Thermometer
4 Minutes
How thermometers work for kids: learn how vibrating molecules and thermal expansion help measure temperature. An English science lesson.
- How vibrating atoms and molecules define temperature
- Thermal expansion demonstrated with a heated water flask
- Why mercury and alcohol are used inside thermometers
What this lesson teaches
A thermometer is an instrument that accurately measures temperature by using the expansion of heated liquids. Temperature is a measure of how fast the tiny atoms and molecules inside an object are vibrating.
Temperature and molecular motion
All objects are made up of tiny particles called atoms and molecules that vibrate constantly, even under normal conditions. When heat is added to an object, these particles vibrate much faster, which is known as high temperature. Slower vibration corresponds to low temperature. Before accurate meters existed, people estimated heat roughly, such as blacksmiths judging hot iron by its color or people sprinkling water onto an iron box to test its heat.
How expansion works
When an object is heated and its atoms and molecules vibrate more rapidly, the distance between those particles increases. This growing gap causes the overall volume of the material to expand. In a demonstration, water placed inside a flask with a marked tube rises upward as it is heated. The faster the particles vibrate from added heat, the further the water expands up the tube, showing how heat can be measured through expansion.
Mercury and alcohol in thermometers
Modern thermometers display readings marked in units of Celsius and Fahrenheit to measure how much a liquid expands:
- Mercury is preferred over water because it absorbs heat much better and expands easily.
- Mercury works across a range from minus 39 degrees Celsius up to 357 degrees Celsius.
- Above 357 degrees Celsius mercury boils, and below minus 39 degrees Celsius it freezes solid.
- Alcohol replaces mercury when measuring extreme cold, allowing temperatures down to minus 100 degrees Celsius to be measured.
Checking for a fever
Thermometers provide exact measurements needed in daily life, such as checking weather conditions or determining whether a person is sick. A normal human body temperature is 98.6 degrees Fahrenheit. A person is considered to have a fever when their body temperature reaches 101 degrees Fahrenheit. Because the liquid expands precisely according to body heat, the thermometer can accurately detect whether a fever is real.