The telemeter scale does not measure speed: it measures distance. It's a distinction I've heard misrepresented for thirty years, even by people who sell chronographs without ever having used them in the field. The physical principle is simple: light travels so fast that it is perceived as instantaneous, sound does not. At 15°C at sea level it travels at roughly 343 metres per second. If you start the chronograph at the flash and stop it at the thunder, the seconds hand indicates directly the kilometres — or whatever units the maker chose — on the scale printed around the dial's periphery. No manual calculation, no app.
The scale is logarithmic, not linear. The graduations crowd together at low values and spread out at high ones, because the relationship between time and perceived distance grows in a way that is not proportional to the visual reading. This makes it less intuitive than the tachymeter, but physically correct. On the bench I've taken apart Minerva pieces from the 1940s and 1950s where the scale printing was extremely fine, almost typographic: a craftsman's precision rarely found today.
Military use is documented as far back as the First World War. Artillerymen used it to estimate the distance of enemy batteries: you saw the gun's flash, started the chronograph, stopped at the detonation. Accurate enough to recalibrate the firing. Pilots and aerial observers made similar use of it. Today the most common application — and the most honest one — remains the meteorological one: estimating the distance of a storm front when you have no instruments to hand.
On historic chronographs the telemeter scale often appears in red or in a colour that contrasts with the tachymeter scale, with which it coexists on the same dial. In models such as the Tissot Telemeter 1938 or certain vintage Minerva pieces it occupies a dedicated ring. On the Poljot 3133 it appears on a rotating internal bezel, operated by an additional crown: a less common solution that allows small manual adjustments. Variants on a fixed external bezel also exist, rare but not unheard of.
One clarification worth making: the speed of sound varies with temperature, pressure and humidity. At high altitudes or in extreme conditions, the reading introduces a non-negligible error. The scale is calibrated for standard conditions; serious technical applications require corrections. For estimating whether the storm is three or ten kilometres away, it is more than sufficient.
