Re: SI and nothing else

[George Spagna <gspagna@rmc.edu>]

At 05:37 PM 9/23/97 EDT, you wrote:
> Another SI note. It is based on the last issue of TPT (September 1997).
>
> On page 324 E. Legendijk observes that the speed of light in the vacuo
> is no longer measurable; its value has been declared to be 299792458 m/s. 
> The unit of length, meter, is defined to make this happen.
>
> I may add that the same is true for the magnetic permeability of empty 
> space. We can measure epsilon_zero (for example, from the force with
> which parallel plates of a charged capacitor attract in a vacuum)
> but we can not measure mu_zero (for example, by using the Biot Savart
> formula). The unit of current is defined to make mu_zero equal to its
> declared value, as David Bowman reminded us. In the old days the relation
> c^2=epsilon_zero*mu_zero was considered to be an experimental fact.

It's even more restrictive than that  -  since c is now a defined quantity,
and the permeability constant (mu-zero) has long since been defined, the
permittivity (epsilon-zero) is now defined.

However -  that should not stop us from measuring these quantities in
laboratory exercises for students.  The techniques and historical insight
are invaluable, even if the results are now moot in terms of defining
physical quantities.
George Spagna           **********************************************
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