From Glossary of Meteorology
A combination maximum thermometer and minimum thermometer.
The tube is shaped in the form of a "U" having a bulb at either end. One bulb is filled with a clear liquid that expands or contracts with temperature variation, forcing before it a short column of mercury having iron indices at either end. The indices remain at the extreme positions reached by the mercury column, thus indicating the maximum and minimum temperatures. The indices can be reset with the aid of a magnet.
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