Copper and zinc plates connected by micro-ammeter; your hand completes the circuit.
Clean copper and zinc sheet metal stock is cut into strips or pads. Alligator clip a zinc strip and a copper strip and plug the leads into a modern multimeter. DC 2V scale should cover the 1.09V that we expect from a zinc/copper battery at standard conditions of 1 Molar electrolyte and room temperature.
Our fingers are a network of electrolytic conductors, with more or less conductance (depending on moisture and salt). Pressed to the metal strips, adjacent fingers complete the circuit and the meter reads a value, usually closer to .5V than 1V.
The dependence of voltage on the electrolyte concentration is logarithmic, 30 mV per electron/factor of 10 in concentration. Substitution of other electrolyte species gives a variable but reliable voltage output from the different metals.
The finger skin conditions change and the output changes. Pressure and sweat are both important factors, leading this effect into service as one kind of lie detector, part of the polygraph, called skin conductance.