From AMS Glossary
A mercury-in-glass thermometer that records temperature upon being inverted and thereafter retains its reading until returned to the first position.
It consists of a conventional bulb connected to a capillary in which a constriction is placed so that upon reversal the mercury column breaks off in a reproducible manner. The mercury runs down into a smaller bulb at the other end of the capillary, which is graduated to read temperature. A 360° turn in a locally widened portion of the capillary serves as a trap to prevent further addition of mercury if the thermometer is warmed and the mercury expands past the break-off point. The remote-reading potentialities of reversing thermometers make them particularly suitable for use in measuring temperatures at depths in the sea. In this application, both protected thermometers and unprotected thermometers are used, each of which is provided with an auxiliary thermometer. They are generally used in pairs in Nansen bottles. They are usually read to 0.01°C, and after the proper corrections have been applied, their readings are considered reliable to 0.02°C. Details of the correction procedure are given in Lafond's Tables.