Bathythermograph (BT)

A bathythermograph (BT) is an instrument that records seawater temperature against depth as it descends through the water column, producing a temperature profile on a single drop. The original mechanical BT, developed in the 1930s, scribed a temperature trace onto a smoked or coated glass slide using a bourdon-tube thermal element and a pressure-driven stylus. The expendable bathythermograph (XBT) replaced it: a free-falling thermistor probe transmits temperature over a thin copper wire while depth is inferred from a known fall-rate equation. XBTs let ships profile the upper ocean underway without stopping, and they built much of the historical upper-ocean heat-content record.