Dear All, (01)
John says "All units of measure are ultimately defined by some process of
measurement." I think that this is true, but we can go further and say that
in many cases the kind of quantity is also defined by the process of
measurement. This becomes untrue only when the quantity is embodied in a
physical law. An example of this is temperature. Temperature is not merely a
phenomenon defined by the differential thermal expansion of mercury and
glass or the behaviour of electrons in a thermocouple junctions, because we
have the laws of thermodynamics. If we didn't it would be no different from
Rockwell C harness, which defines both the measurement method and the kind
of quantity. (Ed is unhappy about including kinds of quantity like Rockwell
C harness because of the absence of a physical law, but I believe we have no
choice.) (02)
There are two very important cases where the measurement method also defines
the quantity - WGS84 position and TAI time. There are many possible
definitions of position - i.e a choice of frame. WGS84 defines a particular
frame, within which the earth is more or less at rest, and specifies a way
of identifying a position within that frame. (03)
Similarly there are many possible definitions of time - i.e. a choice of
gravitational potential and path for the clock. TAI defines both a
particular time, and specifies a way of identifying an instant within that time. (04)
Best regards,
David (05)
At 11:11 07/08/2009 -0400, you wrote:
>Not sure I agree. Anything which is a unit at least implies that there
>could be a multiple of that unit (theological concepts aside), which
>implies counting which is a form of measurement.
>
>On the other hand, getting into concepts of measurement and
>observation themselves will bloat an ontology of units beyond
>recognition. Certainly there are methods of measurement for properties
>such as hardness which are only faintly comparable in detail, but I
>would hope that concepts such as scales / coordinate systems would
>help with this, since some of those can be mapped precisely from one
>to the other, but not all of them even if they purport to represent
>the same property.
>
>Josh
>
>On Aug 7, 2009, at 9:30 AM, Mike Bennett wrote:
>
>> True, but we need to also remember that not all units are units of
>> physical measurement, i.e. not all measured amounts of something are
>> measured amounts of some physical thing. In particular, the concepts
>> of
>> "Amount", "Unit" and so on are applied equally to amounts of money and
>> units of currency. Though you could argue that apart from the US
>> Dollar,
>> units of currency are defined by some process of measurement (the rate
>> against the US Dollar).
>>
>> Therefore I would hope that the fundamental properties at the core of
>> this, are not prematurely committed to the physical world. Rather,
>> there
>> are amounts and units, and there are physical amounts and physical
>> units
>> which are kinds of these, as well as currency amounts and currency
>> units
>> which are kinds of these (and all the other interesting variations we
>> have seen). Perhaps things like hardness would fit into that kind of
>> framework more easily than if we pre-commit to a physical-only view of
>> measurement and units.
>>
>> Mike
>>
>> John F. Sowa wrote:
>>> David,
>>>
>>> All units of measure are ultimately defined by some process
>>> of measurement. The meter was formerly defined by a comparison
>>> with a standard held in a vault in Paris. Now it's defined as
>>> a wavelength of light emitted by a certain type of atom under
>>> certain conditions.
>>>
>>> There is no difference, in principle, between the definition of
>>> 'meter' or the definition of 'standard candle':
>>>
>>> The Standard, or International, Candle is a measurement
>>> of light source intensity. It was originally defined as
>>> a one-sixth-pound candle of sperm wax, burning at the rate
>>> of 120 grains per hour. This intensity of light was standardized
>>> in 1921 in terms of incandescent lamps, and candles are no
>>> longer used for reference. (From Britannica online)
>>>
>>> The definition of 'hardness' by various methods is no different in
>>> principle from the definition of any other kind of physical unit.
>>>
>>> John Sowa
>>>
>>>
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>>>
>>
>>
>> --
>> Mike Bennett
>> Director
>> Hypercube Ltd.
>> 89 Worship Street
>> London EC2A 2BF
>> Tel: +44 (0) 20 7917 9522
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>> www.hypercube.co.uk
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>>
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>
>
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> (06)
============================================================
David Leal
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