Maximum and Minimum Thermometer- China.

A Maximum‑Minimum thermometer, also called Six’s thermometer, features a U-shaped glass tube and two recorded-temperature markers. One arm measures the maximum, the other the minimum temperature since the last reset. They are often used in climatology, horticulture, greenhouses, and mechanical labs.

The design typically uses mercury (maximum) and alcohol (minimum), with small magnetic markers indicating the extremes. Mercury-free alternatives using safer liquids also exist.