Measuring warmth
When you want to find out how warm or cold something is, you use a thermometer to find out. A thermometer has a scale that you use to make your measurements. On many thermometers the scale starts at 0°C. This is the temperature at which water freezes. At the top of the scale is the 100°C mark. This is the temperature at which water boils. In between the two marks are a hundred separate marks. Each one stands for 1°C.

Most thermometers are made from a glass tube with a rounded part at one end. This part is called the bulb. It is the part you use to take the temperature of a substance. It must be in contact with the substance you are testing. Many people forget this when they are taking the temperature of water. They put the bulb in the water and leave it for a minute or more, then they lift up the thermometer to read it. When they lift the thermometer, they take its bulb out of the water. This means that the bulb is now surrounded by air and begins to take its temperature. What is being read on the scale is the air temperature not the water temperature.

A thermometer with liquid enclosed in a glass bulb and a long tube is called a liquid in glass thermometer. It works because of the way the liquid behaves when it warms up or cools down. The liquid expands when it gets warmer and contracts or shrinks when it gets cooler. The bulb is full of liquid but the tube has some empty space. This means that when the thermometer bulb is placed in hot water the liquid in the bulb expands and pushes its way up the tube. When the bulb is placed in a cooler situation, the liquid contracts and moves down the tube. You do not read the temperature while the liquid is moving up and down. You wait until the level of the liquid remains steady.

So now you know the two most important things about using a thermometer. Make sure the bulb is touching the substance while you take its temperature and wait for the level of the liquid in the thermometer to stop moving before you take your reading.

Who invented the thermometer?
Galileo. He was a scientist who lived in Italy about four hundred years ago. The thermometer did not look like the ones we use today and it was not accurate. Galileo's thermometer had a glass bulb the size of a hen's egg. There was a glass tube connected to the bottom of the bulb. The tube was as thick as a straw and pointed downwards from the bulb into a bowl of water. When the temperature of the air around the bulb rose, so the level of the water in the straw also rose. When the temperature of the air fell, so did the level of water in the tube. The main problem was that as the air pressure changed, owing to the weather, the water level also went up or down even if the temperature did not change.

How did they make the thermometer more accurate?
A French scientist called Jean Rey turned the thermometer upside down and filled the bulb. This removed the problem of the air pressure but created another problem. The top of the tube was open so water could evaporate from there and make the level fall.
Later, the thermometer was filled with spirit of wine instead of water and the top was sealed so evaporation could not take place.

Some thermometers have a silvery liquid. What is it?
The liquid is mercury. It is the only metal that is a liquid at normal temperatures. It is used because it expands and contracts more evenly than other liquids.

Do school thermometers have mercury in them?
The thermometers in primary schools do not have mercury in them. The reason for this is that mercury is very poisonous and there is a special process needed to clean up mercury if a thermometer is broken and the mercury spills out. In secondary schools they have a person called a technician who is trained to clear up mercury so mercury thermometers are used there. The liquid used in primary school thermometers is alcohol. This is a colourless liquid so it is dyed red or green so that you can see it.

Did the first thermometers have a Celsius scale like thermometers today?
No. At first, scientists could not agree on what to use at the ends of the scales. Some thought that the coldest temperature in winter and the hottest temperature in summer should be used. Other temperatures were considered such as the temperature of ice, melting butter or the insides of a cow!

Who made the first really accurate thermometer?
Gabriel Fahrenheit was a scientist who lived in Holland about three hundred years ago. He was the first person to use mercury in his thermometers and he invented a scale, which had a zero at the coldest substance he could find - a mixture of ice and salt. The top of his scale was the temperature of boiling water. He divided his scale into 212 degrees. When using the scale the letter F was put after the number of degrees. The bottom of the scale was 0°F and the top of the scale was 212°F. The freezing point of water was 32°F and the temperature of the body was 98.6°F. The Fahrenheit scale was used in countries such as the United Kingdom, USA, Canada and Australia but the Celsius scale has been used in the rest of the world and by scientists since it was invented.

When was the Celsius scale invented?
Anders Celsius who was a Swedish scientist invented the Celsius scale. He first described his scale in 1742. He had the boiling point of water at 0°C and the freezing point of water at 100°C. Eight years later, another scientist called Marten Stromer turned the scale upside down so the freezing point of water was 0°C and the boiling point of water was 100°C. At first the scale was known as the centigrade scale because it had a hundred divisions. Today it is known as the Celsius scale.