The current sensing relay senses a current in your furnace's 110V fan power line by proximity. Simply run the fan wire through the top loop of the relay and when the fan turns on, the relay closes, allowing the 24v power on the relay side to flow through and turn the humidifier on (when humidistat controller also calls for it to be on.) View diagrams #2 and #5 here to see how to easily wire it with your humidifier.