This golden donut is likely a streamlined antenna, as we've not seen the fiddly brackets or golden gaskets of yesteryear.

Meanwhile, we've come to the headline feature: new electrical heart sensors (ECG) to complement existing optical sensors.

The Apple Watch is a Lead I ECG, which means it's only measuring voltage between the right and left arms. The first electrode is on the Watch back, pressed against your wrist. The second is the crown, held by your opposite hand's fingers.

These electrodes detect tiny changes in electrical polarization across your skin as your heart beats. The S4 then interprets that data to compute and analyze your heart rhythm.