Certainly David Bowman is correct that we are not determining defined
constants when we measure the speed of light or the permittivity of vacuum.
It probably isn't a bad idea to remember this and to discuss it with
students.
On the other hand, these things were originally measured, sometimes in the
same manner as our students do it. When we use the Coulomb balance I like
to discuss how we are repeating a historical experiment. When the class
gets a wide range (+/- 5%) for a well-defined constant using a
thousand-dollar piece of equipment, it helps drive home the fact that these
are not easy experiments, and the pioneers really worked hard to get the
results we use today.
Also, when we use a variable-plate capacitor and determine the vacuum
permittivity (i.e. air) we initially are checking our instrument just as
David states. But then we switch to other dielectrics and then we really
are making a measurement of the permittivity (or dielectric constant, if you
wish).
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817