Next Up Previous Contents Index
Next: Bang-bang vs. proportional Up: Food and Feedback Previous: Apply negative feedback

Avoid positive feedback

  Suppose the electrician or somebody like him makes a little boo-boo in hooking up the thermostat. Instead of the way we designed it, he wires it like this.

  As long as the temperature stays between 60 and 80, everything will be fine. On the coast of Northern California where I'm writing this book, months may pass before the error is discovered. One bright October day, however, the temperature will creep up to the 80 mark. And then, all Hell breaks loose. With a Whoomp, the furnace lights off and starts doing its best to ``remedy'' the situation. Before long the house is transformed into a broiling pan, a blast furnace, a simulacrum of summer in New Jersey (albeit without the crummy bugs). Should a chilly July day drop the temperature below 60, the inverse scenario plays out. The air conditioner springs into action, commanded by positive feedback to create its own little polar ice cap in your living room. Mush!

Here's how the temperature behaves when the thermostat's wired for positive feedback. As long as the temperature remains between 60 and 80 degrees, all is well. As the sun rises, however, the temperature crosses the 80 degree mark and the furnace starts. This quickly pushes the temperature up near the century mark and the furnace continues to roar, compounding the problem, most of the day. Only in the evening does the rapidly falling outside temperature drag the indoor temperature below 80 degrees, finally shutting off the heat. Unfortunately, the temperature soon falls below 60 and the air conditioner starts. Temperatures plunge and finally hit 40. The only thing that keeps the temperature from rising to the point of ignition or falling until the air is liquefied is the finite capacity of the furnace to heat and the air conditioner to cool, plus leakage of heat to and from the outside.

Again, consider the temperature as a ball rolling over terrain. In the absence of feedback, the terrain was flat. Negative feedback confined the ball to a valley. With positive feedback the ball rests atop a hill:

As with negative feedback, the ball is free to roam within the flat region. Upon reaching the edge, however, rather than being pushed back as negative feedback would do, positive feedback forces it further outside the stable region. In other words, once the ball rolls off the flat crest of the hill, it rolls downhill faster and faster. The system can be returned to the stable region only by pushing the ball uphill against the force of positive feedback.

When things shrug off the bonds of control and start to get weird, positive feedback is usually in the driver's seat. The very word ``feedback'' means, to many people, one of the most common examples of positive feedback: the deafening scream when a microphone  picks up a sound from a speaker and sends it back to the speaker which emits it even louder, which is picked up by the microphone....

Positive feedback makes for humorous anecdotes. But the real world contains, sadly, many examples of positive feedback put in place by well-meaning politicians and businessmen who don't understand the consequences of crossing the wires between noble ends and expedient means. Consider the nuclear arms race.   At the end of World War II, the United States and Britain realised they couldn't afford to match the Soviet army in Europe so they built nuclear bombs and threatened all-out destruction of the Soviet homeland in response to any aggression. The Soviets, faced with the ultimate threat, accelerated their own nuclear program in the belief that only by matching or surpassing the West could they deal on an equal basis. Once the Soviets tested their bomb, hysteria in the West was compounded--now they faced an adversary armed with the Bomb as well as a huge army. The only way to deter the Soviets from using their bomb was to build mo' bigger bombs. And so on, as fear fed on fear, technological developments like the ballistic missile spurred further developments and countermeasures by the other side, the whole process consumed a substantial fraction of all the world's wealth for decades. This tragedy of our age is, at its heart, a case of positive feedback, pure and simple. And like most examples of positive feedback it was stopped (if indeed it has ceased and not just gone on vacation), not by good will, not through mutual understanding and trust, not by the actions of diplomats and politicians, but by the application of stronger negative feedback. The negative feedback in the arms race was economic: the race grew to the point where it virtually bankrupted the participants who were, only then, compelled to slow down.

Once you begin to analyse things in terms of positive and negative feedback, you'll never see things quite the same.

Next Up Previous Contents Index
Next: Bang-bang vs. proportional Up: Food and Feedback Previous: Apply negative feedback

By John Walker