General science interest story. Likely to generate puns. I will keep mine to myself.~ctm
By Adrian Cho Nov. 6, 2018 , 4:05 PM
Like an aging monarch, Le Grand K is about to bow to modernity. For 130 years, this gleaming cylinder of platinum-iridium alloy has served as the world’s standard for mass. Kept in a bell jar and locked away at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, the weight has been taken out every 40 years or so to calibrate similar weights around the world. Now, in a revolution far less bloody than the one that cost King Louis XVI his head, it will cede its throne as the one, true kilogram.
When the 26th General Conference on Weights and Measures (CGPM) convenes next week in Versailles, France, representatives of the 60 member nations are expected to vote to redefine the International System of Units (SI) so that four of its base units—the kilogram, ampere, kelvin, and mole—are defined indirectly, in terms of physical constants that will be fixed by fiat. They’ll join the other three base units—the second, meter, and candela (a measure of a light’s perceived brightness)—that are already defined that way. The rewrite eliminates the last physical artifact used to define a unit, Le Grand K.
The shift aims to make the units more stable and allow investigators to develop ever more precise and flexible techniques for converting the constants into measurement units. “That’s the beauty of the redefinition,” says Estefanía de Mirandés, a physicist at BIPM. “You are not limited to one technology.” But even proponents of the arcane changes acknowledge they may bewilder nonexperts. “Cooler heads have said, ‘What are we going to do about teaching people to use this?’” says Jon Pratt, a physicist at the U.S. National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.
The new SI generalizes the trade-off already exploited to define the meter more precisely in terms of the speed of light. Until 1983, light’s speed was something to be measured in terms of independently defined meters and seconds. However, that year, the 17th CGPM defined the speed of light as exactly 299,792,458 meters per second. The meter then became the measurable thing: the distance light travels in 1/299,792,458 seconds. (The second was pegged to the oscillations of microwave radiation from cesium atoms in 1967.)
The new SI plays the same game with the other units. For example, it defines the kilogram in terms of the Planck constant, which pops up all over quantum mechanics. The constant is now fixed as exactly 6.62607015×10-34 kilogram meters squared per second. Because the kilogram appears in that definition, any experiment that previously measured the constant becomes a way to measure out a kilogram instead.
Such experiments are much harder than clocking light speed, a staple of undergraduate physics. One technique employs a device called a Kibble balance, which is a bit like the mythical scales of justice. A mass on one side is counterbalanced by the electric force produced by an electrical coil on the other side, hanging in a magnetic field. To balance the weight, a current must run through the coil. Researchers can equate the mass to that current times an independent voltage generated when they remove the mass and move the coil up and down in the magnetic field.
The real trickiness enters in sizing up the current and voltage, with quantum mechanical devices that do it in terms of the charge of the electron and the Planck constant. Now that the new SI has fixed those constants, the balance can be used to mete out a slug with a mass of exactly 1 kilogram. The redefinition also effectively makes the quantum techniques the SI standards for measuring voltages and currents, says James Olthoff, a NIST physicist. Until now, the SI has defined the ampere impractically, in terms of the force between infinitely long current carrying wires separated by a meter.
But applying the complex new definitions will baffle anybody without an advanced degree in physics, argues Gary Price, a metrologist in Sydney, Australia, who used to advise Australia’s National Standards Commission. In fact, he argues, the new SI fails to meet one of the basic requirements of a units system, which is to specify the amount of mass with which to measure masses, the amount of length with which to measure lengths, and so on. “The new SI is not weights and measures at all,” Price says.
HT/LdB
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The comments today, from the profound to the silly, have been very entertaining, and I thank you all.
But I still don’t know why 1 pennyweight is equal to 1.2 scruples.
“But I still don’t know why 1 pennyweight is equal to 1.2 scruples.”
Because a pennyweight is a unit of the Troy system, used today only to measure precious metals, and then only in Troy ounces. While the scruple is a unit of the Apothecaries system, no longer used by pharmacists. The grain is the same in all Anglo-saxon customary unit systems, and is 64.7891 mg exactly.
1 pennyweight (dtw) = 24 grains
1 scruple = 20 grains
therefore, 1 dtw = 1.2 scruples
NIST Handbook 44 Appendix C
“But I still don’t know why 1 pennyweight is equal to 1.2 scruples.”
You only need to remember that it’s 3 barleycorns to the inch, and a longbow arrow should be a cloth yard in length.
I actually used the then alternative, now ‘official’ SI definition of the ampere in the derivation of my ‘new science’ intrinsic capacitance equation for Helmholtz double layer capcitance. Gave a physical quantity for a coulomb of charge, so a physical quantity interpretation of a farad of capacitance defined as a coulomb of charge at an electrical potential of one volt.
The equation, in turn, explained a lot of otherwise scientifically misunderstood experimental results, which in turn proved the fundamentally faulty misunderstanding of effective surface, the correction of which enabled my basic patents and their theoretical plus experimental validation.
When they are at that standard review, they could also define BTU as 1kj. Hardly anyone would notice.
I find it elaborate to define a kilogram in that way, with so many steps,
By the way, how was the first kg produced?
If aliens found out that our unit of mass is defined by a lump of metal kept in a vault in Paris, we will be the laughingstock of the universe. The ideal definition of measurement units must be so fundamental that even aliens can understand them. If I were to define kilogram, second and meter, I will base them on the most common fundamental particle in the universe – the electron.
I define kilogram as a multiple A of electron mass m in kg. Where A = 1/m = 9.109 e31
Convert the electron mass to energy: E = m c^2 where c = speed of light
This gives a gamma ray with frequency: f = E/h where h = Planck constant
I define second as the frequency of the electron energy-equivalent: 1 second = 1.236 e20 cycles
The wavelength of this gamma ray: y = c/f
I define meter as a multiple B of wavelength y in meter. Where B = 1/y = 2.426 e12
Well, since you guys are done with it can I have it for my new garden orb?
But will it change the price of a kilo of weed? Well, if so it is a small price to pay for “settled science.”
No problem. With the outcome.
https://www.google.at/search?client=ms-android-samsung&q=launch+disasters+US+units+EU+norms&spell=1&sa=X&ved=0ahUKEwip5ZmFlvLeAhUBKSwKHUUICw4QBQghKAA&biw=360&bih=560