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What was this truck-mounted widget?

  1. Jan 21, 2010 #1


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    I saw a van downtown that had a spinny widget on top. (The van belonged to a company called Carmichael. It had extension ladders on its roof.)

    Anyway, this widget looked like a black stereo knob but six inches in diameter - a circular base with a bar across it. It spun at several rotations per second, like the short-wave radar bar seen on boats.

    Its rotational speed seemed to be correlated with engine speed but I'm not positive.

    Anyway, my hypotheses:
    - a wind-driven cooling fan for the interior
    - a GPS tracker

    [ EDIT ] Ah. There it is.
    Ironic. I was actually Googling for pictures of van rooves so I could PhotoShop a pic for clarity. And this pic popped up...
    Last edited: Jan 21, 2010
  2. jcsd
  3. Jan 22, 2010 #2


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    Air vent.
  4. Jan 22, 2010 #3


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    I was going to venture a small vent fan.
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