How a Thermometer Works

Video Transcript

"Hi I'm Steve Jones and I'm going to show you how a thermometer works. In fact the thermometer I have drawn here is a liquid in glass thermometer which is the old kind and I acknowledge that there are many new types but the principles are basically the same. In the principle that works with this one is the expansion of the liquid and that depends on temperature. Obviously as this liquid gets larger and expands this liquid inside this thin tube goes up or down depending on the temperature. Then what you need to do is have some standard temperatures and we have a standard temperature at 0 Celsius which is the freezing point of water and 100 Celsius the boiling point of water. So you can easily create a thermometer in this way because this expansion should be uniform. So this is how a liquid in glass thermometer works. But of course not only that there are things called clinical thermometers. Now a clinical thermometer is slightly different. It is basically the same in construction but at the bottom it has a small constriction, it has a small piece where it goes very narrow in this tube and what happens is as it expands it goes up to the reading of the body temperature and then when we remove it from the body the thread breaks and it leaves some of the liquid in the thermometer tube at the top so that you can read the actual reading. It is a special device, that is why you have to shake it to get the liquid to go back into place at the bottom. That is a clinical thermometer. You've also got maximum and minimum thermometers. These were very useful particularly in gardens for determining the range of temperatures which occurred during the night when you weren't there to monitor them and what they had was a mercury thread here and above it was a little piece of very light metal which was fixed by a spring to the side and this would be pushed up the tube and once the reading had been taken as this dropped down this just stuck against the side of the tube and it told you where the mercury had got to and that was the maximum it went to and there was another type which did you the minimum as well. So there are different types of liquid in glass thermometers. Of course we know that there are some thermometers which change, which use the change and resistance of a piece of wire and that depends on temperature too but these types are rather more complex and their workings are not so obvious and not so visible so those type exist and yes have a look at them but in terms of understanding at least understand how the basic thermometer really does work."