The circuit has been running for about 30 hours, and it's interesting to see how the aluminium anode is being eroded. The photo below shows a dark ring where the aluminium has been eaten away (converted into aluminium hydroxide) in the reaction that generates the electrical energy.
Aluminium is probably being consumed elsewhere too, and the dark ring shows the places where it has been consumed the most. It is already evident that the pressure of the spring clip gradually brings more of the can base into closer proximity to the charcoal disc. The diagram below illustrates this (the tissue paper barrier between the charcoal and aluminium is not shown in this diagram).
It's convenient to have the anode "fed" towards the cathode like this. A potential problem is that the connection from the anode to the circuit (crocodile clip in the photo above) is outside the dark ring, and will eventually (quite soon by the looks of things) become separated from the inner part of the anode. To resolve this I've inserted a piece of copper wire under the spring clip and put it into the jaws of the crocodile clip.