From AMS Glossary
(Abbreviated BT.) A device for obtaining, from a ship under way, a record of temperature against depth (strictly speaking, pressure) in the upper 300 m of the ocean.
For a thermal element it has a xylene-filled copper coil, which actuates a stylus through a Bourdon tube. The pressure element is a copper aneroid capsule that moves a smoked glass slide at right angles to the motion of the stylus. A double analog record is thus obtained as the BT is lowered and recovered. This device has generally been replaced by the expendable bathythermograph (XBT).