Telemeter
A telemeter is a fixed scale printed on a chronograph dial or its rim that reads the distance to an event you can both see and hear by timing the delay of the sound. You start the chronograph at the flash, stop at the sound, and the seconds hand shows the distance.
At a glance
- Scale type
- Fixed, chronograph dependent
- Measures
- Distance to a visible, audible event
- Based on
- Speed of sound in air (about 343 m/s)
A telemeter is not a new mechanism, it is a fixed lookup scale built on the speed of sound through air. Light reaches you almost instantly while sound lags, traveling about 343 meters per second, and the scale turns that delay straight into distance so you never multiply in your head.
How you read it
Using it comes down to three steps, all built on timing the gap between flash and sound:
- Start: trigger the chronograph the instant you see the event, such as a lightning flash
- Stop: stop it the instant you hear it, such as the thunder
- Read: the number the seconds hand sits on across the telemeter scale is the distance to the event
This is why a telemeter is useless without a chronograph: the scale only converts a running seconds hand into distance.
Where you find it
It is usually printed around the outer ring of the dial, working off the main seconds hand rather than a separate sub-dial. Among the timing-focused complications, it carries the most military and meteorological history, having been used to estimate the distance to artillery fire or an approaching storm.
For chronographs that can carry this scale, see our guide to the best chronograph watches.
Examples
During a storm you start the chronograph the moment you see the lightning and stop it when you hear the thunder; where the seconds hand lands on the telemeter scale shows roughly how many kilometers away the storm is.
Comparison
Telemeter and tachymeter scales read different things from the same seconds hand.
| Option A | Option B | Notes |
|---|---|---|
| Telemeter scale | Tachymeter scale | A telemeter reads distance from the delay between flash and sound, such as lightning to thunder; a tachymeter reads average speed over a known distance. Both rely on the chronograph but answer opposite questions. |
Related terms
Frequently asked questions
Do you need a chronograph to use a telemeter?
Yes, a telemeter only converts a running seconds hand into distance, it measures nothing on its own. You start the chronograph when you see the event and stop it when you hear it, then read the distance the seconds hand points to on the scale.
What is the difference between a telemeter and a tachymeter?
A telemeter reads the distance to an event you see and hear from the delay of the sound, while a tachymeter reads average speed over a known distance. Both use the same seconds hand and a chronograph, but one answers distance and the other answers speed.
What unit of distance does a telemeter show?
A telemeter scale is usually graduated in kilometers, since sound travels about 343 meters per second in air, so it covers roughly one kilometer in about three seconds. Older scales graduated in miles also exist; the unit is marked on the dial itself.