Insert one copper and one zinc or galvanized steel electrode into a potato, at least one inch apart. Electrodes can be copper pennies, heavy gauge wire, nails or metal strips. Wrap a strand of copper wire around the two electrodes to connect them.
Clip one alligator clip to each electrode. Alternately, strip a few inches of insulation off the end of two lengths of insulated wire, one copper and one zinc or galvanized steel. Wrap the wire securely around the electrodes, copper to copper and steel to steel.
Attach or touch the lead from the steel or zinc electrode to the negative terminal of the multimeter. Connect the lead from the copper electrode to the positive terminal.
Read the voltage output on the multimeter. Adjust the connections until you get a reading.