14 DAY TRIAL // A thermometer is a device that can be used to measure the temperature of a certain object. Some of the most widely used and popular thermometers are the mercury and alcohol ones, although over the years several other types have been built, based on different physical changes that occur with the variation of temperature.
Alcohol and mercury thermometers
The mercury thermometer was originally invented by the German physicist Daniel Gabriel Fahrenheit and basically consists of a volume of mercury being trapped inside a glass envelope. Most of the mercury mass is located in the so-called bulb (both alcohol and mercury thermometers are known as bulb thermometers) that connects to a much narrower tube inside which the mercury can expand. The remnant volume of the narrow tube that is not occupied by mercury ca be either vacuum or nitrogen gas.
As we all know, the volume of a certain compound varies according to its temperature. This is exactly the phenomenon exploited by the alcohol and mercury thermometers. As temperature drops, the volume of the mercury shrinks, and it expands as the temperature increases. The level at which the meniscus of mercury rises in the narrower tube indicates exactly the temperature of the mass of fluid in the bulb of the thermometer, when compared against a scale.
Alcohol thermometers have basically the same construction and operation, albeit in this case the mercury is replaced with an organic liquid, usually ethanol, toluene or kerosene, died in an easily observable color. As a result, alcohol thermometers are rather restricted in the upper part of the scale to a maximum of 78 degrees Celsius (ethanol thermometers), limiting their application only to the measuring of body or atmospheric air temperature.
By contrast, the mercury thermometer is somehow limited in the lower part by its solidification point, which is about -38.83 degrees Celsius. Also, because mercury is toxic for the human body, lately, in some countries the use of mercury thermometers has been banned, determining manufacturers to resort to alternative liquid metal alloys.
Bi-metallic strip thermometer
Another notable type of thermometer is the bi-metallic strip one, which works on the same principle as bulb thermometers do, although this time by using a solid instead of a liquid. The most important part of this constructive type of thermometer is of course the bi-metallic strip, basically, a strip created from two differently expanding metals joined together at each end through different techniques.
In normal temperature, the two metal strips have the same length; however, when the temperature rises they start to expand, each at a different rate. This means that one of the metal strips will end up being slightly longer than the other, but since they are joined together at each end they cannot expand normally and eventually curve the strip out of the original shape. This mechanical movement can then be measured, through several mechanisms, to accurately extract the temperature of the bi-metallic strip.
Electric resistance thermometer
As I said earlier, the list of thermometer types currently available commercially is practically endless and it would take a considerable amount of time to explain every one of them, so I will try to keep things short. Another type of thermometer worth mentioning is the electric resistance one, which works by measuring the electrical resistance of a thermoresistor - an electronic device whose electric resistance varies according to its temperature. Once the characteristics of the thermoresistor are known and since measuring the electrical resistance is not that big of a deal, it is relatively easy to convert a temperature reading into a displayed value.