Thanks, John (Denker), for your response to my recent post. Now I see more
clearly where you're coming from. Your statement that the primary standard
is valid at only one point, and your method for calibrating a secondary
scale with multiple "calibrated points" is clearly explained and well done.
I still don't think I am as worried as John about the negative implications
of using F=ma for defining the "concept of force." However, I now agree
that a spring scale used as John suggests is at least viable for defining
the concept of force.
But I need to do more thinking about practicality, accuracy, accessibility,
etc. As I think about these things I necessarily have to intermix the
notions of "concept of force" with the notions of "SI units." Here are some
of the questions I am dealing with, and some of these are the same questions
the IUPAP Commission on Units has to consider.
(1) Is it a good idea to have a "concept of force" that uses a different
method than that used to define the "unit of force?"
(2) If a spring scale with a single mark were used as the primary standard
for force, does that satisfy the usual criteria we impose for primary
standards?
(2a) Doesn't it add a new "base unit," and haven't we tried real hard to
limit the base units to the smallest number possible?
(2b) Like the primary mass standard, the force standard would rest with a
single entity that has to be secured away in a safe manner and used
occasionally to certify secondary standard scales. This makes the primary
standard inaccessible to ordinary scientists. We've gotten away from that
with time and length, but haven't found any good way to avoid this for mass.
Now, adding another single-entity primary standard would seem a step in the
wrong direction.
(2c) Although the F=ma definition of 1 newton is not easily performed with
high accuracy, I'm not sure the spring scale is any better. Friction,
temperature, readability of the mark, etc. are problems.
(2d) Preservation of the standard is questionable. Does the primary
standard suffer fatigue as it is used to calibrate the secondary standards?
By questioning how wise it would be to use a spring scale as a primary
standard for force (from an operational and accuracy viewpoint) as I have
done in points 2 above, we then return to point 1. If the scale does not
satisfy the criteria we want for primary standards, is it wise to use a
different method for defining the concept than we use for defining the
standard unit?
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