Re: Temperature Scales

[brian whatcott <inet@INTELLISYS.NET>]

At 14:38 11/16/00 -0500, you wrote:
> The Celcius temperature scale is based on the freezing and boiling points of
> water. The temperature range was divided up into 100 degrees. Does anyone
> know how the Fahrenheit scale was determined?
> -tony

There is no basis of absolute certainty with which to provide an
objective answer to this question of scaling temperature.
One thing is certain - the ability to decide one or more temperature
standards depended on the ability to measure such temperatures.

 There is an assertion that Florentine sealed thermometers using a
liquid alcohol thread date from the 1650's.
In fact Callendar claimed that the Duke of Tuscany's thermometers
were intended to represent degrees as one thousandth parts of the
reservoir's volume.
(Ethyl Acohol, having a volumetric thermal expansivity around 0.108%
 per degC would provide a centigrade type scale if this were so)

Because this material has a liquid range of 156K to 352K or -117C to 79C
it is reasonable to suppose that an elevated standard at the boiling
point of water might be out of reach for this sort of instrument.

There seems to be general agreement that the lower standard used by
Fahrenheit as the zero of his scale, was a mix of ice and common salt.
This would give a freezing point depression of 21.2 degC at the
eutectic (according to Lange). We would reckon the temperature at
which water freezes as about 38 degF on this basis. But finding
the eutectic is not easy, and he may have used a mix more similar
to 1 gm mol per 100 gm giving  18.6 degC depression or less.

 Less clear is the progression of his scaling of an upper standard -
it is thought he initially set the oral temperature at a 12 degree
mark, but later multiplied the scaling so that the oral temperature
(X8) became 96 degrees. He read of the constancy of boiling temperature
 of water, a report of the Royal Academy, Paris and it was this
research interest that led to his selecting a mercury in glass sealed
thermometer(1714).
 Rationalizing the scale over 180 degrees from the new upper point
of 212 deg seems to have been the final step in the wide adoption
of this meteorological instrument maker's Dutch product, described
in Phil Trans 1724. He also made worthy hygrometers.





brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!