This thermal switch turns on the blower fan when the water is boiling. The switch is simply wired to the terminal block on the side of the humidifier control box. The switch is normally open and closes when heated.
The switch body measures 5/8" in diameter and 3/8 thick. The stud is threaded for an 8-32 nut. The nut and a plastic washer is included. The two spade terminals are 1/4".
For a wiring diagram see page 5 of the Skuttle Wiring Manual.