Zero Point of Thermometer

Date: 04/06/2002 at 08:09:59
From: Mark 
Subject: Zero point of thermometer

What is the zero point of a thermometer? Is it 0 degrees of any 
temperature scale, or 0 Kelvin, or -273.15 Kelvin? I know how to 
convert the scales to the other scales. Just what is the zero point?
How would you find the zero point on a thermometer?

Date: 04/06/2002 at 09:27:34
From: Doctor Tom
Subject: Re: Zero point of thermometer

Hi Mark,

That's a good question.

In every temperature scale, the temperature of a substance rises by 
one degree when a specific amount of thermal energy is added to the 
substance. This "specific amount" differs, of course, with different 
scales. Since the Centigrade steps are bigger than the Fahrenheit 
steps, the amount of energy you need to add to go up one degree 
Centigrade is more than the amount needed to go up one degree 
Fahrenheit.

But the idea is this: if you start at any temperature and add that 
amount of energy, the temperature goes up one degree. If you add that 
amount again, the temperature goes up another degree, etc.

Remember that it's thermal energy only that counts. If you're melting 
ice, for example, you add a lot of energy, but the temperature doesn't 
rise, since the energy you're adding is to overcome the heat of fusion 
of ice - the energy required to convert ice at freezing to water at 
freezing.

So temperature goes up linearly with added thermal energy, but where 
is the zero point?

The first reasonable system was the Fahrenheit system. Since the 
inventor of the system had no idea of absolute zero, he just chose for 
his zero the coldest temperature that was, at the time, available in a 
laboratory (the temperature of ammonium chloride in ice water). Then 
he needed to pick another point and assign it a number, and everything 
else would be fixed. He chose human body temperature as 100 degrees 
(obviously he had a very slight fever when he measured this, since 
normal body temperature is around 98.6 degrees Fahrenheit.)

Of course this was long ago, and soon colder things were found, so 
negative temperatures were required.

Centigrade was more scientific, in a sense, since its endpoints were 
chosen in a purely physical way: 0 degrees for freezing pure water and 
100 degrees for boiling pure water (at standard pressure - sea level, 
or 760mm Hg).

Of course Centigrade also needed negative temperatures.

Later, the study of thermodynamics showed that there is an absolute 
zero. In other words, you can't just keep removing thermal energy 
forever, and once it's all gone, the temperature can't go lower. This 
is VERY cold: -273.15 degrees Centigrade.

A more reasonable temperature in some sense, is the Kelvin scale, 
where absolute zero is chosen to be zero. But since all the scientific 
data were/are in terms of one-degree steps in the Centigrade scale, 
0 degrees Kelvin is chosen to be -273.15 degrees Centigrade, and the 
freezing point of water is chosen to be 273.15 degrees Kelvin.  
(Actually, I think Kelvin is technically defined with the triple point 
of water which is .01 degree different from the freezing point at
standard pressure. The triple point is where solid, liquid, and 
gaseous water all exist at equilibrium, and this determines not only a 
temperature and a pressure, but is very easy to duplicate and does not 
require a pressure measurement.)

There's also something called the Rankind scale that's like the Kelvin 
scale but uses steps of the Fahrenheit size.  It's hardly ever used.

To convert from Centigrade to Kelvin, add 273.15 degrees.

To convert from Centigrade to Fahrenheit, use this formula:

   C = (5/9)(F-32)

So the situation is a little strange, but we're stuck with it for 
historical reasons. Perhaps an even better way to do it would be to 
define 0 degrees as absolute zero and 100 degrees as the triple point 
of water. But then the size of a degree would change, and all 
scientific publications that have ever been written that mention a
temperature would be out of date, and that would certainly not be 
worth the trouble.

- Doctor Tom, The Math Forum
  http://mathforum.org/dr.math/