An Electrochemical Enigma
In 1936, workers building a railway near Khujut Rabu, a village just outside Baghdad, unearthed a peculiar artifact. The object, a terracotta pot about 14 centimeters high, held a cylinder of rolled copper sheet. Inside the copper, an iron rod was suspended, held in place at the top by a bitumen plug. The find was first described in 1938 by Wilhelm König, the German director of the National Museum of Iraq. He noticed signs of corrosion on the metals that suggested an acidic agent had once been present inside the jar.
The object's construction strongly resembles a simple galvanic cell, a foundational form of electric battery. The iron rod acts as the negative electrode (anode), and the copper cylinder is the positive electrode (cathode). If the jar were filled with an acidic electrolyte—such as vinegar or grape juice, both available in the region at the time—an electrochemical reaction would generate a direct current. Numerous modern experiments with replicas have confirmed this. Depending on the electrolyte used, a single jar can produce between 0.8 and 2.0 volts. The Discovery Channel's MythBusters team connected ten replicas in series using lemon juice and generated over 4 volts of electricity.
Debating an Ancient Power Source
König’s hypothesis, published in 1940, was that these devices were used for electroplating—applying a thin layer of gold or silver onto other metal objects. Archaeologists have not found irrefutable evidence of electroplated artifacts from this period in Mesopotamia. Objects that König believed were electroplated are now generally considered to be fire-gilded using mercury. The dating of the jars is also uncertain. Though initially attributed to the Parthian period (c. 250 BCE – 224 CE), the style of the pottery is more consistent with the later Sassanian period (224–651 CE).
Other proposed uses for a low-voltage electrical source include a mild form of electrotherapy for pain relief, a practice known to the ancient Greeks who used electric fish. Another idea suggests the jars could have been embedded within a metal statue to deliver a small shock to a person who touched it, creating a mysterious or divine effect during a religious ritual.
The widely accepted alternative theory dismisses the battery hypothesis altogether. Many archaeologists believe the jars were simply storage vessels for sacred papyrus or parchment scrolls. Similar containers, though without the iron rod, have been found at the nearby ancient city of Seleucia containing decomposed scroll remnants. In this scenario, the scroll would be wrapped around the central rod and placed inside the copper cylinder for protection, with the bitumen plug providing a seal. The acidic residue found inside could be the result of the papyrus decomposing over centuries. The original artifacts were held at the National Museum of Iraq, but their current whereabouts are unknown following looting in 2003.