What is a Thermometer?
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Thermometers measure temperature, by using materials that change in some way when they are heated or cooled. In a mercury or alcohol thermometer the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit (used in the United States) or Celsius (used in Canada) and Kelvin (used mostly by scientists).
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Early History
Several inventors invented a version of the thermoscope at the same time. In 1593, Galileo Galilei invented a rudimentary water thermoscope, which for the first time, allowed temperature variations to be measured. Today, Galileo's inventioni is called the Galileo Thermometer, even though by definition it was really a thermoscope. It was a container filled with bulbs of varying mass, each with a temperature marking, the buoyancy of water changes with temperature, some of the bulbs sink while others float, the lowest bulb indicated what temperature it was.
In 1612, the Italian inventor Santorio Santorio became the first inventor to put a numerical scale on his thermoscope. It was perhaps the first crude clinical thermometer, as it was designed to be place in a patient's mouth for temperature taking. Both Galilei's and Santorio's instruments were not very accurate. In 1654, the first enclosed liquid-in-a-glass thermometer was invented by the Grand Duke of Tuscany, Ferdinand II. The Duke used alcohol as his liquid. However, it was still inaccurate and used no standardized scale.