A bathythermograph (BT) is a device for obtaining, from a ship under way, a record of temperature against depth and 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 was generally replaced by the expendable bathythermograph (XBT).

This object is a bathythermograph MK 5. It has a torpedo-shaped body with 6 fins and is in a grey-painted wooden case. The instrument has a fitting for a bathythermographic slide, and a catch release for a pin to mark the slide. The top of the bathythermograph is marked 'Hawker Siddeley' and it was made by S. G. Brown Limited.

Until the development of the bathythermograph in 1938, the thermocline (the layer of rapid temperature change between surface and deep water) had not been understood. The bathythermograph was able to show the structure of this layer more readily than could be achieved by measurements using a water bottle and thermometer.