Although the principle
of the reversing thermometer had been described by scientists as
early as 1845, this Negretti and Zambra thermometer was the first
thermometer to accurately determine the temperature at great depth
and return to the surface and retain its readings. As such, it is
considered the first modern reversing thermometer. This was the
reversing thermometer sent to scientists and used on the Challenger
This thermometer was housed in a helical mounting mechanism meant
to cause the reversing thermometer to flip at the required depth.
A helical screw would measure the depth on the way down and release
the mounting at the desired depth.