Monthly Archives: February 2019

Maintaining standards: The triple-point cell

Our society depends on standards in countless, mostly invisible ways.   If we can’t agree on how to measure weight, length, temperature, or time we can kiss manufacturing and our manufactured world with its specified, engineered, and interchangeable parts goodbye.   Actually creating and maintaining these standards is hard and the methods change over the years.  The meter was once defined in relation to earthly distances until the realization that the earth changes over time.  Now it is defined in relation to the speed of light which we are pretty sure does not.  The second used to be defined in terms of the day, now it is based on a fundamental property of the cesium atom.  The kilogram has just been redefined in terms of Planck’s constant which then turns into a combination of the second and the meter.

Making any of these measurements is difficult and requires a lot of fancy equipment, often involving lasers, vacuum chambers, electromagnets, and/or racks of electronics.  Here’s how NIST’s new F2 atomic clock works schematically and here’s a package from its inventor on the details.   As the F2 becomes a practical albeit sophisticated standard, even fancier methods are under development for the future.

The Kelvin, fundamental unit of temperature, is a nice exception to this complexity.  It is defined in relation to the triple point of water; that temperature at which the liquid, solid, and vapor phases of isotopically controlled, gas and contaminant-free water are in equilibrium.  Measure this and the Kelvin is 1/273.16 of that.   The aptly named triple point cell requires appropriate water, a skilled glassblower, and some patience.   Thermometers can be calibrated against this standard within and across laboratories.

The Fluke Corporation, despite its name, has long been a respected supplier of a wide variety of test and measurement equipment and they sell such a triple point cell.  In the right hands, it can allow the temperature of 0.01C (the Centigrade and Kelvin are equivalent) to be measured with an uncertainty better than ± 0.0001 °C.  Here’s Fluke’s Matt Newman showing how it is done and not a laser to be seen.

Addendum 20 February 2019: The Kelvin has also been redefined as of November 2018.  It is now tied to Boltzmann’s constant, k.  NIST says that not much will change for the moment since the triple point cell is a known, reliable tool.


Guiding Waves to Guiding Light: The vacuum tubes among us

Before the laser came the maser and before that the radar that let civilization live long enough to create the other two.  We think that vacuum tubes have been completely overcome by their solid-state, fully integrated and integratable semiconductor rivals but they soldier on in niches where very high powers have to be sent out of antennas either to other antennas or to scatter back from targets.  Here’s a superb old video explaining the ‘klystron‘, a name fragrant with the aroma of old school pulp science fiction.   They’re still in use as are Traveling Wave Tube Amplifiers (TWTAs, pronounced ‘tweetas’) along with a few other devices that are coming up on nearly ninety years of life.

The Bell System is long gone but its manufacturing arm, the Western Electric Corporation, still has a website and offers products under its old banner.  Its ‘Historic Technical Library’ section is a goldmine of references.  Under ‘Western Electric Technology’ we can learn how to use our Picturephones and read the classic 1965 monograph, Principles of Electron Tubes.  The latter delves rigorously into the business of taking small radio signals and amplifying them to for communications, science, or surveillance.   Both klystrons and TWTAs get detailed treatment.  Fittingly, the final chapter is on gas lasers featuring the ever popular helium-neon variety  with only a brief mention of the carbon dioxide laser invented about the time the book would have gone to press.

It is easy to forget how the development of lasers and nonlinear optical devices came as logical outgrowths of the earlier work at much longer wavelengths – storing power in one medium and exchanging it to another all by playing games with resonances and the speed of light.   The Handbook allows the reader to rediscover these links, often for the first time.  It is not also surprising that places strong in the one such as Stanford; home of the brothers Varian, Edward Ginzton, and William Hansen of klystron fame, became so strong in the other with Schawlow, Hänsch, Siegman, Byer, and Harris. Of course, Bell Labs also falls into that category but it hardly bears repeating since it was so strong in so many areas.

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