A thermometer is an instrument used to measure temperature. Please read about how different thermometers work and check out our brilliant thermometer resources!

What is a thermometer?

Find out all about what a thermometer is and how it works!

How a thermometer works for kids

A thermometer is an instrument used to measure temperature. You can measure the temperature of many different things with a thermometer, such as the temperature of a room to see how hot or cold it is, the temperature of a person to tell if they are sick, or the temperature of food to check if it is cooked.

The temperature is usually displayed in degrees Celsius, but it can also be Fahrenheit. This is because Celsius and Fahrenheit are two different scales for measuring temperature.

The main types of thermometers are new digital thermometers, including infrared and older mercury thermometers.

Mercury thermometers

Mercury thermometers look like sealed glass tubes with a liquid inside and numbers marked on the side to display the temperature. The liquid is called mercury, perfect for measuring temperatures because it changes from a solid to liquid very easily and is the only metal in liquid form at room temperature. In addition, mercury has a high boiling point, so it is suitable to measure higher temperatures.

How do mercury thermometers work?

  1. When the tip of the mercury thermometer touches the material it is measuring; the material conducts heat energy to the mercury.
  2. Then the mercury expands as it becomes liquid and rises the tube.
  3. The place where the mercury stops on the scale is where you can take the temperature reading.

Handy! But these thermometers are being phased out because the mercury is toxic if the thermometer breaks.

Digital thermometers

Digital thermometers use an electronic computer chip to tell the temperature. The tip of a digital thermometer is sensitive to heat, so it can accurately measure the temperature and tell the computer chip what the measurement is.

Often used to take medical readings, digital thermometers are the most accurate way to test someone’s temperature to see if they are ill.

How do digital thermometers work?

  1. When the temperature changes, the sensor inside a digital thermometer notices a difference in electronic reading from the resistor (part of an electric circuit that limits the flow of electricity).
  2. This change in resistance is then converted into a change in temperature.
  3. A number then gives a digital reading of the temperature on the screen.

This process takes about 30 seconds, which is faster than mercury thermometers, as they take 3 minutes to heat up and give you a reading!

Infrared thermometers

Infrared thermometers are digital thermometers that can measure temperatures from a distance. They can be used when the object you want to measure is very fragile or dangerous to be near. Infrared thermometers can measure temperatures between a few centimeters or many miles away.

How do infrared thermometers work?

  1. Infrared thermometers detect radiation from objects caused by the molecules inside objects moving around. The higher the temperature, the faster the molecules move and the more infrared radiation they emit.
  2. The radiation from the object is measured and turned into heat by an infrared thermometer.
  3. The heat is then turned into electricity and sent to a detector, producing a number on display to indicate the object’s temperature.

These thermometers have been widely used during the COVID-19 pandemic to detect if people have a high temperature from far away, so people don’t have to be too close together.

What are thermometers used for?

Thermometers are super helpful for lots of different things! For example, you may notice a thermometer on your classroom wall to check the room’s temperature or see one in the kitchen to help with cooking. People use thermometers to check if food, such as a roast, is cooked, if hot oil is ready for frying food, or if sugar is melted or burned. Thermometers also check people’s temperatures to see if they are sick with a fever.

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