Gene:
My definition of coherence is the same as that presented in any
internationally recognized document. There are seven base units and from
them other units are formed coherently. End of the story.
Prefixes cannot destroy coherency as they have nothing to so with it.
Perhaps you might try to consider that prefixes are simply international
representation for (power) numbers, just like 10^3 or 10 ^-6 are. A "10^6"
is the SI "M". End of the story. (I hope. Just in case, consider that a
megagram is a million of grams just as a microgram is a millionths of a
gram.
Stan
Now, that was the reply concerning the possibility of a confusion with the
size of a mass - there is none. For generations this is how it was and I do
not recall any people who were using (not studying) metric units to be
confused. That is why there is little chance this will ever be changed
(despite my fighting for it - and why not!). The confusion entered the
picture past 1960 - with the spread of the coherent newton, joule, watt, ..
. However, in my teaching many thousands of students from all walks of life,
it took me little to explain the kg anomaly (to those who needed to know)
and equip attendees with full understanding of the issue. A few strategic
exercises helped the memory to avoid the 1000 factor potential error. You
must remember that people who live with I-P units are used to a plethora of
anomalies - those few in SI are almost nil by comparison. Metric people do
not even know of them, just like the I-P practitioners ignore ours.
Here is what I would write into SI 10 on the subject of the mass unit and on
prefixes. Notice that the volume to mass relationship with water is the most
important thing to remember for practical use.
Pg 8, par 3.3.2.5: Reword the second sentence to: The kilogram is also the
name of the artifact that serves as the international standard for
calibration. The unit of mass was originally set on the mass of water
contained in 1 cm3 and named the gram, symbol g. A change was made later to
make the artifact 1000 times bigger, or a kilogram. A new name was not
coined - thus the double meaning: an artifact and a unit. This lead to the
potentially confusing anomaly whereby multiples, that is units with
prefixes, still combine with the gram, not the kilogram. Thus 1000 kg is 1
Mg and not the logical but obviously unnecessarily complicated 1 kkg.
The original definition of the gram makes it easy and useful to remember
that 1 Mg of water fills
1 m3 volume, 1 kg fills 1 dm3, 1 g fills 1 cm3, 1 mg fills 1 µm3, etc.
In engineering work, particularly in calculations, it is important to
remember that DERIVED UNITS are formed with the kg, it being the base unit.
The kilogram, not the gram, figures in dimensional analysis for such units
as newton, joule, pascal, and with quantities such as specific heat.
On the prefixes:
Prefixes are symbolic names (e.g., kilo, mega, milli) that express powers of
10. They are international names for numbers (e.g., million = mega).
Prefixes were devised to shorten lengthy numbers, an alternative to the 10n
notation (e.g. 5000 m or 5x103 m shortened to 5 km) and to the creation of
new names (e.g. 3520 yards renamed 2 miles). SI currently recognizes 20
prefixes; all are listed in the Table 5. Notice that the majority of
prefixes represent the powers of 103. When applied, prefixes precede the
unit and are written together with it (e.g., kilogram or kg). Being names
for numbers, they of course combine with any unit just as a number would.
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[email protected]>
Sent: 08 Jun 26, Thursday 22:41
Subject: [USMA:41258] Re: tonne
Stan,
With respect to SI, there is one and only one *coherent* SI unit for each
physical quantity. e.g. kilogram for mass, newton for force, joule for
energy, watt for power, etc.
The Mg is certainly *not* a coherent SI unit of mass, neither is a
kilowatt (kW) a coherent SI unit of power.
Applications of SI prefixes to SI coherent units do, in fact,
make multiples and submultiples of coherent SI units incoherent with
respect to all the other coherent SI units.
The kilogram is the only exception for historical reasons.
SI prefixes are certainly destroyers of coherence in favor of scaling SI
coherent units up or down to enable convenient expressions of numerical
values without exaggerating implied precisions.
How does your definition of "coherence" differ from the above CGPM concept
of coherence?
Gene.
---- Original message ----
Date: Wed, 25 Jun 2008 08:01:24 -0400
From: "Stan Jakuba" <[EMAIL PROTECTED]>
Subject: Re: [USMA:41217] Re: tonne
To: <[EMAIL PROTECTED]>, "U.S. Metric Association" <[email protected]>
Gene:
I suggest you check on the definition of "coherent." And perhaps also on
what a prefix is as it has nothing to do with coherence.
Stan
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[email protected]>
Sent: 08 Jun 24, Tuesday 13:33
Subject: [USMA:41217] Re: tonne
Stan,
More simply; A "tonne" is a "metric ton" is 1 000 kg in coherent SI.
The
Mg has never been widely used and is not coherent or appropriate for
masses of 1 000 kg and more.
I object to promotion of the Mg on grounds of incoherence with the SI
unit
of mass, the kilogram.
Gene.
