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The Baghdad Battery, sometimes referred to as the Parthian Battery, is the common name for a number of artifacts created in Mesopotamia, possibly during the Parthian or Sassanid period (the early centuries AD). These jars were probably discovered in 1936 in the village of Khuyut Rabbou’a, near Baghdad, Iraq. These artifacts came to wider attention in 1938 when Wilhelm König, the German 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 galvanic cells, perhaps 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.
The artifacts consist of approx. 130 mm (5 in) tall terracotta jars (with a one and a half inch mouth) containing 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 bitumen plugs or stoppers, and both rod and cylinder fit snugly inside the opening of the jar which bulges outward towards the middle. The copper cylinder is not watertight, so when the jar was filled with a liquid containing citric acid, 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 scholars[who?] to believe lemon juice, grape juice, or vinegar was used as an acidic agent to jump-start the electrochemical reaction with the two metals.
König thought the objects might date to the Parthian period (between 250 BC and AD 224). However according to Dr St John Simpson of the Near Eastern department of the British Museum, their original excavation and context were not well recorded (see stratigraphy), so evidence for this date range is very weak. Furthermore, the style of the pottery (see typology) is Sassanid (224-640).
Most of the components of the objects are not particularly amenable to advanced dating methods. The ceramic pots could be analysed by thermoluminescence dating, but this has apparently not yet been done; in any case, it would only date the firing of the pots, which is not necessarily the same as when the complete artifact was assembled. Another possibility would be ion diffusion analysis, which could indicate how long the objects were buried.
Copper and iron form an electrochemical couple, so that in the presence of any electrolyte, an electric potential (voltage) will be produced. König had observed a number of very fine silver objects from ancient Iraq which were plated with very thin layers of gold, and speculated that they were electroplated using batteries with these being the cells. After the Second World War, Willard Gray demonstrated current production by a reconstruction of the inferred battery design when filled with grape juice. W. Jansen experimented with benzoquinone (some beetles produce quinones) and vinegar in a cell and got satisfactory performance.
However, even among those who believe the artifacts were electrical devices, electroplating as a use is not well regarded today. Paul Craddock of the British Museum said “The examples we see from this region and era are conventional gold plating and mercury gilding. There’s never been any untouchable evidence to support the electroplating theory.” The gilded objects which König thought might be electroplated are now believed to have been fire-gilded (with mercury). Reproduction experiments of electroplating by Dr Arne Eggebrecht consumed “many” reproduction cells to achieve a plated layer just one micrometre thick. Other scientists noted that Dr Eggebrecht used a more efficient, modern electrolyte; using only vinegar, the battery is very feeble.
An alternative, but still electrical explanation was offered by Paul Keyser. It was suggested that a priest or healer, using an iron spatula to compound a vinegar based potion in a copper vessel, may have felt an electrical tingle, and used the phenomenon either for electro-acupuncture, or to amaze supplicants by electrifying a metal statue.
Skeptical archaeologists see the electrical experiments as embodying a key problem with experimental archaeology; such experiments can only show that something was physically possible, they say nothing about whether it actually occurred. Further, there are many difficulties with the interpretation of these artifacts as galvanic cells:
- the bitumen completely covers the copper cylinder, electrically insulating it, so no current can be drawn without modifying the design;
- there are not any wires or conductors with them;
- no widely accepted electrical equipment is associated with them. (Controversial stone reliefs depicting arc lights have been suggested, however the voltages obtained are orders of magnitude below what would be needed to produce arc lighting);
- a bitumen seal, being thermoplastic, is excellent for forming a hermetic seal for long term storage. It would be extremely inconvenient however for a galvanic cell, which would require frequent topping up of the electrolyte (if they were intended for extended use).
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 would not be at all surprising if any papyrus or parchment inside had completely rotted away, perhaps leaving a trace of slightly acidic organic residue.
If one is to put a piece of copper and a piece of iron half-submerged in vinegar in a closed jar, the iron on the upper part gets a black coating by reacting with the fume of the vinegar. The upper part of the copper similarly will get a green coating. Both materials are still in use as painting colors today, but manufacturing them was probably not the purpose of the device, because for this end the two metals don’t need each other’s company. But in the same time, in the vinegar the lower part of the iron gets a copper coating. It is a galvanic reaction (the iron gains a negative charge which attract copper ions) but the two metals don’t need to touch each other or be connected electrically whatsoever.
For what purpose could they use the coppered iron? Hardly anything. It is possible that they were just fascinated by the phenomenon and studied it for a later possible use.
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