Together with the Habicht brothers, in the years around 1908 Einstein developed this measurement device, which he often designates in his letters as “our little machine.” In order to be able to measure extremely small electrical currents, the potential multiplier amplifies the voltage by a factor of 10,000. Around 1920, Walter Gerlach measures the contact voltages of metals with this device. This study serves to confirm experimentally Einstein’s paper on the photo-effect.