This Aprilaire Manual Humidifier Control is designed for low voltage service to control Aprilaire humidification equipment.  An increase in relative humidity expands the nylon ribbon which opens the control switch to stop operation of the humidifier.  A decrease in relative humidity reverses the process and closes the control switch.  The Aprilaire 4655 Control can be mounted on a wall in the living space, or on the return air duct.