The chattering noise level died out about three decibels at a time. Lou
said, "When a steel magnet is etched in acid, the north pole shows
selective etching!"
I shook my head. "Lou," I said, "we don't know whether Venus has a
magnetic field, whether it is aligned to agree with the Earth's--nor
even whether the Venusians have discovered the magnetic compass."
"Oh, that isn't the reference point," said Lou Graham. "I'm quite aware
of the ambiguity. The magnetic field does have a vector, but the arrow
that goes on the end is strictly from human agreement."
"So how do you tell which is the north pole?"
"By making an electromagnet! Then using Ampere's Right Hand Rule. You
grasp the electromagnet in the right hand so that the fingers point
along the winding in the direction of the current flow. The thumb then
points to the north pole."
"Oh, fine! Isn't that just the same confounded problem? Now we've got
to find out whether Harla is equipped with a right hand complete with
fingers and thumbs--so that we can tell him which his right hand is!"
"No, no," he said. "You don't understand, Tom. We don't need the right
hand. Let's wind our electromagnet like this: We place the steel bar
horizontally in front of us. The wire from 'Start' leaves us, passes
over the top of the bar, drops below the bar on the far side, comes
toward us on the under side, rises above the bar on the side toward us,
and so on around and around until we've got our electromagnet wound.
Now if the 'start' is positive and the 'end' is negative, the north
pole will be at the left. It will show the selective etching in acid."
I looked at him. "Lou," I said slowly, "if you can define positive and
negative in un-ambiguous terms as well as you wound that electromagnet,
we can get Holly home. Can you?"