Tool of the Times

Early in the twentieth century, oceanographers used a device called a bathythermograph (BT) to record water temperature beneath the surface on glass slides coated with smoke and oil to. Invented in the late 1930’s, the re-usable probes preserved information that proved critical to the U.S. Navy’s growing fleet of submarines and anti-submarine activities. As the metal BT dropped through the water, a bellows contracted under the increasing pressure, moving the attached slide along one axis. Another  component changed shape as temperature changed, moving a stylus that scratched a line in the coating. The resulting plots could then be interpreted with gridded readers, as in this 1940 photo. (Photo courtesy of Woods Hole Oceanographic Institution Archives)