When this device is wired with your Aprilaire models 400, 600 or 700 it will turn on a "Filter Change Reminder Light" as needed.  Your humidifier has a filter (also sometimes called a pad) that needs to be changed once or more per year, depending which model you have, if you have hard or soft water, and how long your heating season is. Using this current sensing relay will turn on the humidifier indicator light to let you know it is time to change the filter - you don't have to remember.

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.