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The measurement of temperature we use in Europe is based on the centigrade scale.

0°C = the freezing point of water

100°C = the boiling point of water


Types of Thermometer

The Alcohol Thermometer

This was the first to be invented and it is still in use today. The column of alcohol is quite wide, coloured (usually red) to make it easy to read. It is often used for measuring air temperatures which range between - 20°C and +50°C. This is fine because alcohol freezes at - 80°C and boils at +78°C.

The Mercury Thermometer

This is the most commonly used laboratory thermometer because temperatures above +78°C often need to be measured. Mercury freezes at - 39°C, so for very low temperatures it is not so useful. Mercury boils at + 357°C which is the upper limit of the thermometer.

The Clinical Thermometer

When taking the temperature of a liquid or of air, normally the thermometer should be kept in the substance whilst you take the reading. For a human or animal body this is not possible. A special mercury thermometer was developed so it could be taken out of the body to read the temperature.

The clinical thermometer is a special thermometer which has a sharp bend in its tube that is narrower than the rest of the tube.

To use a clinical thermometer, it is put into the body, either in the mouth (under the tongue) or in the anus. The thermometer is left there for one minute. The mercury rises past the bend to indicate the body temperature. When the thermometer is removed from the body, the mercury shrinks as it cools and draws back into the bulb. However, the column of mercury breaks at the kink and allows you to read the temperature. To get the mercury column back past the bend, a quick shake of the thermometer is all that is needed.



Fact File No.3

The normal body temperature is 37°C.

One of the lowest recorded temperatures on Earth was -89,2°C at Vostok, Antarctica, on 21 July 1983.

One of the highest recorded temperatures on Earth was 56.7°C in Death Valley, California, USA on 10 July 1913.


Some interesting temperatures




Temperature at the Sun's surface


Temperature of a Bunsen burner flame


Iron melts


Gold melts


Silver melts


Water boils


Highest recorded temperature on Earth


Human body temperature

+18 to +20

Room temperature


Water freezes

- 37

Car antifreeze freezes

- 89,2

Lowest recorded temperature on Earth

- 273

Absolute zero (see below!)


Absolute Zero

In the 19th century, physicists started to investigate very low temperatures. In fact, they wanted to find out what was the lowest temperature possible. For this they started using a new scale of degrees called Kelvin, after Lord Kelvin, a famous British scientist at that time. This scale uses the same units as the Celcius scale, but it starts at ABSOLUTE ZERO, the temperature at which everything freezes solid, even air. Absolute zero = O K (there is no ° symbol in the Kelvin scale) = - 273°C.




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Temperature is a measure of the hotness or coldness of an object.


How can we measure temperature?

By touch

Your body is a temperature measuring instrument. By simply touching an object, you can tell if it is hot or cold. However, this does not tell you how hot or cold it is.

Using a thermometer

Scientists are always interested in measuring things, so it is not surprising that a devise was invented to measure temperature. It is called a thermometer.



A mercury thermometer using the centigrade scale


The inventor of the first thermometer is thought to have been a very famous Italian scientist called Galileo Galilei. Galileo was born in 1564. His first thermometer was a very simple one. Like most of the thermometers in use today, it worked on the idea that as things get hot they expand and as they cool down they contract.

Galileo's thermometer was "open" at one end which made it sensitive to air pressure changes. The first "closed" thermometer was invented by Ferdinand II Grand Duke of Tuscany in 1641. He invented the alcohol thermometer. His invention started a thermometer making tradition in Florence. The thermometers made in Florence in the 17th century were so good that some of them were still being used 200 years later.

Alcohol is a good liquid to use in a thermometer because it remains a liquid over most of the normal temperatures found on the Earth's surface. You will sometimes use alcohol thermometers in school. The alcohol is often coloured red or green in these thermometers so that the liquid can be seen more easily. However, alcohol is not much use at hot temperatures because it boils at about 80°C, which is quite a low temperature.

For higher temperatures a different liquid is needed. The Florentines experimented with mercury which boils at a much higher temperature than alcohol, at 357°C. The main problem with mercury thermometers is that they need a very fine tube, so it is difficult to see the level of the liquid inside them.

Now that accurate thermometers had been made to measure the temperature, scientists discussed what units they should use to measure temperature in. It is not surprising that everyone had their own particular ideas about this. By the 18th century as many as 35 different temperature scales had been developed. Fortunately, scientists like to standardise their measurements and use only one type of unit to measure things by. All we have left today are two scales of temperature in everyday use, (Fahrenheit and Celcius), and one scale used by many scientists, especially physicists, who deal with really low temperatures (Kelvin).


The Fahrenheit scale

The Fahrenheit scale is named after Gabriel Daniel Fahrenheit, a Dutch instrument maker who was famous for the quality of his thermometers. In 1714 he put forward a temperature scale which was based on the human body temperature. Originally, human body temperature was 100 on the Fahrenheit scale, but it has since been adjusted to 99°F.

Fahrenheit temperatures are written like this: 32°F

The Fahrenheit scale is still used in a few countries, such as the USA, but scientists all over the world usually use the other two scales.


The Centigrade scale

In 1740, Anders Celcius, a Swedish astronomer, proposed a temperature scale which was based upon the freezing point of pure water (which he set at 0 degrees) and the boiling point of pure water (which he set at 100 degrees). He divided the temperatures in between 0 and 100 degrees into 100 equal units. This explains why the Celcius scale is also called the Centigrade scale (centi - grade = 100 parts). Celcius temperatures are written like this: 37°C.


Centigrade-Fahrenheit Conversion

The two scales, Fahrenheit and Celcius naturally give different numerical results for the same measurement:





Freezing point of water



Boiling point of water



Body temperature




To convert Fahrenheit to Celcius:

X°F =

(X-32) / 9 x 5

or X minus 32, multiplied by five and then divided by nine.


To convert Celcius to Fahrenheit:

X°C =

(X x 9) / 5 + 32

or X multiplied by nine, then divided by five, then add 32 to the result.


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