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Because of mercury when it thermometer gets hot the liquid(expands) and when it gets colder  compacts and move downwards 
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The most commonly used liquid in thermometres is mercury.The thermometric property of mercury is that, It increases with a rise in temperature.The temperature range is from,-30degree celcious to 350 degrees celsious.When the thermometre gets hotter, the liquid inside it rises upwards and tries to expand. When the thermometre gets cold, the mercury compacts and goes downwards.

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