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🌡️ How does a thermometer measure temperature?

You take the glass stem out from under your tongue. The silver thread sits at 38.7. Nothing was poured in — the liquid got bigger.

lessons

~10 min

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🔬 Science

subject

Adults

level

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What you’ll learn

Expansion turns warmth into motionExplain how liquid expansion in a capillary, and unequal metal expansion in a bimetal strip, become a readable motion on a calibrated scale.A bulb and a thin stem, or two metals that expand unequally, turn warming into movement. The scale is an empirical map, and mercury is being retired.
Thermocouples turn a difference into voltageExplain that the Seebeck voltage lives along two dissimilar wires in a gradient, and that a reference junction or cold-junction compensation completes the reading.A few microvolts per degree come from the wires, not the weld. Ice baths or connector sensors supply the missing end of the difference.
Infrared sensing reads emitted radiationExplain how emitted infrared, emissivity, and field of view shape a noncontact temperature estimate.Every surface glows in thermal infrared. The gun estimates temperature from that glow, if emissivity and the viewed spot match its assumptions.

Questions this course answers

Why does a small volume change become a visible climb in a liquid-in-glass thermometer?

Most of the liquid sits in the bulb. The extra volume can only travel along a thin bore, so height change equals added volume divided by that tiny area.

Where does the useful thermocouple voltage actually come from?

NIST and the BIPM guide locate the Seebeck voltage along the wires wherever they sit in a temperature gradient. The weld completes the circuit at the place you want to measure.

Why can a shiny metal kettle fool an infrared thermometer even when the kettle is hot?

NASA frames this as two unknowns, temperature and emissivity. Without a good assumption, or a high-emissivity patch such as tape, the number can look precise and still be biased.

NIST lists mercury-in-glass thermometers as typically useful up to about how many degrees Celsius?

NIST gives a typical mercury range of about -38 °C to 400 °C. Organic-fluid bulbs usually top out near 150 °C but can go much colder.

Grounded in trusted sources

NIST, How Do You Measure Air Temperature Accurately?, https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-air-temperature-accurately
NIST, Mercury Thermometer Alternatives: Hg Alternatives, https://www.nist.gov/pml/sensor-science/thermodynamic-metrology/mercury-thermometer-alternatives/mercury-thermometer-0
NIST, Mercury Thermometer Alternatives: Thermocouple, https://www.nist.gov/pml/sensor-science/thermodynamic-metrology/mercury-thermometer-alternatives/mercury-thermometer-5
NIST, Precise Temperature Measurements with Invisible Light, https://www.nist.gov/news-events/news/2019/05/precise-temperature-measurements-invisible-light
NOAA Global Monitoring Laboratory, General Radiation Information, https://gml.noaa.gov/grad/about/rad.html
OpenStax, College Physics 2e, 13.2 Thermal Expansion of Solids and Liquids, https://openstax.org/books/college-physics-2e/pages/13-2-thermal-expansion-of-solids-and-liquids
BIPM, Guide to Secondary Thermometry: Thermocouple Thermometry, Part 1, https://www.bipm.org/documents/20126/41773843/Thermocouple_Thermometry_Part1.pdf
NASA NTRS, Risch, Understanding Radiation Thermometry – Part II, https://ntrs.nasa.gov/citations/20150021315

Every Wunder lesson is built from real, reputable sources — never invented.

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