A simple, robust, and high precision mechanical square-wave polarograph is described which has a very high signal-to-noise ratio, and thus greatly reduced interference due to "capillary response".With this instrument reversibly-reduced ions such as Cd²⁺ and Pb²⁺ at concentrations down to about 2×10^-8M can be detected.This instrument also includes a device with which the i-t curve in each half period of the square wave voltage can be obtained.It is a convenient tool for the investigation of the mechanism of electrode processes.