We're all familiar with the kind of feedback that occurs in an engineering system when a public address microphone is placed too near a loudspeaker; the output from the speaker is picked up by the microphone and amplified further, which in turn makes the output from the speaker even louder, and so on. This positive feedback loop ultimately results in an ear-splitting howl.

Less commonly known is the negative feedback loop in which feedback operates as a component in a self-regulating system. A room thermostat is a good example. Each thermostat contains a thermometer, which measures the temperature of the air in the room, as well as an instrument enabling the user to set the desired temperature. Most important, the thermostat contains a mechanism that compares the desired temperature with the actual temperature; if the reading on the thermometer is below the desired temperature, this mechanism sends a signal to turn on the heating system. When the temperature in the room reaches the desired temperature, the signal to the heating system is turned off.