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c.1890
mercury, opal glass, wood
This is a Pocket max–min thermometer, dating from around 1890, a variant of the Six’s thermometer, invented by James Six in the 1780s. Two scales: Minima (on the left) and Maxima (on the right), calibrated in degrees Fahrenheit on an opal glass plate. Such instruments were used to register daily fluctuations of temperature, especially in horticultural, botanical and agricultural observations, as well as in meteorological services of the 19th and 20th centuries. Another name sometimes encountered is Thermomètre Dimenuon, which denoted a pocket maximum–minimum thermometer of Six’s system. These instruments were also called registering thermometers, since Six’s thermometer automatically records the maximum and minimum temperatures that have occurred during a given period (usually 24 hours), while at the same time indicating the present temperature. The bottom end of the index on the right registers the maximum temperature, while the minimum temperature is registered by the bottom end of the index on the left. The present temperature is shown by the mercury column.
The case is made of characteristic burl wood (most likely birch burl). This timber is prized for its figure with numerous swirls and “eyes,” and was often used for fine boxes and scientific instruments of the 19th century. Inside is a recess for the magnet. The lid closes tightly, indicating a travel or field format.
In the U-shaped glass thermometer tube at the bottom lies a small column of mercury, acting as a piston. On either side of it are columns of liquid (toluene or kerosene). At the upper ends of the tube are expanded reservoirs. In the liquid columns float two metallic index rods (min and max), which are pushed along by the movement of the mercury “piston” and then remain in place until returned by a magnet. These rods record the highest and lowest temperatures until reset.
Compact, elegant, and scientifically refined, this pocket max–min thermometer represents both the ingenuity of Six’s invention and the craftsmanship of late 19th-century instrument making.
