Beautiful Realization

“Whaddaya mean, Sy, ‘charge and resistance and voltage all playing beautiful together‘? How’s that beautiful?”

“It is when they play together in a Kibble Balance, Vinnie. That beautifully-designed device solved the realization problem for two of the revised fundamental standards of measurement. Here’s the one for electricity.”

“That’s odd. It says ‘electric current’ but the number’s about charge. And I don’t see anything in there about voltage or resistance.”

“True. The electronic charge e is one of our universal constants. It and the speed of light and Planck’s constant h are the same on Mars as they are here on Earth. Take a cesium-based laser from Earth to Mars and its frequency doesn’t change. That’s why the revisions are measure-anywhere standards, no need to carry something to Paris to compare it to a physical object.”

“This is another one of those definition tricks, isn’t it? Like the cesium frequency — we defined the second by saying it’s the time required for so-and-so many waves of that light beam. Here, it’s not like someone measured the charge in coulombs, it’s we’re gonna make the coulomb exactly big enough so when we do measure an electron it’ll match up.”

“You’re not wrong, Vinnie, but it’s not quite that arbitrary. Lots of people did measure the electron against the old standard. This number represents the most accurate estimate across all the measurements. The standards board just froze it there. It’s the same strategy they took with the other six fundamental standards — each of them sits on top of a well-known constant.”

“Like Newton’s gravitational constant?”

“Sorry, Al, not that one. It’s universal, alright, but it’s only known to four significant figures, nowhere near the 8-or-better level the metrologists demand.”

“So tell us about the beauty part, Sy.”

I grab some paper napkins from the dispenser at our table. Al gives me a look. In his opinion Vinnie uses way too many of those and he doesn’t want it to spread. “Just using what I need to make a point, Al. Vinnie, I know you like pictures better than algebra but bear with me.”

“Yeah, you went through the kilogram thing pretty quick what with the garlic and all.”

“Oooh, yeah.” <scribbling on the first napkin> “Well anyway, here’s a sketch of the Kibble Balance rigged for weighing but let’s just pay attention to the parts in the dark blue oval. That zig-zag line labeled RK is a resistor that exploits the quantum Hall effect. Quantum math says its resistance is given by RK=h/e2. That’s exactly 25812.80756 ohms.”

“That a lot more digits than gravity.”

“Nice catch, Al. Now the second component in the oval is a quantum voltmeter. If you put a voltage V across it, the Josephson junction inside passes an alternating current whose frequency is f=V/CJ, where CJ=h/2e.” <scribbling on the second napkin> “Put another way. the frequency tells you the voltage from V=f×CJ and that’s the same as V=f×h/2e.” <scribbling on the third napkin> “The current iW going through RK is V/RK and that’s going to be iW=(f×CJ)/(RK)=f×(CJ/RK)=f×(h/2e)/(h/e2)=(f/2)×e. You with me?”

“Gimme a minute… You’re saying that the current is going to be half some frequency, which we can measure, times the charge on an electron. Yeah, that makes sense, ’cause the current is electrons and you got us counting electrons. Hey, wait, what happened to the h?”

“Canceled out in the fraction, just the way that e canceled out in the fraction for the kilogram.”


“Better than cute, it’s beautiful. The same equipment, the Kibble Balance plus a gravimeter, gives you the realization of a kilogram depending only on h, AND the realization of the ampere depending only on e. Once you know the standards for time, which depends only on that cesium frequency, and for length, which depends only on time and the speed of light, you can get standards for mass and electric current in the NIST lab here on Earth or up on Mars or anywhere.”

“Almost anywhere.”

“What’s your exception?”

“In space, where there’s no gravity.”

“Einstein covered that with his Equivalence Principle.”

~~ Rich Olcott


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