"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?"
Lou turned to Teresa Dwight. "Has this Harla fellow followed me so far?"
She nodded.
"Can you speak for him?"
"You talk, I hear, he reads me. I read him and I can speak."
"Okay, then," said Lou Graham. "Now we build a Le Clanche cell. Ask Harla does he recognize carbon. A black or light-absorbing element. Carbon is extremely common, it is the basis of life chemistry. It is element number six in the periodic chart. Does Harla know carbon?"