From our smartphones, smart speakers, TVs, radios, disc players, and car sound systems, it’s an enduring and enjoyable presence in our lives. Now that recorded sound has become ubiquitous, we hardly think about it. Speakers at the Gaithersburg ceremony included IEEE Senior Member Kent Rochford, acting NIST director Senior Member Tony Ivanov, chair of the IEEE Washington (D.C.) Section Member Steven Jefferts, an NIST physicist and Fellow Clark Nguyen, president of the IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society. Lyons’ clock was honored on 8 August with ceremonies at the National Institute of Standards and Technology, in Gaithersburg, Md., and at the site of the former NBS building, in Washington, D.C. An example is the U.S. NIST-F1 atomic clock. Modern atomic clocks first cool the atoms to near absolute zero by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. The new definition meant that seconds were measured by counting atomic oscillations, and minutes and hours became multiples of the second rather than divisions of the day. That changed in 1967, when the second was redefined as the duration of 9,192,631,770 energy transitions of the cesium atom. In 1955 Louis Essen, a physicist at the U.K. National Physical Laboratory, outside London, built the first atomic clock accurate enough to be a time standard.īefore atomic clocks, the second was defined by dividing astronomical events, such as the solar day or the tropical year, into smaller parts. Based on the frequency of the microwaves emitted by the ammonia molecule, the clock was not accurate enough to be used as a time standard, but it did prove the concept. Lyons and his NBS colleagues began designing their atomic clock in 1947 and had a working prototype by 1948. But atomic clock research at Columbia was halted during World War II. For his work, Rabi received the 1944 Nobel Prize in physics. That was close to the number that later would define the second. He first measured a cesium atom’s resonance frequency in 1940, estimating the frequency at 9191.4 megacycles. Rabi in 1939 informally discussed with scientists at the National Bureau of Standards his idea of applying the team’s molecular beam magnetic resonance technique as a time standard.

Most of the concepts that led to atomic clocks were developed by physicist Isidor Isaac Rabi and his colleagues at Columbia in the 1930s. Early experiments in the 1930s and 1940s were made possible by rapid advances in quantum mechanics and microwave electronics. Thomson wrote in the second edition of the Elements of Natural Philosophy, published in 1879, that hydrogen atoms, sodium atoms, and others were “absolutely alike in every physical property” and “probably remain the same so long as the particle itself exists.”Ītomic clock experiments didn’t begin until nearly 60 years after the correspondence between Maxwell and Thomson. In 1879 he wrote to electricity pioneer William Thomson, suggesting that the “period of vibration of a piece of quartz crystal” would be a better absolute standard of time than the mean solar second (based on the Earth’s rotation) but would still depend “essentially on one particular piece of matter” and therefore would be “liable to accidents.” Maxwell theorized that atoms would work even better as a natural standard of time. Physicist James Clerk Maxwell was perhaps the first to recognize that atoms could be used to keep time. The circuits were accurate enough to measure and record variations in the Earth’s rotation, but they were still limited in performance and sensitive to environmental changes. It kept time according to the mechanical resonance of vibrating crystals of piezoelectric material-which created electrical signals with a precise frequency. In the 1920s the quartz crystal oscillator circuit was invented. A BRIEF HISTORY OF TIMEKEEPINGįor thousands of years the reference for timekeeping was the Earth’s rotation rate-which was limited in accuracy. Administered by the IEEE History Center and supported by donors, the milestone program recognizes outstanding technical developments around the world. This month the atomic clock received an IEEE Milestone.

When were clocks invented