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Measuring temperature

An old wall thermometer Temperature is a measure of how much heat energy a substance or object contains. A slice of apple pie at 40°C contains more heat energy than a same-sized slice of the same pie at 30°C. Temperatures can be measured accurately using devices called thermometers. Older thermometers used mercury, the only metal that is liquid at room temperature. Because mercury is poisonous, many types of thermometer nowadays usually use some type of liquid alcohol.


Medical thermometers, one showing calibrations in degrees Celsius (right), the other a modern version with a digital display...Read More >>Medical thermometers, one showing calibrations in degrees Celsius (right), the other a modern version with a digital display (left).

Thermometers

When a substance gets hotter, it expands to a greater volume. Nearly all substances have this property, which is called thermal expansion. It is the basis of how thermometers work. In a thermometer, liquid is enclosed in a tall, narrow glass (or plastic) tube. When the temperature increases, the liquid increases in volume, and the height of the column of liquid in the tube rises. The height is proportional to the increase in temperature. Marks, called calibrations, alongside the tube, give a measure of temperature according to a certain scale.

The word "thermometer" was first used in 1624, by the French mathematician Jean Leurechon, who was describing one with a scale of eight degrees.

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