The Ultimate Thermometer

The substance, water, without which life would be impossible, has been used to measure all temperatures, in effect.

When science, serious science began, it was realised that some sort of temperature scale would be necessary. Mr Faranheit came along with an arbitrary system. Then Mr Celsius saw common sense and invented another system, first calling it "Centigrade" before being modified to "Celsius".

He defined it very simply; with water.

"At normal temperature and pressure, let pure melting ice be zero Centigrade, and pure boiling water be one hundred Centigrade."

Just to make things easy. To convert from one to the other was fairly easy:

Celsius to Fahrenheit (°C × 9/5) + 32 = °F
Fahrenheit to Celsius (°F - 32) x 5/9 = °C

Or, in words,

°F to °C Deduct 32, then multiply by 5, then divide by 9
°C to °F Multiply by 9, then divide by 5, then add 32

Most Americans use Deg F (except for the scientists), whilst almost everyone else uses Deg C (because Celsius is the Universal unit of temperature as adopted by the Scientific Community - it is simpler).

One may have noticed that in the definition of zero and one hundred Celsius, zero is not simply "ice", nor is one hundred simply "steam".

There is a very good reason for this. Thermodynamics again. If you take a block of ice and heat it, it's temperature goes up like this:



There are five bits to it: three sloping bits and two flat bits. The sloping bits are easy. Over time, as more heat gets put into the substance, the temperature rises, the first slope is the temperature of ice rising, the second slope, that of water, the third, that of steam, as more and more heat is supplied.

But what about the flat bits? This is the interesting part. The flat parts of the graph are where the temperature stays constant because all the heat (therefore energy) is used in breaking chemical bonds, the first flat bit from ice to water (solid to liquid), the second flat bit from water to steam (liquid to gas).

Celsius used this simple fact to define his fixed points, the two flat bits, where the temperature is always the same.

So, if you want something to be cooled to EXACTLY zero degrees Celsius, stick it in a bag of MELTING ice, or if you want something to be EXACTLY 100, put it into a container of boiling water.

That is why water could be called "The Ultimate Thermometer"

( It is quite clear that the bigger the sample of ice, the more heat you have to put in to break the two types of chemical bonds. There is a measure for this was well, it is called "Specific Heat Capacity", but that is another story. )

Great Things are Afoot

Units. What are they? They are the means by which we measure how much of something we have got.

How far can this car go?

What time is it?

How much is a can of coke?

How much beer have I drunk tonight?

Knowing the answers to these questions enables us tell whether or not we can get there, late or not, have time to buy as many Coca-Cola's we need, how many beers I can safely consume before I go home, and how far it is to the nearest ReHab.

Of course, science has to be a little more exact than saying "a few", "enough, or "too many".

In fact, we have to be very precise about everything we measure.

The first example may be the simplest: Distance. How far away is that tree? The ancient civilisations used a "cubit",the distance from the tip of the fingers to the elbow of a regular guy.

Old England invented the "foot" consisting of twelve "inches". The "yard", being three "feet".

The trouble with these from a physics point of view was that there was no universal standard "foot" upon which everyone agreed.

The same applied to weight and the measurement of time.

After a nit of huffing and puffing (it was called the "cgs" system (centimeter - gram - second)), an international organisation called "SI" (Systeme Internationale) based in France, decided once and for all to actually define "length", "weight" and "time".

The Meter - the length of a platinum-Iridiumn bar in a lab at Sevres.

The Kilogram - a certain quantity of water (more in a minute).

The Second - 9,129,631,770 vibrations of a Caesium atom.

The history is fascinating. The thing about the decimal system is that all quantities are measured in multiples of ten, being prefixed by Latin words signifying how many tens:

Deca - 10
Kilo - 1,000
Mega - 1,000,000
Giga - 1,000,000,000
Tera - 1,000,000,000,000

and if you want to go smaller:

deci - 0.1
centi - 0.001
milli - 0.0001
micro - 0.000001
nano - 0.000000001
pico - 0.000000000001
femto - 0.000000000000000001

(This is just a selection of about 30 in all, but are the most common)

Doing a length conversion,
One "inch" was equal to about 2,54 centimeters
one "foot" was equal to about 1.5 decimeters.

The kilogram was deliberately defined to be the mass (weight) of exactly one cubic decimeter of water at 20 deg centigrade.

This was also defined as the "liter". However, it is not entirely exact. a cubic deciliter of water actually holds 1.0000028 (I forget how may zeroes) liters of water, due to an oversight in the definition, which remains ever since.

The meter was redefined as the length of the equator divided into 40,000 equal bits, and then re-redefined as a specific number of wavelengths of radiation from a specific atom. It was subsequently re-re-redefined as "the length of the path traveled by light in vacuum during the time interval of 1/299 792 458 of a second. ("299 792 458" is the number of meters light travels in one second in free space)

This system was first called the "MKS" system (Meter-Kilogram-Second),which was then revised to include units of electricity (the Amp) and energy (the Joule) amongst other things.

This system, which remains today, is called the SI system.

Simple, Innit?