negative feedback
Oct. 5th, 2006 06:37 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
miriam_eNo, I don't mean when someone criticises you. Negative feedback is an engineering term that describes an incredibly common thing. Life uses it all the time, and so do engineers.
Thermostats switch the heater off when the temperature exceeds a set amount, and back on when the temperature drops. Steam engines use a centrifugal governor to close a steam valve when the rotational speed exceeds a limit, and open it again when speed is lower. The halogen gas in special light bulbs takes up vaporised tungsten, and instead of letting it condense out on the cold glass, it preferentially deposits it at very hot sites -- the hottest, thinnest parts of the filament, where tungsten atoms are boiling off ['boiling' is the wrong word, but you get the idea].
Two things got me thinking about negative feedback today.
One was when, instead of working, I was watching birds fly. Have you noticed how birds keep their wings in a V shape? That uses a very simple, but delightfully elegant form of negative feedback. That simple shape keeps the bird's flight stable. If the bird gets destabilised and tipped to the side, then the force upward on the more horizontal wing is greater than the upward force on the more vertical wing, which automatically rights the bird without it having to do anything. Isn't that beautiful?
The other thing that made me think about negative feedback was I was thinking about how we stay upright (I was actually working this time). We don't need to think about how we do it because most of it is done by a feedback loop between our leg muscles and our spinal cord. When a muscle stretches it sends a message to the spinal cord which tells the muscles on the other side of the leg to relax and simultaneously, another message to the stretching muscles to tighten up. This maintains your position without having to send much info to the brain. The brain can then concentrate on finer adjustments without dealing with the low-level stuff.
Thermostats switch the heater off when the temperature exceeds a set amount, and back on when the temperature drops. Steam engines use a centrifugal governor to close a steam valve when the rotational speed exceeds a limit, and open it again when speed is lower. The halogen gas in special light bulbs takes up vaporised tungsten, and instead of letting it condense out on the cold glass, it preferentially deposits it at very hot sites -- the hottest, thinnest parts of the filament, where tungsten atoms are boiling off ['boiling' is the wrong word, but you get the idea].
Two things got me thinking about negative feedback today.
One was when, instead of working, I was watching birds fly. Have you noticed how birds keep their wings in a V shape? That uses a very simple, but delightfully elegant form of negative feedback. That simple shape keeps the bird's flight stable. If the bird gets destabilised and tipped to the side, then the force upward on the more horizontal wing is greater than the upward force on the more vertical wing, which automatically rights the bird without it having to do anything. Isn't that beautiful?The other thing that made me think about negative feedback was I was thinking about how we stay upright (I was actually working this time). We don't need to think about how we do it because most of it is done by a feedback loop between our leg muscles and our spinal cord. When a muscle stretches it sends a message to the spinal cord which tells the muscles on the other side of the leg to relax and simultaneously, another message to the stretching muscles to tighten up. This maintains your position without having to send much info to the brain. The brain can then concentrate on finer adjustments without dealing with the low-level stuff.