Re: Boiling

[Larry Smith <Larry.Smith@SNOW.EDU>]

At 10:50 AM -0600 3/11/01, brian whatcott wrote:

> Reviewing the progress of thermometry like this:
> 1) a centigrade system between fixed points provided by a specified
> freezing mixture and mean human body core temperature (due to Fahrenheit)
> 2) a centigrade system between fixed points provided by the
> freezing point of pure water, and the boiling point of pure water
> at a pressure of 1 atm  (due to Celcius)
> 3) a linearly divided scale with one physical fixed point which
> is very repeatable - the triple point of pure water, and a lower point
> which can be repeatably and closely approached, scaled so as to
> approximate as far as possible with a simple zero shift, an existing
> temperature scale in which many important results were
> historically expressed.
>
> I find it difficult to drum up any of Jim's righteous indignation
> on behalf of the worthy teams of international scientific
> bureaucrats who promulgate standards and definitions which are
> intended above all to make critical scientific units more repeatable,
> and more standardized in all countries and laboratories.
> This is worthy work, no doubt - but it is not religion, in my view.

I think Jim's indignation was that the latest definition (#3 quoted by
Brian) happened half a century ago and the physics books still say water
boils at 100 degrees C.  Yes, the true value is close to 100, but the fact
that it isn't exactly 100 ought to be in the physics books; Jim shouldn't
be forced to go to a PChem book for this.

But I would add that Serway does address this, so maybe Jim's sample of
current physics texts was too small.
"T_c_ = T -273.15" and
"Early gas thermometers made use of the ice point and steam point according
to the procedure just described.  However, these points are experimentally
difficult to duplicate.  For this reason, a new temperature scale based on
a single fixed point with b equal to zero was adopted in 1954 by the
International committee on Weights and Measures. ... The triple point of
water ... was chosen as a convenient and reproducible reference temperature
for this new scale.  This triple point occurs at a temperature of
approximately 0.01 degree C and a pressure of 4.58 mm of mercury.  On the
new scale, the temperature of water at the triple point was set at 273.16
kelvin,....  This choice was made so that the old temperature scale based
on the ice and steam points would agree closely with the new scale based on
the triple point.  This new scale is called the thermodynamic temperature
scale, and the SI unit of thermodynamic temperature, the kelvin is defined
as the fraction 1/273.16 of the temperature of the triple point of water."
(p 535 in the 4th edition)

Larry