DNA supercoiling on agarose gel -> reference needed - (Nov/15/2009 )

hi!

so we did a restriction digest on plasmid dna and then ran it on agarose gel, nothing complicated.
Now i'm writing the report and the tutor specifically asked to answer why there are several bands instead of one on the lane of the control.
This is obviously caused by different states of supercoiling, but i need a reference for this and i'm having a really hard time finding one as most people seem to tread this as common knowledge.

maybe someone had to to something similar and still remembers where he/she found a good reference fot this, a nod in the right direction would be much appreciated

This reference looks good.