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an instrument for comparing the readings of two clocks.
The two disks of a chronoscope have narrow, uniformly spaced radial slits: one disk has ten slits numbered from 0 to 9, and the other has 100 slits numbered from 0 to 99. A flash lamp is positioned behind the disks, which rotate on a single axis with angular velocities of 1 and 10 revolutions per second, respectively. The flash lamp fires upon reception of electrical signals from the clocks being compared, illuminating the slits and numbers above the lamp at the given instant. A set of fixed index marks permits the observer to determine readings down to one-tenth of a division of the second disk, which corresponds to a nominal precision of 0.1 millisecond. The difference in the readings of various clocks may be determined with high accuracy by feeding electrical pulses from the clocks to a chronoscope. Photographic chronoscopes are equipped with a device for photographing the disk readings.
Chronoscopes were commonly used in astronomy, physics, and experimental biology until the mid-20th century; they have been replaced by more advanced electronic instruments.