If it was a little black box with 2 yellow wires coming out of it, it is a current sensing relay that will activate the humidifier when the furnace blower comes on. It has a metal bracket with a scew going through the top of it. The common wire for the blower motor gets wrapped 2 times around the bracket and will cause it to sense the current of the blower motor. It probably is a no. 50 relay made by april aire. You dont even need one for that furnace as on the circuit board in the blower compartment there is a terminal marked ACC. You use this terminal to power the transformers 110v side. It is only powered when the blower comes on.