National Physical Laboratory

Atomic Timeline 

Essen Timeline - 1955

Birth of Atomic Time

Louis Essen and Jack Parry design and build the world's first caesium atomic clock at NPL. Essen invites Director Edward Bullard 'to come and witness the death of the astronomical second and the birth of atomic time' 


Essen Timeline - 1957


Schematic of the first caesium atomic clock 


9 billion beats per second

NPL and the US Naval Observatory complete a three-year study comparing the astronomical second with the frequency of Essen's clock. One second is found to be equal to 9,192,631,770 cycles of the transition frequency of a caesium atom 


Essen Timeline - 1962

Transferring time

Using the Telstar 1 satellite, NPL and the US Naval Observatory carry out the first two-way transatlantic clock comparisons. Two-way satellite time transfer goes on to become one of the most precise and accurate ways of comparing frequency standards 


Essen Timeline - 1967

Time redefined

The SI second, previously based on the Earth's motion, is redefined in terms of the caesium transition frequency measured using Essen's clock in 1958. The caesium atom now underpins the definition of time

Photo: Members of the Consultative Committee for the Definition of the Second - Essen is third from the left. Courtesy of BIPM. 


Essen Timeline - 1967a


Essen (centre) and Sir Alan Cook (right) hand over a rubidium atomic clock for control of the MSF time signal to Rugby Radio Station 


Essen Timeline - 1972

Introducing the leap second

Leap seconds are introduced into the global time scale, Coordinated Universal Time (UTC). This ensures the world's time scale stays in sync with the Earth's rotation rate, which fluctuates unpredictably and, in the long term, is slowing down

Find out more about leap seconds

Photo: One of NPL's atomic clocks shows a leap second making a 61-second minute in 1994 


Essen Timeline - 1976


NPL designs and installs fail-safe brakes for the chiming mechanism of Big Ben after a mechanical failure 


Length redefined

The SI metre is redefined in terms of the SI second as 'the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second'. This enables scientists to accurately measure length using time

Find out more about NPL's speed of light research 


Essen Timeline - 1985

NPL going solo

The Royal Greenwich Observatory (RGO) announces that it will no longer maintain an atomic time scale and is allowing its atomic clocks to run down. This decision leaves only NPL operating an atomic time scale in the UK 


Essen Timeline - 1991

Hydrogen clocks

NPL adds a hydrogen maser to its atomic clock ensemble, greatly improving the performance of the UK's time scale, UTC(NPL). Hydrogen masers, which use the hydrogen atom as their frequency standard, offer increased stability over caesium clocks 


Essen Timeline - 1996


Jon 'Doctor Who' Pertwee visits the NPL atomic clocks as part of a BBC Radio 2 science programme 


Essen Timeline - 2001


Part of a femtosecond comb (a laser producing pulses of light of extremely short duration) used to measure the frequencies of NPL's optical clocks 


Essen Timeline - 2004

The next generation

NPL announces the world's most accurate optical frequency measurement, made in a trapped laser-cooled strontium ion. Optical atomic clocks could form the basis of a future redefinition of the SI second

Photo: The optical cavity used to stabilise the laser for the NPL strontium ion optical clock 


Time from NPL

The MSF Radio Time Signal, also known as 'The Time from NPL', moves from Rugby Radio Station to Anthorn Radio Station in Cumbria. The broadcast provides an accurate and reliable source of time, based on UTC(NPL), to radio-controlled clocks across the UK 


Essen Timeline - 2011

World's most accurate clock

NPL's caesium fountain atomic clock - the UK's primary frequency standard - is revealed to be the most accurate timekeeper in the world, achieving an accuracy equivalent to losing or gaining only one second in 138 million years 


Beginning of NPLTime®

NPL launches a new service, NPLTime®, providing trusted time for a new age of financial services. NPLTime® offers a precise time signal, directly traceable to Coordinated Universal Time (UTC) and independent of GPS, delivered through optical fibres

Find out more about NPLTime® 


Essen Timeline - 2014


A trap for a single ytterbium ion used by NPL to set new limits on the time variation of fundamental physical constants and form the basis of the next generation of atomic clocks 


Essen Timeline - 2015

Future of atomic time

The new caesium fountain atomic clock NPL-CsF3 becomes operational, allowing continuous operation of a primary clock at NPL. Work continues on the next generation of optical atomic clocks at NPL, which should achieve accuracies equivalent to losing or gaining one second in the age of the universe



Please note that the information will not be divulged to third parties, or used without your permission