A telemeter scale is usually a printed dial scale that works with a chronograph to give a rough distance to something the wearer can both see and hear. It had a serious job: artillery soldiers used it to judge how far away enemy guns were.
How does a telemeter scale work?
The wearer starts the chronograph at the moment they see an event, such as a gun’s muzzle flash. They stop it when the sound of that event reaches them. The chronograph hand then points to a figure on the telemeter scale. That figure is the rough distance to the event. In most watches no extra mechanism is involved. The chronograph does the timing, and the scale printed on the dial turns the elapsed time into a distance.
How accurate is a telemeter?
A telemeter gives an estimate, not a precise measurement. It cannot measure distance as closely as a laser rangefinder. It also needs two things from the event: the wearer must see it, and the wearer must hear it. A gun’s flash followed by its report is the classic case. An event that makes no sound, or one that is heard but not seen, gives the scale nothing to work with.
Not to be confused with
The tachymeter is a different scale with a different purpose. A tachymeter works out average speed over a distance that is already known. A telemeter works out the distance itself, from an event the wearer sees and then hears. Put simply, a tachymeter answers how fast, and a telemeter answers how far. Both terms turn up in descriptions of chronographs, so it pays to check which one a watch carries.
Where it comes from
The English word comes from the French télémètre. Selligue coined that term for a device that measures distance. Artillery soldiers used the scale. They started the chronograph when they saw an enemy gun’s muzzle flash, then used the reading to judge how far away the gun was. Because of that military use, telemeter scales were especially common on chronographs made from the 1930s to the 1950s.
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