A Conversation for Units of Measurement

Still having my problem ...

Post 1

Jeremy (trying to find his way back to dinner)

The Entry says that there are 7 SI base units. The first one mentioned is the meter (m), which is described as the distance that light travels in a certain part of ... A SECOND. ?????

How do I get the second? It is defined as the duration of a certain number of periods of a atomar transition (sorry for being unprecise, but you'll know what I mean).

Why has a meter its own definiton. Wouldn't it be more logical to derive the meter as the distance that the light travels during 30,6633...etc. periods of radiation of the transition between the two hyperfine levels of the ground state of 133Cs?

In my opinion the meter is a derived unit, at least logically. Please correct me if I'm wrong (and I suppose I must be wrong, as millions of scientists can't be wrong ... smiley - winkeye)


Still having my problem ...

Post 2


Actually you aren't wrong. 'Base unit' doesn't imply that the unit is independent. It is simply a matter of taste. Right now people are discussing about abolishing the candela as a base unit for example.

However your definition of the metre would create a new base unit: the speed of light.smiley - winkeye

Still having my problem ...

Post 3


Surely the speed of light (in a vacuum) isn't a unit as much as a physical constant.

Glad to see this as an edited entry finally, Cefpret. Also an interesting note:
I see you spelt "metre" the English way. Now recently some British exam board tried declaring that its GCSE paper answers must use the "international" (i.e. American) spellings of scientific terms. e.g. meter, sulfur. After complaints from teachers, not to mention the fact that examiners would have no clue as to which is the "correct" spelling without looking most terms up, this was abandoned. Candidates will not be penalised for using the U.S. spellings, however.

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