There are many kinds of thermometer. Most thermometers have a small amount of liquid in a ‘bulb' at the bottom of a long, thin tube. The tube is surrounded by thick glass so that the thermometer doesn't break easily. The liquid in a thermometer is usually mercury (a substance the colour of silver, which is very easy to see inside the tube) or alcohol.

The alcohol is coloured with a dye so that the liquid is easily seen. When the liquid in the thermometer bulb is heated, it expands (its volume increases). It therefore rises up the tube. The case or thick glass around the narrow tube of the thermometer is graduated in degrees Celsius. By 'reading' the scale we find the temperature of the substance surrounding the bulb of the thermometer.

par Claude Foumtum