Gel smiling and crying

[Joe Nomen]

Occassional during electrophoresis the bands tend to concave
(smiling/crying).  Sometings the bands are straight but narrow near the end
of the gel.  I've been told this is due to irregular current and/or 
temperature changes in the running buffer.

Can someone elaborate a more detailed explaination to me?  Or point me to an
article or textbook & chapter and I read it myself.

Thank you. 
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There are two versions of history, the one printed in books and what really
happened. ;^)