It consists of a valve connected to a hollow sealed float by means of a lever, mounted near the top of the tank. The float is often ball-shaped, hence the name "ballcock". The valve is connected to the incoming water supply, and is opened and closed by the lever which has the float mounted on the end. When the water level rises, the float rises with it; once it rises to a pre-set level, the mechanism forces the lever to close the valve and shut off the water flow. This is an example of negative feedback and of proportional control.