Re: A weighty subject

[brian whatcott <inet@INTELLISYS.NET>]

At 20:59 10/14/99 -0400, Michael Edmiston wrote:
> ... admit that people really do say that thermometers
> measure temperature, and voltmeters measure potential difference, and
> balances measure mass.

People say that the astrology column predicts tomorrow's events too,
but are you representing a glorious tradition of physicists, or are you
 in fact a vox populi? (I know the answer perfectly well: it is no
accident that the folks long in the business can see your merits...)


> And all you can come up with for whether an electronic balance will
> properly re-calibrate on the moon is to say that we might lose
> readership if we get into too many engineering details?  What a
> wimp-out!

Hehehe....

> For an electronic null-type balance, there is no mechanical/engineering
> limitation that would prevent it from working in any gravitational
> field from greater than zero up to slightly above earth-g. ...


Here's an easy little test of how tolerant your devices are
 to a little static variation in the g vector.

 Select a digital pan scale.
 Fashion a wire loop which hangs off the pan.
Tie a wire hook to a 250 gram balance mass.
   Turn the scale upside down and auto zero it.
 Then hook on the 250 gram mass.
     This is simply a change of sign from +1 to -1g
Let me know how your "mass" balance does!


/snip/

> I also do not understand your insistence that a device isn't really a
> mass-measuring device if it cannot measure mass in a free-fall or in
> field-free space.  A mercury manometer cannot measure pressure
> differences in free-fall or in field-free space.

You are really getting my point now. It is my position that when
we dream that lab instruments are directly measuring some lab variable
of interest, often enough we are wrong - you even more than I perhaps? -
unless we think about the quantity which we are using as the token:
a manometer, we may come to realise sooner or later, does not indicate
pressure difference between the two arms: instead it measures  a
function of a pressure difference, a density and a value of g.

Weren't you *shocked* when you found out that your lab barometers
could not sometimes be successfully filled to 29.92 inches against a
Torricellian vacuum? WHY would a barometer fill only to 28 inches or
 so, on a standard day?

      Were you prepared to ask about your proxy? Could you see that
if g did not vary, and the aneroid barometer announced a standard day,
that your pressure measuring device was suddenly measuring the reduced
density of gas saturated mercury?   You are so prepared now, I wager.


> ....
> Finding some environmental conditions under which an instrument does
> not work does not invalidate the fact that it works under a different
> set of conditions.
>
> Michael D. Edmiston


I often puzzled to see why the old hands here advocated some slightly
 heterodox sounding concept -  e.g that energy may not be localizable,
 - or that it may not be globally conserved, that (mechanical)
 potential energy mgh was a slippery concept, and so on.

The reason I was puzzled, I expect, was because *I* do not have to
steer young people through potential minefields of faulty
conceptualization, but I can imagine that *they* do, and they squeeze
away from precipitous falls along mountain paths up which I do not
have cause to climb.

But this 'weight' topic is, I am amazed to find, within my experience:
I see the teachers, and the engineers who have placed "weight"
in an exalted conceptual place which they guard, are very often
in trouble.
They stored the concept too close to the idea of mass, so that the
m & w colors run and merge together, to use a metaphor.

In contrast, the folks introduced to force and mass and acceleration
and work and energy and distance  with their ridiculously clear
connective equations can sail through estimations that need rocket
 scientists of the old school.
(Literally! THAT is the concept I am offering here.)

And these fortunate few, may never have heard more than a passing
reference to weight. That's how unimportant it is, in my view.

Sincerely

brian whatcott <inet@intellisys.net>
Altus OK