This is a long time favourite, but it took my long time before I could finally add one to my collection. It was one of the "must haves" I read about when I started collecting two years ago. It's a Slava medical watch. You wait for the seconds hand to be in vertical position (at 12 or at 6) and count 15 heart beats. The inner scale then tell the pulse. This one is mechanical, but there is also a quartz version.