(soft music) - [Narrator] You're feeling woozy.

Maybe your forehead feels hot.

You stick a thermometer in your mouth but it comes back 37°C or it comes back 38°C.

You're running a fever, call in sick.

But if it comes back at 40°C, get yourself to the ER.

We have a lot of confidence that we're measuring temperature accurately.

One or two little degrees can literally mean the difference between calm nerves and a medical scare.

Knowing temperature is important beyond your health too, whether it's setting the grill for the perfect burger or figuring out what hot outfit will keep you cool at a summer cosplay event.

You can often proceed in form because of our trust in modern thermometers.

If I told you thermometers don't measure temperature, would you find your trust shaken?

Thermometer means measurer of heat but that's the same thing as temperature anyway, right?

Well, scientifically speaking, no.

But knowing about heat can tell you about temperature, if you're clever.

Temperature describes the average energy of the molecules in a system, while heat describes the transfer of energy between systems that are at different temperatures.

Thermodynamics tells us how heat moves around in its first three laws.

Zero: heat is the same regardless of how it's measured.

One: heat cannot be created or destroyed.

And two: heat moves from hotter to colder.

Yeah, that list starts at zero because that part was figured out after one and two.

Anyway, armed with these laws, we can understand how thermometers work to make them as useful to us as possible.

Consider the bulb type.

That liquid-in-a-glass-tube thing you know and love.

Imagine it's in the freezer.

The thermometer loses heat to the surroundings.

That means the molecules in the liquid slow down and the liquid contracts, pulling its level down the scale.

Move it over to the sauna and the thermometer gains heat.

This causes the molecules in its liquid to move faster, so the liquid expands and the level moves up the scale.

When the thermometer is steadied out, both it and the surroundings are the same temperature, which means, thanks to thermodynamics, the thermometer tells us what the temperature is.

We've been playing around with simple up/down thermometers for hundreds of years.

What up, Galileo, Fahrenheit and Celsius?

Some clever physical chemists and engineers have given us all kinds of thermometers you see today.

So, let's take a little tour.

Many people associate trusty bulb thermometers with the mercury.

Before we learned that mercury exposure could lead to health problems, this liquid metal was preferred because it expands and contracts at a constant rate and stays liquid over a useful range.

And it sort of looks like the T-1000, which is very cool.

But finding silvery bulb thermometers is rare these days.

That's not exactly a new trend because even before mercury thermometers, some early adopters used red wine because its alcohol content protected it from freezing and it was easier to see in glass than quicksilver.

Nowadays, most bulb thermometers use ethanol that's dyed red and sometimes blue.

One of the most common electrical thermometer is the thermistor.

These are a resistor that conducts electricity differently as temperature changes.

Thermistors are made from metal oxides like manganese or, say, iron or semiconductor materials like germanium or silicon, depending on the temperature range.

And they're often embedded in a glass bead with wires sticking out.

These can be made at very low cost, meaning they're often what's inside a digital thermometer that you might have stuck in a mouth recently, checking if you or someone else has a fever.

It's worth noting that electricity-based thermometers are still using thermodynamics but using moving electrons, electricity, gets us into the quantum realm.

Sound a little Sci-Fi?

Then check out this action at a distance option.

Some metalloids and metals produce electricity when light shines on them.

Explaining this got Einstein a Nobel Prize, and the world got light sensors and solar cells.

All objects give off a sort of light called infrared radiation or IR.

Indium, germanium, and other semiconductors will produce electricity when IR hits them.

So, pointing them at, say, a child's aching ear or a simmering pot of stew, the semiconductor detector converts the radiation readings to temperature on a digital readout.

You may also recognize infrared sensors in the form of heat-seeking cameras used in hunting, warfare, and cryptozoology missions to track down Bigfoot.

Measuring temperature through radiation readings can reach far.

Satellites scanning Earth measure its temperature from thousands of miles up, so satellite thermometers in orbit are essential to understanding our climate.

And one day, they could routinely tell us about other planet's climates.