Give Now  »

Noon Edition

How Thermometers Work: Temperature and Volume

They tell us how to dress in the morning and when to stop roasting a turkey. When we catch a cold, they tell us how sick we are. Thermometers are an everyday part of our lives, something we usually take for granted.

Temperature affects human beings, making us shiver when we're cold or sweat when we're hot, but it also affects every other physical substance in the universe.

All About Volume

The most common effect has to do with volume, or the amount of space something fills up.

Generally speaking, increasing temperature will cause most substances to expand, while decreasing temperature will cause them to contract.

Atoms And Size

The reason for this is that everything is made of tiny bits of matter called atoms. Even if something isn't moving, its atoms are never completely still. The atoms continually wiggle around and bump into each other. As a substance gets hotter, its atoms start jostling about more vigorously.

This extra jostling pushes the atoms farther apart, causing the whole substance to slightly increase in size. If the substance cools off, its atoms stop pushing against each other so vigorously. This makes the substance shrink again.

How Thermometers Work

This connection between temperature and volume is what makes most thermometers work. The most common type of thermometer is an airtight glass tube with a reservoir bulb at one end.

The reservoir is filled with a colored liquid. As the temperature increases, the liquid expands, forcing itself partway up the narrow tube. The hotter it gets, the higher the liquid climbs.

Numbers printed along the tube translate the liquid's volume into temperature, and you learn whether to wear a sweater or a short sleeved shirt.

Support For Indiana Public Media Comes From

About A Moment of Science