## The Weight is Over, The Kilogram is Settled

Metrology is the study of weights. A scientist who studies metrology is called a metrologist. Metrologists, physicists, and other specialists, for decades, have been trying to tackle an issue concerning the mass of a kilogram. When you think about it for a second, this is a rather significant undertaking since the kilogram is the international standard by which all mass in the universe is measured.

The kilogram is not just a concept, theory, or arbitrary measurement. It is based on an actual physical object – a platinum and iridium cylinder that, for 126 years, has defined the kilogram from a high-security vault outside Paris. Like all objects over time, they slowly shed mass, and since 1889, the actual mass of the object has diminished by 60 micrograms. This is a problem the scientific community has been aware of for a very long time.

Gathering last week in Paris, the 104th meeting of the International Committee for Weights and Measurements (CIPM) was satisfied that after years of experiment it has found a new method to define the kilogram. As reported by Nature magazine:

The breakthrough comes in time for the kilogram to be included in a broader redefinition of units — including the ampere, mole and kelvin — scheduled for 2018. And this week, the International Committee for Weights and Measures will meet in Paris to thrash out the next steps.

Research teams, using two completely different methods, achieved results that are both precise enough, and in sufficient agreement, to topple the physical definition. One was to manufacture near-perfect spheres of silicon crystal and literally to count the atoms using X-ray diffraction. The other was to develop an insanely accurate electromagnetic scale (a watt balance) and from the electric charge measured reverse ferret via Planck’s constant to Avogadro’s number.

The agreement will lend stability to the measurement of a kilogram, and no longer will we have to base the international measurement on a decaying object.

The metric system now encompasses seven base units: mass (kilogram), distance (metre), time (seconds), electric current (ampere), temperature (Kelvin), substance (mol) and luminosity (candela). Of those seven, six are based on universal constants of nature. The kilogram had been the last holdout.

Correction: Kilogram is a measurement of mass, not weight as was originally posted.

Evan,

“Weight” an extrinsic property of a thing, it is a measure of the gravitational force on a mass, whose SI units are Newtons (N).

“Mass” is a fundamental intrinsic property of a thing whose SI units are kg.

In your article you often used “weight” where the correct term is “mass.”

What universal constant of nature exactly corresponds to a second? I thought the closest thing to a second was several thousand pulses of a cesium atom, and that wasn’t even accurate over months and years.

Sorry, but no cigar. The kilogram originally had a definition in terms of other physical definitions, as this short article from NIST states.

Prefixes

Outside

Rules

Background

Units

Bibliography

Constants,

Units &

Uncertainty

home page

Historical context of the SI

meter kilogram second ampere kelvin mole candela history of SI

Unit of mass (kilogram) Abbreviations: CGPM, CIPM, BIPM

At the end of the 18th century, a kilogram was the mass of a cubic decimeter of water. In 1889, the 1st CGPM sanctioned the international prototype of the kilogram, made of platinum-iridium, and declared: This prototype shall henceforth be considered to be the unit of mass. The picture at the right shows the platinum-iridium international prototype, as kept at the International Bureau of Weights and Measures under conditions specified by the 1st CGPM in 1889.

It was lacking because it would necessary to specify a temperature of the water, hence the change to the platinum-iridium block.

All this fluff is about the adoption a new, more stable, standard kilogram that does not change over time.

It really is imperative that we use the correct terminology and not mix weight and mass as if they are interchangeable. The kilogram defines canonical mass. The interplay of that mass in a gravitational field defines weight and is dependent on both mass and the local gravitational acceleration. Move the standard kilogram from Paris to Boulder and its weight likely changes but its mass does not.

“the actual weight of the kilogram has diminished by 60 micrograms” is wrong both in concept and attendant units.

I absolutely agree- this is one of my pet peeves. I once read something there some wrote “kilograms of force”, rather than the correct Newtons.

