Tachymeter scale
A tachymeter scale, often labelled Tachymeter or Tachymètre, converts an elapsed time into a rate per hour. On a chronograph it commonly indicates average speed over a known distance. It may appear on the dial or a surrounding ring. It works with the timing function, rather than acting as a sensor continuously measuring speed.
For a one-kilometre measurement, start the chronograph at the beginning of the distance and stop at its end. Longines gives one kilometre in 30 seconds as an example, producing a reading of 120 km/h. The calculation is 3600 divided by elapsed seconds. Similarly, 45 seconds over one kilometre gives 80 km/h. If the measured distance is one mile, the reading is miles per hour. The scale and its range must suit the timing interval.
Do not confuse a tachymeter with a pulsometer, calibrated for a specified number of heartbeats. When identifying an older chronograph, photograph the scale's wording, number sequence and position, then compare them with the correct reference. The scale alone proves neither a particular calibre nor originality. A worked calculation also does not establish that an older watch still measures correctly; the chronograph function requires separate assessment.
