Tachymeter (watch)

It can also be used to measure the frequency of any regular event in occurrences per hour, such as the units output by an industrial process.

As a second example, if it takes 20 seconds to travel one unit of distance, then the average speed on the watch used for the purpose of the picture only is 180 units of distance per hour (examine the picture of the watch which is here to simplify the idea, actual tachymeters may vary slightly).

For events that happen either very quickly or slowly, one can adjust the sixty-second tachymeter scale commonly found on watches.

Some watches, not common, have "wraparound" or "scroll" scales, which extend the readings to lower speeds, typically 45 units.

This allows two additional modes of use: The tachymeter bezel can be aligned with a free running second hand, and, more subtly, can be used to find the average speed over longer times/distances.

Glance at the position of the minute hand on the tachymeter scale 60 units of distance later, and average speed will be indicated.