Many windmill manufacturers made tank regulators for their windmills and the Aermotor Company was no exception. The general principal of a tank regulator is that once the tank was full of water, the windmill would be automatically furled by this device to make it stop pumping, and then when the water level in the tank dropped, the mill would start pumping again.
According to a company brochure, This regulator is simple in design and reliable in operation. It can be set up just above the pump where the operating lever will connect with the pump rod. It stops the windmill when the tank is full and starts it again when the water falls a few inches. If the operating wire is properly supported, the tank may be several hundred feet away.