Vacuum switches apply the vacuum signal to a diaphragm that is opposed by a combination of spring forces. When the signal force is greater than the spring force it reaches its set-point (also called “On” pressure), and the internal mechanism moves and changes the electrical state. When the switch trips, the normally open circuit closes and the normally closed circuit opens. The switch will remain in this state until the vacuum signal moves back to its reset point (also called “Off” pressure). The difference between the “On” and “Off” pressure is defined as the Differential or Hysteresis of the switch.