Lemon-powered clocks work by using the process of electrolysis. The lemon juice is an acidic electrolyte, which is then connected in a circuit through a metal electrode. There must be two different metals present to produce an electric charge; zinc and copper are common. Otherwise, an outside electric source would have to be present to induce electrolysis. The two metals produce the current necessary to charge the electrolytes, thus allowing the process of electrolysis (separation) to occur and electricity to flow enough to power a clock.
Process of Electrolysis
Electrolysis is present in all electrical processes; it is the flow of electrical current through a substance known as an electrolyte conductor. All conductors have ions that are fast moving, meaning they are molten or mobile. To induce electrolysis, a circuit must be created to create an electric charge. An external source of electricity (which must be present to initiate the process) passes through an electrode that is the go-between of the electricity and the electrolyte (liquid containing moving ions) from which the electrolyte ions absorb or lose electrons. The ions that gain and lose electrons lose their charge and move away from the electrolyte. It is a process that chemically separates an element, resulting in the release of energy that is carried by the roaming electrons throughout the circuit, thus powering a clock, battery or light. The process of electrolysis is most notably used for separating hydrogen from oxygen in water.
Commercial Lemon Clocks
In most lemon-powered clocks, the electrode conductor is either a small peg of mixed metals or two separate metals connected together. There are many commercial lemon-powered clocks that use small plugs made out of copper and zinc, into which the lemon is then stuck. The connection induces electrolysis, and the energy flows through hidden wire to power the (usually analogue) clock.
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Homemade Lemon Clocks
The well-known grade school science fair project is a little less romantic in appearance, using a string of lemons pierced by pins or paper clips wrapped in aluminum foil that are connected by a copper wire that forms a connected circuit through a clock. The electrolyte (lemon acid) is present, the two metals are present, a closed circuit is created; electrolysis can occur, thus powering a clock (though for a very limited amount of time). Lemons are not the only things that can be used to power a clock using electrolysis. Any liquid electrolyte, such as salt water, is effective.