It is used to measure the maximum
temperature within a period (usually daily). The most common maximum thermometer is the
mercury-in-glass thermometer with a constriction in the bore below the lowest graduation.
When temperature rises, the expansion force pushes the mercury through the constriction.
When temperature falls after reaching the maximum value, mercury in the bulb contracts
but the thread of mercury cannot fall back to the bulb due to the constriction.
As a result, the mercury thread breaks at the constriction leaving the thread
indicating the maximum temperature reached sine the last reset. In practice,
the maximum thermometer rests horizontally inside a screen with bulb on left side.