A few days ago, I sat in a medical recliner at the University of California, San Francisco. A cardiologist placed 10 stickerlike electrodes onto my limbs and chest and then connected the wires to a dated-looking contraption with a screen and a keyboard on a cart.

About a minute later, a printer produced a chart of my heart’s electrical activity on red graph paper. The procedure I had undergone was an electrocardiogram, or an EKG, which is used to diagnose cardiac problems like arrhythmia and heart attacks.

I took the test to gain a better understanding of the implications of the Apple Watch Series 4, Apple’s new smart watch, which will become available Friday. For the first time, the watch includes an electrical heart sensor that will eventually work with an app that takes EKGs. When the EKG app, which has been approved by the Food and Drug Administration, is released this year, we will be able to place a finger on the watch’s crown to measure the electrical charges across our hearts.

The Apple Watch’s EKG won’t be nearly as comprehensive as the one produced by a traditional electrocardiograph, which hooks up to multiple parts of the body, like the one the cardiologist used on me. The watch is a single-lead EKG device, meaning it will record one angle of the heart’s electrical signals — enough to collect data about arrhythmia but not to diagnose a heart attack.