Six's thermometer is popular among gardeners, and for use in greenhouses. It's purpose is to record the maximum and minimum temperatures reached since the thermometer was last read, typically around 24 hours. Generally speaking the minimum temperature occurs during the night and the maximum temperature occurs during the day.
The thermometer consists of a fairly large cylindrical bulb A which originally contained alcohol but now oil of creosote is more common. This is connected by a U – shaped stem to a second bulb nearly full of the same liquid.
The bend of the U contains mercury. Two scales are provided, one against each limb of the tube so that the temperature may be read against either of the mercury levels. Resting on each of the mercury columns are small steel indexes with light springs to hold them in position in the stem. Expansion or contraction of the liquid in A causes a movement of the mercury thread. Consequently, one or other index is pushed by the mercury and left in the extremest position reached.
The index on the left indicates the minimum position reached and that on the right indicates the maximum position reached. After reading the maximum and minimum temperatures, a magnet is used to bring the springs back into contact with the mercury.