When I visited Selfridge’s Apple display last week I chatted to a very knowledgeable assistant who explained the way the Apple Watch measures your pulse. I hadn’t thought much about this, assuming it was just some sort of pressure sensor, and didn’t expect it to be very accurate. It turns out there is real wizardry here. Later I found the full explanation in 9to5Mac.com. If I tell you the heart rate sensor employs something called photoplethysmography you will want to read all about it here.