How does thermometer work?

Does it feel boiling hot to you today or is it just me? And how we can tell? If I say today's hotter than yesterday and you disagree, how can we settle the argument? One easy way is to measure the temperature with a thermometer on both days and compare the readings. Thermometers are simple scientific instruments based on the idea that metals change their behavior in a very precise way as they get hotter (gain more heat energy). Let's take a closer look at how these handy gadgets work.
Mercury thermometers

The simplest thermometers really are simple! They're just very thin glass tubes filled with a small amount of mercury—a rather special metal that's a liquid at ordinary, everyday temperatures. When mercury gets hotter, it expands (increases in size) by an amount that's directly related to the temperature. So if the temperature increases by 20 degrees, the mercury expands and moves up the scale by twice as much as if the temperature increase is only 10 degrees. All we have to do is mark a scale on the glass and we can easily figure out the temperature.

How do we figure out the scale? Making a Celsius (centigrade) thermometer is easy, because it's based on the temperatures of ice and boiling water. These are called the two fixed points. We know ice has a temperature close to 0°C while water boils at 100°C. If we dip our thermometer in some ice, we can observe where the mercury level comes to and mark the lowest point on our scale, which will be roughly 0°C. Similarly, if we dip the thermometer in boiling water, we can wait for the mercury to rise up and then make a mark equivalent to 100°C. All we have to do then is divide the scale between these two fixed points into 100 equal steps ("centi-grade" means 100 divisions) and, hey presto, we have a working thermometer!

Dial thermometers

An illustration of a typical bimetallic strip dial thermometer showing how an expanding metal coil moves the pointer up the scale.

Not all thermometers work this way, however. The one shown in our top photo has a metal pointer that moves up and down a circular scale. Open up one of these thermometers and you'll see the pointer is mounted on coiled piece of metal called a bimetallic strip that's designed to expand and bend as it gets hotter (see our article on thermostats to find out how it works). The hotter the temperature, the more the bimetallic strip expands, and the more it pushes the pointer up the scale.

Artwork: How a dial thermometer works: This is the mechanism that powers a typical dial thermometer, illustrated in a patent by Charles W. Putnam from 1905. At the top, we have the usual pointer and dial arrangement. The bottom artwork shows what's happening round the back. A bimetallic strip (yellow) is tightly coiled and attached both to the frame of the thermometer and the pointer. It's made up of two different metals bonded together, which expand by different amounts as they heat up. As the temperature changes, the bimetallic strip curves more or less tightly (contracts or expands) and the pointer, attached to it, moves up or down the scale. Artwork from US Patent 798,211: Thermometer courtesy of US Patent and Trademark Office.

Electronic thermometers

One problem with mercury and dial thermometers is that they take a while to react to temperature changes. Electronic thermometers don't have that problem: you simply touch the thermometer probe onto the object whose temperature you want to measure and the digital display gives you an instant temperature reading.

Electronic thermometers work in an entirely different way to mechanical ones that use lines of mercury or spinning pointers. They're based on the idea that the resistance of a piece of metal (the ease with which electricity flows through it) changes as the temperature changes. As metals get hotter, atoms vibrate more inside them, it's harder for electricity to flow, and the resistance increases. Similarly, as metals cool down, the electrons move more freely and the resistance goes down. (At temperatures close to absolute zero, the lowest theoretically possible temperature of −273.15°C or −459.67°F, resistance disappears entirely in a phenomenon called superconductivity.)

Electronic medical thermometer.

An electronic thermometer works by putting a voltage across its metal probe and measuring how much current flows through it. If you put the probe in boiling water, the water's heat makes electricity flow through the probe less easily so the resistance goes up by a precisely measurable amount. A microchip inside the thermometer measures the resistance and converts it into a measurement of temperature.

The main advantage of thermometers like this is that they can give an instant reading in any temperature scale you like—Celsius, Fahrenheit, or whatever it happens to be. But one of their disadvantages is that they measure the temperature from moment to moment, so the numbers they show can fluctuate quite dramatically, sometimes making it difficult to take an accurate reading.

Measuring extreme temperatures

If you want to measure something that's too hot or cold for a conventional thermometer to handle, you'll need a thermocouple: a cunning device that measures temperature by measuring electricity. And if you can't get close enough to use even a thermocouple, you could try using a pyrometer, a kind of thermometer that deduces the temperature of an object from the electromagnetic radiation it gives off.

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