A maximum-minimum thermometer works by registering the maximum and minimum temperatures over a certain time period by using two scales set in liquid mercury. Two bulbs filled with alcohol or oil regulate the movement of the mercury.
Maximum-minimum thermometers are typically U-shaped parallel tubes of glass. One side registers the minimum temperature, while the other registers the maximum temperature since its last reading. The bend at the bottom of the thermometer contains the liquid mercury, which moves up and down based on contractions of the oil or alcohol located in the two bulbs at the top of the thermometer. The contractions of the alcohol or oil are the result of thermal changes in the environment causing it to expand or contract.
The oil or alcohol pushes the mercury in both tubes to record certain temperature readings, which are marked by steel indexes in the tubes that are held in place by small springs. The indexes are located on the surface of the mercury and move when the mercury is pushed by the oil or alcohol. When the temperatures reach their minimum and maximum, the steel indexes remain in place. This allows for the readings of both the minimum and maximum temperatures simultaneously without constant monitoring of the thermometer. The steel indexes can be returned to the surface of the mercury by using small magnets.