“No,” Jake said, stepping into the laboratory. “I’m an astronomer.”

“So what are you doing down here?” Younger asked, almost truculently. “We don’t do any stargazing here.” His voice had an adenoidal twang and an accent that sounded to Jake like Boston or maybe Down Maine.

“I came to see this MHD generator you’re working on,” Jake said.

Rogers smiled boyishly and gestured. “Well, there it is.”

The MHD generator was hardly impressive. The man-tall stack of copper plates stood in the middle of the lab. A squat cone-shaped contraption that looked to Jake a little like a rocket nozzle was stuck into one side of the pile and a stainless steel tube ran out from the other. A tangle of metal pipes and plastic tubing coiled all over the apparatus like the arms of an octopus. Thick wiring festooned the whole assembly, hanging from the ceiling, snaking along the floor. Jake recognized a big green tank of liquid oxygen off in one corner, a thin whiff of white vapor seeping from its top. Beside it stood what looked like a coal hopper, blackened with soot. That’s the smell, Jake realized: burning coal.

“That’s it?” he asked.

“That’s the rig,” Rogers said genially. Pointing, he explained, “Powdered coal and liquid oxygen in at this end, they’re burned in the combustion chamber, the plasma shoots through the channel inside the magnet there and produces a dozen kilowatts or so.”

“Fourteen, this morning,” one of the technicians called from the other side of the generator.

“You said plasma?”

“Ionized gas,” Rogers answered. “It’s hot enough for some of the atoms to have their electrons knocked off them, so the gas becomes electrically conducting.”

“I’m an astronomer. I know what plasma is,” Jake said. “The stars are plasma. Most of the universe is plasma.”

Roger chuckled good-naturedly. “Yeah, right. But the plasma we get from burning coal is only lightly ionized.”

Younger spoke up. “We add potassium to the mix in the combustion chamber to increase the ionization.”

“And the plasma gives up electrical energy as it goes through the magnetic field?” Jake asked.

Rogers said, “Right. The plasma’s our armature, like the coil of copper wire in a normal power generator.”

Jake looked from Rogers’s friendly smile to Younger’s cantankerous scowl. “An armature,” he said, questioningly.

Patiently, Rogers explained, “In a conventional power generator, you spin a bundle of copper wires in a magnetic field. The copper conducts electricity, and as it spins through the magnetic field it generates an electrical current.”

“Faraday figured that out in the eighteen-thirties,” Younger added.

“And the bundle of copper wires is the armature,” Jake said, a little uncertainly.

“Right,” Rogers agreed. “In the MHD generator, though, the stream of hot plasma is the armature.”

With growing understanding, Jake said, “I get it. But the conductivity of the plasma must be a lot lower than the conductivity of copper wire.”