This is an image of a mercury bulb thermometer. The temperature is measured by reading the number next to the thin black line that goes partly up the yellow tube.
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Image courtesy of Wikipedia Creative Commons
Thermometers measure temperature. "Thermo" means heat and "meter" means to measure. You can use a thermometer to measure the temperature of many things, including the temperature of the air, the temperature of our bodies, and the temperature of the food when we cook. Temperature is a measure of the hotness and coldness of an object.
Thermometers usually have a bulb at the base and a long glass tube that goes out of the bulb. The glass tube of a thermometer is filled with alcohol or mercury. Both mercury and alcohol grow bigger when heated and smaller when cooled. Inside the glass tube of a thermometer, the liquid has no place to go but up when the temperature is hot and down when the temperature is cold. Numbers on the glass tube that mark the temperature when the line is at that point.
The thermometer measures temperatures in Fahrenheit, Celsius and another scale called Kelvin. Fahrenheit is used mostly in the United States, and most of the rest of the world uses Celsius. Kelvin is used by some scientists.
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