Chronographs may have one or more graduated scales, which allow particular measurements to be taken. The most common is the tachymeter scale, used to calculate the average speed of a vehicle along a one-kilometre stretch. Usually positioned along the outermost perimeter of the quadrant, it shows written down the possible average speeds of a hypothetical vehicle, which decrease the more the time taken by the vehicle to cover the unit space increases. Another type of scale is the telemetry scale, which allows the distance of an immediately visible and subsequently audible phenomenon to be measured, such as lightning and thunder or the burst and sound of a firework. At the basis of telemetry is the relationship between the speed of sound in the air (333.33 m/s) and the speed of light (approximately 300 million m/s). This means that, by counting the seconds that elapse between the visual event and the sound event, it is sufficient to multiply this interval by 333.33 and thus obtain the distance at which this event occurred, in metres. Taking the example of the thunderstorm, once the lightning is seen, one starts the chronograph equipped with a telemetric scale, generally placed on the bezel and expressed in miles or kilometres; one stops the measurement at the moment one hears the thunder. In this way, the distance to the place where the lightning struck can be read on the telemetric scale, without making any calculations.
