The pots contain copper and zinc plates which become covered by weak sulphuric acid when the plates are pushed down.. The plates are connected by a switch either to an electric motor which turns a striped disc or to a set of red LED's (Light Emitting Diodes).

Click to enlarge

Click to enlarge
   Description of the Experiment   

The sulphuric acid in the exhibit is very dilute - much weaker than the sulphuric acid in car batteries.
When the acid is between the plates, they behave like a battery, so the motor turns round or the red LED's flash on and off, depending on the switch.

Any pair of metals separated by any conducting liquid will behave as a battery. Most combinations of metals and liquids are not very good because they foul up chemically very soon and stop producing an electric current. Good batteries have to be carefully engineered to avoid this problem. (Strictly, we should call the thing a cell; it becomes a battery of cells when there are several connected together.)
The copper, zinc, sulphuric acid cell is not very good for long periods of use. However, the flushing action each time the lever is pushed prevents it fouling up. Because the electrical energy produced by the battery comes from chemical reactions at the plate surfaces, the metals and acid get "used up" and need to be replaced periodically.

   Duration   

The experiments lasts about three minutes.

   Conclusion   

When the juice is between the plates, the citric acid in the juice reacts with each metal, releasing billions of electrically charged atoms of the metal (positive ions) into the liquid.

The ions cannot go far from the metal surface, because they have each left behind one or two electrons in the metal plate and the negative charge of these electrons pulls the ions back again. You may remember that positive and negative electrical charges attract one another.

   Detailed Conclusion   

Some metals react very easily when placed in orange juice (or other acid solution); others not so easily. In fact, zinc is more reactive than copper, so that there are more electrons on the zinc plate than on the copper plate. If the two plates are now connected with a metal wire, electrons will travel along the wire from the zinc to the copper - an electric current flows!

As the current flows, more zinc atoms go into the juice and maintain the imbalance of electrons on the plates so the zinc plate is gradually being eaten away. As electrons arrive at the copper plate, they combine with the charged hydrogen atoms of the acid and produce bubbles of hydrogen gas.

Any pair of metals separated by any liquid which conducts electricity will behave as a battery. Strictly, we should call a pair of different metal plates with liquid between them a cell; it becomes a battery of cells when there are several cells connected together. The first battery ever made used silver and zinc discs separated by salty water!

Most combinations of metals and liquids are not very good because they foul up chemically very soon and stop producing an electric current. In early batteries, someone often had to scrub the metal plates to remove hydrogen bubbles, etc. to keep the battery working! Good batteries these days are carefully engineered chemically to solve the problem.

The copper, zinc, orange juice cell is not very good for long periods of use. However, the flushing action each time the plates are lifted out of the juice prevents it fouling up with bubbles of hydrogen.