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"There's no point in making a mistake unless you understand the mistake so that you donít make it again."
- Alfred Bester

Automatic Light Switch  
  A device that senses if an illuminated room is empty, and turns off the light.  

Waste not, want not - a saying made for the lunar environment.

As they left their joint office, Strong, always penny conscious, was careful to switch off the light. Harriman had seen him do so a thousand times; this time he commented. "George, how about a light switch that turns off automatically when you leave a room?"

"Hmm- but suppose someone were left in the room?"

"Well... hitch it to stay on only when someone was in the room- key the switch to the human body's heat radiation, maybe."

"Too expensive and too complicated."

"Needn't be. I'll turn it over to Ferguson to fiddle with. It should be no larger than the present light switch and cheap enough so the power saved in a year will pay for it..."

From The Man Who Sold The Moon, by Robert Heinlein.
Published by Shasta in 1950
Additional resources -

This kind of switch exists today, of course; they are cheap enough to pay for themselves in a year or so of use.

I don't know if this idea is original to Heinlein or not; commercial light switches for use in homes have been around since the 1890's, if not a bit earlier.

Heinlein continues his justification for such a device, proceeding along in a vein that seems to have some validity today.

Strong objected. "It's no good commercially. Switching of a light when you leave a room is a matter of temperament... If a man hasn't got it, you can't interest him in such a switch."

"You can if the power continues to be rationed. There is a power shortage now; and there will be a bigger one."

Thanks to an anonymous reader, new to, who left this tip on the contact page.

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Additional resources:
  More Ideas and Technology from The Man Who Sold The Moon
  More Ideas and Technology by Robert Heinlein
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