The thermometer and the barometer
The simple thermometer – for measuring temperature.
As liquids heat up, they expand. This property is used in simple thermometers.
The bulb contains most of the mercury (or alcohol) and the tube heading north is actually very thin and holds only a little of the liquid. As the temperature at the bulb increases, so the expanding liquid is forced up the tube. The higher the temperature, the further it goes, so it can be calibrated. The tube has to be closed at the top to stop the liquid from spilling out during transportation, so it also has to be evacuated, otherwise, it would explode as the air above the liquid got compressed.
The Aneroid barometer – for measuring atmospheric pressure.
The box is partially evacuated, so that as the atmospheric pressure increases, the box is squeezed. The remaining air in the box is compressed and resists the squeeze (like squeezing a rubber ball). The chain is pulled via the levers and this turns the pointer.
And we can then read the pressure –
As the pressure indicate the weather, the barometer is commonly used to forecast which way the weather is going.
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