The Changing Definition Of 'Kilogram'

DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."

Comments

Counting Si

[brokenbeaker]

The problem with the single crystal of silicon method, a few years ago, was that there were all these lattice vacany defects cropping up. The formation of such point vacancies is so entropically favoured that I don't think they can ever eliminate them...

Pipe-sizes are not that simple...

[Ashtead]

Threaded pipe dimensions in inches were based on the internal diameter at some point, just to make things even more interesting. Thus, a so-called 1/2" pipe actually has an external diameter of approximately 20 mm, which translates to somewhat more than 3/4". All the common Pipe Thread sizes are this way. There has been some attempt at metricising these, at least within Scandinavia, instead of referring to terms like 1/2" or 3/4" pipe threads, terms like R15 and R20 have been seen instead. That seems to be neither here nor there.

Just to confuse the matter more, in the 1970s, it was common to use metric sizes of threaded copper pipe, which had external diameters in sizes approximating common fractions of inches: 13mm = 1/2", 16mm = 5/8" and 19mm = 3/4" just to mention some of them. These appearently were all threaded with 1mm pitch threads.

Later, these were replaced by true metric pipe sizes with compression fittings or capillary solder fittings. Now the sizes changed again, common ones are 8, 10, 12, 15, 18, 22, and 28 mm. And of course, one needed compression fittings made for 16mm and 19mm also, so as to fit the older pipes...

That's Europe. What I have seen in the US are the commonly found so-called 1/2" copper pipes with solder fittings, this is about 16mm (5/8") in diameter, so I guess they are still using internal diameter measurements. Similarly, the so-called 3/4" pipes appear to have about 21mm outside diameter.

I guess the easiest way to turn these into metric sizes would be to redefine them as 16mm and 21mm and leave it at that. At least the traditional inch-units pipe thread sizes are roughly the same everywhere!

Re:Kilogram?

[sphealey]

Then maybe america should move out of the dark ages sometime.

Funny how when the topic is software or food supplies, everyone jumps in with comments about the dangers of monoculture and the value of diversity in supply, but when the topic is the metric system there can be no deviation from the ONE TRUE FAITH.

Personally, having gone through school at a time when the US was considering a change, and having spent some time in Europe, I have no problem with the metric system. It is more convenient from some tasks, particularly in the chem lab.

But there is nothing inherently superior about a measurement system based on powers of 10. For many tasks, such as woodworking, metric measurements are far more difficult to work with than inches and 1/16th. In fact I would argue that the most "natural" base for a measurement system is 12 as it is evenly divisible by 2, 3, and 4; whereras base 10 is only divisible by 2 and 5. Thirds and fourths are very common divisions of stuff; fifths are not, so a base 12 system is more user-friendly.

That's my 0.02 euro anyway.

sPh