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A swimming race illustrates the simple principles involved in measuring time.This swimmer is competing in a 1,500 metre race and we have an accurate, calibrated wristwatch.We note that at the instant the swimmer touches the edge of the pool our wristwatch reads and 53 seconds.

When I have asked an audience this question they have looked at me incredulously and said, “Starting time?” You cannot know how long the swimmer took unless you knew the time on the wristwatch when the race started.Without the starting time it is Actually, knowing the starting time is still not enough.During the race you have to watch the swimmer and count how many laps he has swum so you know that he has done 1,500 metres.And you have to check to make sure he touches the edge at the end of each lap.

Without these observations you cannot be sure that the time is valid.That is why you need at least two, sometimes three judges to measure the time of the race to the standard needed to enter the record books.It would make no difference how accurate or high-tech the wristwatch was.You could talk about the tiny quartz crystal and the piezoelectric effect used to provide a stable time base for the electronic movement.You could describe the atomic workings of the quartz oscillator and how it resonates at a specific and highly stable frequency, and how this is used to accurately pace a timekeeping mechanism.But without reliable witnesses the accuracy of the watch makes no difference.