Substance expands when heated. And when they cool, they contract. Thus is exactly what happens with the mercury, liquid inside the thermometer. When the weather is hot , the bulb at the end of the tube heats up and so does the mercury. The heated mercury expands and rises up the hollow tube. As the day cools, the mercury also cools. And when this happens, it contracts and drops down the tube. Thus, we are able to read off the both the hot and cold temperature of the day.
The inside of the tube is very small. Thus ensures that even a small increase in temperature can be recorded. When the mercury gets heated by the raising temperature, it expands. This expansion is great compared with the inside of the tube. Hence it causes the mercury to move a noticeable distance up to the tube.
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