He sketched out a two-page proposal for NPL to make a more-practical atomic clock using cesium atoms.Ĭesium is a soft alkali metal that ignites in air and water, so it can be troublesome. Its frequency was unstable, and it could only run for a few hours at a time.

The US clock made headlines, but it did not impress Essen. He spiffed up his equipment for public display by topping it with an ordinary electrical wall clock, and a gold-plated loop filled with ammonia. Lyons based his atomic clock on the 24-GHz resonance of nitrogen molecules. The US had money to build an atomic clock and, in 1947, the US National Bureau of Standards (NBS), now the National Institute of Standards and Technology, put physicist Harold Lyons in charge. However, postwar Britain lacked the needed money. They could measure time down to billionths of a second and be locked to natural resonances of atoms and molecules in the gigahertz range to create atomic clocks stabilized to very high precision.

After the war ended, he measured the speed of light with record nine-digit precision.Įssen also realized that the gigahertz frequencies of microwave electronics could offer the next big advance in time measurement. NPL designated him as its expert on the speed of light to answer questions from industry and government agencies, which needed to know it very precisely for military targeting. The onset of World War II shifted Essen’s work from precision clocks to microwave electronics for military systems. Essen also wrote a thesis on the new clock, which earned him a doctorate. Essen built five copies for official timekeeping, which served accurately and reliably for more than 20 years at the US Naval Observatory, the Royal Observatory in Greenwich, UK, and elsewhere. In 1936, NPL made the 100-kHz clock its primary standard of time and frequency. Essen had mastered the technology so well he was able to design and build a new clock resonating at 100 kHz, a frequency much more useful for the young radio industry. NPL asked Essen to finish the oscillator, and his improvements made it a laboratory standard. They were working on a 20-kHz quartz ring oscillator to measure radio frequencies in 1932 when a bout with pneumonia killed Dye at the young age of 44.

They got along well, and Dye’s quest for accurate measurement rubbed off on Essen. After hearing of a plum position-junior scientific assistant-at the National Physics Laboratory (NPL) near London, he applied, and was pleased to start in 1929 at an annual salary of £175.Įssen’s job was to assist David Dye, who directed NPL’s division of electrical standards and measurements. But he had also grown tired of living with his cash-strapped parents in Nottingham. With a scholarship, he was able to continue postgraduate study there in physics. In 1928, the 19-year-old graduated with first-class honors from University College Nottingham in the UK. Indeed, poverty in childhood didn’t hold Essen back. In one obituary, Royal Society colleague Sir Alan Cook called Essen “a modest and somewhat self-effacing man devoted to the pursuit of the highest precision in electrical measurements.” The Guardian newspaper was more succinct, dubbing Essen a “time lord.” By the time of his death in 1997 at age 89, however, he was a Fellow of Britain’s Royal Society, a reflection of his long and distinguished science career. Such practical humility was characteristic of Essen, who had grown up poor in a house that lacked indoor plumbing. “Rather than being flattered, he felt it would duplicate one of the most fundamental units of measurement to widespread confusion.” Soon after the debut of Essen’s clock, astronomer Gerald Clemence suggested naming the atomic second the “essen.”īut Essen would have none of it, writes his son-in-law Ray Essen in Revolutions in Time, a biography based on the elder Essen’s unpublished memoir, Time for Reflection. The significance wasn’t lost on the science and people of Essen’s time, either. We also can credit Essen with pinning down the speed of light to nine-digit accuracy, without which we couldn’t dream of robotic cars or spacecraft steering themselves with pinpoint precision. Essen’s clock has everything to do with the next time you don’t get lost-GPS relies on exceedingly accurate time measurements. The fast pace of our tech-driven lives depends on precision timekeeping. That is not just an esoteric exercise in fussy academic physics. Louis Essen’s 1955 invention of the first practical atomic clock changed the basis of timekeeping from the periodic motion of the Earth, as recorded by astronomers, to the periodic motion of electrons in cesium atoms as measured by physicists.

Atomic clocks with accuracy