Any device that measures a time interval. The term is usually restricted to measurements that have a small percentage error. Early time measurements used the human pulse rate or counted the number of swings of a pendulum for small time intervals. Astronomers used a regulated rotating drum to measure the time intervals of short changes in objects in the sky. Physicists did the same in the laboratory. The term chronograph seems to have stuck even when the device is digital rather than a graph. The term chronometer is often used interchangeably with chronograph, although again the result may be an electronically produced number rather than a meter reading.