Abstract: A device for the measurement of time at which the physical state of a fluid medium is modified, comprises a vertical cup which contains the fluid and the necessary reagents, and in which a ferromagnetic ball is placed and is driven in rotation by an external magnetic field. The cup has at its lower part an annular travel path for the ball and it comprises a detection device which delivers an electric signal whenever the ball passes in front of it which is associated with a measurement unit determining the variation of the time interval between two consecutive passages of the ball.