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Deep within Iraq, hidden in the town of Khujut Rabu, lies a terracotta pot measuring around 6 inches, or 140 millimeters, tall, with the opening measuring 1.5 inches, or 38 millimeters, tall. Discovered in 1936, a cylinder made of a rolled sheet of copper was placed inside the pot, with a single iron rod held inside the copper cylinder. Asphalt separated the iron rod from the copper cylinder, likely because the cylinder was not watertight. The pot itself suffered weather exposure and corrosion. But, little did archaeologists at the time know that this artifact would completely shift our understanding of electric currents and archaeology.
Wilhelm König, an assistant at the National Museum of Iraq, discovered the battery at the museum in 1938 and was fascinated by it. König believed that the artifact was from the Parthian period, specifically from 250 BCE to 224 CE, although later analysis proved that the artifact was likely from the Sassanid dynasty, which lasted from 224 CE to 640 CE. However, König had another theory to form the entire argument around this artifact. After examining silver objects from Iraq which were plated in gold, König believed that the objects were electroplated. Electroplating is a process in which a metal coating is placed onto a solid object by using an electric current. This current oxidizes the ions, positively charged atoms, in the metal object, thus allowing the metal coating to cover the solid object. Following his discovery, König published a paper with the hypothesis that the artifact was a galvanic cell used for electroplating gold onto silver. A galvanic cell, for clarification, is an electric cell that generates electric currents through simultaneous and spontaneous oxidation reactions.
Additional evidence supporting the electric use of this “battery” came from the fact that the corrosion on the artifact likely came from an acidic substance being placed in the jar, with the substance being suspected of having been wine or vinegar. The presence of the acidic agent led to speculation that the liquid was used to produce electrolytes from the acidity of the liquid. This would then create an electric current due to the difference between the electrode potential of the copper and the iron. Electrode potential, if it was not already known, is another word for the electromotive force of a galvanic cell that is built from an electrode and another electrode that has not been characterized and usually appears at the boundary between an electrolyte and an electrode, with electromotive force being a word to describe the transfer of energy to an electric circuit by a measure of volts.
After WWII, American engineer Willard Gray created a reconstruction of the battery and filled it with grape juice, which ended up creating an electric current. Another engineer, W. Jansen, performed the same experiment except with vinegar and benzoquinone, which is produced by certain types of beetles, instead of grape juice and achieved the same results. On a 2005 episode of the show MythBusters, it was shown that the battery, when lemon juice was placed inside a replica of the pot, 4 volts of electricity were produced and, when 10 of these jars were linked, produced enough power to electroplate a coin and deliver a current to acupuncture needles. Other theories suspected that the battery produced electric currents not because it was meant to function as a battery, but because it was used for methods like electroacupuncture, in which a small current of electricity is transferred to small acupuncture needles.
Despite these results, however, numerous pieces of evidence and observations have disproved the theory that the “battery” was used as a battery. For example, since the iron rod was visible outside of the asphalt but the copper was not, it would have been impossible to complete a circuit with the battery since a wire would not be able to connect with the copper. Additionally, the asphalt seal was thermoplastic, which meant that at low temperatures, the asphalt would become solid and hard to remove. This would be inconvenient for the galvanic cell as the amount of electrolyte in the battery would need to be manually adjusted just like any battery, which would be difficult to do with the asphalt sealing off the opening to the “battery”. However, the piece of evidence that proves that the artifact was not a battery was that König was confused about how the silver objects were covered with gold. The objects were, in fact, fire-gilded. Fire-gilding is a process where a combination of gold and mercury is applied to a metal surface and, using fire, the mercury is evaporated, leaving only the gold. Electroplating wouldn’t be invented until 1805, over 1 millennium after the creation of the artifact.
Now that we have the evidence to prove what the “battery” is not, we need to know what it is. The answer to that question is relatively simple: storage vessels. The artifact was designed in a similar way to other artifacts used to store sacred scrolls. These scrolls came from Seleucia near the Tigris River. On the other side of the river is the city of Ctesiphon, which is within close distance of Khujut Rabu, where the artifact was discovered. Additional evidence comes from the corrosion on the vessels, which signifies that the parchment and papyri inside had rotten away. This would also tend to leave an acidic residue within the vessel, which explains the presence of an acidic substance on the artifact. The copper cylinder and the iron rods are also covered by this explanation, as they were likely in the pot to keep the scrolls in place. As of now, this is the most reasonable explanation for what the artifact was because, as stated by archaeologist Elizabeth Stone, “I think it's not a battery. I think the people who argue it's a battery are not scientists, basically. I don't know anybody who thinks it's a real battery in the [archaeology] field.”
When the United States commenced the 2003 invasion of Iraq, thousands of items and artifacts from the National Museum of Iraq were stolen, including the Baghdad Battery. Despite extensive searches for the artifact, it has never been found. It is likely that with current advances in archaeology, archaeologists could have finally put the debate around the artifact’s function to rest; however, that future will likely never come to fruition. Nevertheless, we can still marvel at the simple purpose of the Baghdad Battery. The archaeology world would be forever changed by how the simple artifact taught archaeologists to not give too much credit to ancient civilizations and to recognize that, in some cases, the simplest explanation is sometimes the best.
If you want to learn more about how archaeology and history connect with technology, check out more of STEM-E's articles!
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