I am going to disagree with most of the remarks Brian Whatcott made
about my recent posting. I am going to respond to these in order of
importance, most important first.
* * * *
Brian says that balances don't measure mass; they measure force.
I say this is semantics, and I suspect Brian will eventually agree with
me if I give enough examples.
Brian, what does a thermocouple measure? Won't virtually everyone say
it measures temperature? Are you going to insist that it measures the
Seebeck Voltage?
What does a mercury thermometer measure? Won't virtually everyone say
it measures temperature? Are you going to say it measures the
expansion/contraction of mercury?
What does an analog voltmeter measure? Won't virtually everyone say it
measures potential difference? Are you going to say that it actually
measures current? Or maybe you're going to say it measures magnetic
force?
I can sit here and think of these kinds of examples all afternoon.
Almost everything we measure is measured by way of some change that is
easily observed, but this observed change is in something proportional
to the actual thing being measured. We don't measure temperature
directly, we measure the effect of temperature on something else. But
people don't get uptight about saying we're measuring temperature when
we're really measuring the volume and pressure of a gas thermometer.
So why are you getting picky about mass measurement and holding it to
a different standard than virtually everything else we do?
Balances are designed to measure mass just like voltmeters are designed
to measure potential difference. Saying that balances measure mass is
no more wrong than saying voltmeters measure potential difference.
Unless the manufacturer errs, or the user errs, an instrument measures
what it is designed to measure. The reason I started this whole
discussion was to point out that the user might be erring if he thinks
an "electronic scale" is measuring force or weight, because it might
actually be measuring mass. And I intend this last sentence with the
same semantics as I am using in this and the previous several
paragraphs.
* * * *
Brian says that "balance" need not apply to equal-arm balances. I say
it depends upon who you ask. The "Academic American Encyclopedia" (one
that I have found extremely reliable) says about the word balance: "The
term is properly applied to an instrument that opposes equal weights in
two pans suspended from the ends of a lever that has its fulcrum
precisely in the middle." But the "McGraw Hill Encyclopedia of Science
and Technology" (also a reliable source) says that the word balance
historically applied to equal arm balances, but "In contemporary usage,
the word 'balance' may be used, but the method is quite different from
the classical two-pan design." It also says, "There are many types of
balances: spring type scales, single-lever deflection types, precise
two-lever-arm designs, torsion designs, substitution designs, and
complete electronic weighing systems."
By the way, all the language in the McGraw hill source uses language
that says we are measuring mass. For example, it says, "Electronic
weighing systems not only measure mass, but also perform calculations
such as average weight, standard deviation, and moisture percent."
At one time I would have said that balances measure mass and scales
measure force. But I am quite sure that I cannot make that distinction
any more. The electronic age, and public usage of the words, have
completely muddied this distinction. That's why I asked, in a recent
post, if we need to invent some new names.
* * * *
Brian says: "It seems not to be known by Michael in particular that a
quite popular current design (Ohaus) has the load pan balanced
coaxially by a magnetic balance coil. (or is he also agreeing with this
concept? - it is not clear from his text.)"
I have actually designed and built electronic balances. And we have an
Ohaus "balance" using the design Brian mentions. In our labs I just
counted 15 electronic "balances" that measure mass in one way or
another. I believe these use roughly 5 different methods of operation
ranging from strain gauges on a load cell to electronically nulled
balance beams, and including the coaxial design mentioned by Brian. I
didn't think it was necessary to describe all possible designs, and I
specifically stated that "...the exact mechanism is not so
important..."
* * * *
When I said that the instrument would-be/could-be re-calibrated on the
moon. Brian said probably not, because the engineers might not have
included that much latitude (6:1) in the operation of the calibration
or capability of the balance. I actually thought about that, and I
don't know the answer. For the microprocessor controlled,
electromagnetically driven null-detector balances I definitely disagree
that the engineers would have excluded this capability because of cost
constraints. It wouldn't cost a penny more to make this work just fine
on the moon. No mechanical redesign of any type would be needed.
However, this possibility might be excluded by software. Perhaps the
balance is supposed to be a "smart balance" and detect abnormal
situations and then signal an error code. Hence, it might not
re-calibrate itself on the moon. But that would only be because the
software might be programmed to expect a null current for the
calibration mass within a specific range; say, 100 milliamps +- 10
milliamps; else flag an error condition.
However, if the software was not designed to detect and exclude
abnormally low null currents in the electromagnet, this balance would
work just fine on the moon, and would measure MASS accurately there.
(Actually its precision would be less because the calibration curve
would have a much smaller slope, and the digital to analog converters
(or vice-versa) would be operating over a reduced range.) It would
also work on any planet with a smaller g than earth. However, it might
not work on Jupiter, because the electronics might not have sufficient
magnet-current capability to arrive at a null condition when the
standard mass (or unknown masses) are "placed on the pan" on a planet
with high g.
* * * *
When I claimed that an electronic balance re-calibrated on the moon
will behave similarly to an equal-arm balance, Brian simply claimed
"No."
I was hoping it would be obvious what I meant there, but apparently
not. What I meant was that a properly re-calibrated electronic balance
will properly measure mass on the moon, just like an equal-arm balance
will properly measure mass on the moon, and in contrast to a
non-re-calibrated simple spring scale which will not properly measure
mass on the moon.
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