I thought the exact same thing. We should also not confuse the terms precision with accuracy. Which it seems the author does in this case. Precision is how close you’re values are to each other, as in how close is the spread of your gun shots when you’re aiming at a bullseye target and accuracy is how close you are to the “true” value, as in how close did you get to the bullseye.

Thank you Steven. My thoughts exactly.

The Kilogram’s mass is found by dividing its weight by the Earth’s gravitational acceleration, which varies by location. The standard kilograms gain and lose mass due to slight residues and erosion. Both of these result in errors in our measurement of mass, the former due to changes in gravitational acceleration with location and the latter due to slight actual changes in the standards’ mass. Since we use these as, you know, standards, any change can throw our measurements off.

If, for example, the Kilogram’s weight was measured as one kilogram somewhere with a gravitational acceleration of 9.8 m/s^2, and you measure something’s weight in those kilograms somewhere where the gravitational acceleration is 9.81 m/s^2, you will go an incorrect result.

+1 to Steven’s comment. It might seem pedantic but when we are discussing precision to the level of counting individual atoms, it becomes paramount to understand exactly what is being measured.

Also metrology is not the study of weights. It’s the study of measures. As someone who works with some metrologists I had to point that out. 🙂

My basic understanding was that the kilogram was the weight of 1 litre of pure water at sea level. Is this wrong?

“the actual weight of the kilogram has diminished by 60 micrograms” is wrong both in concept and attendant units.

Can you explain and/or can you dumb this down for me, please?

Good news that the last outlier is being replaced!

On a side note, metrology is not the science of weights, but of measurement. And kilogram, second, meter, ampere, kelvin, mol and candela form the International System of Units (SI), not the metric system.

The first sentence is incorrect. Metrology is the science of measurement not simply the study of weights. As your last paraph highlights, measurements include distance, time, current & etc. Credibility in journalism requires at least feigning accuracy.

If you’re a science writer and you write an article containing the phrase “weight of a kilogram”, then you shouldn’t be a science writer.

Only a theorical remarc for been exact. Metrology is the study not only of weights but of all mesures.

I’m not sure that “reverse ferret” is the correct term to describe the watt balance. Nice writeup of BIPM Watt Balance project here: http://www.bipm.org/en/bipm/mass/watt-balance/

This explains how my dealer has been ripping me off.

Steven Buczkowski. well if the mass decays it will loose mass, which would make it weigh less i.e apply less pressure on whats beneath it. mass and weight is not interchangable but if you know the gravitational force thats affecting you, you can easily calculate your mass by your own weight and vice versa. maybe they should have written “The kilogram has lost the mass of 88 micrograms on earth” but it really just says the same.

“Metrology is the study of weights” Actually, it’s the science of measurement. Not just weight, anything that can be measured. http://bit.ly/1M6XSLv.

Now if only the USA would adopt ithe Kg and litres as standard measures….

Metrology is not just the study of weights, but of all measurements. As stated by the BIPM:

“Metrology is the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.”

Isn’t the metric system based around water at its densest? 1 gram of water is 1 mL of water which takes up the space of 1 cc. Celsius is based on water’s freezing and boiling points, which leads us to Kelvins.

Metrology is the study of measures, not just weight or mass.

Metrology is the study of measurement, not just weight. and a kilogram is not a weight. It is a mass.

There is a beautiful video on Youtube by Veritasium that explains all about the history of the kilogram and the solution to this problem.

Interesting article, but…

Weight is not mass.

Mass is not weight.

Kilogram is a unit for mass.

Newton is a unit for weight. 1 newton = 1 kilogram * meter / (second^2)

Reverse ferret? Is that a thing?

Metrology is the study of measurement not only weights.

Are you sure that “the kilogram is the international standard by which all mass in the universe is measured”?

Have you checked with all other sentient species?

There may be a bigger story here than you realise!

Regards

Sean

If that object really defined the kilogram, then it didn’t get 60 micrograms lighter – the rest of the universe just got 0.000006% heavier