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Thermometer: Definition, Types & Examples

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  • 0:00 What Is a Thermometer?
  • 1:19 Liquid, Solid, and Gas…
  • 2:50 Other Types of Thermometers
  • 3:59 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Whenever you get the forecast for the day, check your body temperature, or use the air conditioning, you are using a thermometer. Learn about the different kinds of thermometers, and discover how they work.

What Is a Thermometer?

You come into contact with thermometers on a daily basis. You use one when you want to know how hot or cold it is outside (even if you don't take this measurement yourself, someone else does and sends the information to news outlets and the weather application on your phone). A clinician takes your body temperature when you have a doctor's appointment, and the thermostat in your home keeps a running tab on how hot or cold it is inside your house.

The word thermometer comes from the Latin thermo-, meaning 'heat,' and -metrum, meaning 'measure.' So, a thermometer is a scientific tool for measuring temperature. The temperature of an object or substance refers to how hot or cold it is.

A thermometer contains two essential elements: a sensor that detects a change in temperature and a device that communicates that temperature to the person reading it.

Thermometers work by utilizing the physical properties of various substances. For example, liquids and solids expand as they heat up and contract when they cool down. Similarly, gases increase in pressure as they heat up and decrease as they cool down. Using known temperature points of a substance (such as the freezing and boiling points of a liquid), you can make a thermometer that translates these physical changes into a number that we understand as temperature.

Liquid, Solid, and Gas Thermometers

Thermometers Using Liquid

Many thermometers are made using liquids. The mercury-in-glass thermometer was the very first, invented by Daniel Gabriel Fahrenheit in 1714. A mercury thermometer consists of liquid mercury in a glass bulb attached to a thin, enclosed tube. As the temperature increases, the mercury expands. Since the mercury is confined, the only place for the mercury to expand is up into the tube. The mercury level rises and indicates the temperature of the surroundings. Since mercury is highly toxic, it is rare to find mercury thermometers outside of science labs these days. Instead, you can find alcohol thermometers that work on the same principal.

Thermometers Using Solids

Thermometers can also be made using solids. A spring thermometer works on a similar principal to the liquid thermometers. It consists of a coil of metal that expands when it heats up and contracts when it cools. The spring is connected to a pointer that moves along a numbered scale as the coil expands or contracts. You can find spring thermometers in older thermostats.

Thermometers Using Gas

Many industrial thermometers utilize gas in a metal or glass enclosure. This enclosure is attached to a pressure gauge that translates the pressure of the gas into a temperature reading. As the gas heats up, the gas molecules move faster and hit the side of the container more often. This leads to an increase in pressure, and the gauge registers a higher temperature. As the gas cools, the molecules slow down, the pressure decreases, and the gauge goes back down.

Other Types of Thermometers

The two types of thermometers that you are likely to encounter most often are digital thermometers and liquid crystal thermometers. These are the kinds of thermometers that you find at the local drug store when you want to know if you or your child has a fever.

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