Curt Fischer <crf3@po.cwru.edu> wrote in message news:<3EE60448.1DC66970@po.cwru.edu>...
> Brett Robson wrote:
> 
> > >
> > >The last I heard, the Avoirdupois pound was defined as being the weight of
> > >27.7015 cubic inches of distilled water at 62 degrees F with the barometer
> > >being at 30 inches.
> > 
> > 1. avoirdupois pound is not an imperial pound
> 
> You're right here.  Congratulations.  Don't let it go to your head
> though....
> 
> > 2. your definition is pound-force not pound
> 
> A pound force is a pound.  So what are you talking about?

No, a pound force is a recent spinoff from the pound, which has always
been a unit of mass.

So new, in fact, that the pound force is uniquely idenitifed by that
name as the one based on the avoirdupois pound, the only one of all
the hundreds of different pounds used at various places and times
throughout history that has spawned a unit of force that has seen any
significant use.

For example, unlike pounds avoirdupois and unlike kilograms, the troy
pounds and troy ounces are always units of mass--there are no troy
pounds force or troy ounces force.  Consider the other pounds still in
use, especially informally, in many different places in Latin
America--the ones redefined back in the 19th century to replace many
different old pounds, the ones defined as exactly 500 g or half a
kilogram.  Units of mass, of course.

Pounds are, since a 1959 international agreement, by definition
exactly 0.45359237 kg.  We no longer have independent standards for a
pound.  In the United States, for the 66 years prior to that
international agreement, our pound had been defined as a slightly
different exact fraction of a kilogram.

Pound force, OTOH, don't even have an official definition.

As the newcomer, the pound force should be identified as such, and
should use a symbol to distinguish it from the normal pounds as units
of mass.  American Society for Testing and Materials, Standard for
Metric Practice, E 380-79, ASTM 1979:

      3.4.1.4 The use of the same name for units of force 
    and mass causes confusion.  When the non-SI units
    are used, a distinction should be made between force 
    and mass, for example, lbf to denote force in 
    gravimetric engineering units and lb for mass.

> > 3. your definition is /less/ accurate than using a standard object
> >    (perhaps this would be an interesting homework project for you. Discuss a
> > practical way of maintaining as constant the 3 variables, volume, temp, and
> > atmospheric
> >     pressure. Be sure to mention the effect of measurement on values)
> 
> Umm, why couldn't you use any number of commercially available devices
> to maintain the desired temperature, volume, and pressure?

You can't measure the volume to the precision you can compare the mass
to the standard kilogram.

It is very difficult to keep the temperature throughout the water all
the same, and not fluctuating enough to be anywhere near the precision
you can get with a balance.

Gene Nygaard