Re: Why are there 2 different temperature measurement units?

Hello Niklas,

A very good question.  When you say "2 units", I assume you mean Celsius
and Fahrenheit - but did you know there is even a third unit, called the
Kelvin?  

Well, all these units were invented by different people for different
reasons at different times in history, so let's take a look.  Actually,
many people in history have tried to invent ways of measuring temperature,
but the first one that most of us have heard of was Daniel Gabriel
Fahrenheit (1686-1736).  He spent most of his life inventing measuring
instruments and he tried to devise a temperature scale which was divided
according to things from everyday experience - very cold water, very hot
water, human body temperature and so on.  He had to adjust his scale
several times so in the end it turned out to be a rather strange scale
where water freezes at 32 and boils at 212.  However, his system became
popular in Europe and when people from Western Europe migrated to America
and founded the United States, they took the system with them.  In fact,
you will still hear people describe old units  like Fahrenheit, feet,
pounds and so on as "British units" - but this is because British people
took that system of measurement with them to the USA when they migrated -
not because the units are commonly used in Britain today.

Next up, we have a man named Anders Celsius (1701-1744).  He invented a
scale where the difference between freezing and boiling water is divided
into 100 parts.  This scale used to be called the centigrade scale, because
centi- is Latin for 100 and sometimes you will still hear people say
"degrees centigrade".  The funny thing is that originally the scale was
reversed - 100 was the freezing point and 0 was the boiling point, but
another scientist, Carl Linnaeus (1707-1778) later suggested it should be
the other way around.  The centigrade scale was renamed Celsius in honour
of Anders Celsius and people decided that 0 -100 was more sensible than 32
- 212, so the system became very popular and is widely used in many
countries today.

Now Kelvin - which is named after the famous scientist Lord Kelvin
(1824-1907 - his real name was William Thomson).  He discovered that there
is a very, very cold temperature which is so cold that you cannot get any
colder.  He called this temperature "absolute zero" and he decided that
there should be a new temperature scale which started with 0 at this
coldest possible temperature.  Of course you can convert between all of
these scales and it turns out that absolute zero - 0 Kelvin - is about
minus 273.15 degrees Celsius, or minus 459.67 degrees Fahrenheit.  It's
hard to imagine how cold that is, but at that kind of temperature, gases
like hydrogen or oxygen or helium become liquids, or even freeze to solids!

OK - you ask a very good question, "why don't we just use one?"  I'm afraid
there is no one simple answer, there are lots of reasons.  Scientists like
to use the Kelvin scale, because it makes their calculations easier and it
tells us a lot about the relationship between temperatures and pressures,
especially when we are studying gases.  But in everyday life, most people
will use either Celsius or Fahrenheit.  This might be because they feel
more comfortable with one or the other, perhaps because of how old they are
and when they grew up.  Or it might be for reasons that are social,
cultural, historical or political - in most countries the government
decides what the national system of measurement should be.  Most of Europe
has adopted what is called the SI system (Celsius, metres, kilograms and so
on), but the USA stays with the "British system" - perhaps because of those
first settlers over 200 years ago!  The important thing in science is not
to mix up your units.  I don't know if you have heard of a spacecraft
called the Mars Climate
Orbiter?  It was sent to Mars about 5 years ago but the mission failed
because some of the engineers were using "British Units" and some used the
SI system, so their calculations were wrong and the spacecraft crashed!

Here are some links to the things I talked about:

History
of thermometers
Celsius
Fahrenheit
Kelvin
Brief History
of Temperature Measurement