The inner workings of the Baghdad Battery
Fill the jar with an acidic liquid, such as vinegar or fermented grape juice, and you have yourself a battery capable of generating a small current. The acidic liquid permits a flow of electrons from the copper tube to the iron rod when the two metal terminals are connected. This is basically the same principle that was discovered by Galvani 2,000 years later and that Volta successfully harnessed into the first modern battery a few years later.