A telemeter makes it possible to measure the distance of a phenomenon which is both visible and audible, like the lightning and thunder in a storm. The chronograph hand starts at the instant the phenomenon is seen (lightning); it is stopped when the sound is heard (thunder).
The position on the scale shows at first glance the distance in kilometres separating the phenomenon from the observer (or the user from the storm, in this example). Calibration is based on the speed of which sound travels through the air, which is approximately 340 meters per second or 1,115 feet per second.
The timepiece also features a tachymeter (base 1000) at the centre of the dial in a colimaçon, snail shape, which allows the user to measure a length of time up to three minutes instead of one minute in a traditional chronograph scale around the dial.
The tachymeter scale is to show the speed of a moving object, such as a car, over a known distance. As the moving car passes the starting-point of the measured course, whose length corresponds with that used as the basis of calibration, the owner starts the chronograph hand and stops it as the car passes the finishing-point.