In 1936, in the village of Khuyut Rabbou'a (near Baghdad) in Iraq, archaeologists discovered some peculiar ancient vessels, each containing an iron bar and a cylinder of rolled-up copper sheeting.
They languished in a museum's storage for two years, until Wilhelm König, director of the National Museum of Iraq, found them and became fascinated. In 1940, he published a paper speculating that they may have been galvanic batteries, capable of electroplating gold onto silver objects.
The Copper sheeting and iron rod in the Baghdad batteries form an electrochemical couple, so that an electrical voltage will be produced in the presence of an electrolyte. Lemon juice, grape juice, vinegar and other acidic substances are generally thought to have been used in the Baghdad batteries.
"The Discovery Channel program MythBusters determined that it was indeed plausible for ancient people to have used the Baghdad Battery for electroplating or electrostimulation. On MythBusters' 29th episode (which aired on March 23, 2005), ten hand-made terracotta jars were fitted to act as batteries. Lemon juice was chosen as the electrolyte to activate the electrochemical reaction between the copper and iron. (Oddly enough, it was discovered that a single lemon produced more voltage than one of the batteries when using copper and zinc.) However, the batteries which they reproduced did not produce a substantial amount of energy and had to be connected in series in order to achieve a 4 V potential drop and test the theories.
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. Instead of linking the cherubim's golden wings to the low power batteries, an electric fence generator was connected. 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, due to the apparent lack of knowledge of electricity, any form of electrical sensation from them could equate to the divine presence in the eyes of ancient people."
Regarding the observation that using a lemon actually worked better than lemon juice, it may well be that the creators of the batteries actually used several fruits connected in a series, which itself may have also been immersed in lemon juice - electric kool-aid, indeed. Some other item may have been used as a electrolyte which has not yet been considered.
The Rods of Ra site suggests a connection between the Baghdad batteries and mysterious Egyptian ornamental rods such as those found belonging to Pharaoh Pepi II. Another company, Egyptian Healing Rods also sells modern replicas.