1 metre is defined as the distance between two marks drawn on a platinum - iridium rod(at zero degree celsius) kept at international bureau of weights and measures and one kilogramme is defined as the mass of a cylindrical piece of platinum - iridium alloy kept in the same place...why cant any other metal or alloy be kept instead of this platinum - iridium alloy...please mention the source of answer...
2007-06-07 02:29:17 · 4 answers · asked by jr 2 in Science & Mathematics ➔ Other - Science
Platinum is one of the most non-reactive metals around. It doesn't rust, etc. Iridium is closely related to platinum, and from a practical standpoint, makes platinum harder and more machinable. In fact, platinum iridium alloy is what most "platinum" jewelry is made from. Thus, the end result is something hard, able to have been constructed within exacting tolerances, and non-reactive with the atmosphere.
Oh--and you're looking at an old book--the meter has been redefined in terms of physical constants, today, it is defined by the International Bureau of Weights and Measures as the distance travelled by light in absolute vacuum in 1/299,792,458 of a second.
See http://en.wikipedia.org/wiki/Meter
2007-06-07 04:25:44 · answer #1 · answered by supastremph 6 · 0⤊ 0⤋
A standard is chosen for all SI units just so that everyone can know what the units stand for in any part of the world. So, the people sitting at International System of Units said that 1 kg will be defined as the mass of this platinum-iridium alloy piece that we have created and this became a standard. Every weight is compared to that alloy piece and given a value corresponding to it. If something weighs 2x the alloy piece, it is said to be 2 kg and so on. Its just that to measure something, you need a yardstick. Suppose you pick up a rod and say,"From now on, this will be the unit of length." and for some reason, people follow you. Every length will be measured by comparing the length of the that thing by the rod. That's what SI is about. They define standards and we are supposed to follow them.
2016-05-18 23:23:15 · answer #2 · answered by ? 3 · 0⤊ 0⤋
I would assume that it is because this alloy has the most favorable coefficient of thermal expansion of any metal that they knew of at the time.
In other words, it expands and contracts the least when exposed to differences in temperature (the rod does not change size).
2007-06-07 02:33:22 · answer #3 · answered by Randy G 7 · 0⤊ 0⤋
It changes very little with respect to time; non corroding. It may change less with temperature, but I'm sure it would have to be a the same temp it was laid down at. But, in the end, it will last forever, and that is a fitting thing for what it is.
2007-06-07 02:33:56 · answer #4 · answered by mike453683 5 · 0⤊ 0⤋