How does a thermometer work? A traditional thermometer works by knowing how much mercury will expand at different temperatures. An oven thermometer works by knowing how much a piece of metal will expand when heated. And a modern thermometer uses a thermistor, which has a different electrical resistance at different temperatures.
The idea behind the traditional thermometer has been around for at least two thousand years. The ancient Greek inventor Philo of Byzantium, whom we have encountered before, did experiments with heated water in an airtight tube. He was only doing experiments and was not trying to gauge the temperature of things. He also did not have access to the glass he would need to make a tube, so he would not have been able to see the moving water line inside the tube. Advances in the 17th century were made that allowed people to measure temperatures. Many of them used water in a sealed tube, which is not great for this purpose. When heated, water expands too much. Plus, it sticks to the glass tube due to surface tension. It also freezes below 0℃ and boils above 100℃, making it impractical for measuring more extreme temperatures. Water also doesn’t expand at a uniform rate. It actually gets denser when heated from 0 to 4℃, before getting less dense from there onwards. But the increase of the expansion increases with heat, so it is not easily measurable. In 1714, a physicist called Daniel Gabriel Fahrenheit hit on the idea of using mercury. Mercury is much better than water because it has a freezing point of -38.83℃ and a boiling point of 356.73℃. That range is outside most of the uses of a mercury thermometer at the time. Mercury is also silver, which makes it much easier to see in a glass thermometer than water would be. Mercury is also a very good conductor of heat and responds to temperature changes much more quickly than water does. Mercury doesn’t stick to the glass walls of the tube, and it expands at a uniform rate. Mercury can be a problem if you drop the glass thermometer because Mercury is highly toxic. A thermometer only contains about half a gram of Mercury, but it can still be dangerous if not handled properly.
So, how does a mercury thermometer work? There is a vacuum tube with a bowl at the bottom, filled with mercury. As the bowl of mercury is heated, the molecules in the mercury move more, slowly spreading out, and the mercury expands. Mercury expands at a rate of 0.018% for every 1℃ increase in temperature, making it very easy to measure. Interestingly, the glass also expands with the heat, which has to be taken into consideration when calibrating the thermometer.
A mercury thermometer is all well and good if you are not going outside the band of temperature where mercury is liquid, but if you want to measure higher, say, with an oven, there is another method. Thermometers in ovens use bimetallic strip thermometers. They also work on the principle that materials expand when they are heated, but, in this case, they use metal. There are two strips of metal connected to each other. They are usually copper and steel because copper expands more quickly than steel when heated. When the oven gets hot, the copper expands and bends the steel upwards. This movement increases or decreases with the amount of heat. The movement can be calibrated and connected to a display to show the temperature.
The final type of thermometer are modern digital thermometers. They don’t work by using expanding materials. They work by using the level of electrical resistance in a material. When a metal is heated, the thermal energy makes the atoms move a lot more. An electrical current passes through metal as free electrons, and they flow from one side to the other, heading for the positive charge. The conductivity of the metal depends on how much vibration there is because more vibration will scatter the free electrons. The more vibration there is, the lower its conductivity is. When a metal is heated, the atoms move more, and there is a greater chance that they will scatter the free electrons, lowering the metal’s conductivity. The metal is often a metal like platinum, and the voltage passing through it at a baseline temperature can be measured. As the temperature increases or decreases, that voltage will change, and the thermometer can use that difference to work out the temperature. And this is what I learned today.
Sources
https://uk.rs-online.com/web/content/discovery/ideas-and-advice/digital-thermometers-guide
https://www.bestbuy.com/discover-learn/how-do-thermometers-work/pcmcat1657137246492
https://www.advancedenergy.com/en-us/about/news/blog/how-does-a-digital-thermometer-work
https://home.howstuffworks.com/therm.htm
https://tameson.com/pages/bimetallic-thermometer
https://en.wikipedia.org/wiki/Thermometer
https://knyamed.com/blogs/resources/why-mercury-is-used-in-thermometers
https://en.wikipedia.org/wiki/Mercury-in-glass_thermometer
https://portal.ct.gov/deep/p2/mercury/guidance-for-managing-broken-mercury-fever-thermometers
https://www.bbc.co.uk/bitesize/guides/zm2kvk7/revision/8