A sphygmograph.

The scale sometimes found on a chronograph that measures pulse rate.

A scale on the watch's bezel graduated in indications representing heartbeats (for the most part 30). The readindgs from this scale work in conjunction with chronograph function, producing the number of heartbeats in one minute. This function proves to be quite helpful for people involved in athletic activities.

A scale on a chronograph watch for measuring the pulse rate.