Maximum thermometer is also a mercury-in-glass thermometer having a constriction near the bulb end. When the temperature of air rises, the mercury in the thermometer expands and forces its way into the stem past this constriction. But when the bulb cools none of the mercury above the constriction will get back into the bulb and then the length of the mercury remains the same. The end of the mercury thread farthest from the bulb registers the highest temperature reached in a day.
The liquid used in minimum thermometer is alcohol in which a dumb-bell-shaped index is set. When the temperature falls, the alcohol column drags the index towards the bulb end but when the temperature rises the alcohol column expands and runs past the index without disturbing it. Thus the end of the index farthest from the bulb gives the lowest temperature attained in a day.