The next step toward controlling something is choosing the objective, expressed in terms of the numbers we're measuring. Suppose we'd like to arrange things so the house never gets colder than 60° Fahrenheit nor warmer than 80° . We can modify the thermometer by adding electrical contacts at the 60 and 80 degree points on the scale, like this.
When the needle touches one of these contacts, it will close a circuit. The circuit remains closed as long as the temperature is outside the range of 60 to 80 degrees. We can wire this modified thermometer up to a pair of indicator lights. When the temperature drops below 60° the ``Too cold!'' light will come on, and when it exceeds 80° , the ``Too hot!'' indicator illuminates.
This apparatus monitors the temperature, detects when it's outside the range we're happy with, and lets us know what the problem is (too hot or too cold). We can think of this refinement as having drawn limit lines on the temperature graph, like this.
Whenever the temperature is above the top line, the Too hot! light will go on, and whenever it falls below the bottom line, Too cold! will be signaled.