Maximum and minimum thermometer. The two columns are connected by a 'U'-bend at the bottom which contains mercury. The left column (minimum) leads to an alcohol reservoir. As the temperature increases, the alcohol expands, pushing the left side of the mercury down, forcing the right side up. As the temperature decreases, the alcohol contracts, allowing the mercury to return. Small steel pointers in each column (near central button) are pushed by the mercury to record the displacement limits. Here the thermometer has recorded a maximum of 24 Celsius and a minimum of -3 Celsius, read from the base of each pointer.