How the Kilogram Has Put on Weight

You might want to check here. Yep, I've missed a few 'already-started' ones myself.

One does have to allow for the partial buoyancy of the material in air, which is around 0.01% at everyday temperatures and pressures with variability based upon the weather conditions prevailing at the moment of measurement.

A few micrograms aren't going to matter for all practical purposes!

I think that the precision standards in 1880s was not as good as today and difference 80 proto-types may exist.

As when Gregorian in 17th or 18th cnetury corrected Julian Calendar and now a difference of one "Leap-Second" is detected. Nothing Astonishing.

Moe precision in units and behaviors are expected.

I think buoyancy is for both sides/ trays or (Material and scale).

Obviously they should be doing this on the Moon. Or Mars. For chrissake, couldn't they have weighed one of Curiosity's tires or something?

Well, depending on how much mass earth loses(, or gains) per year, would the kilogram not change also?

Good point.

Just like the rest of us, just sitting around doing nothing but looking pretty, we tend to gain weight. I suppose we will have to send the kilogram to the Gym so it can loose that unwanted weight....

This is what the original poster suggests -Gym

The Mass of the Standard Kilogram might not change in Value unless some mass is added or removed physically. What might change is the value of Gravity pull of the earth, which affects the Weight on earth (different from Mass). I thing that we should be talking of the gravity acceleration force that might have changed ??!!

If they're depending on gravity to calibrate the Standard Kilogram, they've already flunked Physics 101.

Do you care to elaborate?

I actually was looking into this a couple of days ago. Searching "mass versus weight" provides numerous sources, but Wikipedia describes it reasonably well. The first paragraph:

"In everyday usage, the mass of an object is often referred to as its weight though these are in fact different concepts and quantities. In scientific contexts, mass refers loosely to the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight refers to the force experienced by an object due to gravity. In other words, an object with a mass of 1.0 kilograms will weigh 9.8 newtons (newton is the unit of force, while kilogram is the unit of mass) on Earth (its mass multiplied by the gravitational field strength). Its weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass."

Regarding the definition of "mass", the Wiki page is helpful:

"In physics, mass [...], more specifically inertial mass, is a quantitative measure of an object's resistance to acceleration"

Exactly. A kilogram of mass is kilogram of mass on the Moon (where it weighs less), on Jupiter (where it weighs more) and even in orbit (where it weighs nothing).

Calibrating the Standard kilogram by means of its weight is an exercise in futility.

Why? Because that amount of mass has that specific weight only at that particular spot and at that particular time. Why? Because Earth's gravity is not uniform - it varies over space and time, thanks to Earth's shape and density which are neither uniform nor static. In a million years from now the strength of Earth's gravitational pull where you're sitting at this moment will be somewhat different, as will the weight of the Standard Kilogram. However, the mass of the Standard Kilogram would still be one kilogram.

Weight is a not a measure of mass, it is a measure of force; the product of mass times acceleration; in this case, acceleration due to gravity. When you stand on your bathroom scale, you are not measuring your mass, you are measuring both.