|
In 1742 the Swedish astronomer
Anders Celsius published a paper in the "Kungliga Swenska Wetenskaps
Academiens Handlingar", the Annals of the Royal Swedish Academy
of Science, entitled "Observations on two persistent degrees
on a thermometer". This paper is the origin of the Celsius temperature
scale.
After providing
a background on the various ways of expressing temperature used at
that time, Celsius presents an account of his experiments with two
"fixed points" for a temperature scale: The temperature
of thawing snow or ice and the temperature of boiling water.
The idea to use
the freezing point of water as a temperature calibration point was
not new and had already been suggested by Reamur and Newton as well
as by the less well-known contemporary scientists Martins, Weitbrecht
and Poleni. Celsius, however, used the melting point rather than the
freezing point, i.e. thawing snow or ice rather than freezing water.
"This experiment",
Celsius writes, referring to placing a thermometer in thawing snow,
"I have repeated many times during two years in all winter months
and all kind of weather, and during different barometric changes,
and always found precisely the same point on the thermometer... When
it was strong winter I have taken the cold snow into my room and put
if in front of the fire until it began to thaw... "
Furthermore, using
one of Reamur's thermometers, Celsius finds, within the experimental
error, the same freezing point in Uppsala (latitude 60 oN) as in Torneå
(latitude 66 oN) as Reamur found in Paris.
The second calibration
point was more complicated. "As far as the second fixed point
is concerned," Celsius writes, "it is well known that once
water has began to boil, it will not take up any considerable degree
of heat even if one continues the boiling for long; thus the mercury
in the thermometer will end at the same point, despite the objections
of Mr. Taglini." However, the intensity of the boiling might
affect the calibration point to a certain degree and Celsius proposes
a standardised method for the determination. He also notes that when
the thermometer is taken out of the boiling water the mercury level
at first rises somewhat before it retracts. The explanation, Celsius
suggests, is that the glass tube contracts before the mercury starts
to cool off.
The second factor
that affects the boiling point of water is the air pressure. This
was already observed by Fahrenheit, and Celsius reports on series
of experiments to confirm this observation. His conclusion is that
"the height of the thermometer in boiling water always is proportional
to the height of the barometer, thus 8 "points" on the thermometer
I use correspond to a change of one inch in the barometer reading;
a thermometer which is somewhat more sensitive or have large degrees
can be used as a barometer when put into boiling water, and would
be easier to bring along when travelling at sea or land, especially
on high mountains."
Finally Celsius
proposes a standardised procedure for the calibration of a thermometer:
1. Put the cylinder
AB of the thermometer (i.e. the bulb) in thawing snow and mark the
freezing point of water C, which should be at such a height over the
cylinder at A that the distance AC is half the distance between C
and the water boiling point mark D.
2. Mark the boiling
point of water D at a pressure of "25 tum 3 linier" (approximately
755 mm).
3. Divide the distance
in 100 equal parts or degrees; so that 0 degree corresponds to the
boiling point of water D, and 100 to the freezing point of water C.
When the same degrees have been continued below C all the way down
to A the thermometer is ready.
Celsius thus put
the boiling point at 0 degrees and the freezing point at 100. This
was, however, soon to be reversed. It is often claimed that Carl von
Linné (Carolus Linnaeus) instigated the reversal, but it appears
more likely that the responsible person was Daniel Ekström, who
manufactured most of the scientific instruments, including thermnometers,
used by both Celsius and Linné.
<<
Back to Resources List
|
