A reversing thermometer in the upright (not reversed) position. The mercury rises from the mercury reservoir (blue) through the capillary section (cyan and red) into the glass cylinder and upper reservoir (magenta).When the thermometer is turned upside down (reversed), the mercury column ruptures at the appendix in the capillary section (between the red and blue). The red and magenta mercury settles into the upper reservoir, while the cyan mercury returns to the blue reservoir. The level of the magenta/red mercury can then be read against the scale and indicates the temperature.
The green thermometer is an auxiliary thermometer which gives the ambient temperature. It is used to check the temperature when the main thermometer is read on deck of the ship. Its reading allows to correct for the expansion of the glass tube that occurs due to the warmer ambient temperature on deck. The overall accuracy thus achieved is 0.02°C. If multiple reversing thermometers are used in parallel the accuracy can be increased to 0.01°C.
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