Chemistry: This Day in Science History - August 24 - Louis Essen and Atomic Clocks

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This Day in Science History - August 24 - Louis Essen and Atomic Clocks Aug 23rd 2011, 22:05 On August 24, 1997, Louis Essen passed away. Essen was a British physicist who together with Jack Perry brought us the first atomic clock in 1955. Atomic clocks are the most precise method to measure the passage of time. Essen's atomic clock was accurate to one second in 2,000 years. Atomic clocks are based around quartz crystals. These crystals can be made to vibrate by applying an electric field to them, an effect known as piezoelectricity. Most crystal driven clocks are tuned to resonate at a particular frequency and never calibrated again. An atomic clock's crystal is constantly calibrated. Essen's clock uses the outer electrons of a cesium atom to tune the crystal. Cesium has 55 electrons and the last two electrons have a slight energy difference. The difference is between their spin values which have different magnetic properties from each other. The energy difference is equivalent to a frequency of 9192631770 cycles per second. Part of the clock is an oven that evaporates cesium atoms from a small sample. Each evaporated atom contains an outer electron in either of the two energy states. The higher energy atoms are separated magnetically from the lower energy atoms. The clock's crystal is tuned as close as possible to the frequency difference between the two energy states. The crystal's oscillation is used to drive radio waves at the lower energy cesium atoms. The radio waves will excite the lower energy electrons into the higher energy state. The excited atoms are collected magnetically and counted by a detector. If the counted value changes, the crystal's frequency is changed until the counter reads the maximum value again. This constant tuning gives the atomic clock its accuracy. Atomic clocks could use many other substances as a reference frequency. Atomic clocks have been made from hydrogen, rubidium and ammonia. Cesium is used because it was used in the first clock that worked. Atomic clock technology has progressed to an accuracy of one second loss in six million years. Find out what else occurred on this day in science history.

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