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The forerunner of the thermometer, generally held to have been invented at the beginning of the 17th century. It normally consisted of a small flask with an extremely elongated neck. The (partial) vacuum, obtained by warming the flask before inverting the neck into a reservoir, drew liquid (usually water) into the tube. The level then fluctuated with variations in temperature (and pressure). Although the height of the column may have been marked by threads or similar devices, a thermoscope differed from a thermometer in having neither a fixed, graduated, and calibrated scale nor a truly evacuated, sealed tube.

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