You're exactly right in thinking that as the iron wire is made shorter, its
resistance would decrease, and it would get hotter. By moving the clip closer
and closer to the battery terminal, you are effectively making the iron wire
shorter, but without having to cut it. Current does not flow through the
piece of iron wire that is past the clip -- it only flows through the piece
of iron wire that is between the clip and the battery terminal. So the closer
you move the clip to the terminal, the shorter the piece of iron wire that
the current flows through.
Hope this helps.
Exploratorium Teacher Institute
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