behind, and suddenly the two rulers disagree. It's still the same tub of water. But while it's moving, it isn't the same height everywhere. Your body is a tub like that: it's mostly salty water, which carries current, but nowhere near as freely as copper.

Your heart is the hand doing the pushing. With every beat, heart muscle cells switch on, one patch after another, sending current spreading through your body. While that current flows, the voltage runs higher on one side of you and lower on the other.

An ECG never measures the voltage at one spot. It measures the gap between two electrodes, called a voltage difference. Let's ride along with a single heartbeat. The spark starts at a tiny spot in the heart's upper right, sweeps through the two upper chambers, the atria, and the line rises in a small bump.

Then it rushes into the two thick lower chambers, the ventricles. That's the strongest push, and the line shoots up into a sharp spike. Finally the ventricles reset, and the line rolls over one gentle hill. A bump, a spike, a gentle hill: that's the pattern on every ECG printout.

Move the electrodes, and it's like watching the tub from another side: the waves change shape. That's why a standard ECG records twelve leads, looking at one heart from twelve directions. How big is that gap? About one millivolt, roughly one fifteen-hundredth of a AA battery.

The machine has to amplify it many times over before it can draw that line. Here's an odd thing. Clip the electrode at the wrist or at the shoulder, and the trace looks almost the same. Your arm is like a dead-end pipe sticking out of the tub. Nothing flows inside it, so it's the same height from end to end.

Put both electrodes on the same arm, and the difference is zero. You'd see nothing. That's why a smartwatch ECG asks you to hold the crown with a finger from your other hand. The back of the watch touches one wrist, your finger brings in the other arm. Join the two arms, and both ends of the tub are covered.

Where else have you seen something that only works when both your hands touch it?