The metaphor he chose is remarkable, because it is true that at one time the church had an immense influence on the well being of the population by preaching the right, and righting the wrong, very much like a thermostat, which regulates air temperatures by supplying a feedback. A thermometer measures shows temperature, but takes no action. Martin Luther King Jr. used this metaphor to hint that the church should not be simply sitting around watching (like a thermometer), but should take action to correct injustice (like a thermostat).