[time-nuts] Updating the unit of,time: the second.

Tom Van Baak tvb at LeapSecond.com
Tue Jun 4 11:52:58 EDT 2019

Mike, Attila, Rick,

 > Which caesium beam standards were available in 1956?

The Atomichron, made by the National Company. This was the first 
commercial cesium standard; about 50 were made. Attila, you saw one at 
my house when you visited last year. It's about 7 feet tall. The one I 
have was used by NBS(NIST), then made its way to a remote cabin in 
Minnesota for a couple decades, then to a garage in Minneapolis, where I 
bought it and drove it to Seattle. Atomichron photos here [1].

The hp 5060 and 5061 came much later.

 > The picture of the beam tube is only a small fraction of the clock 
itself. There are multiple racks full of RF equipment not shown

Correct. Better views of Essen's cesium clock and laboratory here [2]. 
Note also the Atomichron in the background of figure 6.

Since you are interested in the history of atomic clocks, especially 
cesium beam clocks, I highly recommend these papers:

"History of early atomic clocks"
Norman F Ramsey

"Essen and the National Physical Laboratory’s atomic clock"
Dale Henderson

"Atomic time-keeping from 1955 to the present"
Bernard Guinot, Elisa Felicitas Arias

"The classical caesium beam frequency standard: fifty years later"
Jacques Vanier, Claude Audoin

"Fifty years of commercial caesium clocks"
Leonard S Cutler

The above come from:

Special issue of Metrologia: “Special issue: fifty years of atomic 
time-keeping: 1955 to 2005”,
Volume 42, Number 3, June 2005.

See also:

"History of Atomic Frequency Standards: A Trip Through 20th Century Physics"
Arthur O. McCoubrey

"Atomichron: The Atomic Clock from Concept to Commercial Product"
Paul Forman

The First Atomic Clock Program: NBS, 1947-1954
Paul Forman

A fine collection of clear photos and historical PDF here:


And finally,

also found here: https://tf.nist.gov/general/pdf/2907.pdf
or here: https://tf.nist.gov/general/pdf/2917.pdf
"A Historical Review of U.S. Contributions to the Atomic Redefinition of 
the SI Second in 1967"
Michael A. Lombardi


[1] http://www.leapsecond.com/museum/nc2001/


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