Water conductivity meter, Kohlrausch bridge (WTW, Wissenschaftlich-Technische Werkstatten GmbH). 1962.

Device used to measure the lake water conductivity. It could be used in the laboratory or in the field, with a battery power supply, visible at the bottom of the figure. Apparatus and power supply are housed in a wooden case with a compartment for storing the electrode (right). The Kohlrausch bridge is the AC version of the Wheatstone bridge.