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My Crooke's Radiometer Revisited

For those who put up with my blathering about what might be screwing up my 10+ year old radiometer a few weeks ago, here's "the rest of the story."

I gave up and plopped down $8.95 (Plus S&H) for a brand new radiometer. It's now sitting on the kitchen window sill spinning like an NPR news anchor when a little bit of sun falls on it.

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Interestingly, the cardboard disk glued on its base bears eggsackly the same wording in the same type font as my old one, right down to the bottom line where the old one declared "Made in China" and the brand new one says "Made in India".

I tucked the old radiometer away where my heirs will find it and undoubtedly chuck it out. I just couldn't bring myself to bust it open to see whether the needle pivot was no longer as sharp as it needed to be and maybe creating too much friction drag.

I guess I'll never really know just why the old radiometer "wore out". I'll just have to blame it on old age.

Sort of like what this h*mo sapiens is finding out about himself, as I get older everything either dries up, leaks or breaks.

Jeff

Reply to
Jeff Wisnia
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I missed that whole thread. Those things need to be in a vacuum high enough that the free path of the gas inside is long compared to the dimensions of the rotor (I don't know if that means 100% or 1%, just 'long'). Perhaps it just leaked.

Reply to
Tim Wescott

I missed the thread too, but there are TWO versions of Crooke's that turn in OPPOSITE directions. At high vacuum, the reflective sides get more impulse from light and turn away. At low vacuum, the black side gets hotter and the air molecules leave the hot side faster so the black side turns away (slower). At some intermediate vacuum the forces balance and it doesn't turn.

Reply to
Nick Hull

Oooh -- I want the _pair_. Isn't the impulse from light pressure very weak, though? Can the high-vacuum version be made cheaply?

Reply to
Tim Wescott

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