A maximum/minimum thermometer contains a mixture of mercury and alcohol.
The mercury sits in the u-bend of the thermometer.
The bulb at the top of the tube reading the minimum temperature contains alcohol and the bulb at the top of the tube reading the maximum temperature contains a vacuum.
On the minimum side the expansion of the mercury is restricted by the contracting alcohol, on the maximum side the expanding mercury can expand more freely into the vacuum.
At any given time both thermometers should record the same temperature.
However, during the day they would have recorded the maximum and minimum temperature - a steel marker should indicate these temperatures.
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