91–100 of 155 posts
And with that, I mourn the obsolescence of my last remaining "absolute unit" joke and will have to develop something equally satisfying based on dressed lumber to replace it.
What about the imperial units? Are those defined against metric as well?
>Despite the greatest of precautions, every time the standard kilo was handled — for example, to compare it to another unit that could then be used to calibrate instruments — it would shed some atoms and its mass would be slightly changed. Over its lifetime, that standard kilo is estimated to have lost about 50 micrograms. Why did the standard kilo ever need to be handled? Just store it on a balance and only handle t…
The balance still would need to be balanced.
The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
Your intuition is correct -- it is possible to build more-precise intercomparison tools at the gram scale than the kilogram scale, but until next Monday, our units are referenced to Le Grand K, and any gram-reference must subdivide the absolute standard. After Monday, you're free to build your own gram-scale Kibble balance and call it an absolute standard.
>Despite the greatest of precautions, every time the standard kilo was handled — for example, to compare it to another unit that could then be used to calibrate instruments — it would shed some atoms and its mass would be slightly changed. Over its lifetime, that standard kilo is estimated to have lost about 50 micrograms. Why did the standard kilo ever need to be handled? Just store it on a balance and only handle t…
As far as I know, the mechanism by which Le Grand K is drifting with respect to all of the national references is not known.
To leave it on a balance (which must be operated/checked/maintained) would expose the K to much more risk.
I've always been mildly irritated that the name of the SI mass base unit has a prefix. Second, liter, meter, Newton, kilogram. Were I emperor of the world I would slide the name scale up so the mass equal to 2.205lbs was called a gram.
I share your consternation with the kilogram's name, but it is almost as fundamental to our units as Franklin's implicit, and perhaps unfortunate, choice to call the charge of the electron negative.
Furthermore, the pound is a unit of force. One can find locations (poles and equator) on Earth's surface where the measured weights of an object can differ by at least 0.5% [1], making any definition of mass based upon weight troublesome.
Defining a unit of weight in terms of a unit of time and a unit of length. Explain that to your grandma! :)
Earlier quoted context omitted.
>The definition of units are done in the most convenient and precise way possible. testing my scale with a mirror that contains a kg of photons doesn't sound very convenient.
You were also never going to test your scale with a weight that is stored in a vault in Paris. So far you would test with a replica of a replica of that weight, in the best case (getting a copy directly from the local standards body). With the new definition in principle anybody can create an object that weighs exactly one kg and sell it to you.
Earlier quoted context omitted.
The reason the mole is in the SI puzzled me too. It seems that the reason is that chemists don't care how many atoms there are in their reactions. It would be inconvenient: fractions of atoms don't make sense, and the numbers are far from human scale. They just want an agreement, so that even if they don't know the exact number of atoms in their test tube, they know they all have the same number. That's what units ar…
That is what has happened, essentially. The mole is atomically precise, and will never change.