Most of you are aware of what feedback is. For those who aren't, it's what you get when you feed the output of a system into the input of the same system. There are two types. Positive feedback is what happens when the output reinforces the input. You know it as the howl you get when shoving the microphone of the PA system into the speaker. Positive feedback generally is unstable. The other kind, negative feedback, happens when feeding the output back in reduces the input. This generally results in stability.
Not always, though. The LA and I have a light on our balcony. It's on a light sensor as well as the switches, so when the switches are on, it still won't light unless it's dark. However, there is a negative feedback loop which occurs at a certain stage of the evening. "Oh", says the sensor, "it's dark. I should allow the light to come on." This occurs, and "Oh", says the sensor, "it's not dark. I should turn the light off."
It runs at about 2Hz. Negative feedback producing instability. Who'd have thought?
I'm attempting to distract myself. It's sort of working.