cubic VVV

[Chris Waigl]

James Smith wrote:

>No, actually a liter is the volume of exactly 1 kg of
>pure water at 3.98 °C and under a pressure of 1 atm.
>It is also exactly ten deciliters.
>
It can't be both. It would also be improper to define an SI unit in two
different, (potentially and actually) conflicting ways.

>However, because
>of a measurement error early in the merification
>process, 1 liter equals 1.000027 cubic decimeters.
>Within the limits of accuracy for most real-world
>measurements, the difference is negligible.
>Accurate current reference works give 1 liter =
>1.000027 cubic decimeter = 1000.027 cubic centimeters
>and 1 milliliter = 1.000027 cubic centimeter.
>
>
What you are describing is the state of affairs between 1901 and 1964.
In 1964, the liter reverted to its original definition as exactly 1
cubic decimeter = 1000 cubic centimeters = 1/1000 cubic meter.

Meanwhile, in 1960, the meter, too, was redefined without reference to
the meter bar (which itself was build with reference to a measurement of
the earth's meridian; an alternative reference to periods of a pendulum
was used, too, in the 18th century); this definition was again revised
in 1983. The current meter definition relies on time measurement only
(plus a universal constant).

>As I understand - but I may be mislead on this part of
>the tale - in the original metric scheme there was no
>liter, the cubic centimeter was the standard of volume
> and the liter came to be because of this measurement
>error.
>
>
In my understanding, the original liter definition was precise (and
identical in its formulation with today's, i.e. it didn't rely on any
measurements), but it was the kilogram definition that depended on the
precise measurement of the mass of 1 liter of pure water at a given
temperature and atmospheric pressure. That's where the measurement error
struck. Even today, with the iridium cylinder that serves as the
kilogram standard, the kilogram is still the iffiest SI base unit. There
are several proposals for a redefinition, one of which will without
doubt be adopted one day.

(The liter is not the standard of volume. That's the cubic meter.
"Liter" is just a shorthand for 1/1000 m^3.)

Chris Waigl

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