How Does a Thermometer Work? Why the Red Line Goes Up and Down

A line that moves by itself

Look at an outdoor thermometer on a cold November morning. A thin red line stops beside a number on the scale. Bring the thermometer inside, and after a few minutes the line has crept higher. Nobody pushed it. So what made it move?

The answer is a cause-and-effect chain. Science Buddies, writing for Scientific American, explains that a thermometer like this has a liquid-filled reservoir at the end of a narrow tube. When the liquid warms, it expands and rises in the tube. When the liquid cools, it contracts and falls. A higher level means a higher temperature.^[2]

Cause: things grow when they warm

To expand means to take up more space. To contract means to take up less. Science Buddies says scientists call this thermal expansion and contraction: matter tends to change its volume when its temperature changes. Volume is the amount of space something fills.

Not everything changes by the same amount. According to Science Buddies, gases change volume the most, liquids change a medium amount, and solids change too little to be useful in an everyday thermometer. That is why the moving part of a thermometer is a liquid, not a piece of solid metal or glass.^[2]

Effect: the bulb and the narrow tube

Most of the liquid sits in the bulb, the round reservoir at the bottom. When the bulb warms, all that liquid expands at once. It cannot spread out sideways, because the glass holds it in. The only way out is up the tube.

The tube is very narrow on purpose. Science Buddies says a narrower tube makes a more accurate thermometer. Here is one way to picture it: if you pour one spoonful of water into a wide bowl, the level barely changes, but the same spoonful poured into a thin straw makes a tall column. A tiny change in the bulb becomes a movement you can see.^[2]

Making the numbers

A tube of moving liquid only tells you warmer or colder until someone adds a scale. Chris Woodford, a science writer for the website Explain that Stuff, describes how it is done. The maker marks the liquid level at two fixed points: the temperature of melting ice and the temperature of boiling water. Then the space between the marks is split into equal steps.

On the Celsius scale, water freezes at 0 °C and boils at 100 °C, so there are 100 equal steps between them. Woodford explains that the scale works because the liquid expands by an amount that is directly related to the temperature change. The scale was devised by Anders Celsius, a Swedish scientist.

The scale keeps going below zero. In an Ontario winter, the line might stop at −10 °C, which is read as "minus ten degrees Celsius". Below zero, the bigger the number after the minus sign, the colder it is, so −10 °C is colder than −5 °C. To read any thermometer, keep your eye level with the top of the liquid. Looking from above or below can make the line seem to sit at a different mark.^[1]

Why the liquid is often red

Older thermometers often used mercury, a silvery metal that is liquid at room temperature. Woodford notes that mercury is toxic. It is safe while sealed inside the glass, but a broken thermometer could let it out. Many thermometers now use alcohol, usually coloured red, which Woodford says is generally safer.

Alcohol has another advantage in a cold country. Woodford says pure alcohol freezes at a much lower temperature than mercury: about −114 °C, compared with about −40 °C for mercury. A frozen liquid cannot rise or fall, so an alcohol thermometer can measure colder air.^[1]

Thermometers without liquid

A digital thermometer works in a completely different way. Woodford explains that it uses a metal probe. As the metal warms or cools, it lets electricity pass through it more or less easily. A tiny computer chip measures that change and turns it into a number on the screen. Electronic thermometers respond almost instantly, though their readings can jump around a little.^[1]

Measuring the weather fairly

Weather services want their numbers to describe the air, not the sunshine on a hot sidewalk. The Bureau of Meteorology, Australia's national weather agency, keeps its thermometers in a Stevenson screen. This is a white box with slatted sides called louvres. The louvres let air flow through, while the box shields the thermometer from rain and direct sunlight. The white paint helps the box absorb less heat.

Placement matters too. The Bureau sets its screens over grass, not concrete, and away from buildings and trees, which can reflect or absorb heat. The thermometer sits 1.2 m above the ground, because the Bureau says the temperature near the ground can differ by as much as 7 °C from the temperature 2 m up. Many of its stations now use electronic sensors that send their readings automatically.^[3]