A Six's thermometer functions by using alcohol to expand or contract the mercury inside to display both the highest and lowest temperatures of a given time period. There are bulbs at the top of each arm of the tube, one above each indicator. The bulb above the minimum indicator tube typically contains alcohol while the bulb atop the maximum indicator tube contains a vacuum.
Six's thermometers are an excellent way to record both the highest and lowest temperatures in a day without going outside and making note of the temperature repeatedly. Before each reading, the thermometer must be reset with iron indexes being moved back into position at each end of mercury section with the aid of a magnet. The minimum temperature is recorded from the left column of the tube while maximum temperature is recorded from the right column. The key difference between regular thermometers and Six's thermometers, besides the ability of the latter to record maximum and minimum temperatures together, is that in regular mercury thermometers it is the expansion of mercury which records the temperature. However, in Six's thermometers, it is the expansion of alcohol or creosote that records the temperature variation while the mercury simply displays it.