Dust motes

Sitting in a status meeting this morning, I found myself staring at the particles of dust illuminated by the beam of the overhead projector. They were all moving in the same direction — toward the lens, like moths drawn to the light. And I realized that when I’ve watched dust in a projector beam in the past, it has always moved in that direction. There must be a physical reason for this.
Here’s my theory: The projector heats the air around it, which rises. Cooler air then flows toward the projector to replace the departed warm air. Therefore, once a projector has been on long enough to heat up, the air in front of it will always be moving toward the lens, carrying dust with it.
Unfortunately, I haven’t had the opportunity to test this theory. The simplest way would be to light a couple of cigarettes, hold them near the projector, and observe the behavior of the smoke. But you can’t do that where I work. A bowl of dry ice would probably work just as well, but how do I explain bringing that to a status meeting? Guess I’ll have to figure out another method.

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