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Me09 Electricity by redox reaction of the two Cola can metals

In the previous experiment with metals of a Cola can aluminium was a better electron donor than iron: Its reaction with hydrochloric acid was stronger.
In the following experiment you will observe that aluminium can also indirectly transfer electrons in an electric circuit.
What you need: Cola can (recycling icon FE), scissors, sand paper, 2 insulated copper wires with  4 crocodile clips, digital multimeter, fruit (apple), salt water.
Experiment: Cut a piece of steel 15 x 60 mm (Fe) from a Cola can, sandpaper its upper and lower ends, connect it to the crocodile clip of the red wire. Fix the closure of the Cola can to the white wire.
Clip the white wire to the lower terminal of the multimeter, the red one to the terminal in the middle.
* Use the free ends of the two metals to puncture an apple (left), to dip it into salt water (middle) and to press it between thumb and fore finger (right).
Observations: The multimeter reads voltages between 0.1 und 0.3 Volt. A current flows between the two metals.
Explanations: These three divices are Galvanic Cells. The details of the chemical reactions you will learn by the following experiment.