Thermometers

~ how do we know how hot something is? ~

1

What is a thermometer?

A thermometer is a tool that measures temperature — how hot or cold something is. The classic kind is a liquid thermometer: a thin glass tube with a colored liquid inside (often red-dyed alcohol). The liquid moves up the tube when warm, and back down when cool. We read the temperature from where the top of the liquid sits on the scale.

100° 75° 50° 25° the bulb the tube red liquid the scale (holds most of the liquid) (liquid rises and falls here) (expands when warm) (degrees Celsius)
good to know
Old thermometers used mercury (a silvery liquid metal). Today most liquid thermometers use coloured alcohol because mercury is poisonous if it spills. The science is the same — both are liquids that expand a tiny bit when warmed.
2

How does the liquid know to move?

Remember: when you heat something, its particles move faster. Faster particles bump each other harder and push apart, so the liquid takes up more space — it expands. When you cool it down, particles slow and pack closer, so the liquid takes up less space — it contracts.

In a thermometer, the bulb's liquid is squeezed so the only way for it to expand is to push up the narrow tube. That's why the line goes up when it gets hotter.

↓ click in the chamber to add a flame and watch the liquid rise ↓
the big idea
The red line on a thermometer is just particles taking up more room when they get warmer. Every thermometer in the world works on this same idea: heat → particles move more → liquid expands → level rises.
1742
3

Anders Celsius's clever idea

Anders Celsius
1701 – 1744
Swedish astronomer

To measure temperature, you need numbers — but where do you start counting? In 1742, Anders Celsius picked two reference points that anyone with a kitchen could check.

Celsius worked at Uppsala University in Sweden, where he also studied the aurora borealis (the northern lights) when he wasn't busy inventing temperature scales.

his clever choice

Then he split the space between them into 100 equal degrees. That's why we call it the Celsius scale and write degrees with the symbol °C (so 22°C means 22 degrees Celsius).

a glass of ice water
0°C
water is freezing / ice is melting
a kettle of boiling water
100°C
water is turning into steam
wait, what?!
Celsius's original 1742 scale was actually backwards! He set 0° as the boiling point and 100° as the freezing point. After he died, scientists (probably his colleague Carl Linnaeus) flipped the scale to the version we use today. Imagine if hotter meant smaller numbers!
why these two reference points?
Because they're easy to recreate. A scientist in Sweden and a scientist in Australia can both fill a glass with ice water or boil a kettle and get the same temperature. That makes the scale universal — every Celsius thermometer in the world agrees.
4

Where ice, water, and steam live

Drag the slider to change the temperature. Watch what happens to water: below 0°C it freezes solid, between 0°C and 100°C it stays a liquid, and above 100°C it boils into a gas.

cold hot
20°C
water is a liquid
key temperatures to remember
  • 0°C — freezing point of water (ice ↔ water)
  • 100°C — boiling point of water (water ↔ steam)
  • 22°C — comfortable room temperature
  • 37°C — your body temperature
← previous: why does cold pull? → next: expansion, ice & engineering
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