Foudroyante (rapid seconds display)
A foudroyante displays fractions of a second. Jaeger-LeCoultre's Calibre 391 has a hand making one revolution in six steps per second when the chronograph is activated.
This provides a reading in sixths of a second on that model. Such display resolution is not a guarantee of the watch's daily timekeeping accuracy. The number of steps, dial position and relationship with the chronograph must be checked for the particular movement.
When cataloguing, record calibre and function alongside the model reference. A small subdial alone does not establish a foudroyante. Distinguish the rapid display from ordinary small seconds and from other ways of displaying seconds. Compare its behaviour with the manufacturer's instructions and state whether the function was tested. Specifications for Calibre 391 should not be extended to older Jaeger-LeCoultre watches or other brands without separate documentation.
