Power Switching Devices Apply or Interrupt Electric Power To A Controlled Device. Basic Types Of Switching Devices Are Electro-Mechanical Contactors (Relays) That Switch Electric Power Through Physical Contacts, or Solid State Relays, And SCR Contactors That Switch Power Electronically, And Mercury Relays (Contactors) That Switch Power By Changing The Mercury Level In A Tube Reservoir With A Plunger Activated By A Solenoid Coil.