The builder nodded rapidly. "All right. But why doesn't the electricity
affect the walls themselves? I thought they were made of steel."

The engineer glanced through the dead-light at the reddish disk of the
Earth, hazy and indistinct at a distance of forty million miles. "It
isn't steel; it's a non-magnetic alloy. Besides, there's a layer of
crystalline sulphur between the alloy and the vacuum space."

"The vacuum is what keeps out the cold, isn't it?" Jackson knew, but he
asked in order to learn more.

"Keeps out the sun's heat, too. The outer shell is pretty blamed hot on
that side, just as hot as it is cold on the shady side." Smith seated
himself beside a huge electrical machine, a rotary converter which he
next indicated with a jerk of his thumb. "But you don't want to forget
that the juice outside is no use to us, the way it is. We have to change
it.

"It's neither positive nor negative; it's just neutral. So we separate
it into two parts; and all we have to do, when we want to get away from
the earth or any other magnetic-sphere, is to aim a bunch of positive
current at the corresponding pole of the planet, or negative current at
the other pole. Like poles repel, you know."

"Listens easy," commented Jackson. "Too easy."

"Well, it isn't exactly as simple as all that. Takes a lot of apparatus,
all told," and the engineer looked about the room, his glance resting
fondly on his beloved machinery.