What Thermometers Do

• To understand how a thermometer works, look to the word itself. Thermo means "to heat" and meter means "to measure," and that is exactly what it does. Thermometers are tools that are often used to determine one's body temperature. They are also used to measure the heat in a car's engine, in cooking, in meteorology, in science and other industries.
  
  

Thermometer's Parts

• Traditional thermometers consist of a glass tube filled with either colored alcohol or mercury set alongside a series of numbers. They rely on a concept called thermal expansion, which basically means that things tend to expand when they get hotter and contract when they get cooler.
  
  

Changes in Density

• In the case of liquids and gases, more temperature means less density. For example on a hot day, the liquid in the thermometer will rise due to it becoming less dense. The opposite is true on a cold day, as the liquid becomes dense and takes up less space.
  
  

Measurement Intervals

• By itself, this concept is not that useful in determining the exact temperature. So certain points on the thermometer are measured according to the expansion of the liquid at certain temperatures. For example, a Fahrenheit thermometer could be measured at 0 degrees and 100 degrees, and then the rest of the temperatures can be marked off at regular intervals.
  
  

Digital Age

• However, most modern thermometers used in doctors' offices and around the house are digital. Instead of displaying the temperature using liquid against a glass tube, digital thermometers use a digital screen. The basic apparatus at the root of these more advanced thermometers is the thermistor.
  
  

Electrical Resistance

• Digital thermometers use the concept of electrical resistance to determine temperatures. Thermistors are semiconductor devices that are sensitive to temperature. As temperature increases in a thermistor, the thermal energy rearranges its charges and allows it to conduct more electricity. A simple computer within the digital thermometer calculates temperature based on the changes in the thermistor's electrical resistance. This process happens relatively quickly. When the end of a digital thermometer is placed in a patient's mouth, the temperature is normally determined and displayed in less than a minute.
  
  

Other Kinds of Thermometers

• Spring thermometers are based on the same concept as liquid thermometers, but they use the thermal expansion of a metal coil instead of alcohol or mercury. Optical thermometers measure infrared light emissions to determine temperature.