The clinical thermometer
The thermometer used for fever has a very short scale. Why?
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Step 1 of 4
- Liquid rises past a narrow kink in the tube.
- Warmth pushes it higher, past the kink.
- The kink blocks liquid from flowing back down.
- The reading holds until a firm shake.
The rule
Body temperature stays near 37 °C, so the scale is short.
A clinical thermometer measures body temperature. Its scale runs from about 35 °C to 42 °C.
A kink is a narrow part of the tube. It sits just above the bulb. Liquid can go up past the kink. The liquid cannot flow back.
Out of the mouth, the liquid cools and would sink. The kink stops it. So the reading holds. A firm shake sends the liquid back down.
Some thermometers have mercury, a shiny liquid metal, in the bulb. Mercury is harmful if the glass breaks. So many clinical thermometers are now digital. They show the number on a screen.