Re: Are dimensions any use in an MKSA-only context?

robert bristow-johnson wrote:
One will have to think carefully what happens in a lab when measuring
the units from the definitions. The definition of the second is dictated
by the fact that there are cesium clocks readily available, and length
can be accurately measured using lasers.

and a standard of mass, not dependent on a prototype like what exists
in Sevres France, can be done with a Watt Balance.

Yes, that prototype is difficult to maintain, due to environmental effect that causes it mass, despite being kept under very diligent circumstances in a locked valve.

presently they use
that to measure hbar in terms of our present meter, kg, and second.
but they could instead *fix* hbar to what our present best guess is,
and then use the watt balance to define what a kilogram is.

likewise (but this would not be so well accepted), instead of using a
Cavendish-like apparatus to measure G, in terms of the existing meter,
kg, and second, with the meter and kg already defined, and using the
Cs-133 clock as a *transfer standard* (not a primary standard of
time), they could then define G to be what our present best guess at G
and use the Cavendish apparatus to define what a second is, in terms
of Cs-133 cycles.

That would not work, I think, because the Cavendish apparatus is not exact enough.

By contrast, there are a lot of Cesium atomic clocks around, so it is easy to get reference points.

I think one might also want to use as little physics theory as possible when defining these units. So just referring to some constants set to certain values without indicating which type of experiment to define the units is less desirable at this stage of our physical knowledge. Look at how long time it took to define the meter via a fixed value of c. Perhaps fixing /h is the next candidate, as it can be measured accurately.

i think this might be behind what is irritating Jan, motivating what
started this thread.

Not really: he asked for the relevance of physical dimensions in MKSA only teaching. There appears to be not muchm therfore there were not initial answers.

But the question is really: why have MKSA only teaching, when the physical theory to use is by means not fixed at that? It is convenient to be able to switch between different dimension systems - should the students not know about that? And the MKSA system might well change in the future.

Hans

.

Relevant Pages

  • Re: Comparing offset of multiple refclock/servers
    ... decide whether the goal is to compare the clocks against each other (open loop) or the to assess the performance of the synchronized system. ... Once upon a time the power of an unintentional radiator was limited to 15 microvolts per meter at a distance lambda / 2 PI, where lambda is the wavelength in meters. ...
    (comp.protocols.time.ntp)
  • Re: What Is _MOTION_?
    ... clocks cannot measure time directly. ... much like the meter. ... the "real rod length" versus the ... from the standpoint of the observer shooting the beams. ...
    (sci.physics.relativity)
  • Re: What Is _MOTION_?
    ... clocks cannot measure time directly. ... > hypothesizing the Lorentz contraction. ... > Time is a useful tool, much like the meter. ... from the standpoint of the observer shooting the beams. ...
    (sci.physics.relativity)
  • Re: Everything that can go wrong
    ... clocks and they added up to over £600 on their own (cost to fix whole ... £43.92 Cover Meter Lower ... £71.13 Speed and Tacho glass + pilot cover ... and the whole bike was going for £450. ...
    (uk.rec.motorcycles)