I'm trying to build a very simple, inexpensive way to sense airflow in a duct. I saw a device that was about the size and physical appearance of a spark plug, the lower portion of which protruded into a duct and appeared to function by being contacted by a swinging metal gate which was moved by the air flowing by the device when a fan was running. The portion of the device outside the duct had a single cable attached, much like a spark plug wire The purpose of the device was to insure that the fan was running. Any idea what such a device might be called?

Sounds like some sort of sail switch. Do a web search for sail switch images, see if you can find a device that looks like what you have.