>> MICHAEL: Uh... He took the temperature water freezes at and said that's zero and then he took the

temperature it boils at and says that's a hundred. And he figured a hundred

was a good amount of demarcations to make in between the two. Right?

>> DEREK: Yeah. Except that's not what he did. >> MICHAEL: Really?

>> DEREK: Celsius never devised nor used the scale that now bears his name.

>> MICHAEL: Are you kidding me? >> DEREK: No, Michael, I'm not!

This is the Swedish town of Uppsala, located 70 kilometers north of Stockholm.

So this is the house where Celsius lived. In 1741 he was professor of astronomy at

Uppsala University. And this is the first ever Celsius thermometer... or is it?

>> This is the first scale of Anders Celsius added onto... or, added onto an old

thermometer.

So we have Delisle' scale on the left hand side and Celsius' new scale in the right.

>> DEREK: These few markings show that Celsius came up with

the idea of separating the freezing and boiling points of water by 100 degrees.

He made his first observations with the scale on Christmas Day 1741. But have you

noticed something strange about this scale?

I mean, 50 is marked in the middle but the numbers increase down towards the

bulb of the thermometer. So, Celsius had his scale upside down.

He set zero degrees at the boiling point of water and a hundred at the

freezing point.

Why would he do this? Well, for one thing

Celsius was just following the convention of the other scale on the

thermometer. Delisle' scale also had zero degrees for the boiling point of

water increasing down to 150 for his freezing point. And a likely reason both

of them used upside down scales is because they avoid negative numbers.

In Sweden it gets much colder than freezing but never warmer than boiling water so

you don't have to worry about pesky minus signs and this helps avoid logbook

errors.

I think it would be really weird if you had water boiling

zero degrees and freezing at a hundred. Wouldn't that be strange?

Although it might seem strange today, there is no objectively good reason for

preferring an ascending scale over a descending one for measuring degrees of

something, like hot or cold.

In fact, when Celsius died of tuberculosis in 1744, he was still using

this inverted scale.

So who reversed it? Who do we have to thank for the modern Celsius temperature scale?

Well in 1745, just a year after Celsius' death

a new column appears in the Uppsala temperature record using the modern

scale and at the top it's got the heading "Ekström" Now Ekström was the

instrument maker at Uppsala. In 1747 another column is added with the heading

Strömer who was Celsius' successor as professor of astronomy.

Again, it's got the same modern scale. But it's another professor at Uppsala who

claims that he reversed the scale:

That's the famous biologist Carl Linnaeus. He says he reversed the scale

when he ordered a thermometer from Ekström for his greenhouse.

Whoever it was, we know that by 1745 there was an operational thermometer at

Uppsala University with a scale that we now all know as the Celsius temperature scale.

Except... this was not the first such thermometer ever created. In 1743 the

year before Celsius died, a French scientist working independently in Lyon

created a thermometer with zero degrees at the freezing point of water and a

hundred at the boiling point.

his name was Jeane Pierre Christin. So why isn't it called a degree Christin instead

of a degrees Celsius?

Well, for a long time this temperature scale wasn't referred to using either of

their names and instead it was just called the "centigrade scale," meaning a

hundred steps.

The problem with this was "centigrade" has other meanings in French, Spanish and

Italian, where a grade specifically refers to one one-hundredth of a right

angle. So to eliminate this confusion the International Bureau of Weights and

Measures in 1948 decided to rename the centigrade scale after a scientist

bringing it into alignment with other temperature scales like Kelvin

and Fahrenheit. They chose Celsius possibly due to a popular 1800's German

chemistry textbook by a guy named Berzelius. In it, Berzelius identified

Celsius as the first to devise a temperature scale with zero at water's

freezing point and a hundred at its boiling point propagating the myth that

Celsius created this scale. So is Celsius undeserving of having his name

immortalized on weather maps around the world?

Well, no. I mean making two marks and deciding on a number of degrees in

between them that is the easy part which others could, and as history shows, did do...

but what Celsius did was he established which physical processes could reliably

produce a fixed temperature. At the time when he was working, there were some 30

different temperature scales in use. Some thermometers had 18 different scales on

them, some more reliable than others. For fixed reference points one scale used the

deepest cellar of the Paris observatory,

others used the melting temperature of butter, the internal temperatures of

certain animals, or the hottest day in summer in Italy, Syria or Senegal. These

dubious scales combined, with inconsistent thermometer construction

made recording accurate temperatures nearly impossible, not to mention sharing

those temperatures with other scientists elsewhere. Celsius solved this problem in

a number of ways: he demonstrated that melting snow maintains a fixed temperature

regardless of latitude or ambient pressure. He also determined the precise

relationship between boiling point of water and the ambient pressure, so that

thermometers could be calibrated under any conditions.

He made it possible to establish a universal reliable system of temperature

measurement and this is why Celsius' name is a unit of temperature, even

though the scale itself was not created by him and in fact today the Celsius

scale is no longer defined by either of water's phase transition points, instead

it is based off the Kelvin scale and that scale is defined by setting the

triple point of water,

where solid, liquid, and gas all exist in equilibrium at exactly 273.16 Kelvin.

This fixes the size of a degree Kelvin, which is exactly the same as the size of

a degree Celsius. The zero point of the Celsius scale is set a 273.15 Kelvin.

And what all this means is that today pure water boils at

99.974 degrees Celsius and freezes at negative

0.0001 Celsius. This precision in a nearly universal system of temperature

measurement is thanks to a huge number of scientists

so the C after the degree symbol stands not only for Celsius or Christin

or Carl Linnaeus, but for the community of scientists whose work over centuries

made a scale so robust that we take it for granted.

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