Venus 170 Chronograph from 1940
WWII Chronograph with a Venus 170 movement. Black dial, radium markings, last watchmaker service: Feb-2020 . On amazing vintage dial it is a telenetric scale that allows the measurement of the distance seperating the observer from the place of an optical/acoustical phenomenon. This scale is based on the speed of sound through the aire which is approximately 340 metres per second.The moment one percieves a flash of light i.e. the artillery gunner seeing the shell he has fired explode (or a flash of lightning as Rob's example) the stop watch is started. On hearing the noise of the shell exploding the stop watch is stopped. The position of the stop watch hand is then pointing to a number on the telmetric scale, that number indicates the distance in kilometres, between the observer and the phenomenon.