Electrocardiogram

An electrocardiogram is the test that picks up the heart's electrical activity. And electrical here is literal: the heart puts out electrical impulses, and the machine detects them and writes them down as a tracing, the way someone with a notepad jots down, one by one, the knocks coming through the wall.

That tracing tells you several things at once. Whether the heart's rhythm is steady, holding the same beat all the way through, or irregular. How strong those electrical impulses are. And what their timing is, meaning the order they arrive in and how much space falls between one and the next.