As substances get hotter, their molecules move around more rapidly and they may take up more space. A thermometer contains a liquid that expands as it gains heat energy. This causes the level of the liquid to rise in a narrow tube. A scale beside the tube allows the temperature to be read.
The simplest thermometers really are simple! They’re just very thin glass tubes filled with a small amount of silvery liquid (typically mercury—a rather special metal that’s a liquid at ordinary, everyday temperatures). When mercury gets hotter, it expands (increases in size) by an amount that’s directly related to the temperature. So if the temperature increases by 20 degrees, the mercury expands and moves up the scale by twice as much as if the temperature increase is only 10 degrees. All we have to do is mark a scale on the glass and we can easily figure out the temperature.
How do we figure out the scale? Making a Celsius (centigrade) thermometer is easy, because it’s based on the temperatures of ice and boiling water. These are called the two fixed points. We know ice has a temperature close to 0°C while water boils at 100°C. If we dip our thermometer in some ice, we can observe where the mercury level comes to and mark the lowest point on our scale, which will be roughly 0°C. Similarly, if we dip the thermometer in boiling water, we can wait for the mercury to rise up and then make a mark equivalent to 100°C. All we have to do then is divide the scale between these two fixed points into 100 equal steps (“centigrade” means 100 divisions) and, hey presto, we have a working thermometer!
Not all liquid thermometers use mercury. If the line you see in your thermometer is red instead of silver, your thermometer is filled with an alcohol-based liquid (such as ethanol). What’s the difference? Mercury is toxic, although perfectly safe if it’s sealed inside a thermometer. However, if the glass tube of a mercury thermometer happens to break, that potentially exposes you to the poisonous liquid inside it. Alcohol thermometers are generally safer for that reason and they can also be used to measure lower temperatures (because alcohol has a lower freezing point than mercury; it’s about ?114°C or ?170°F for pure ethanol compared to about ?40°C or ?40°F for mercury).
Picture Credit : Google