Although these days it’s a relatively simple
thing to measure the temperature of anything,
it is quite difficult to define exactly what temperature
We know from experience that hot things (from
the oven say) will eventually cool down and that
cold things (from the freezer) will warm up. What
is happening is that in both cases the object
is reaching equality (technically it is called
thermal equilibrium) with its surroundings (the
air around it). We can therefore define temperature
as that quantity which is the same for both things
(the initially hot/cold object and the air) when
they have settled to thermal equilibrium.
There have been many different scales and devices
created for measuring temperature. The three most
commonly used today are the Fahrenheit,
(or Centigrade) and Kelvin
of the first attempts to make a standard temperature
scale happened about AD 170. Galen, in his medical
writings, proposed a standard "neutral"
temperature made up of equal quantities of boiling
water and ice. On either side of this temperature
were four degrees of heat and four degrees of
cold. Crude maybe, but at least it was an attempt
to make heat measurable.
The earliest devices used to measure temperature
were called thermoscopes. They consisted of a
glass bulb having a long tube extending downward
into a container of coloured water.
So where did all this Celsius, Centigrade, Fahrenheit
and Kelvin stuff come from?
It's a tale that starts with three scientists;
a Dane, a Dutchman and a Swede at around the beginning
of the 18th century and ends with a Belfast-born
'Scot' in the middle of the 19th century. Very
cosmopolitan. Their names were Roemer, Fahrenheit,
Celsius and Thomson. It begins to sound familiar.
So were Roemer and Thomson the only ones to miss
out in historical terms because I've never heard
of Roemer or Thomson degrees? Not really. Roemer
had other strings to his bow, as they say, and
Thomson changed his name.
Roemer was an astronomer and was the first person
to measure the speed of light by observing the
satellites of Jupiter - not an insignificant contribution
However he was also one of the first to successfully
produce a "temperature measuring thingy"
(a thermometer to you and me). He created a glass
tube with a glass bulb at the end - pretty much
as we have today. He filled it with strong wine,
coloured with saffron to make it easier to see
or less likely to be drunk? He knew that temperature
could be measured by recording the volume of the
liquid by seeing how far up the thin tube the
liquid rose when heated to different temperatures
(liquids expand when they are heated). In practice
he took a measurement of freezing water, another
of boiling water and then divided the scale into
seven parts. He added an eighth part, as big as
the others, below the freezing point and called
it zero degrees. His boiling point was at 60 degrees.
Why would he choose 60 degrees as the top end?
Could it have been anything to do with there being
60 minutes in an hour?
Sixty is actually a pretty convenient number
for doing simple arithmetic. It is easy to calculate
fractions of it because it has 10 factors (2,3,4,5,6,10,12,15,20,30)
- numbers which can divide into it exactly. However,
50 only has 4 factors and 100 only 7. That probably
helps explain why there are 60 minutes in an hour
and so Roemer was just following common sense
So now he had a scale divided into 8 parts going
from 0 to 60 - so each part was 7.5 degrees. The
freezing point of water was at 7.5 degrees. On
this scale his experiments showed that human body
temperature was pretty constant and equal to about
Enter Mr Fahrenheit. Roemer showed him one of
his new thermometers which used his temperature
scale but only measured up to about the temperature
of the human body, which on his scale remember
was 22.5 degrees. Building such a device makes
sense - after all how often do you need to measure
temperatures higher than that? Fahrenheit realised
the potential of Roemer's device but didn't like
fractions - who does? So he set about refining
it. First of all he divided Roemer's scale more
finely (to get rid of the fractions) so that one
degree in Roemer's scale now equaled 4 new Fahrenheit
degrees. He reset the zero degrees to be exactly
where a mixture of ice, water and salt freezes.
I suppose that was about the coldest thing he
could easily produce and so now he had a scale
from 0 (ice, water, salt) to 90 (human body, =
22.5 x 4) degrees, with pure water freezing at
30 (= 4 x 7.5) degrees.
He was happy with that for a while but eventually
figured that mercury was a better liquid to deal
with in these devices rather than Roemer's alcohol,
which was of variable quality from thermometer
to thermometer, and also he didn't like dividing
by 3 rather than 2 so he reset the scale again
so that water froze at 32 degrees and the human
body temperature was 96 degrees.
After Fahrenheit's death it was realised that
human body temperature can vary and is therefore
not a good scale point so once again the boiling
point of water was used and set to 212 degrees
(close to Fahrenheit's original value of 205 degrees)
for convenience of dividing the scale.
Meanwhile, a fresh start had been made by the
Swedish scientist Celsius - he had no hang-ups
about the number 60 and built a device where there
were 100 degrees between the freezing and boiling
points of water. Sounds an obvious thing to do,
right? All the best ideas sound obvious afterwards.
One final twist in the tail - Celsius actually
set freezing point at 100 degrees and boiling
at 0 degrees. All scientists can be mad once in
their lives. That was reversed after his death
to be the more natural way around we know today
and in 1948 the temperature scale was officially
named the Celsius scale (the term centigrade had
come into common use) at the Ninth General Conference
of Weights and Measures.
So where does Thomson come into all of this?
William Thomson was born in Belfast in 1824, but
his family moved to Glasgow when he was six. He
went to Glasgow university at the ripe old age
of 10, still a record. After doing some fundamental
work on heat and temperature he devised a temperature
scale similar to that of Celsius. However, his
scale started at -273.16 °C. There was a very
good reason for that. He had worked out that that
was the temperature at which matter (molecules
would have no kinetic energy and therefore no
movement. For this reason this temperature is
referred to as 'absolute zero' and is the coldest
anything in the universe can get.
Notice that the unit of temperature on Kelvin’s
scale is one kelvin NOT one degree, so it is different
in that respect from Celsius and Fahrenheit. The
correct expression is therefore, ‘the temperature
of the Sun’s surface is about 6000 kelvin’
not ‘the temperature of the Sun’s
surface is about 6000 degrees kelvin’.
For his work in physics, Thomson was knighted
and eventually created Baron Kelvin of Largs,
which is where the unit of temperature gets its
So there you are, you can take your choice and
'Phew it's hot today, it's 89 degrees Fahrenheit'
'Phew it's hot today, it's 32 degrees Celsius'
'Phew it's hot today, it's 305 kelvin'