Baghdad Battery

 The Baghdad Battery, sometimes referred to as the Parthian Battery, is the common name for a number of artifacts created in Mesopotamia, during the early centuries AD, probably discovered in 1936 near Baghdad, Iraq. These artifacts came to wider attention in 1938 when Wilhelm König, the director of the National Museum of Iraq, found the objects in the museum's collections. In 1940, König published a paper speculating that they may have been used for electroplating gold onto silver objects. This interpretation continues to be considered as at least a hypothetical possibility. If correct, the artifacts would predate Alessandro Volta's 1800 invention of the electrochemical cell by more than a millennium.

Description
 The artifacts consist of terracotta pots approximately 130 mm (5 in) tall with a one-and-a-half-inch mouth. Each contains a copper cylinder made of a rolled-up copper sheet, which houses a single iron rod. At the top, the iron rod is isolated from the copper by plugs or stoppers, and both rod and cylinder fit snugly inside the opening of the jar, which bulges outward toward the middle. The copper cylinder is not watertight, so if the jar was filled with a liquid, this would surround the iron rod as well. The artifact had been exposed to the weather and had suffered corrosion, although mild given the presence of an electrochemical couple. This has led some to believe that lemon juice, grape juice, or vinegar was used to generate an electric current from the copper and iron electrodes.

Original Theories
 König thought the objects might date to the Parthian period (between 250 BC and AD 224). However, according to St John Simpson of the British Museum, their original excavation and context were not well-recorded, so evidence for this date range is very weak. Furthermore, the style of the pottery is Sassanid (224-640).

 The artifacts strongly resemble another type of object with a known purpose — namely, storage vessels for sacred scrolls from nearby Seleucia on the Tigris. Those vessels do not have the outermost clay jar, but are otherwise almost identical. Since it is claimed these vessels were exposed to the elements, it is possible that any papyrus or parchment inside had completely rotted away, leaving a trace of slightly acidic organic residue.

Testing
 The idea that the battery could have produced usable levels of electricity has been p ut to the test at least twice.

 On the 1980 British Television series Arthur C. Clarke's Mysterious World, a recreation of the battery was filled with grape juice to produce half a volt of electricity, demonstrating that the battery could electroplate a silver statuette in two hours. However, doubt has recently been cast on the validity of these experiments.

 The Discovery Channel program MythBusters determined that it was indeed plausible for ancient people to have used the Baghdad Battery for electroplating. In 2005, ten hand-made terracotta jars were fitted to act as batteries. Lemon juice was used to activate the electrochemical reaction between the copper and iron. Connected in series, the batteries produced four volts of electricity.

 The show's research staff proposed three possible uses: electroplating, medical pain relief (through acupuncture), and religious experience. It was discovered that, when linked in series, the cells indeed had sufficient power to electroplate a small token. For acupuncture, the batteries produced a "random" pulse that could be felt through the needles; however, it began to produce a painful burning sensation when the batteries were grounded to two needles at once. For the religious experience aspect of the batteries, a replica of the Ark of the Covenant was constructed, complete with two cherubim. When touched, the wings produced a strong feeling of tightness in the chest. Although the batteries themselves had not been used, it was surmised that any form of electrical sensation from them could equate to the divine presence in the eyes of ancient people. In the end, the Baghdad Battery myth was found plausible on all three accounts.