Man Who Sold the Moon / Orphans of the Sky by Robert A. Heinlein

power?”
    “Be practical. You aren’t the Atomic Energy Commission.”
    “I might stick a windmill on the roof.”
    “That’s better, but still not good. Now get busy with that knot in the end of your spinal cord. I’ll start some coffee. This is going to be another all-night job.”
    He grinned at her. “O.K., Carrie Nation. I’m coming.”
    She smiled happily at him. “That’s the way to talk.”
    He rose and went over to her, slipped an arm about her waist and kissed her. She relaxed to his embrace, but when their lips parted, she pushed him away.

    As the first light of dawn turned their faces pale and sickly, they were rigging two cold light screens face to face. Archie adjusted them until they were an inch apart.
    “There now—practically all the light from the first screen should strike the second. Turn the power on the first screen, Sex Appeal.”
    She threw the switch. The first screen glowed with light, and shed its radiance on the second.
    “Now to see if our beautiful theory is correct.” He fastened a voltmeter across the terminals of the second screen and pressed the little black button in the base of the voltmeter. The needle sprang over two volts.
    She glanced anxiously over his shoulder. “How about it, guy?”
    “It works! There’s no doubt about it. These screens work both ways. Put juice in ’em; out comes light. Put light in ’em; out comes electricity.”
    “What’s the power loss, Archie?”
    “Just a moment.” He hooked in an ammeter, read it, and picked up his slide rule. “Let me see—Loss is about thirty percent. Most of that would be the leakage of light around the edges of the screens.”
    “The Sun’s coming up, Archie. Let’s take screen number two up on the roof, and try it out in the sunlight.”
    Some minutes later they had the second screen and the electrical measuring instruments on the roof. Archie propped the screen up against a sky-light so that it faced the rising Sun, fastened the voltmeter across its terminals and took a reading. The needle sprang at once to two volts.
    Mary Lou jumped up and down. “It works!”
    “Had to work,” commented Archie. “If the light from another screen will make it pour out juice, then sunlight is bound to. Hook in the ammeter. Let’s see how much power we get.”
    The ammeter showed 18.7 amperes.
    Mary Lou worked out the result on the slide rule. “Eighteen-point-seven times two gives thirty-seven-point-four watts or about five-hundredths of a horsepower. That doesn’t seem like very much. I had hoped for more.”
    “That’s as it should be, kid. We are using only the visible light rays. As a light source the Sun is about fifteen percent efficient; the other eighty-five percent are infrared and ultra-violet. Gimme that slip-stick.” She passed him the slide rule. “The Sun pours out about a horsepower and a half, or one and one-eighth kilowatts on every square yard of surface on the Earth that is faced directly towards the Sun. Atmospheric absorption cuts that down about a third, even at high noon over the Sahara desert. That would give one horsepower per square yard. With the Sun just rising we might not get more than one-third horsepower per square yard here. At fifteen-percent efficiency, that would be about five-hundredths of one horsepower. It checks—Q.E.D.—what are you looking so glum about?”
    “Well—I had hoped that we could get enough sunpower off the roof to run the factory, but if it takes twenty square yards to get one horsepower, it won’t be enough.”
    “Cheer up, Baby Face. We doped out a screen that would vibrate only in the band of visible light; I guess we can dope out another that will be atonic—one that will vibrate to any wave length. Then it will soak up any radiant energy that hits it, and give it up again as electrical power. With this roof surface we can get maybe a thousand horsepower at high noon. Then we’ll have to set up banks of storage batteries so that we can store